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

MATERIALS TRANSACTIONS Vol. 53 (2012), No. 1

  • PREFACE

    pp. 1-1

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    DOI:10.2320/matertrans.MPR2012901

  • Interrupt Shearing Test for Evaluating Effect of Large Shear Deformation on Evolution of Microstructure into Ultrafine Grains

    pp. 2-7

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    DOI:10.2320/matertrans.MD201119

    A new physical simulation method for evaluating the effect of shear deformation on the evolution of a microstructure is proposed. This method utilizes a high-speed compression testing machine and is capable of simulating the formation of fine grained steels in the transformation route. The outline of the proposed method, which is based on shearing and named the ‘interrupt shearing test’, is presented. The result of the preliminary application of the newly proposed test at an elevated temperature is shown. It becomes clear that the large shear deformation reproduced by the proposed method is capable of producing fine grained steels with a grain size of around 2 µm surrounded by high angle grain boundaries (HAGBs), although controlled cooling immediately after plastic deformation to accelerate transformation into finer grains is not applied in the present test.
  • Deformation Behavior in Tubular Channel Angular Pressing (TCAP) Using Triangular and Semicircular Channels

    pp. 8-12

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    DOI:10.2320/matertrans.MD201107

    In this paper, two different (triangular and semicircular) channel types were investigated in tubular channel angular pressing (TCAP) suitable for deforming cylindrical tubes to large strains without changing the tube dimensions. To examine the effects of the channel geometry on the strain distribution and deformation behavior during the TCAP process, finite element method (FEM) simulations and an analytical model were employed. The FEM results demonstrate that equivalent plastic strains of 2.15–2.9 and 2.35–2.6 were developed after applying one pass TCAP in the triangular and semicircular channels, respectively. The mean values of the equivalent plastic strains were almost identical for both cases, but the strain through the thickness with semicircular channel was more homogeneous than that in the triangular channel. Tube thinning in the early stages of the process was observed as a result of tensile circumferential strains, but this can be compensated by the back pressure effect resulting from the next shear zones and also compressive circumferential strain resulting from the decreasing tube diameter. While the strain values for both channel types were similar, the required load for the semicircular channel was lower than that of the triangular channel.
  • Production of Al/Al2O3 Nanocomposites through Consolidation by High-Pressure Torsion

    pp. 13-16

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    DOI:10.2320/matertrans.MD201128

    Aluminum nanocomposites containing 30 vol%Al2O3 were produced by severe plastic deformation using high-pressure torsion (HPT). HPT was conducted at room temperature under a pressure of 6.0 GPa for disk samples (Disk-HPT) and 3.0 GPa for ring samples (Ring-HPT). For comparison, an alternate rotation (Cyclic-HPT) was also adopted to check any difference in the microstructural development. Ring-HPT showed a more uniform dispersion of Al2O3 particles in the Al matrix and higher hardness values than those obtained by Disk-HPT. Agglomeration of Al2O3 particles was observed for any condition of HPT but the agglomerates were smaller in size, less in volume and more finely distributed in Ring-HPT than in Disk-HPT.
  • Comparative Analysis of Plastic Flow and Grain Refinement in Pure Aluminium Subjected to Simple Shear-Based Severe Plastic Deformation Processing

    pp. 17-25

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    DOI:10.2320/matertrans.MD201113

    In the present work, effects of loading scheme and strain reversal on structure and hardness evolution have been studied by using high pressure torsion (HPT) and twist extrusion (TE) techniques. High purity aluminium (99.99%) was processed at room temperature up to a maximum total equivalent strain of εmax ≈ 8 by TE, and HPT in monotonic and reversal deformation modes with strain increment Δεmax = 1. Minimum subgrain sizes reached in this study were 1.6 µm for TE and 1.1 µm for HPT. It was revealed that microstructural change with straining was a common consequence of severe plastic deformation (SPD) processing and was not affected significantly by the loading scheme. Among the SPD methods used in this study, HPT in monotonic regime produced the smallest grain size, while the most homogeneous microstructure was obtained by TE due to specific vortex-like flow field imposed by the tool geometry.
  • Changes in Strength and Microstructure of Cu (100) [001] Single Crystals Caused by Accumulative Roll-Bonding

    pp. 26-29

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    DOI:10.2320/matertrans.MD201116

