MATERIALS TRANSACTIONS
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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678

MATERIALS TRANSACTIONS Vol. 45 (2004), No. 3

  • Thermal Characteristics and Intermetallic Compounds Formed at Sn-9Zn-0.5Ag/Cu Interface

    pp. 606-613

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    DOI:10.2320/matertrans.45.606

    The thermal characteristics of various Sn-based solder alloys and intermetallic compounds (IMCs) formed at the Sn-9Zn-0.5Ag/Cu interface have been investigated by using differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy, energy dispersive spectrometry, transmission electron microscopy and electron diffraction. The melting ranges of the Sn-37Pb, Sn-9Zn and Sn-3.5Ag alloys are 179.5∼191.0, 195.5∼208.1 and 220.4∼227.8°C, and the heats of fusion are 104.2, 163.9 and 151.0 J/g, respectively. When 0.5 mass% Ag is added to the Sn-9Zn alloy, the melting temperature of the solder alloy increases from 195.5 to 196.7°C, but the melting range and heat of fusion decrease from 12.6 to 11.3°C and 163.9 to 74.7 J/g, respectively. The IMCs formed at the Sn-9Zn-0.5Ag/Cu interface are determined as a scallop-shaped Cu6Sn5 near the solder alloy, a flat Cu5Zn8 close to the Cu substrate and Ag3Sn particles between the Cu substrate and Cu5Zn8 layer. The Cu6Sn5 is bi-structural, namely, hexagonal and monoclinic, which is caused by the Ag dissolution in the Cu6Sn5 layer.
  • Thermodynamic Study of Phase Equilibria in the Sn-Ag-Bi-Zn Quaternary System

    pp. 614-624

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    DOI:10.2320/matertrans.45.614

    The phase equilibria in the Sn-Ag-Bi-Zn quaternary system have been studied experimentally and using thermodynamic calculations. The determined values of the thermodynamic parameters of the Sn-Ag-Zn and Sn-Ag-Bi systems were applied in the calculation of the phase diagrams. Thermodynamic evaluation of the Sn-Bi-Zn and Ag-Bi-Zn systems was performed by considering a two-phase separation of the liquid phase. The phase boundaries in some vertical sections of the quaternary system were determined using differential scanning calorimetry to confirm the calculated results. The calculation of the phase diagrams when 10%Bi was added to Sn-Ag-Zn alloy shows that the regions of the primary crystals in the quaternary system did not show a large discrepancy from the Sn-Ag-Zn ternary system. The eutectic temperature decreased to about 203.7°C on addition of Bi to Sn-Ag-Zn alloy. The rate of change was estimated to be 1°C per 1% Bi added. The solidification structure was investigated using scanning electron microscopy and energy dispersive X-ray microanalysis. The microstructure was composed of an Sn-Bi-based eutectic, Ag5Zn8, Ag3Sn, and coarsened Bi of about 50 μm. Based on these results, a non-equilibrium solidification process using the Scheil model was simulated and compared with the observed structures. Our calculations reasonably explain Bi-enrichment in the final solidification zone.
  • Effect of Oxygen on Surface Tension of Liquid Ag-Sn Alloys

    pp. 625-629

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    DOI:10.2320/matertrans.45.625

    The effect of oxygen on the surface tension of liquid Ag-Sn alloys used as one of the main components in lead-free soldering alloys was measured by the sessile drop method. The surface tension of liquid Ag-Sn alloys decreased with increasing tin content. The effect of oxygen was investigated for oxygen partial pressure ranging from about 10-17 to 10-12 atm. The effect of oxygen on the surface tension of liquid Ag-Sn alloys is closer to pure tin rather than pure silver. Thermodynamic simulation using Butler's equation showed that the surface of liquid Ag-Sn alloys is enriched with tin. Accordingly, the effect of oxygen adsorption on the surface tension of liquid Ag-Sn alloys was considered to be closer to that of tin than silver.
  • Some Fundamental Issues in the Use of Zn-Containing Lead-Free Solders for Electronic Packaging

    pp. 630-636

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    DOI:10.2320/matertrans.45.630

    A synergistic approach is applied to address the major concerns about the use of Zn-containing lead-free solders for electronic packaging. Using computational thermodynamics as a predictive tool, the phase stability of the Ag-Al-Cu-In-Sn-Zn system is examined to design a Zn-containing lead-free solder with melting characteristics similar to near-eutectic Pb-Sn solder. Theoretically, it is found that a Sn-0.3 mass%Al-4.2 mass%In-7.8 mass%Zn solder has a melting point (liquidus temperature) of 185°C and a solidification range of 10°C. It is demonstrated that environmentally benign fluxes containing tin-organometallics significantly improve the wetting behavior compared to rosin fluxes used for lead-tin solders. For the Sn-Zn eutectic solder on a Cu substrate at 260°C, it is found that the contact angle is reduced from 150° to about 25° when tin-organometallic fluxes are used instead of rosin flux. Severe accelerated tests (85% relative humidity at 85°C) for up to six weeks show that the mechanical properties of Sn-Zn eutectic solder interconnects are not affected adversely by the environment.
  • Thermodynamic Calculations of Phase Equilibria, Surface Tension and Viscosity in the In-Ag-X (X=Bi, Sb) System

    pp. 637-645

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    DOI:10.2320/matertrans.45.637

    The phase equilibria of the In-Ag-Bi and In-Ag-Sb systems were determined by differential scanning calorimetry (DSC) and electron probe microanalysis (EPMA). Thermodynamic calculations of these systems were also carried out by taking the experimental results into account. The Gibbs energies of the liquid and solid solution phases are described on the basis of the sub-regular solution model, and that of the intermetallic compounds are based on the two-sublattice model. A consistent set of thermodynamic parameters was optimized for describing the Gibbs energy of each phase, which leads to a good fit between calculated and experimental results. In addition, the surface tension and viscosity of liquid phase were calculated on the basis of the thermodynamic parameters obtained in the present assessment.
  • Interfacial Reaction and Morphology Between Molten Sn Base Solders and Cu Substrate

    pp. 646-651

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    DOI:10.2320/matertrans.45.646

    The morphologies and growth of ε(Cu3Sn) and η(Cu6Sn5) intermetallic compounds (IMCs) between a molten Sn base solder and a Cu substrate were experimentally investigated. It is shown that the thickness of the ε(Cu3Sn) and η(Cu6Sn5) compounds decreases with deceasing Sn content and that the order of the growth rate of the compounds on the Cu substrate are as follows: Sn-57(mass%)Bi < Sn-37Pb < Sn-3.5Ag < Sn < Sn-6.7Sb. The growth of these phases basically obeys the parabolic law, but the growth behavior is divided into two stages, the growth rate and morphology of the η(Cu6Sn5) compound are differing from each other in the two-stage. It is suggested that the grooving effect is at least one of the origins of the formation of the scallop morphology of the η(Cu6Sn5) compound.
  • Surface Tension and Density Measurements of Sn-Ag-Sb Liquid Alloys and Phase Diagram Calculations of the Sn-Ag-Sb ternary system

    pp. 652-660

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    DOI:10.2320/matertrans.45.652

    The maximum bubble pressure method has been used to measure the surface tension of pure antimony and the surface tension and density (dilatometric method) of Sn-3.8 at%Ag eutectic base alloys with 0.03, 0.06 and 0.09 molar fraction of antimony at a temperature range from 550 to 1200 K. The linear dependencies of surface tension and density on temperature were observed and they were described by straight-line equations. Moreover, experimental determination of phase diagram and thermodynamic calculations in the Sn-Ag-Sb system were performed and the resulting optimized thermodynamic parameters were used for modeling of the surface tension. In addition, a non-equilibrium solidification process using the Scheil model was simulated and compared with the equilibrium solidification behavior of a Sn-Ag-Sb alloy.
  • Electromigration Effects upon Interfacial Reactions in Flip-Chip Solder Joints

    pp. 661-665

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    DOI:10.2320/matertrans.45.661

