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

MATERIALS TRANSACTIONS Vol. 56 (2015), No. 7

  • PREFACE

    pp. 905-905

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    DOI:10.2320/matertrans.MPR2015903

  • Stability of Long-Period Stacking Ordered Structures at Elevated Temperatures Examined by Multicolor Synchrotron Radiation X-ray Scattering/Diffraction Measurements

    pp. 906-909

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    DOI:10.2320/matertrans.MH201402

    Stability of long-period stacking ordered (LPSO) structures of Mg-Zn-Y ternary alloys at elevated temperatures was examined by multicolor synchrotron radiation small- and medium-angle scattering/diffraction. The LPSO structures directly melted during heating, and the formation of LPSO was observed from supercooled liquid. The temperatures of complete dissolution and the onset of formation of LPSO phases were measured by monitoring the first diffraction peak of 14H, 18R and 10H compositional modulation. The stability of composition modulation in c axis direction was found to be the same as that of L12 cluster ordering in the segregation layer in both heating and cooling processes, i.e., dissolution and formation. Supercooling of LPSO was obtained for the heating/cooling rate of 10 K/min.
  • Phase Relations among D03, α-Mg, and Long-Period Stacking Orders in Mg85Zn6Y9 Alloy under 3 GPa

    pp. 910-913

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    DOI:10.2320/matertrans.MH201413

    The crystal structures of Mg85Zn6Y9 alloy under 3 GPa were investigated between room temperature and 973 K using in situ X-ray diffraction (XRD). In addition, the crystal structures and microstructures of Mg85Zn6Y9 alloys that were recovered after being heated at various temperatures under 3 GPa were also investigated. The in situ XRD analysis results indicated that 10H- and 18R-type long-period stacking orders (LPSOs) were stable below 673 K under 3 GPa; however, the D03 and α-Mg structures, which were not observed at ambient pressure, emerged at 873 K in the sample under 3 GPa. When the temperature was increased to 973 K, the D03 and α-Mg phases fused. The D03 and α-Mg phases were also confirmed to be present in Mg85Zn6Y9 alloy that was recovered after being heated at 873 K under 3 GPa. In contrast, the XRD pattern of the Mg85Zn6Y9 alloy that was recovered after being heated at 973 K under 3 GPa indicated the presence of the 18R-type LPSO, D03, and α-Mg structures. These results indicate that three phases—18R-type LPSO, D03, and α-Mg—were formed during cooling from the molten state at 3 GPa. These results further suggest that the D03, α-Mg, and 18R-type LPSO are energetically very similar under 3 GPa.
  • Microscopic Elastic Properties of Polycrystalline Mg85Zn6Y9 Alloy with Long-Period Stacking Ordered 18R Phase Investigated by Inelastic X-ray Scattering

    pp. 914-916

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    DOI:10.2320/matertrans.MH201409

    Inelastic X-ray scattering measurements were performed on polycrystalline Mg85Zn6Y9 alloy with long-period stacking ordered 18R phase to clarify the microscopic elastic properties. Tiny but clear excitation signals of longitudinal acoustic (LA) modes are observed in the whole Q range measured (Q < ∼16 nm−1). In addition, the transverse acoustic and unknown modes are detectable in the second Brillouin zone (∼13 < Q < ∼26 nm−1). The microscopic sound velocity of the LA modes is similar to the macroscopic value. The microscopic Poisson ratio is 0.22 ± 0.04, slightly smaller than the macroscopic value of 0.26 and usual metal values of ∼0.3, indicating a hard stiffness of the bond angles.
  • Effects of Pre-Strain and Ageing on the LPSO Structure in Mg97Zn1Y2 Alloy

    pp. 917-922

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    DOI:10.2320/matertrans.MH201416

    The effects of pre-strain and ageing on the LPSO structure in Mg97Zn1Y2 alloy were investigated by electron microscopy. It is found that 14H type LPSO structure evenly precipitates from the supersaturated matrix during ageing of solutionized sample. This LPSO structure is observed even at the early stage of ageing, and no precursor phase such as 18R is observed before 14H forms. The 14H structure is kept after a long time ageing. Similar results have been found for ageing of pre-strained sample. Moreover, it shows that the pre-strain could reduce the LPSO plate spacing, since the LPSO plate with one or two structure units increased due to deformation. In both un-deformed and deformed cases, LPSO plate grows by ledge mechanism. The cooperative motion of the structure units in the growth ledge may be due to the elastic interactions between the structure units.
  • Local Strain Fields of LPSO in Mg-Based Ternary Alloys

    pp. 923-927

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    DOI:10.2320/matertrans.MH201404

    L12-type TM6RE8 clusters are distributed in enriched layers of LPSO (long period stacking ordered structure), and shrink in the [0001]Mg direction. The growth mechanism of LPSOs was investigated from the viewpoint of a local strain field. The strength of out-of plane normal strain of the enriched layer varied with the average radius of solute elements in the layer. The first principle calculation showed that there is a relationship between the average atomic radius of solute elements and the structural relaxation in the clusters. Zn conclusion, it is suggested that the difference of average atomic radius between Mg and solute elements controls the degree of the cluster shrinkage by the displacement of the solute elements, and that it is the dominant factor giving rise to the out-of plane normal strains of the enrichment layers.
  • Three-Dimensional Shapes and Distributions of Long-Period Stacking Ordered Structures in Mg97Zn1Gd2 Cast Alloys Characterized by Electron Tomography

    pp. 928-932

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    DOI:10.2320/matertrans.MH201406