    Plate-like copper single crystals with (100) [001] orientation were subjected up to nine cycles of the ARB process. Ultrafine grains less than 1 µm were obtained in the single crystal ARB processed by six cycles. Changes in strength and microstructure of the ARB processed single crystals are compared with those of the ARB processed polycrystals. Origins of high strength of the ARB processed single crystals are discussed.
  • Effect of Deformation Temperature on Microstructure Evolution in ARB Processed Ultralow Carbon IF Steel

    pp. 30-37

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    DOI:10.2320/matertrans.MD201115

    The effect of deformation temperature on the microstructure evolution was investigated in the range from room temperature to 600°C for an ultralow carbon interstitial free steel deformed to high strain by accumulative roll-bonding (ARB). In the whole temperature range, the microstructure was being subdivided by deformation-induced boundaries, and the spacing of such boundaries decreased with increasing the applied strain. The mechanism of this microstructure evolution is known as grain subdivision. In the warm-temperature ARB at 400°C and above, a quite uniform lamellar boundary structure elongated to the rolling direction was obtained after high strain, where the boundary spacing tended to be saturated above strain of approximately 4 and the saturated boundary spacing became smaller when the deformation temperature was lower. It is suggested that the microstructure after high strain deformation is determined by a balance of deformation-induced grain subdivision and restoration processes such as recovery and short-range boundary migration, and that the saturated boundary spacing is explained as a function of the Zener–Hollomon parameter. However, localized shear banding occurred at high strain in the room-temperature ARB, leading to inhomogeneities in the microstructure. It is therefore considered that a moderate deformation temperature as well strain rate has to be chosen to avoid shear localization and obtain homogeneous nanostructures.
  • High-Pressure Torsion for Pure Chromium and Niobium

    pp. 38-45

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    DOI:10.2320/matertrans.MD201131

    Two kinds of body centered cubic (bcc) structure refractory metals, pure Cr and Nb, were subjected to severe plastic deformation through high-pressure torsion (HPT) under applied pressures of 2 and 6 GPa for 2, 3, 4 and 5 revolutions at room temperature. Vickers microhardness is plotted as a function of the distance from the disk center and equivalent strain. It is shown that all hardness values fall on a single curve when they are plotted against equivalent strain for both metals. Vickers microhardness increases with increasing equivalent strain at an early stage of straining and then reaches steady state with the grain size of 200–250 nm in Cr and 240–270 nm in Nb irrespective of the applied pressures. In the steady state, there is no changing in hardness even in applying further straining. Tensile and bending tests show that brittle fracture occurs in Cr but in Nb, the strength significantly increases with some ductility after HPT processing.
  • Strengthening via Microstructure Refinement in Bulk Al–4 mass% Fe Alloy Using High-Pressure Torsion

    pp. 46-55

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    DOI:10.2320/matertrans.MD201127

    A bulk Al–4 mass% Fe alloy processed by extrusion or by a combination of extrusion and successive annealing was subjected to severe plastic deformation (SPD) through high-pressure torsion (HPT) for up to 75 revolutions. Microstructural evolution was examined by optical microscopy and transmission electron microscopy with special attention for the grain refinement in the Al matrix and for the distribution and morphology of Fe-containing intermetallic particles. Solubility of Fe in the Al matrix was studied by X-ray diffraction (XRD). A significant increase in Vickers microhardness was observed because of the development of an ultrafine-grained matrix and a fine fragmentation of the intermetallic particles especially at an early stage of SPD. The strengthening was affected by the initial state of the microstructure prior to HPT, but finally it was saturated at a higher degree of straining.
  • Influence of Processing Regimes on Fine-Grained Microstructure Development in an Al–Mg–Sc Alloy by Hot Equal-Channel Angular Pressing

    pp. 56-62

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    DOI:10.2320/matertrans.MD201108