    Lead-free flip-chip joining processes have attracted the most research interests recently, and inhomogeneous interfacial reactions were observed in the flip chip solder joints. The Cu6Sn5 phase with high nickel solubility, (Cu,Ni)6Sn5 phase, was formed at the substrate side adjacent to the Ni layer in a flip-chip joint with a substrate/Ni/solder/Cu/(Ni,V)/die structure. A Sn/Cu/Sn/Ni/Sn/Cu/Sn couple was prepared to simulate the flip-chip joint. At 200°C, the Cu6Sn5 phase was formed on both ends of the Sn phase at the Cu/Sn and Sn/Ni interfaces for the couple with an electric current stressing or a longer reaction time, but the nickel contents of the two Cu6Sn5 phases are different. The Cu6Sn5 phase at the Ni side has high nickel content and it has almost no nickel at the Cu side. It is concluded that the Cu at the chip-side diffused through the solder phase, reacted with the Ni layer at the substrate side, and the Cu6Sn5 phase with high nickel solubility, i.e. the (Cu,Ni)6Sn5 phase, was thus formed. Electromigration effects significantly enhance the diffusion rate of Cu, but do not alter the phase formation sequence.
  • Vibration Fracture Behavior of Sn-Bi Solder Alloys with Various Bi Contents

    pp. 666-672

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    DOI:10.2320/matertrans.45.666

    This study investigated the resonant vibration properties of Sn-Bi solders with hypoeutectic (Sn-10 mass%Bi and Sn-30 mass%Bi, referred to as 10Bi and 30Bi respectively) and eutectic (Sn-58 mass%Bi, referred to as 58Bi) compositions. Results show that the damping capacity of the 58Bi specimen with continuous distributed eutectic Bi phase was higher than Sn-30Bi, and higher than Sn-10Bi with isolated Bi precipitates. Also, the specimens with higher damping capacity possessed greater vibration fracture resistance under a fixed vibration force. When the vibration was conducted with a constant initial deflection, the vibration fracture resistance decreased in the order of 30Bi, 10Bi and 58Bi. Interfriction at eutectic Sn/Bi phase boundaries can be regarded as an effective mechanism in absorbing vibration energy. In the case of 58Bi and 30Bi, the crack propagated mainly along eutectic-Bi/Sn interfaces and occasionally through the eutectic Bi. As for the 10Bi specimen, the crack propagation path was mainly intergranular and this can be attributed to fast crack linking by Bi precipitates on the eutectic cell boundaries. In addition, microstructural coarseness caused by artificial aging reduced the area and continuity of eutectic Sn/Bi interfaces and thus decrease damping capacity and change the crack propagation morphology.
  • Improvement on Thermal Fatigue Properties of Sn-1.2Ag-0.5Cu Flip Chip Interconnects by Nickel Addition

    pp. 673-680

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    DOI:10.2320/matertrans.45.673

    The thermal fatigue properties of Sn-1.2Ag-0.5Cu (in mass%) flip chip interconnect were improved by a small amount of nickel addition. The thermal fatigue resistance of SnxAg-0.5Cu flip chip interconnects was enhanced by addition of 0.05 mass%Ni, and Sn-1.2Ag-0.5Cu-0.05Ni had longer thermal fatigue life than Sn-1.2Ag-0.5Cu. Cracks developed near solder/chip interface for all the bumps tested. This crack propagation is mainly governed by the nature of the solders themselves because a strain concentrated area was similar for all the tested alloys independent of the chemical contents. From the microstructural observation, fracture in Sn-1.2Ag-0.5Cu-0.05Ni due to thermal strain was a mixed mode, both transgranular and intergranular. From SEM and TEM analyses, fine Ag3Sn and (Cu,Ni)6Sn5 formed network around Sn grains in the initial microstructure of Sn-1.2Ag-0.5Cu-0.05Ni solder. Sn-1.2Ag-0.5Cu-0.05Ni solder joint suppressed coarsening of Sn grains even after thermal fatigue test. Namely, thermal fatigue properties of the Sn-1.2Ag-0.5Cu-0.05Ni solder joint is correlated to its microstructure, and the joint had longer fatigue life in spite of its lower silver content of 1.2 mass% due to both fine Sn matrix in the initial state and suppression of Sn grain coarsening even after thermal fatigue test.
  • Thermal Fatigue Properties of Sn-1.2Ag-0.5Cu-xNi Flip Chip Interconnects

    pp. 681-688

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    DOI:10.2320/matertrans.45.681

    Thermal fatigue properties of Sn-1.2Ag-0.5Cu-xNi (x: 0.02, 0.05, 0.10 and 0.20 mass%) flip chip interconnects was investigated to find out an ideal nickel composition. Sn-1.2Ag-0.5Cu-xNi (x: 0.05, 0.10 and 0.20) had longer thermal fatigue life than Sn-1.2Ag-0.5Cu-0.02Ni. Cracks developed near solder/chip interface for all the bumps tested, and fracture due to thermal strain was a mixed mode, both transgranular and intergranular. Sn grain growth after thermal cycling was suppressed for Sn-1.2Ag-0.5Cu-xNi (x: 0.05, 0.10 and 0.20) solder joints, while Sn grains grew faster for x = 0.02, which suggests that suppression of Sn grain growth enhanced thermal fatigue endurance. Therefore, thermal fatigue properties of Sn-1.2Ag-0.5Cu-xNi (x: 0.02, 0.05, 0.10 and 0.20) solder joint correlated to its microstructure, and the addition of 0.05 mass%Ni is required to enhance thermal fatigue endurance.
  • Low Cycle Fatigue Properties of Ni added Low Silver Content Sn-Ag-Cu Flip Chip Interconnects

    pp. 689-694

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    DOI:10.2320/matertrans.45.689

    The straddle fatigue test has been performed to study the fatigue properties of Sn-1.2 mass%Ag-0.5 mass%Cu-0.05 mass%Ni for flip chip interconnections. The low cycle fatigue resistance of the alloy is equivalent to that of Sn-3 mass%Ag-0.5 mass%Cu alloy, even though the fatigue endurance of Sn-1 mass%Ag-0.5 mass%Cu alloy was poorer than that of the 3 mass%Ag alloy. The alloy has fine microstructure and Ag3Sn intermetallic compound makes a network structure together with fine (Cu,Ni)6Sn5 compound. The microstructure resulted in high cyclic strain hardening exponents, which leaded to good low cycle fatigue endurance of the alloy.
  • The Microstructure, Thermal Fatigue, and Failure Analysis of Near-Ternary Eutectic Sn-Ag-Cu Solder Joints

    pp. 695-702

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    DOI:10.2320/matertrans.45.695

    The electronic industry is making substantial progress toward a full transition to Pb-free soldering in the near future. At present, the leading candidate Pb-free solders are near-ternary eutectic Sn-Ag-Cu alloys. The electronic industry has begun to study both the processing behaviors and the thermo-mechanical fatigue properties of these alloys in detail in order to understand their applicability in context of current electronic card reliability requirements. In recent publications, the solidification behavior of the near-ternary eutectic Sn-Ag-Cu alloys has been reported in terms of the formation of large Ag3Sn plates and their effects on mechanical properties of Pb-free solder joints. Several methods have been employed to minimize the growth of the large Ag3Sn plates in the Sn-Ag-Cu solder joints by controlling the cooling rate during solidification, reducing Ag and/or Cu content, or adding minor alloying elements which reduce the amount of undercooling required for the nucleation of tin dendrites. In the present study, the results of accelerated thermal cycle fatigue tests are reported with the near-ternary eutectic Sn-Ag-Cu alloys of reduced Ag contents. Changes in microstructure and mechanical properties are also discussed by comparing the solder joints before and after thermal cycling.
  • Novel Ultrasonic Soldering Technique for Lead-Free Solders

    pp. 703-709

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    DOI:10.2320/matertrans.45.703