    Three-dimensional (3D) configurations of 14H long-period stacking ordered (LPSO) structures formed in Mg97Zn1Gd2 cast alloys at intermediate stages of the formation process have been studied by single tilt-axis electron tomography using high-angle annular dark-field scanning transmission electron microscopy. Lateral morphology of the 14H LPSO is clearly visualized by reconstructing 3D volumes. An existence of “dent-shaped” area was found in a 3D reconstructed volume for the first time. The edge of LPSO shows a characteristic triangular shape with an angle of 60°, which indicates that the growth front is parallel to {11\bar{2}0}Mg. It is suggested that in-plane irregular or characteristic shapes are related to the lateral growth mechanism of LPSO. Electron tomography has proven to be an indispensable tool to characterize in-plane structural information of LPSO formed in α-Mg matrix.
  • First Principles Calculations of Solute Sweeping and Stacking Faults in Mg-Zn-Y Alloy

    pp. 933-936

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    DOI:10.2320/matertrans.MH201410

    The formation mechanism of the long-period stacking-ordered (LPSO) structure of a Mg-Zn-Y alloy was investigated through energy assessments using first-principles calculations. The solute atoms are swept out from stacking fault regions because of their repulsive interaction with precipitated L12 clusters. The swept-out solute atoms are condensed a few layers away from the stacking-fault regions and accelerate the introduction of other stacking faults. A new scenario in the formation of the LPSO structure is proposed.
  • Phase-Field Modelling on the Formation Process of LPSO Structure in Mg-Y-Zn System

    pp. 937-942

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    DOI:10.2320/matertrans.MH201414

    It has been proposed that a long-period stacking ordered (LPSO) structure is responsible for the excellent mechanical properties of lightweight alloys of Mg-Y-RE (RE: rare earth elements) system. In this study, the phenomenological simulation model for describing the formation process of the LPSO structure in Mg–Y–Zn system is constructed by means of the phase-field method. The results obtained are as follows: The proposed phenomenological model is useful for describing the formation process of LPSO structure, where the enrichment of solute elements Zn and Y on stacking fault region induces the growth of the stacking fault itself. This model is able to calculate not only the formation of lamella structure of LPSO phase but also the growth of single stacking fault region in LPSO phase, simultaneously. It is suggested that the lamella spacing of LPSO structure may depend on the initial condition when a LPSO phase nucleates.
  • In-Situ Observation on the Formation Behavior of the Deformation Kink Bands in Zn Single Crystal and LPSO Phase

    pp. 943-951

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    DOI:10.2320/matertrans.MH201412

    The deformation microstructure and deformation process of the Mg-based long period stacking ordered (LPSO) phase accompanied by the formation of deformation kink bands were examined through dynamical observations during compression tests, and the features were compared to those in Zn single crystals. In both crystals, the formation of deformation kink bands was confirmed in specimens compressed along the direction parallel to the basal plane. The deformation kink bands formed in the directionally solidified (DS) LPSO phase crystals and the Zn single crystals had similar morphologies. Their formation induced plastic strain almost along the c-axis. However, their formation behaviors showed some different features. In the LPSO phase DS crystal, two different migration behaviors of the deformation kink band boundaries existed. The slower migration process was comparable to that observed in Zn single crystals. However, a migration process more than 105 times faster than the slower process was also monitored. The results imply that two different formation mechanisms of the deformation kink band might exist in the LPSO phase crystal.
  • Deformation Behavior of Long-Period Stacking Ordered Structured Single Crystals in Mg85Zn6Y9 Alloy

    pp. 952-956

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    DOI:10.2320/matertrans.MH201415

    Microtension tests were performed on single crystals with different crystallographic orientations, which were prepared using an 18R-type long-period stacking ordered (LPSO) structure in a directionally solidified Mg85Zn6Y9 alloy. Anisotropic plasticity was observed for the LPSO phase specimens. Cleavage cracking occurred on the prismatic planes in a crystal with its loading direction parallel to the basal plane. The cleavage stress for the prismatic plane was determined to be approximately 460 MPa. A crystal that was oriented favorably for gliding on the basal plane exhibited a critical resolved shear stress of approximately 9.4 MPa. When a crystal was loaded along [0001], a sudden load drop was observed at a stress of approximately 250 MPa in the initial linear region of the stress–strain relation. Transmission electron microscopy results after the load drop showed that the deformation microstructure contained a twin-like boundary with a rotation axis along [11\bar{2}0].
  • Deformation Analysis of the Long-Period Stacking-Ordered Phase by Using Molecular Dynamics Simulations: Kink Deformation under Compression and Kink Boundary Migration under Tensile Strain

    pp. 957-962

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    DOI:10.2320/matertrans.MH201408

    The long-period stacking-ordered (LPSO) phase discovered in magnesium alloys is deformed upon the generation of a large number of unique deformation zones, which have no distinct orientation relationships at the deformation boundaries. These deformation zones are considered kink bands, but the mechanisms underlying their generation are not well understood. It has been suggested that the kink bands are responsible for the deformation of the LPSO phase, while simultaneously strengthening the material. In this study, the kink deformation process of the LPSO phase under compressive deformation was investigated through molecular dynamics (MD) simulations. The MD simulations showed that numerous prismatic ⟨a⟩ dislocations were nucleated first, which led to cross-slips towards various basal planes and caused kink deformation. This was followed by the nucleation and motion of a large number of basal dislocations, as well as kink deformations in tandem with the formation of kink bands, which occurred through another process. In addition, the individual dislocations were indistinguishable at kink boundaries. In other words, sharp boundaries were formed. Next, a simulation was performed that applied tensile strain to the model after the compressive deformation described above was implemented on it. This revealed that while kink boundaries with large misorientation angles intermittently migrated because of the tensile strain, the kink bands were not easily removed.
  • Crystal Plasticity Analysis of Development of Intragranular Misorientations due to Kinking in HCP Single Crystals Subjected to Uniaxial Compressive Loading

    pp. 963-972

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    DOI:10.2320/matertrans.MH201403