    Grain refinement during equal-channel angular pressing (ECAP) was studied in a commercial Al–5.8%Mg–0.3%Sc alloy at temperatures from 473 to 723 K (∼0.5–0.8Tm). The samples were quenched in water in every ECAP pass, which is a conventional cyclic process. ECAP to ε = 12 resulted in ultrafine-grained structures developed uniformly at high strains at 473 and 723 K, while processing at 573 K led to the evolution of a duplex grain structure containing partially much coarser grains. In contrast, ECAP process continuously carried out to ε = 12 without interruption at 573 K, resulted in development of a uniform ultrafine-grained structure. Effects of processing regimes on microstructural evolution in the Al–Mg–Sc alloy are discussed.
  • Accelerated Diffusion and Phase Transformations in Co–Cu Alloys Driven by the Severe Plastic Deformation

    pp. 63-71

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    DOI:10.2320/matertrans.MD201111

    The cast Co–5.6 mass% Cu and Co–13.6 mass% Cu alloys were subjected to severe plastic deformation (SPD) by the high-pressure torsion (HPT). The HPT treatment drastically decreases the size of the Co grains (from 20 µm to 100 nm) and the Cu precipitates (from 2 µm to 10 nm). The metastable fcc-Co disappeared, and supersaturated Co-based solid solution present in the as-cast alloys completely decomposed after HPT. Only the phases stable below 400°C remained after severe plastic deformation (i.e. almost pure hcp-Co and fcc-Cu grains). The applicability of the concept of effective temperature originally developed for materials under irradiation for the SPD-accelerated diffusion is discussed.
  • Microstructural Evolution during ARB Process of Al–0.2 mass% Sc Alloy Containing Al3Sc Precipitates in Starting Structures

    pp. 72-80

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    DOI:10.2320/matertrans.MD201125

    Effect of pre-existing precipitates on microstructure evolution during severe plastic deformation was studied. An Al–0.2 mass%Sc alloy was aged at 300 and 400°C for having different sizes of Al3Sc precipitates. The mean precipitate size of Al3Sc was 3.62 and 50 nm for 300 and 400°C aging, respectively. In the as-aged specimens, Al3Sc had coherency with the Al matrix. The three kinds of specimens that were solution-treated (ST), aged at 300°C or aged at 400°C, were then heavily deformed by the accumulative roll bonding (ARB) process up to 10 cycles (corresponding to an equivalent strain of 8.0) at room temperature. After 10 cycles of the ARB process, the specimens showed a lamellar boundary structure having the mean lamellar interval of 0.37, 0.24 and 0.27 µm in the ST, 300°C Aged and 400°C Aged specimens, respectively. Additionally, the fraction of high angle grain boundaries (HAGBs) and the average misorientation between boundaries in the Aged-specimens were both higher than those in the ST specimen ARB processed to the same strain. It indicated that grain refinement during the ARB process was accelerated by the pre-existing precipitates. The reasons for the acceleration in microstructural evolution are considered to be the introduction of shear bands, the enhancement of dislocation multiplication rate and the inhibition of grain boundary migration by the precipitates in the pre-aged specimens.
  • Microstructure and Crystallographic Features of Martensite Transformed from Ultrafine-Grained Austenite in Fe–24Ni–0.3C Alloy

    pp. 81-86

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    DOI:10.2320/matertrans.MD201121

    We studied microstructure and crystallographic features, especially orientation relationship, of martensite transformed from austenite with mean grain sizes ranging from 35 µm to 750 nm in an Fe–24Ni–0.3C alloy. The austenite structures with ultrafine or fine grains were fabricated through intense straining by accumulative roll-bonding process and subsequent annealing. The morphology of martensite transformed from the austenite with various grain sizes was all lenticular type. On the other hand, the martensite plate size decreased with the decrease in the austenite grain size. The orientation relationship of martensite transformed from coarse-grained austenite with mean grain size (d) of 35 µm changed depending on the location within martensite plate: i.e., it was Greninger–Troiano relationship at the center part of each martensite plate, while it changed to Kurdjumov–Sachs relationship near the interphase boundary. In contrast, the whole area of each martensite plate, transformed from fine-grained austenite (d = 2.5 µm) and ultrafine-grained austenite (d = 750 nm), satisfied Greninger–Troiano relationship. Furthermore, high density of dislocations and low angle boundaries within the ultrafine-grained austenite resulted in a large scatter of observed orientation relationship.
  • An Investigation of Cavity Development during Superplastic Flow in a Zinc–Aluminum Alloy Processed Using Severe Plastic Deformation

    pp. 87-95

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    DOI:10.2320/matertrans.MD201106