    Sn-Bi alloy is one of the representative low temperature type lead-free solders. However, the bonding properties of the Sn-Bi solder are not good. The reason for such properties is related to Bi microcrystallines that segregate at the interface between the solder and a Cu substrate. We found that ultrasound improves the bonding strength for the Sn-Bi alloy system solders by dispersing and miniaturizing the Bi crystals. To achieve such dispersion, we invented a novel ultrasonic soldering technique. By using this technique, ultrasound can be applied to printed wiring boards (PWB). Besides the improved bonding strength, we found that the temperature of a PWB is increased by the application of ultrasound to the PWB. Sn-58 mass%Bi solder is melted by the vibrational energy of ultrasound without other heating methods. Moreover, the interfacial layer between the Sn-58Bi solder and the Cu land is homogenized by ultrasound. Also, the interfacial layer between the Sn-8Zn-3Bi solder and a Cu land becomes thinner by ultrasound. We believe that these changes in the interfacial structure improve the mechanical properties of the solders. Therefore, ultrasonic soldering technique will improve the usability and reliability of Sn-Bi alloy system solders.
  • Abnormal Grain Growth of Ni3Sn4 at Sn-3.5Ag/Ni Interface

    pp. 710-713

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    DOI:10.2320/matertrans.45.710

    By the reaction of molten solder alloy with compositions of 96.5Sn-3.5Ag (compositions are all in weight percent unless specified otherwise) with either the thick Cu or the thick Ni substrate at 250°C, the rounded Cu6Sn5 grains formed over Cu and the faceted Ni3Sn4 grains precipitated over Ni. As the soldering time changed from 1 min to 60 min, normal grain growth occurred for rounded Cu6Sn5 grains while abnormal grain growth (AGG) mode was observed for faceted Ni3Sn4 grains. The measured grain size distributions also confirmed the difference between normal grain growth and abnormal grain growth.
  • Effect of Cu Addition to Sn-Ag Lead-Free Solder on Interfacial Stability with Fe-42Ni

    pp. 714-720

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    DOI:10.2320/matertrans.45.714

    The interfacial reactions and the interface microstructures between Sn-3.5Ag-(0.7Cu) lead-free solders and Fe-42Ni substrate were investigated at the reaction temperature of 250°C. Joint strength was also evaluated. The Sn-3.5Ag joint shows the double reaction layers of FeSn2. Ni from Fe-42Ni dissolves into molten Sn-3.5Ag solder during soldering and forms Ni3Sn4 compounds with the eutectic network of Ag3Sn/β-Sn in solder layer during solidification. For the Sn-3.5Ag-0.7Cu jont, the Cu addition in Sn-3.5Ag changes the microstructure of solder and interfacial reaction layers. Ni dissolved into solder melt reacts with Sn and Cu to form a η-phase of Cu-Ni-Sn compound during the soldering without the formations of Ni3Sn4 compound. At the reaction interface, fine particles of FeSn2 can be found near to the reaction layer instead of the faceted compound of FeSn2 in the second reaction layer for the Sn-Ag joint. Sn-3.5Ag-0.7Cu joint increases the joint strength by about 30—40 MPa higher than that of Sn-3.5Ag joints.
  • Microstructures and Shear Strength of Interfaces between Sn-Zn Lead-free Solders and Au/Ni/Cu UBM

    pp. 721-726

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    DOI:10.2320/matertrans.45.721

    Microstructure and shear strength of Sn-Zn lead-free solders and Au/Ni/Cu UBM joint under thermal aging conditions were investigated. The samples were aged isothermally at 373 K and 423 K for 300, 600, and 900 hours. The IMCs at the interface between solder and UBM were examined by FESEM and TEM. The results showed the shear strength was decreased with aging time and temperature. The solder ball with highly activated flux had about 8.2% increased shear strength than that of BGA/CSP flux. Poor wetting and many voids were observed in the fractured solder joint with of the latter flux. The decreased shear strength was caused by IMC growth and Zn grain coarsening. In the solder layer, Zn reacted with Au and then was transformed to the β-AuZn compound. At the joint interface, although AuZn grew first, γ -Ni5Zn21 compounds were formed with aging time.
  • IMC Growth and Shear Strength of Sn-Ag-Bi-In/Au/Ni/Cu BGA Joints During Aging

    pp. 727-733

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    DOI:10.2320/matertrans.45.727

    The growth kinetics of intermetallic compound (IMC) layers formed between Sn-3Ag-6Bi-2In ball-grid-array (BGA) solder and Au/Ni/Cu substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0 to 100 days. A quantitative analysis of the IMC layer thickness as a function of time and temperature was performed. The intermetallic layer exhibited a parabolic growth at the given temperature range. Because the values of the time exponent (n) are approximately 0.5, the layer growth of the IMC was primarily controlled by diffusion over the temperature range studied. The apparent activation energy value calculated for the Sn-Ag-Bi-In/Au/Ni/Cu BGA joint was 64.8 kJ/mol. Also, the reliability of the solder ball attachment was characterized by mechanical ball shear tests. The brittleness of the solder joints increased with increasing aging temperature and time, and the fracture occurred within the IMCs and Ni layer. The deterioration of the solder ball shear strength was found to be predominantly caused by the formation of the IMC layer.
  • Influence of the Interfacial Reaction Layer on Reliability of CSP Joints Using Sn-8Zn-3Bi Solder and Ni/Au Plating

    pp. 734-740

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    DOI:10.2320/matertrans.45.734

    In order to investigate the use of Sn-8Zn-3Bi solder as a potential substitute for Sn-Pb solder, which has a lower melting point than Sn-Ag family solders for CSP assembly, we studied mechanical properties of CSP joints plated with varying thicknesses of Au and Ni on Cu pad. Joint strength and other mechanical properties were evaluated in relation to reflow peak temperature. The combination of 0.05 μm Au plating thickness and reflow peak temperature of 498 K resulted in the best joint reliability in the as reflowed condition and also after aging treatment. The joint was founded to have thin Ni3Sn4 type interfacial reaction layer that included Cu and Zn between the solder and the Ni plating. This interfacial structure was shown to improve the joint strength.
  • Effect of Iron Plating Conditions on Reaction in Molten Lead-Free Solder

    pp. 741-746

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    DOI:10.2320/matertrans.45.741

    To demonstrate the dissolution of plated iron in molten lead-free solder and the effect of iron-plating conditions on reactions in molten lead-free solder, the reaction test between Sn-3.0Ag-0.5Cu (mass%) solder and plated iron was performed. An Iron-plated copper plate, which was made by electroplating iron onto an oxygen-free copper substrate, was used as the test piece. The reaction test was carried out by using an oven in normal air. The solder was placed on the test piece and it was put into an oven held at 400, 430 and 460°C. The interface between the solder and plated iron was particularly examined. It was found that the intermetallic compound of FeSn2 was formed at the interface regardless of the plating conditions. The results showed that the grain size of plated iron decreased with the increased current density and the dissolution thickness of plated iron in molten lead-free solder increased with the increased current density in the rack plating. In the barrel plating, the grain size was rather small in all the test pieces and the dissolution thickness was rather thick. Thus it has been made clear that the dissolution of plated iron in molten lead-free solder is attributable to the grain size of plated iron.
  • Influence of Noble Metal Coating on Wettability of Copper Substrate by Sn-Ag Eutectic Solder

    pp. 747-753

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    DOI:10.2320/matertrans.45.747