    The mechanism and the effective factors for the development of intragranular misorientations due to kinking is studied by a crystal plasticity finite element method. A single crystal with hexagonal close-packed (HCP) structure in which only basal slip system is activated is used as a model material. To activate basal slip system, the initial crystal orientations are set to be the ones whose basal planes are slightly deviated from the compressive direction. The result shows that basal slip and the development of intragranular misorientations are sometimes localized near the center of the specimen depending on the initial deviation angle, strain hardening rate, and strain rate sensitivity. The mechanism is discussed in terms of the nonuniform stress distribution and lattice rotation. The effect of slight changes in the boundary conditions shows significant effect on the positions of slip localization. In summary, the present numerical results suggest that there are a number of effective factors for the development of the intragranular misorientations due to kinking including initial crystal orientation, material parameters, and boundary conditions.
  • PREFACE

    pp. 973-973

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    DOI:10.2320/matertrans.MPR2015902

  • Bonding Characteristics of Underfilled Ball Grid Array Packaging

    pp. 974-980

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    DOI:10.2320/matertrans.MI201407

    In recent years, ball grid array (BGA) package has been widely used in the portable electronic device, which can offer miniaturization and increased functional density. Thus, the failure chances of package under shock and vibration environments have been increased. In this work, we investigate the effect of underfill properties on bonding characteristics of underfilled BGA packaging. Three kinds of underfill materials with different additive content were formulated. Dynamic mechanical analysis (DMA), tensile test and shear test were conducted to examine the material properties of underfill resin such as the storage modulus (E′), glass transition temperature (Tg), mechanical and adhesion strength. In addition, the three-point bending test was conducted for underfilled BGA assemblies to investigate the mechanical reliability of BGA interconnects. The results indicate that the underfill resin should have a proper Tg to ensure the mechanical properties, and modulus of toughness of underfill resin acts as an important factor rather than mechanical strength and E′ (Stiffness) to ensure the mechanical reliability of BGA assembly.
  • Joining of Silver Nanowires by Femtosecond Laser Irradiation Method

    pp. 981-983

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    DOI:10.2320/matertrans.MI201405

    The silver nanowires with diameter around 100 nm and length up to 10 µm were successfully synthesized by a simple polyol method. After the irradiation of fs laser, the silver nanowires melted at special spots because of the surface plasmon effect, which provide possible strategy for nanojoining. Silver nanowires could be joined together at the contact point as a result of the antenna effect with slight deformation. When the gap between two silver nanowires is larger, the self-generated nanoparticles in the middle of silver nanowires performed as medium to form nanojoints.
  • Ag Nanowire and Nanoplate Composite Paste for Low Temperature Bonding

    pp. 984-987

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    DOI:10.2320/matertrans.MI201414

    Nanopastes based on noble metals for low temperature bonding are currently of great interest. We have developed Ag nanowire and nanoplate composite paste. Copper-copper joining has been achieved using solid state sintering of nanopastes. We show that an enhanced bonding strength can be achieved by integrating Ag nanoplates into Ag nanowire pastes. Ag nanowire and nanoplate composite pastes are capable of being a low-temperature interconnect material potentially for interconnection in lead-free microcircuits, flexible electronic packaging and sensing applications.
  • Lead Free BGAs Soldered with SnPb36Ag2 Solder

    pp. 988-991

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    DOI:10.2320/matertrans.MI201409

    Electronic components shrink in their dimensions due to space restrictions on electronic printed circuit boards (PCB). This is only possible on a limited scale for integrated circuits. Their number of connectors increases due to the growing complexity of the semiconductors. However the miniaturisation of the connectors is limited because of handling constrains. To overcome this dilemma the connectors of the ICs have been moved from the sides to the bottom where a matrix of solder balls forms the connection to the PCB, so called Ball Grid Arrays (BGA). Due to legislative reasons the balls of these components are made of lead free solder. However, there are still applications where tin lead solder is mandatory. It was the goal of the investigation to work out the process parameters to solder lead free BGAs with tin lead solder and to evaluate the degradation of this mixed technology.
  • Joining of Pure Copper Using Cu Nanoparticles Derived from CuO Paste

    pp. 992-996

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    DOI:10.2320/matertrans.MI201410

    A paste containing CuO particles and polyethylene glycol 1000 as a reducing solvent has been applied to joining pure Cu in electronic applications, and the bondability of the joints and bonding mechanism were investigated. Based on a combination of thermogravimetric and differential thermal analysis, pressurization in the bonding process was determined to be started at temperatures near the exothermal peak of 320°C. Pressurization started at a temperature of 320°C, with the 11 MPa shear strength of the Cu-to-Cu joint being 2.4 times greater than a joint pressed at room temperature. During the bonding process, CuO particles were not directly reduced to Cu, but were instead first reduced to Cu2O nanoparticles, which were subsequently reduced to Cu nanoparticles, and an oxide film of a Cu substrate was also reduced, thus ensuring a direct connection between a sintered Cu layer and substrate. The shear strength increases with holding time. Moreover, the shear strength of a joint created with CuO paste and a holding time of 15 min (20 MPa) is in fact higher than what can be achieved using a conventional lead-rich Pb-5Sn solder, thus making it well-suited for use in electronic applications.
  • Preparation and Sealing of Polymer Microchannels Using Electron Beam Lithography to Pattern Absorber for Laser Welding

    pp. 997-1001

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    DOI:10.2320/matertrans.MI201402

    Laser welding can make very precise joints in plastics products, both in terms of joint location and amount of heating applied. These welding methods allow complex products such as microfluidic devices to be made, where channels and structure resolution below 100 µm is regularly used. However, to date, the dimension of welds made using lasers has been limited by the focus spot size that is achievable from the laser source. Theoretically, the minimum spot size possible from a good quality laser beam is comparable to the wavelength of the radiation emitted. Practically, with reasonable focal length optics, the spot size achievable is a few factors larger than this. The resulting weld even larger than this. The narrowest welds feasible to date have therefore been 10 to 20 µm wide using a near-infrared laser source.
    The aim of this work was make welds less than 10 µm wide in PMMA thermoplastic, using EB lithography to prepare laser absorber tracks and channels, followed by laser welding to carry out welds of the order of 1 µm wide. This technique should allow welds to be made below the resolution limit of the near-infrared laser.
    Welded joints with a width of 1 µm have been achieved and channels with a width of 5 µm. The procedure was based on the principle of transmission laser welding using a thin coating of infrared absorbent material at the joint interface. The coating was patterned using electron-beam lithography to obtain the required resolution in a reproducible manner, and that resolution was retained after the transmission laser welding process. The joint strength was ratified using larger scale samples. The results demonstrate that plastics products could be made with a high density of structure with resolution below 1 µm, and that welding can be applied without excessively heating regions outside the weld lines. This may be applied to smaller scale sensor and analysis chips, micro-bio and chemical reactors using either liquids or gases, and to microelectronic packaging.
  • Shear Strength Degradation of Pb-Free Solder Joint with Mounted Location in Automobile

    pp. 1002-1006

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    DOI:10.2320/matertrans.MI201420

    Due to the End-of-Life Vehicles, car electronics must be Pb-free starting in 2016. Since the operation environment of these electronics changes with the mounted location, an understanding of the degradation behavior of the Pb-free solder joint is essential to ensuring long-term reliability. As such, this study focused on examining the degradation behavior and the dependence of the shear strength and crack length on the thermal cycling number under cabin (−40 to +85°C, 1500 cycles) and engine (−40 to +125°C, 3000 cycles) room conditions. We measured the shear strength and crack propagation length of R2012 and R3216 ceramic chip resistors as a function of thermal cycling number, and analyzed the degradation behavior with chip component size under two thermal cycling conditions. These results revealed that degradation occurs faster under engine room conditions than under cabin conditions. After 2500 thermal cycles, the crack length of the solder joint and the shear strength propagated to at least 95% and decreased by 78% compared to the as-reflowed bonding length and strength, respectively.
  • Molecular Dynamics Simulation of the Effect of Carbon Nanotube Chirality on Nano-Joining with Gold Particle

    pp. 1007-1009

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    DOI:10.2320/matertrans.MI201403

    The behavior of gold atoms depending on the CNT chirality in a nanojoining process is studied by molecular dynamics simulation. The deformation regularity and the diffusing characteristic of the gold particle during the joining process, as well as the C-Au bonds distribution in the final joint are studied. Our results show that when joining with higher spirality CNT, gold particle tends to deform more. With the CNT more similar to armchair type, the gold particle as a whole displaces more. In the final joint, the total bonds number decreases from typical armchair CNT to typical zig-zag CNT. However, the bonds distribution in detail is irregular from joint to joint, which is the consequence of lattice structure of both materials.
  • Interfacial Nano-Mechanical Properties of Copper Joints Bonded with Silver Nanopaste near Room Temperature

    pp. 1010-1014

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    DOI:10.2320/matertrans.MI201412

    Sintering of nanomaterials at low temperatures has been demonstrated as an alternative for flexible electronic packaging. Silver nanowires were synthesized via polyol method and used as filler material for bonding copper to copper near room temperature. The experimental results indicated that both silver-to-silver and copper-to-silver formed metallurgical bonds. The elastic modulus and nano-hardness of copper joints at the copper-silver interface were characterized using nanoindentation. A transition layer at the interface was observed and its thickness was determined. Sintered silver filler material showed good elasticity both inside and out of the transition layer.
  • Copper-Based Nanostructured Coatings for Low-Temperature Brazing Applications

    pp. 1015-1018

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    DOI:10.2320/matertrans.MI201419

    This feasibility study demonstrates the possibility to apply nanostructured filler materials for novel low-temperature brazing applications by exploiting the size-dependent melting behavior of metals and alloys when confined to the nano-scale regime. As an example, a copper-based nanostructured brazing filler is presented, which allows metal brazing of coated Ti-6Al-4V components at 750°C, much below the bulk melting point of copper (1083°C). The copper-based nanostructured brazing fillers can be produced in the form of coatings and free-standing brazing foils. The nano-confinement of Cu is abrogated after brazing and, consequently, the brazed joints can be operated well above their reduced brazing temperatures.
  • Solid-Liquid Interdiffusion Bonding of Copper Using Ag-Sn Layered Films

    pp. 1019-1024

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    DOI:10.2320/matertrans.MI201422

    Newly developed bonded materials and fabrication processes are expected to firmly bond copper leads to SiC chips for application in next generation power modules. Solid-liquid interdiffusion bonding of copper was performed using Ag-Sn layered films. Microstructural development and mechanical properties of bond layers were investigated. The bond layer grew at the thin film interfaces because of the solid-liquid interdiffusion. Cu6Sn5 and Ag4Sn or Ag3Sn phases were formed at the initial bonding stage, and subsequently, Cu3Sn formed between the Cu6Sn5 and Cu as both bond time and temperature increased. Finally, the bond layer was primarily composed of Ag4Sn and Cu3Sn. The hardness and Young’s modulus of Ag4Sn were much lower than those of Cu3Sn. The growth of the Cu3Sn layer that formed between Ag4Sn and Cu could be limited by optimizing the design of the faying surface.
  • Improvement of Joint Reliability of Sn-Ag-Cu Solder Bumps on Cu by a Laser Process

    pp. 1025-1029

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    DOI:10.2320/matertrans.MI201421