    The influence of thin coatings (70 nm) of Au, Ag and Pd on the wettability of a Cu substrate by a Sn-3.5Ag eutectic solder was investigated using the contact angle measuring system combined with meniscograph tester. The wettability of the Cu substrate with Sn-3.5Ag was improved by the Au and Ag coatings, while reduced by a Pd coating, especially in terms of the contact angle. Namely, the contact angles were 29—30° on the Au coated-Cu substrate, 34—35° on the Ag coated-Cu substrate, 49—52° on the Pd coated-Cu substrate and 42—45° on the uncoated Cu substrate. On the wetted area, each coating layer dissolved into the solder and the Cu-Sn intermetallic compound was formed at the solder/substrate interface as well as on the uncoated Cu substrate, but the coating metal layers remained on the unwetted area. The difference in wettability of the noble metal coating on the Cu substrate was found to be attributed to the difference in the substrate-flux interfacial tensions (γsf).
  • Growth Kinetics of Reaction Layers in Flip Chip Joints with Cu-cored Lead-free Solder Balls

    pp. 754-758

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    DOI:10.2320/matertrans.45.754

    The growth kinetics of the reaction layers formed in the flip chip joints using Cu-cored Sn-5 mass%Ag balls was investigated. In particular, the effect of Ni coating over the Cu core was investigated on the microstructures of flip chip joints with Ni/Au plated Cu pads after reflow soldering and the subsequenct heat exposure at 373 K, 398 K and 423 K. A Ni-Sn reaction layer formed at the Cu core/Sn-5Ag interface in the joint using the Cu-cored Sn-5Ag ball with Ni coating over the Cu core with reflow soldering. A similar Ni-Sn reaction layer formed at the joint interface between the Sn-5Ag solder and the Ni/Au plated Cu pad with reflow soldering. The growth rates of those Ni-Sn reaction layers following the subsequent heat exposure were slower than that of the Cu-Sn reaction layer formed at the Cu core/Sn-5Ag interface in the joint with the Cu-cored Sn-5Ag ball without Ni coating over the Cu core. In this study, the activation energies of the growth of the reaction layers and the shear load of the joint were also investigated.
  • Application of Thermal Latent Carboxylic Acid Derivatives to Pb-Free Microsoldering

    pp. 759-764

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    DOI:10.2320/matertrans.45.759

    Pb-free microsoldering technology has become important, because complete elimination of lead from electric devices is soon to be enforced. In this work, we studied the application of thermal latent technology of a carboxyl group to Pb-free microsoldering. We found that thermal latent carboxylic acid derivatives could both prevent degradation of a solder alloy under normal conditions and serve as a novel activator when a solder alloy begins to melt. Furthermore, we found that the activator in the flux residue after soldering could be converted into an ester unit which has no influence on electric reliability. Solder paste using thermal latent carboxylic derivatives can be combined to achieve good wettability, high electric reliability and high storage stability, and can be useful for Pb-free microsoldering in response to environmental protection demands.
  • Development of Sn-Based, Low Melting Temperature Pb-Free Solder Alloys

    pp. 765-775

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    DOI:10.2320/matertrans.45.765

    Low temperature, Sn-based Pb-free solders were developed by making alloy additions to the starting material, 96.5Sn-3.5Ag (mass%). The melting behavior was determined using Differential Scanning Calorimetry (DSC). The solder microstructure was evaluated by optical microscopy and electron probe microanalysis (EPMA). Shear strength measurements, hardness tests, intermetallic compound (IMC) layer growth measurements, and solderability tests were performed on selected alloys. Three promising ternary alloy compositions and respective solidus temperatures were: 91.84Sn-3.33Ag-4.83Bi, 212°C; 87.5Sn-7.5Au-5.0Bi, 200°C; and 86.4Sn-5.1Ag-8.5Au, 205°C. A quaternary alloy had the composition 86.8Sn-3.2Ag-5.0Bi-5.0Au and solidus temperature of 194°C. The shear strength of this quaternary alloy was nearly twice that of the eutectic Sn-Pb solder. The 66Sn-5.0Ag-10Bi-5.0Au-10In-4.0Cu alloy had a solidus temperature of 178°C and good solderability on Cu. The lowest solidus temperature of 159°C was realized with the alloy 62Sn-5.0Ag-10Bi-4.0Au-10In-4.0Cu-5.0Ga. The contributing factor towards the melting point depression was the composition of the solid solution, Sn-based matrix phase of each solder.
  • Microstructures, Thermal and Tensile Properties of Sn-Zn-Ga Alloys

    pp. 776-782

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    DOI:10.2320/matertrans.45.776

    The effects of Ga content on the microstructure, thermal behavior and mechanical properties of Sn-Zn eutectic alloy were examined in this study. Results show that Ga was dissolved in both Sn and Zn phases. This gave rise to irregular eutectic structure with misaligned, less distributed massive Zn-rich phase, relatively low melting point, and solid solution strengthening effect. Due to the inhomogeneous dissolution feature of Ga in Sn matrix, Sn-Zn-Ga alloys exhibit a broad melting range and an alternate normal-irregular eutectic structure. Notably, the addition of Ga into the Sn-Zn alloy will improve the tensile strength without reducing the ductility when the Ga content ranges from 0.05 to 1 mass%.
  • Microstructure and Strength of Sn-Bi Coated Sn-3.5 mass%Ag Solder Alloy

    pp. 783-789

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    DOI:10.2320/matertrans.45.783

    Sn-Bi coated Sn-3.5 mass%Ag solder alloy was investigated as a possible low melting temperature solder. Electroplating method was used to form Sn-Bi coated solder alloy on the Sn-3.5 mass%Ag alloy. Sn-Bi coated solders were bumped on the FR-4 substrate and Cu plates by a reflow machine at different temperatures. The die shear strength and microstructure were evaluated with scanning electron microscope (SEM). The compositions of Sn-Bi coated solder, and Bi distribution of solder were studied by electron probe X-ray analyzer (EPMA). The Sn-Bi coated Sn-Ag solder was possible to be bonded at low temperature such as 473-523 K and it is comparable to Sn-37%Pb solder. Intermetallic compounds (IMC's) along the bonded interface were well formed in the coated solder composed of high Bi and the compositions consisted of 59.49 at%Sn-37.20 at%Cu-3.31 at%Ag. Cu6Sn5 and Ag3Sn intermetallics were observed at the interface as well as inside of the solder. Bi segregation in the solder and at the IMC was not observed. The die shear strength was 3.6384 N and increased as coated Sn-Bi thickness increases.
  • Eutectic Alloys in the Sn-Zn-Mg-Al System with Eutectic Temperature of 454 K

    pp. 790-792

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    DOI:10.2320/matertrans.45.790

    We have experimentally found a Sn-Zn-Mg2Sn eutectic alloy with the eutectic composition of 82Sn-10Zn-8Mg (mol%) and with nearly the same eutectic temperature of 454 K as that of the Pb-Sn eutectic alloy. The eutectic alloy was made in air by covering Sn, Zn and Mg with the 59KCl-41LiCl (mol%) composition eutectic melt in an Al2O3 crucible, melting the metals at 1073 K and washing away the alkali-chlorides with distilled water after cooling to room temperature. The as-cast alloy easily oxidized at temperatures above 503 K in air after melting. It was found that the Sn-Zn-(Mg, Al)2Sn eutectic alloys that were made by replacing 0.8 to 4.0 mol% of Mg with Al in the ternary eutectic composition with the same alloying method did not oxidize in air after melting.
  • The Effect of Reduction Capability of Resin Material on the Solder Wettability for Electrically Conductive Adhesives (ECAs) Assembly

    pp. 793-798

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    DOI:10.2320/matertrans.45.793