    Laser soldering has recently been introduced in industry because of its unique properties, which include localized and noncontact heating, a rapid rise and fall in temperature, and easy automation compared to reflow soldering. In this study, the effect of an annealing treatment on the impact strength of solder bumps heated using the laser process was investigated to improve the impact reliability of soldered joints. It was found that, in the as-soldered condition, a thin intermetallic compound (IMC) layer was formed at the interface of the solder bumps on a Cu pad when using the laser process with and without annealing. After aging at 150°C, the impact reliability of the solder bumps heated using the laser process with annealing was superior to that of the solder bumps heated using the laser process without annealing. This was because the IMC grains at the interface clearly grew to a large size. These were effective at preventing Cu atoms from diffusing to the interface of the joints soldered by the laser soldering process and may be the reason that the annealing treatment at 20 W for 5 s prevented the degradation of the maximum load of solder bumps when using the laser soldering at 40 W for 1 s.
  • Effects of Reducing Solvent on Copper, Nickel, and Aluminum Joining Using Silver Nanoparticles Derived from a Silver Oxide Paste

    pp. 1030-1036

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    DOI:10.2320/matertrans.MI201411

    The effects of reducing solvent on copper, nickel, and aluminum joining using silver nanoparticles derived from a silver oxide paste was investigated by thermal analysis, transmission electron microscopy (TEM) observation, and tensile shear testing. A complete weight loss of diethylene glycol (DEG) in a paste occurred during the redox reaction, whereas a polyethylene glycol 400 (PEG) paste retained the PEG solvent until about 300°C due to its longer carbon chains. Residual PEG in the paste reduced the natural oxide film on copper and nickel substrates during bonding, facilitating a direct sinter of silver nanoparticles to these substrates. On the other hand, silver nanoparticles were sintered to the natural oxide film on an aluminum substrate by the DEG paste. The suitability of the reducing solvent for oxide film reduction of the metal substrate during bonding was explained by an Ellingham diagram.
  • Effects of Package Warpage on Head-in-Pillow Defect

    pp. 1037-1042

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    DOI:10.2320/matertrans.MI201404

    Head-in-pillow (HiP) is a BGA defect which happens when solder balls and paste can’t contact well during reflow soldering. Package warpage was one of the major reasons for HiP formation. In this paper, package warpage was measured and simulated. It was found that the package warpage was sensitive to the thickness of inside chips. A FEM method considering viscoelastic property of mold compound was introduced to simulate package warpage. The CTE mismatch was found contributes to more than 90% of the package warpage value when reflowing at the peak temperature. A method was introduced to measure the warpage threshold, which is the smallest warpage value that may lead to HiP. The results in different atmospheres showed that the warpage threshold was 50 µm larger in N2 than that in air, suggesting that under N2 atmosphere the process window for HiP defects was larger than that under air, which agreed with the experiments.
  • Structural Changes of the IMC in Lead Free Solder Joints

    pp. 1043-1046

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    DOI:10.2320/matertrans.MI201408

    The development of Cu–Sn intermetallic compound (IMC) at the solder/Cu joints interface had been studied using five Pb-free solders as SnAg3.0Cu0.5, SnAg3.5Cu0.7, SnAg1.0Cu0.5Bi1.0, SnAg1.5Cu0.7In9.5 and SnCu0.67In2.0 alloys (composition given in mass%). The effects of Bi and In additions on the intermetallic phase formation in the lead-free solder joints with copper substrate were studied. The soldering of the copper plate was conducted at 250°C for 5 s. The solder joint reliability of SnAg3.0Cu0.5 and SnCu0.67In2.0 alloys was assessed with the thermal cycling test in the range from −40°C to 150°C. Altogether 1500 cycles were carried out. The solder joints with SnAg3.5Cu0.7, SnAg1.0Cu0.5Bi1.0 and SnAg1.5Cu0.7In9.5 alloys were subsequently aged at temperatures of 130–170°C for 2–16 days in a convection oven. The joints interface, activation energy, structural integrity were studied on the produced solder joints with optical microscopy, energy dispersive x-ray spectroscopy (EDX) microanalysis. Designed solder alloys, mentioned above, reached the most suitable result for microelectronic. With the increase of the strain rate, the failure mode migrates from the ductile fracture in the bulk solder to the brittle fracture in the IMC layer.
  • First-Principles Study of Cr2N/γ-Fe Interface in High Nitrogen Steel

    pp. 1047-1051

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    DOI:10.2320/matertrans.M2015052

    The first-principles method was employed to perform the convergence calculations on the Cr2N and γ-Fe surfaces respectively, which compose the Cr2N(0001)/γ-Fe(1-11) interface, the stability of Cr2N(0001) surface was also analyzed, to built OT and HCP stacking sequences with N atom terminated interface models. On this basis, the ideal adhesion work, stability and electronic structure of the Cr2N/γ-Fe interfaces were calculated, and the formation mechanism of the interfaces in high nitrogen steel was investigated. The result show that the OT stacking interface has a larger ideal adhesion work, and smaller interface energy over the entire range of Cr chemical potentials, which indicate that the interface with OT stacking structure is more stable than the HCP stacking one. From the electronic structure, there is a strong ionic bond between the N and Fe atoms at the interface region in the OT model, and DOS peaks of the same Fe move to the bonding area apparently as well, so that the interface structure is more stable.
  • Formation of High-Angle Dislocation Boundaries in Drawn Single Crystal Copper Wires

    pp. 1052-1057

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    DOI:10.2320/matertrans.M2014458

    The formation mechanism of the high angle boundaries in drawn single crystal copper wires was investigated using electron backscattering diffraction. Two stages for formation of high angle boundaries were found: at low strains, the high angle boundaries correspond to the boundaries between different fiber texture components referred to the drawn direction of wires while the high angle boundaries start to present within one fiber texture at high strains. The critical strain for the two-stage transformation depends on the initial orientations parallel to axis direction of single crystal wires. Such difference in formation of high angle boundaries with increasing strain is associated with the deformation homogeneity.
  • Enhanced Age Hardening in an Al-Mg-Si Alloy Using High-Speed Compression

    pp. 1058-1062

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    DOI:10.2320/matertrans.L-M2015810

    The age hardening of an Al-Mg-Si alloy was enhanced by the effect of high-speed (105 s−1) compression prior to aging. This enhancement of the age hardening is brought about by the formation of vacancy clusters during high-speed compression. High resolution transmission electron microscopy reveals that these vacancy clusters form stacking fault tetrahedra. Following peak aging, vacancy clusters and aging precipitates coexist in the grain interior. The high aging hardness obtained following high-speed compression is most probably due to the combined effects of vacancy and precipitation hardening.
  • Thermal Effects on Lifetime Evaluation of Adhesiveless Copper–Polyimide

    pp. 1063-1066

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    DOI:10.2320/matertrans.M2015041

    Adhesiveless materials are commonly used for electric or portable products that require high flexibility during operation. This structure incorporates a copper substrate and polyimide film. For life-time studies, the key aspect to evaluate is the effect of the thermal factors. In general, techniques to measure the life-time of the materials involve storing samples over a long term period in order to monitor changes; however, this is time-consuming. The aging test is an alternative method to predict and evaluate the changes in the mechanic properties over a short term period; yet the test conditions influence the accuracy of the results. The temperature and relative humidity (RH) are the major parameters for the test, and the test conditions of 85°C/85% RH and 150°C will be discussed in this paper. It is observed that the lifetime of rolled-annealed copper foil with polyimide film is significantly higher than metal films produced by other common methods, and the results are explained using the analysis of the fracture mechanics. From comparing the test data with samples stored for three years, the best prediction was achieved with conditions of 85°C/85% RH; with errors of less than 5%. The best flexibility was achieved when the optimum thickness of the copper and polyimide was determined.
  • Prediction Method of Low Cyclic Stress-Strain Curve of Structural Materials

    pp. 1067-1071

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    DOI:10.2320/matertrans.M2015085

    Based on a novel miniature tensile-fatigue device, a novel prediction method of low cyclic stress-strain curve for structural materials with cyclic hardening behavior was investigated. The prediction method was based on a modified exponential fitting equation, which was verified more appropriate to describe the low cyclic stress-strain curve than the known Hollomon equation. On basis of the fitting equation, the low cyclic stress-strain curve could be validly predicted via merely knowing the material’s engineering stress-strain curve and the stabilized cyclic stresses at two strain amplitudes. The feasibilities of the fitting method and prediction method were verified by the low cycle fatigue tests on extruded AZ31B magnesium alloy, 7075 aluminum alloy and H63 copper zinc alloy. The predicted and actual stabilized cyclic stresses were approximately close to each other. From engineering standpoint, the prediction method would reduce the testing work amount of cyclic stress-strain curve to a great extent.
  • Post-Weld Heat Treatment Effects on Hardness and Impact Strength of Aluminum Alloy 6061 Friction Stir Butt Weld

    pp. 1072-1076

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    DOI:10.2320/matertrans.M2015074

    Butt joint friction stir welding (FSW) of aluminum alloy 6061-T651 rolled plate was prepared and the weld specimens were post weld heat treated (PWHT) by solution heat treatment and aging. The hardness, toughness and fracture surface of the PWHT and as-welded specimens were investigated and their relationships were discussed. The effects of FSW are found to soften the nugget zone and thermomechanical affected zone in the as-welded condition, however, the PWHT can provide a full strength recovery in the whole weld specimen. Results from the analysis of relationship of the hardness-tensile strength found that the tensile strength (TS) is linearly proportional to the Vickers hardness number (VHN) with a TS to VHN ratio of about 2.75. However, the linear proportion with a negative slope of the hardness-toughness relationship is observed with the characteristic toughness constant of about 40 J for AA 6061. From fracture surface investigation, the unique fracture characteristics comprising double V grooves and double sharp hills with strips equivalent to the cup and cone fracture of the tensile fracture of medium ductile alloys are observed.
  • Application of Grid Increment Cluster Expansion to Modeling Potential Energy Surface of Cu-Based Alloys

    pp. 1077-1080

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    DOI:10.2320/matertrans.M2015024

    We demonstrate the applicability of extended cluster expansion technique, GICE, to calculation of a potential energy surface (PES) at discrete position in terms of atomic arrangement with an example of Cu and Cu-Ti binary system on fcc lattice. We find that the proposed CE successfully predicts total energy within error of 0.5 meV/atom for Cu and 1.2 meV/atom for Cu-Ti with respect to DFT calculation, which indicates that this method can model the PES and possesses potential to formulate physical properties in terms of atomic arrangement.
  • Influence of Interfacial Reaction on Wear Resistance of Aluminum Alloy/SiC Composites Fabricated by Low Pressure Infiltration Process

    pp. 1081-1086

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    DOI:10.2320/matertrans.F-M2015809

    Hybrid-MMCs have been fabricated by a low pressure infiltration method, using aluminum alloy as matrix, SiC fiber and SiC particle as reinforcements. Influences of orientation of fibers, interfacial reaction and matrix property on the wear resistance of Hybrid-MMCs have been investigated. The 3D-distribution of SiC fibers in the Hybrid-MMC are effective to prevent SiC particles from dropping out during the wear test. Thin reaction products at the interface between matrix and SiC fiber or SiC particle strengthen the interface bond, which results in an increase in the wear resistance of the Hybrid-MMC. The wear resistance can be improved by the formation of interfacial reaction layer and an age-hardening of the matrix. Volume expansion of the matrix accompanied by the age-hardening of the matrix increases the interface bond strength. The strengthened interface bond protects the reinforcements from dropping out and results in a superior wear resistance of the MMC.
  • Evaluation of the Biocompatibility of a Ti-Ta-Sn Alloy Using Cell Cultures

    pp. 1087-1091

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    DOI:10.2320/matertrans.M2015062