    Electrically conductive adhesives (ECAs) have been developed as an alternative to traditional Sn-Pb solders for electronic and optoelectronic applications. However, there are critical limitations such as the low conductivity, and unstable contact resistance. These limitations have seriously hindered the broad applications of ECAs. In order to overcome these limitations, a new formulation using Pb-free conducting filler particles was proposed. Our previous study proved that the metallurgical interconnections among conducting filler particles and between particles an the conducting pads were established by process control. In particular, it was found that the wetting behavior of conducting fillers in ECAs is one of the main mechanisms for the establishment of conduction paths. In this study, we propose the fundamental concept of an assembly process using fusible Pb-free conducting filler particles. Also, the effect of the reduction capability of base resin material on the solder wetting property was investigated. As candidates for ECA compositions, two types of resin materials and two lead-free solders were investigated. The reflow temperature profiles for each ECA formulation were determined using a differential scanning calorimeter (DSC) dynamic scan. The wettabilities of the lead-free solders were investigated using an optical microscope with a CCD camera and a microfocus X-ray television system. It was found that the developed resin material with an intrinsic reduction capability shows good wettabilities in both lead-free candidates.
  • Formation of a Self-Interconnected Joint using a Low-Melting-Point Alloy Adhesive

    pp. 799-805

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    DOI:10.2320/matertrans.45.799

    The self-interconnection process is one of the promising methodologies for joining novel materials and assembling micro-electronic devices. Basic experiments fabricating micro joints using conductive adhesives with low melting point solders, such as Sn-In eutectic alloy, were demonstrated as an alternative method to conventional soldering or using the usual adhesive joining used in electronic assembly. The joint morphology, the formation of electrical conduction interconnections, and the self-interconnection characteristics were examined by optical and X-ray transmission microscopy. The behaviors of melting fillers such as aggregation, coalescence, and wetting were found to occur during joint formation. Especially on the copper line patterns of a glass-epoxy substrate, selective adhesional wetting of the melting alloy, enhanced by the oxygen-reduction capability of resin and capillary phenomena, was the main driving forces of the self-interconnection of joints. The alignment, joint height, and volume fraction of the filler need to be set correctly for a successful adhesion to be achieved.
  • TiO2 Nanoparticle Suspension Preparation Using Ultrasonic Vibration-Assisted Arc-Submerged Nanoparticle Synthesis System (ASNSS)

    pp. 806-811

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    DOI:10.2320/matertrans.45.806

    This paper proposes a new method—the ultrasonic vibration-assisted arc-submerged nanoparticle synthesis system for preparing TiO2 nanoparticle preparation. In addition to the influence of the ultrasonic amplitude and various process variables such as breakdown voltage, pulse duration, current, and dielectric liquid temperature on particles, this paper will also discuss the relationship between the pH value and the surface potential of the suspension. Experimental results have shown that the proposed method can successfully prepare anatase type TiO2. The TEM image further shows a mean particle size below 10 nm, and that nanoparticle suspension cannot reach a state of stable suspension and starts aggregating under a pH value below 6.4.
  • Giant Magnetoimpedance in (Co0.85Fe0.06Nb0.02Ni0.07)75Si10B15 Amorphous Ribbon

    pp. 812-817

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    DOI:10.2320/matertrans.45.812

    The impedance and impedance-phase in (Co0.85Fe0.06Nb0.02Ni0.07)75Si10B15 amorphous ribbon strongly depend on the dc magnetic field. The change ratios of magnetoimpedance ΔZ(H)/Z(0), magnetoresistance ΔR(H)/R(0) and magnetoreactance ΔX(H)/X(0) intersect at frequencies, where the change ratio of the magnetoimpedance-phase Δθ(H)/θ(0) is zero. There are two intersecting points, which occur near the frequencies of peak and valley of ΔZ(H)/Z(0), respectively. With increasing dc fields, the intersecting frequencies as well as the peak-frequencies of magnetoimpedance shift to higher frequencies. The simulation of magnetoimpedance for magnetic ribbons was done considering the combination influence of skin effect and relaxation permeability spectra. The negative linear magnetoimpedance at low frequencies, the peak-phenomenon of impedance with field at high frequencies, as well as the intersection of the magnetoimpedance, magnetoresistance and magnetoreactance were well simulated.
  • Morphological Evolution of Grain-Boundary SiO2 in Internally Oxidized Cu-Si Bicrystals

    pp. 818-823

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    DOI:10.2320/matertrans.45.818

    Cu-0.78 mass%Si bicrystals are internally oxidized at various conditions and morphological evolution of amorphous SiO2 formed on a grain boundary (GB) is observed. After the reaction of Si atoms with oxygen, a film-like SiO2 phase with holes is initially formed on GB of Cu. At temperatures used for the internal oxidation, the holes rapidly grow and the morphology of the film-like SiO2 changes to a particulate equilibrium shape through formation of two-dimensional SiO2 network and its subsequent breakup. During the morphological evolution, the volume of SiO2 on GB is conserved. The morphological evolution of SiO2 on GB is caused by diffusional flow of matter to decrease the sum of Cu/SiO2 interface and Cu grain-boundary energies. The activation energy Q of the morphological evolution is obtained as Q ≈ 290 kJ/mol. Among the values of activation energies reported in the previous studies on diffusion in SiO2, Q ≈ 290 kJ/mol is close to that for the volume-diffusion of SiO in SiO2, QSiO ≈ 268 kJ/mol. This supports that the morphological evolution of SiO2 on GB is controlled by the volume-diffusion of SiO in SiO2.
  • Flat Cavities formed in TZP caused by Superplastic Deformations at High Strain-Rates and Their Effect on Elongation

    pp. 824-832

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    DOI:10.2320/matertrans.45.824

    Cavitation in a superplastic 3Y-TZP pulled under different temperature and strain-rate conditions was analyzed by means of SANS techniques. In high strain-rate deformations, the formation of crack-like flat cavities with their flat surfaces lying roughly normal to the tensile axis was found, in addition to conventional cavities slightly elongated along the tensile direction having been frequently observed in superplastically deformed 3Y-TZP. Coalescence of the nearby crack-like flat cavities during the high strain-rate deformations gave rise to the generation of large brittle-like cracks, which led to an earlier fracture without apparent occurrence of necking. A mechanism for the formation of the flat cavities and their effect on the elongation are discussed.
  • Computer Simulation of Formation Energy and Migration Energy of Vacancies under High Strain in Cu

    pp. 833-838

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    DOI:10.2320/matertrans.45.833

    Recently it has been reported that many vacancy clusters are generated at sawtooth-like fractured tips of thin foils. The elastic strain of the torn portion is more than 10% before the fracture, which is expected to cause the generation of many vacancy clusters. In this paper, the formation and migration behaviors of vacancies in Cu under high elastic strain from 10% compression to 20% elongation were studied by computer simulation using the effective medium theory (EMT) potential. The model lattice was elastically deformed along the ‹110› and ‹100› directions. Poisson's ratio was determined to minimize the total energy. After full relaxation of the lattice by the static method (Newton-Raphson method) under fixed boundary conditions, a vacancy was introduced and the change of the total energy (formation energy) was calculated. The migration energy of vacancies was obtained as the total energy difference between the model lattice with an atom at the lattice point and the atom at the saddle point. High strain dependence of these energies is obtained. For example, the formation and the migration energies of vacancies are 1.21 eV and 0.79 eV in the absence of deformation, respectively. The formation energy in the ‹100› deformation is 1.09 eV and 1.13 eV by 10% compression and 10% elongation, respectively. The migration energies and the migration distances vary with the migration direction. For example, the migration energy for the shortest migration distance in the ‹100› deformation is reduced to 0.45 eV and 0.26 eV by 10% compression and 10% elongation, respectively. While that for the longest migration distance increases to 1.29 eV by 10% compression. These results are explained by the configuration of neighboring atoms nearest to the vacancy.
  • Notch Size Effects in the Fatigue Characteristics of Al-Si-Cu-Mg Cast Alloy

    pp. 839-843

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    DOI:10.2320/matertrans.45.839

    Influences of the microstructure, micro defects and the stress concentration factor on fatigue characteristics were investigated for a JIS-AC4B alloy containing 6.79%Si, 2.93%Cu, 0.17%Mg and 0.59%Fe, and for an iron free Al-Si-Cu-Mg alloy. Solidification microstructures consist of dendritic α phase, eutectic Si, Al2Cu and Mg2Si phases in both alloy specimens and a few gas and shrinkage porosities appear in every specimen; while Fe compound modified by Mn appears among the dendrites in the AC4B alloy. Rotating bending fatigue tests were carried out on specimens with notches of 2, 1, 0.3 and 0.1 mm radius. Both AC4B and Al-Si-Cu-Mg alloys show the same fatigue sequence when the notch size is larger than 1 mm, indicating that the gas and shrinkage porosities act as the origins of cracking and thus govern the fatigue characteristics. Contrarily, when the notch radius becomes smaller than 0.3 mm, so that the stress concentration factor becomes larger than 2.4, the AC4B alloy has a higher fatigue strength than the Al-Si-Cu-Mg alloy, indicating that Fe-compounds may retard crack propagation.
  • Evaporation of Phosphorus in Molten Silicon by an Electron Beam Irradiation Method

    pp. 844-849

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    DOI:10.2320/matertrans.45.844

    The evaporation behavior of phosphorus in molten silicon during electron beam irradiation was investigated with the aim of producing solar grade silicon (SOG-Si) from metallurgical grade silicon (MG-Si) by a sequential metallurgical process. Batch experiments showed that the evaporation rate of phosphorus increased in proportion to the power of the electron beam and phosphorus content. The phosphorus removal rate was controlled by free evaporation from the molten silicon surface. Electron beam irradiation makes it possible to secure a higher temperature at the free liquid surface, which results in more efficient dephosphorization. A continuous flow experiment indicated that the phosphorus concentration at the outlet increased when the silicon feed rate was raised, which was attributed to the fact that the hearth residence time of the molten silicon was proportionally shorter. Thus, the flow of molten silicon in the hearth did not behave as a complete mixed reactor flow type reaction, but was close to a plug flow type reaction. With a 150 kg scale pilot manufacturing plant, MG-Si containing about 25 mass ppm of phosphorus was successfully purified to P < 0.1 mass ppm.
  • Removal of Metal Impurities in Molten Silicon by Directional Solidification with Electron Beam Heating

    pp. 850-857

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    DOI:10.2320/matertrans.45.850

    An industrial-scale pyrometallurgical method of removing metallic impurities from metallurgical grade silicon (MG-Si) was developed as an element technology in a sequential purification process for manufacturing high-purity silicon for solar grade silicon (SOG-Si) by segregation of metallic impurities during solidification. Metallic impurities were removed from MG-Si using an electron beam heating equipment. Molten silicon was supplied continuously at a constant mass to a water-cooled copper mold and was allowed to solidify gradually in an unidirectional manner from the bottom upward. This process is termed directional solidification. The iron concentration after solidification can be expressed by Pfann's and Burton's equations, and was reduced from an initial 1500 mass ppm to below 10 mass ppm. Aluminum removal was excessive, presumably due to vaporization to the gas phase. Above a certain height in the ingot, it became impossible to remove metallic impurities by partition during directional solidification. This phenomenon showed a correlation with the concentration of enriched iron in the silicon pool. The mechanism of metallic impurity removal was estimated based on visual examination of the solidified structure and EPMA. The iron concentration profile of ingots and critical purification height were estimated experimentally using a 20 kg scale device and verified as being applicable on an industrial scale in experiments with 150 kg scale equipment. Solar grade silicon was test-produced by this process and showed satisfactory quality for solar cell use.
  • Boron Removal in Molten Silicon by a Steam-Added Plasma Melting Method

    pp. 858-864

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    DOI:10.2320/matertrans.45.858

    Oxidation and removal of boron from molten silicon by a steam-added plasma melting method was investigated as an important part of a sequential metallurgical process for producing high-purity solar grade silicon (SOG-Si) from commercially available metallurgical grade silicon (MG-Si). Experiments were carried out with the mass of silicon per charge varied in the range from 0.6 to 300 kg, corresponding to the laboratory scale to industrial scale. Boron was removed to [B] < 0.1 mass ppm, which is the permissible boron content for SOG-Si. The deboronization rate was proportional to the steam content, 3.2th power of the hydrogen content of the plasma gas, boron content of the molten silicon, and area of the dimple formed by the plasma gas jet, and was inversely proportional to the mass of the molten silicon. A thermodynamic study showed that preferential oxidation of boron in molten silicon is positively related to higher temperatures, supporting the conclusion that this plasma method, which causes a local increase in the temperature of the reaction surface, is in principle advantageous.
  • Beneficial Effect of Sulfur State on High-Temperature Steam Oxidation Resistance in High Chromium Ferritic Steels

    pp. 865-869

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    DOI:10.2320/matertrans.45.865

    The presence of sulfur (S) at an impurity level in high chromium (Cr) ferritic steels improves remarkably high-temperature steam oxidation resistance. However, it still remains unknown which S state in the steels gives such a beneficial effect. There are two possible S states in the steels; one is the soluble S state in a solid solution of the steel, and the other is the precipitated S state which occurs through the sulfide formation or the S segregation in grain boundaries. Either state appears depending on the S content and the heat-treatment temperature. In this study in order to elucidate the effective S state, high-temperature steam oxidation resistance was investigated with high Cr ferritic steels, by changing the S states in them by proper heat treatments. As the result, it was found that the precipitated S state operated more effectively to the improvement of steam oxidation resistance, as compared to the soluble S state.
  • Filament and Droplets Formed Behind a Solid Sphere Rising Across a Liquid-Liquid Interface

    pp. 870-876

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    DOI:10.2320/matertrans.45.870

    An understanding of the dynamic behavior of non-metallic inclusions such as bubbles and alumina passing through an interface between molten steel and slag is of essential importance for producing clean steel. Model experiments were carried out in this study using water and silicone oil as the working fluids. The behavior of a solid sphere rising through an interface between stratified two liquid layers and the associated deformation of the interface were observed with a high-speed video camera. A filament-like column of the lower liquid was formed behind the sphere rising in the upper liquid layer. Many droplets were generated due to breakup of the column. Empirical equations were proposed for parameters characterizing the shape and size of the column and droplets.
  • Agglomeration of Copper Thin Film in Cu/Ta/Si Structure

    pp. 877-879

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    DOI:10.2320/matertrans.45.877

    Copper agglomeration in Cu(100 nm)/Ta(50 nm)/Si structure deposited by ion beam deposition was examined. Copper thin films were annealed at 650°C for 1 to 60 min in hydrogen atmosphere. The surface morphology, crystalline microstructure and electrical resistivity were investigated by field-emission scanning electron microscopy, X-ray diffraction and Van der Pauw method, respectively. Experimental results revealed that nucleation and growth of voids ocurred in the copper film annealed for 5 min. Further annealing made the film a connected island structure and then isolated island structure.
  • Effect of Heat Treatment on Microstructure and Properties of Semi-solid Chromium Cast Iron

    pp. 880-887

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    DOI:10.2320/matertrans.45.880

    Semi-solid processed 18% Cr and 27% Cr cast irons were produced by using copper cooling plate and metal mold. A series of experiments were carried out to clarify the effect of heat treatment on microstructure, hardness, wear properties, and corrosion characteristics. The results show that 27% Cr alloys possess better abrasive resistance than 18% Cr alloys due to higher carbide volume under all the conditions when tested by dry sand rubber wheel with silica abrasive. However 18% Cr alloys show higher wear resistance than 27% Cr alloys when using super hard alloy disk against the specimen plate surface. The harder disk indent into the carbides, leads to spalling and pitting, and therefore greater wear rate. For the corrosion test result, 27% Cr alloys have better corrosion resistance than 18% Cr alloys as a result of higher chromium content. A combination of semi-solid processing and heat treatment improves wear resistance and corrosion resistance of chromium cast iron.
  • Effect of Al on Bulk Amorphization and Magnetic Properties of FeSnPSiB Alloys

    pp. 888-892

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    DOI:10.2320/matertrans.45.888