    The purpose of this study was to determine the suitability of a newly developed Ti-Ta-Sn alloy for use as a metallic biomaterial. The in vitro cell toxicity was determined by testing the corroding metal solutions using cell culture. Besides the cell adhesion rate, cell proliferation and colony formation were tested on the metal plates. Results of the cytotoxicity tests for higher concentrations of the corroding metal solutions (32 ppm and 64 ppm) revealed that the toxicity for U937 macrophages was lowest for the Ti-Ta-Sn alloy, followed by SUS316L and Co-Cr-Mo, with Ni-Ti being the most toxic. The Ti-Ta-Sn solution showed no cytotoxicity, even at a concentration of 64 ppm. The Co-Cr-Mo and Ni-Ti solutions showed high cytotoxicity for L929 cells. The cytotoxicity that was measured from the lactate dehydrogenase (LDH) release was highest for the Co-Cr-Mo alloy, followed by Ni-Ti, SUS316L, and Ti-Ta-Sn that showed the lowest value. Besides, the success rate of cell adhesion was highest for the Ti-Ta-Sn alloy after culturing for 6, 12, and 24 h. The cell proliferation tests showed that the cell proliferation speed and the relative cell proliferation rate after 3 days were both highest on Ti-Ta-Sn plates. Colony formation was highest on the Ti-Ta-Sn plates, and it was lowest on Co-Cr-Mo and SUS316L plates. These results demonstrated the suitability of the Ti-Ta-Sn alloy for use as a metallic biomaterial at the cellular level.
  • Performance Analysis of Thermoelectric Modules Using Polyhedron Elements

    pp. 1092-1095

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    DOI:10.2320/matertrans.E-M2015808

    Our previous work showed that the utilization of polyhedron elements has better advantages than the parallelogram elements in the thermoelectric (TE) generation. Especially a high efficiency of converting heat into electricity can be achieved at an optimal shape. This study proposed new TE module configurations consisting of polyhedron elements, and examined the TE performance of them by conducting the finite-element analysis. The simulation results show that the performance of TE module in the case of symmetrically arranging the polyhedron elements is slightly higher than that of arranging elements in parallel because of the more homogeneous heat flux and current density. The heat transfer and electric resistance, respectively, depend on the module configuration and element shape, and affect the TE performance simultaneously. TE performance increases significantly when the internal resistance becomes smaller and the heat diffuses slower.
  • Enhanced Room-Temperature Stretch Formability of Mg–0.2 mass%Ce Alloy Sheets Processed by Combination of High-Temperature Pre-Annealing and Warm Rolling

    pp. 1096-1101

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    DOI:10.2320/matertrans.L-M2015811

    Mg–Ce alloys exhibit good cold rollability. However, their room-temperature formability is known to be almost the same as that of commercial Mg alloys. In this study, Mg–0.2 mass%Ce alloy sheets were processed by repeated high-temperature pre-annealing and subsequent warm rolling. The Mg–Ce alloy sheet processed by pre-annealing at 773 K, rolling at 573 K, and then final annealing at 423 K exhibited a significantly increased Erichsen value of 8.0, which is comparable to the Erichsen values of commercial Al alloys. On the other hand, the reference material obtained by pre-annealing at 673 K, rolling at 673 K, and then final annealing at 423 K had a low Erichsen value of 3.1. The former sheet exhibited a basal texture with lower texture intensity and a larger basal pole inclination angle tilted toward the rolling direction. Extensive twinning contributed to this texture modification. In addition, fewer local deformation bands were observed in this sheet. The pre-annealing at 773 K, which promoted recrystallization, is thought to have resulted in the annihilation of local deformation bands. It is suggested that the enhancement of the room-temperature formability of the Mg–Ce alloy sheets could be attributed to the synchronous effects of the texture modification and the suppression of the formation of local deformation bands.
  • Electronic Properties and Stability of Graphene Oxyradical Systems

    pp. 1102-1106

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    DOI:10.2320/matertrans.M2014465

    An investigation about the electronic properties of a series of graphene patches with O-bonding to zigzag or armchair edges is performed by density functional theory (DFT). The stability, orbital hybridization, spin density, HOMO and LUMO energy for 4a4z-O, 5a5z-O and 6a6z-O graphene oxyradicals are discussed. The 4a4z-z2, 5a5z-z3 and 6a6z-z3 are the most stable structure in their individual graphene oxyradicals systems and the corresponding C=O bond length is about 0.1231 nm. This shows that the structure with O-bonding to central positions of zigzag edges is the most stable one indicating its “safe harbor” status. Meanwhile, spin density changes obviously after O-bonding to zigzag edge of graphene. As the presumptive outer effects, folding along an axis at z3 position would deprive the “safe harbor” status with O-bonding to zigzag edge. This inspires the exploration of new ways in absorbing or storage energy behavior and intermediate of combustion can be understood better.
  • Electrodeposition of Magnetite on Carbon Steel in Fe(III)-Triethanolamine Solution and Its Corrosion Behavior

    pp. 1107-1111

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    DOI:10.2320/matertrans.M2015016

    A dense and adhesive magnetite layer was successfully electrodeposited on a carbon steel in the Fe(III)-TEA solution. Electrochemical tests for the carbon steel and magnetite were conducted in an alkaline solution. These tests using the adherent magnetite specimens produced by electrodeposition is a new method to investigate the corrosion behavior of magnetite. The corrosion resistance of magnetite is superior to that of the carbon steel. When the magnetite and carbon steel are electrically contacted, the magnetite and carbon steel play the role of cathode and anode because the corrosion potential of the magnetite was higher than that of the carbon steel. If the magnetite and carbon steel are galvanically contacted to the equivalent area ratio (1 : 1), the corrosion rate of galvanic coupled carbon steel will increase by about 3.5 times than that of non-coupled carbon steel.
  • Precipitation Structure of Al–10 mass%Si–0.3 mass%Mg Alloy Produced by High Pressure Die Casting and Permanent Mold Casting with T5 Treatment

    pp. 1112-1119

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    DOI:10.2320/matertrans.M2015100