    Structure and magnetic properties of melt-spun multi-component Fe78−xAlxSn2P12 Si4B4 (x = 0, 3, 4, 5 at%) amorphous ribbons and copper-mold-cast rods were investigated. The improvement of thermal stability was recognized with the replacement of Fe by 3∼5 at% Al in these Sn-containing alloys. The super-cooled liquid region (ΔTx), defined by (TxTg) where Tx is the crystallization temperature and Tg the glass transition temperature, increases from 46.5 K as x = 3 to the maximum value 50.2 K as x = 4. With increasing Al content, magnetic properties of annealed Fe78−xAlxSn2P12Si4B4 (x = 3∼5 at%) amorphous ribbons are: saturation magnetization decreases from 1.22 to 1.18 T; coercive force decreases from 3.3 to 2.2 A/m, maximum permeability increases from 137000 to 208000, Curie temperature lowers from 591.1 K to 578.3 K; the electrical resistivity increases from 175 to 188 μΩ-cm; and core loss is about 0.14 W/kg (50 Hz, 1.1 T external magnetic field). Bulk amorphous rods with 1 mm diameter were successfully demonstrated for the composition Fe74Al4Sn2P12Si4B4 by a copper mold casting method. Due to the combination of good glass forming ability and superior soft magnetic properties, these alloys will find potential applications in industries.
  • Simulation of Droplet Ejection for a Piezoelectric Inkjet Printing Device

    pp. 893-899

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    DOI:10.2320/matertrans.45.893

    A computer-aided analysis system has been developed in this study to simulate the formation, ejection, and impact of liquid droplet in a piezoelectric inkjet printing device. The effects of voltage pulse of the piezoelectric actuator and surface tension of the liquid on the droplet ejection phenomena are evaluated by the simulation system. The droplet ejection is a fluid flow phenomenon, which involves two phases (liquid and gas) and free surface evolution of the liquid. The computer simulation system is based on a SOLA (Solution Algorithm) scheme coupled with VOF (Volume of Fluid)/PLIC (Piecewise-Linear Interface Construction) and CSF (Continuum Surface Force) models. To simplify the simulated system, only the nozzle region of the inkjet printing device is considered. The behavior of the piezoelectric diaphragm is imposed on the simulation system by a moving boundary, which is determined by the velocity profile corresponding to the driving voltage. The effects of the surface tension of the liquid are evaluated by the CSF model. The results show that comprehensive images of the inkjet dynamics, which include the evolution of the droplet; from formation, ejection to impact, can be revealed by the simulations. The effects of surface tension of the liquid on the size and flying velocity of the droplet as well as the formation of the satellite droplets can also be evaluated by the simulation system.
  • Preparation of IrO2 Thin Films by Oxidating Laser-ablated Ir

    pp. 900-903

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    DOI:10.2320/matertrans.45.900

    Highly conductive and transparent IrO2 thin films were prepared on SiO2 substrates by embedding laser-ablated Ir thin films in IrO2 powders at 973 K for 10.8 ks in O2. The binding energies of Ir 4f7/2 for the as-deposited Ir and the IrO2 thin films were 61.1 and 62.1 eV, respectively. The shift of Ir 4f7/2 binding energy implied the oxidation from Ir to IrO2. The resistivity of the IrO2 thin films at room temperature was 4.0 × 10−7 Ωm. The optical transmittance of IrO2 thin films can reach to 60—80% in the wavelength range of 400—800 nm.
  • Effect of Mg on the Sintering of Al-Mg Alloy Powders by Pulse Electric-Current Sintering Process

    pp. 904-909

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    DOI:10.2320/matertrans.45.904

    Utilization of deoxidization mechanism of magnesium (Mg) is an effective method to remove the oxide films at aluminum (Al) alloy powder surface in pulse electric-current sintering (PECS) process. The continuous amorphous oxide film at Al alloy surface are broken and removed by deoxidization of Mg. Crystalline particles of MgAl2O4 or MgO, or both of them, are formed, which depend on Mg content in Al alloy powder and sintering temperature. After that the metal/metal contact is caused, and solid state sintering of Al alloy powder is facilitated. The electrical resistivity and tensile properties of powder compacts are improved by Mg addition. Based on the analyses of electrical resistivity, tensile properties and microstructures of the sintered specimens, optimum amount of Mg addition to improve the sintering properties of Al powder is determined to be 0.3—2.5 mass%.
  • An Air Cavity Formed Behind a Poorly Wetted Sphere Penetrating into a Water Bath

    pp. 910-917

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    DOI:10.2320/matertrans.45.910

    Uniform dispersion of fine particles of poor wettability in molten steel is of essential importance for enhancing the efficiency of the steel refining processes. Although model investigations on the dispersion of fine particles of good wettability have extensively been carried out, those for fine particles of poor wettability are rather limited. In this study the behaviors of three kinds of solid spheres penetrating into a water bath were observed with a high-speed video camera. The wettability was changed by coating hydrophilic or hydrophobic material. The well-known crown was formed behind a solid sphere of good wettability, while an air cavity was formed behind a solid sphere of poor wettability. Empirical equations were derived for some parameters describing the shape and size of the cavity.
  • Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion

    pp. 918-924

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    DOI:10.2320/matertrans.45.918

    The shear extrusion processing combined with bulk mechanical alloying is proposed to yield the n-type Bi-Te-Se material from elemental granules. It has well-developed texture so as to improve the electric conductivity and thermoelectric properties. The shear extrusion processing of (Bi2Se3)0.05(Bi2Te3)0.95 alloy green compact can afford the preferred orientation factor of anisotropic crystallographic structure: F = 0.67. The electric resistivity of (Bi2Se3)0.05(Bi2Te3)0.95 is controlled to be 0.491 × 10−5 (Ωm), which is 0.2 times lower than that of hot-pressed specimen. Maximum power factor is achieved to be 3.31 × 10−3 (W/mK2) even without any dopants. The bending strength of the material produced in this work is also improved to be 166 MPa, 1.7 times higher than that of conventional hot-extruded specimens.
  • Determination of Trace Elements in High-Purity Molybdenum by Solid-Phase Extraction/ICP-MS

    pp. 925-929

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    DOI:10.2320/matertrans.45.925

    We attempted a simple pretreating method consisting of solid-phase extraction using bonded silica gel with benzenesulfonic acid (SCX) as the solid-phase sorbent to determine trace elements in pure molybdenum samples by means of inductively coupled plasma mass spectrometry (ICP-MS). Molybdenum was anionized by adding hydrogen peroxide solution to a sample decomposed with acid, and separated from cation trace impurities that had been kept in the chemically bonded silica gel of the ion-exchange type. The target elements retained in the solid phase were eluted with a small amount of dilute nitric acid. In this method, some trace elements, such as Be, Al, Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Cd, In, Tl, Pb and Bi were determined by ICP-MS using the eluate. The detection limitations (3σ), as follows: Be 0.028, Al 2.64, Mg 1.9, Cr 0.20, Mn 0.13, Fe 3.85, Co 0.019, Ni 0.48, Cu 0.084, Zn 0.25, Ga 0.092, Cd 0.014, In 0.059, Tl: 0.027, Pb 0.044 and Bi 0.012 ng/g (ppb).
  • Shape Memory Effect and Crystallographic Investigation in VN Containing Fe-Mn-Si-Cr Alloys

    pp. 930-935

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    DOI:10.2320/matertrans.45.930