    The precipitation structures in as-cast and T5-treated specimens of high pressure die casting and permanent mold casting in Al–10 mass%Si–0.3 mass%Mg alloys were investigated using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM). No precipitates were observed in any of the as-cast materials. Fine needle-shaped precipitates, coarse rod-shaped precipitates, and granular precipitates were observed in all peak-aged specimens, which are thought to be the main cause of the increase in hardness during T5 treatment. These precipitates became finer near the eutectic phases than inside the α-Al phase cells, because the contact interface between the α-Al phase and the eutectic Si phase can be considered to become an area of vacancy sink and inhibit the growth of the precipitates. It became clear that almost all the precipitates in the die cast and the permanent mold cast specimens of this alloy were not due to the as-cast heat; rather, they were generated by the T5 treatment.
  • Turning Machinability of Short Alumina Fiber Reinforced Aluminum Alloy Composite Using Carbide Tool

    pp. 1120-1126

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    DOI:10.2320/matertrans.M2015013

    The possibility of turning fiber-reinforced aluminum alloy composites using a carbide tool was examined. Two types of short alumina fibers, which have the same fiber size, but a different chemical composition and hardness, were used as the reinforcements. The composites were fabricated by squeeze casting. Optical microscopy revealed that the fibers were randomly arranged in the alloy matrix. Fiber reinforcement decreased the cutting force and feed force of the aluminum alloy. The lower the hardness of the fiber in the composite, the lower the cutting force and feed force of the composite. The roughness of the machined surface was significantly decreased by the fiber reinforcement under every cutting condition. Observation of the chip formed on the machined surface indicated that the decrease in the surface roughness by the reinforcement was due to the suppression of the built-up-edge formation. Roughness values of the machined surface of the composite were similar to those when a diamond tool was used. The decrease in the hardness of the fibers in the composite had a significant effect on providing the long tool life.
  • Investigation of Submicron Powder Fabricated Cr50Cu50 Alloys Using Various Vacuum Hot-Press Sintering Temperatures

    pp. 1127-1132

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    DOI:10.2320/matertrans.M2015088

    The aim of this study is to explore the two different constituents of submicron-sized copper and chromium powders as Cr50Cu50 alloy materials. The research imposes various vacuum hot-press sintering temperatures (950°C, 1000°C, 1050°C and 1100°C) and pressures maintained at 12 MPa for 1 h, respectively. The experimental results show that the optimal parameters for the hot-press sintering of Cr50Cu50 alloys are 1050°C at 12 MPa for 1 h. The relative density reaches 96.09% and the apparent porosity decreases to 0.12%. Moreover, the hardness and TRS (transverse rupture strength) values increase to HV0.2 198.82 (HRB 91.07) and 910.04 MPa, respectively. The results of this study also indicate that the closed pores are effectively reduced and the mechanical properties of the Cr50Cu50 alloys are dramatically improved by increasing the temperature of the hot-press sintering process. Moreover, the optimal hot-press sintered Cr50Cu50 alloys also possess a dense microstructure and good electrical conductivity. The resistivity is decreased to 5.89 × 10−6 Ω·cm and the ICAS is enhanced to 29.27%.
  • Influence of Boron Addition on the Hot Ductility of Low-Carbon Aluminum-Killed Steel

    pp. 1133-1139

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    DOI:10.2320/matertrans.M2015069

    The hot ductility of low-carbon aluminum-killed steel with different boron contents was investigated. The hot tensile experiment was conducted using Gleeble3500 simulator. The reduction of area of each specimen was determined to estimate the quality of hot ductility. The changes of precipitates in studied steel were calculated by Thermo-Calc software. The microstructures of specimens were observed using optical microscopy (OM), sanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on the experimental results, the fracture types of specimens were analyzed. The results show that the hot ductility improves with increase in boron content. In the experimental condition of this study, the combining ability of boron with nitrogen is greater than that aluminum and nitrogen in the energetics point of view. Coarse BN particles decrease the effect of pinning grain boundary by fine AlN. Boron in solid solution plays a role of inhibiting the austenite/ferrite transformation. The improvement of hot ductility is caused by the formation of coarse BN and existence of boron in solid solution.
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  • Separation of Covering Plastics from Particulate Copper in Cable Wastes by Induction Electrostatic Separation

    pp. 1140-1143

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    DOI:10.2320/matertrans.M2015138

    A plate-type induction-electrostatic separator unit has been designed and assembled to study the removal of particulate copper from cable scrap which contains plastics for recycling. From test results, a copper rejection of 99.95% and plastics recovery of 98.8% could be successfully achieved under conditions of over an 437.5 V/cm electric field, relative humidity of less than 35%, and a splitter position of 4 cm horizontal distance and 25 cm vertical distance. A relationship to predict the recovery of plastics has been developed through regression analysis based on factors identified by analyzing free falling trajectories of particles under gravity and electrostatic attractions.
  • Effect of Sn on Thermal Conductivity of Mg-5Zn Based Alloys

    pp. 1144-1146

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    DOI:10.2320/matertrans.M2015123

    For the enhancement of high temperature performance of the Mg-Zn based alloys, Sn element has been alloyed. However, the effect of Sn on the thermal conductivity of the Mg-Zn based alloys has been barely studied. In this study, pure Mg and Mg-5Zn alloys are alloyed with a 1, 3, and 5 mass% of Sn, respectively, and the heat capacity, thermal diffusivity and thermal conductivity of the alloys are then evaluated in a temperature range of 298 to 573 K. In the Mg-Sn binary alloys, the heat capacity and thermal diffusivity gradually decrease with increasing the volume fraction of the Mg2Sn phase. In the Mg-Zn-Sn based alloys (ZT alloy), since the adding Sn element is mostly located at the MgZn2 phases, thermal diffusivity is not affected with an increment of Sn content. Furthermore, the thermal conductivity of the ZT55 alloy at elevated temperatures is slightly higher than that of the ZT53 alloys.

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