    An industrial application of Fe-based shape memory alloys for joining the pipes in tunnel making constructions requires the proof stress over 400 MPa and 3.5% shape recovery strain. To meet such standards in civil engineering, the Fe-base shape memory alloy containing high-density of coherent VN precipitates has been developed by designing the composition of the Fe-28Mn-6Si-5Cr (mass%) alloy containing 1.5 vol% VN compounds. In order to make clear the mechanism of enhancement of the proof stress and shape recovery strain, crystallographic investigations were carried out using optical microscopy, X-ray diffraction and high-resolution electron microscopy. The {111}-composed octahedral shaped VN precipitates are formed coherently with a cube-to-cube orientation relationship with the matrix, i.e., (001)P || (001)M and [100]P || [100]M. The precipitates are surrounded by the misfit dislocations at every 8 layers of the (100) planes, which is in agreement with the estimation of the lattice mismatch between the precipitate and the matrix phase. The enhancement of shape recovery is explained by the reversible movement of transformation dislocations left around the precipitates in connection to the residual stress yielded by martensite transformation around the precipitates. It is also suggested that the recovery strain could be proportional to the total number of precipitates.
  • Fracture Toughness of JLF-1 by Miniaturized 3-Point Bend Specimens with 3.3—7.0 mm Thickness

    pp. 936-941

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    DOI:10.2320/matertrans.45.936

    A small specimen test technique is required to evaluate the fracture toughness values of several millimeter thick plates of structural materials and to maximize the use of very limited space for materials irradiation in intense neutron sources like IFMIF. In view of several advantages of three-point bending (3PB) over compact tension (CT), miniaturized 3PB specimens with 7.0, 5.0 and 3.3 mm thickness were prepared from a Japanese low activation ferritic steel, JLF-1, which is a candidate first wall and fusion blanket material.Elastic-plastic fracture toughness tests by the unloading compliance method at room temperature and plane-strain fracture toughness tests at 77 K were conducted in general accordance with the ASTM standards. Emphasis was focused on the determination of the actual J-value for crack initiation, JIN, for reliable fracture toughness evaluation with the 3PB specimens. The obtained values of JIN at room temperature and KIC at 77 K were 100—120 kJ/m2 and 20—22 MPam1/2, respectively, exhibiting little dependence on specimen size. By combining the experimentally obtained data with the plane-strain FEM analysis, a method was proposed to estimate JIN from a load-displacement curve measured for a single specimen. The method is applicable to heavily irradiated materials with little ductility.
  • Improvement of the Surface Layer of Steel Using Microwave Plasma Nitriding

    pp. 942-946

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    DOI:10.2320/matertrans.45.942

    Microwave plasma was employed for the nitriding of SCM445 steel and characteristics of the steel obtained by this process were compared with those of steel treated by conventional ion nitriding. The latter showed a large amount of globular shaped nitride compounds around 1-2 μm size precipitated on the steel surface and surface coarsening after ion nitriding. However, microwave plasma nitriding successfully produced a nitrided steel without a compound layer and a diffusion layer thickness of more than 0.5 mm was achieved at gas composition of 30%N2-70%H2 (500°C, 2 h). Also, a smooth surface was also achieved. Microwave plasma nitriding was capable of producing just the diffusion layer with a good surface condition for subsequent deposition of a protective coating.
  • Adherence of Porcelain Veneered on Titanium with an Intermediate Plasma-Sprayed Zirconia Layer

    pp. 947-952

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    DOI:10.2320/matertrans.45.947

    Porcelain veneered on pure titanium with an intermediate plasma-sprayed ZrO2 bond coat layer was investigated in this study. The pure porcelain coating veneered on Ti was used as a control. The specimens were subjected to four-point bending test to measure the bonding strength. The fractograph, microstructure, surface roughness, and phase of plasma-sprayed ZrO2 bond coat were examined. The phase constituents at the location of adhesive failure of the fractured specimens were investigated by thin-film X-ray diffractometry. Experimental results reveal that the bonding strength of the coating with the intermediate layer was increased by 55.7% that was accompanied by a significant decrease of the area of adhesive failure in the fracture. The ZrO2 provided a higher surface roughness to enhance the mechanical bond at the interface of porcelain/ZrO2, meanwhile it acted as a sufficient oxygen diffusion barrier at 800°C to prevent the oxidation of Ti at the interface of ZrO2/Ti.
  • Electron-Phonon Mechanism of ω Phase Transformation

    pp. 953-957

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    DOI:10.2320/matertrans.45.953

    We have first calculated the driving force of ω phase transformation (PT) based on the electron-phonon interaction Hamiltonian. The nucleation rate of PT has also been investigated in the interaction system. The condensation of one single phonon makes a great contribution to the formation of charge density wave (CDW) whose amplitude is related to the wavevector of phonon. Considering the electron-phonon interaction and the condensation of phonon, Double Sine-Gordon equation of atomic phase angle has been proposed to study the nonlinear characteristics of ω PT and the electron-phonon coupling mechanism is suggested as the main mechanism of nucleation.
  • Effect of ECAP Strain on Deformation Behavior at Low Temperature Superplastic Regime of Ultrafine Grained 5083 Al Alloy Fabricated by ECAP

    pp. 958-963

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    DOI:10.2320/matertrans.45.958

    A series of tensile testing was carried out on the ultrafine grained 5083 Al alloy, which was fabricated by equal channel angular pressing (ECAP) with different ECAP strains, at low temperature superplastic (LTS) temperature of 548 K. This investigation was aimed at examining the effect of the ECAP strain inducing different microstructure in the alloy on the deformation mechanisms at LTS regime. The most distinguishable microstructural evolution by increasing the ECAP strain from ∼4 to ∼8 was an increment of a portion of high angle boundaries while the (sub)grain size remained almost unchanged. The sample after 4 passes (a strain of ∼4) did not exhibit LTS, but superplastic elongations were obtained in the sample after 8 passes (a strain of ∼8). An analysis of the mechanical data in light of the standard deformation mechanisms revealed that deformation of the sample after 4 passes was governed by dislocation climb while grain boundary sliding attributed to LTS of the sample after 8 passes. The difference of the deformation mechanisms in the present case was discussed in terms of the microstructures developed under different ECAP strains.
  • Synthesis of Au/TiO2 Core-shell Structure Nanoparticles and the Crystallinity of TiO2 Shell

    pp. 964-967

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    DOI:10.2320/matertrans.45.964

    The synthesis of Au/TiO2 core-shell structure nanoparticles was carried out by the hydrolysis of TOAA (titanium oxide acethylacetonate) in gold sol ethanol solution with water. It was possible to achieve 1 nm thick TiO2 shell on the surface of gold particles. The morphology and the crystallinity of TiO2 shell were investigated by TEM and UV-vis absorption spectrometer. The crystral structure of TiO2 shell was amorphous because the surface plasmon band of gold nanoparticles appeared only when the radioactive ray was irradiated on the TiO2-coated gold sol ethanol solution.
  • Influence of Sulfate Ions on Conversion of Fe(OH)3 Gel to β-FeOOH and α-Fe2O3

    pp. 968-971

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    DOI:10.2320/matertrans.45.968

    Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and chemical analyses were used for characterizing the effect of sulfate ions on the conversion rate of condensed Fe(OH)3 gel, prepared from FeCl3 and NaOH solution, to β-FeOOH and α-Fe2O3 particles by aging at 100°C. Fine particles and supernatant solution were sampled from the aged suspensions containing colloidal particles. XRD profiles and TEM observation showed that β-FeOOH was formed from the gel and subsequently converted into α-Fe2O3. It was revealed that the addition of sulfate ions accelerated the β-FeOOH formation from the gel, but delayed the conversion from β-FeOOH to α-Fe2O3. The concentration changes of sulfate and chloride ions in the supernatant solution phases were in agreement with the conversion from Fe(OH)3 gel to β-FeOOH and α-Fe2O3 particles.
  • Fractal Characteristics of the Solidification Process

    pp. 972-975

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    DOI:10.2320/matertrans.45.972

    Assimilating the solid-liquid interface to a binary domain described by a set of coupled equations, one studies the solidification process. The numerical solutions of the thermal breather, the thermal breather pair and the thermal cluster type are associated to a virtual crystallization germ, a stable crystallization germ and a crystalline grain, respectively. The fractal characteristics of the solidification process are analyzed by means of fractal dimension dynamics.

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