    A Zn–22% Al eutectoid alloy was processed by equal-channel angular pressing for 8 passes at 473 K and then tested in tension at 473 K over a range of strain rates. A highest elongation of 2230% was recorded at a strain rate of 1.0 × 10−2 s−1 representing high strain rate superplasticity. Quantitative cavity measurements and a cavity volume analysis were undertaken to investigate the growth of internal cavities during superplastic flow. The results demonstrate a clear transition from the superplastic diffusion growth of cavities at the lower elongations to plasticity-controlled cavity growth at the higher elongations. A cavity growth diagram provides an excellent description of the cavity growth processes including the transition from superplastic diffusion growth to plasticity-controlled growth at cavity radii larger than ∼2.1 µm.
  • Retardation of Softening of Ultrafine-Grained Copper during Low Temperature Annealing under Uniaxial Tensile Stress

    pp. 96-100

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    DOI:10.2320/matertrans.MD201117

    It is known that pure metals having ultrafine grains (UFGs) exhibit softening and grain coarsening at temperatures about one third of the melting temperatures. When such low-temperature annealing (LTA) of UFG copper is conducted under uniaxial tensile stress, the retardation of the softening is found to occur compared with that without stress. Observations of the microstructure and texture analysis indicate that the softening and the grain coarsening are attributed to recrystallization, and the presence of uniaxial tensile stress during LTA retards recrystallization. High voltage transmission electron microscopy revealed that the dislocation density of UFG copper annealed under uniaxial tensile stress is lower than that after LTA without stress. The retardation of recrystallization is associated with the reduction of the dislocation density of unrecrystallized UFG copper induced by dynamic recovery.
  • Fatigue Crack Growth and Related Microstructure Evolution in Ultrafine Grain Copper Processed by ECAP

    pp. 101-108

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    DOI:10.2320/matertrans.MD201123

    Fatigue crack growth tests were carried out using centre-cracked tensile (CCT) specimens of ultrafine grained (UFG) copper, aiming at clarifying microstructure evolution around the fatigue crack tip and at better understanding of fatigue crack propagation mechanisms. The fatigue crack growth tests revealed that UFG copper had the lowest threshold stress intensity factor range ΔKth. On the other hand, the crack growth rates were almost identical among the UFG and conventional specimens tested at higher ΔKI regime. From the microstructure observation after the fatigue crack growth test, significant grain growth was detected in the intimate vicinity to the fracture surface in UFG copper. The size of the grown grains in the UFG copper increased with increasing stress intensity factor range and with the size of the cyclic plastic zone.
  • Effects of SiO2 Particles on Deformation of Mechanically Milled Water-Atomized SUS304L Powder Compacts

    pp. 109-115

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    DOI:10.2320/matertrans.MD201120

    Ultrafine-grained (UFG) materials show increased strength and hardness, but usually exhibit limited ductility. It is imperative to improve the mechanical properties in terms of ductility and toughness. Bimodal grain size distribution is an effective way to retain high strength, while improving ductility. SUS304L water-atomized powders were mechanically milled (MM) and consolidated by hot isostatic pressing (HIP). A bimodal structure could be observed in the sintered compacts. After sintering of the water-atomized powders, many silicon oxide particles disperse uniformly in the ultrafine-grained regions and at the UFG/CG interface. The ultrafine-grained regions improve the strength and hardness, while the coarse-grained regions provide work-hardening to maintain significant uniform plastic deformation. However, compared with bulk materials without silicon oxide particles, the silicon oxide particles existing in bimodal structure compacts lead to impaired inter-particle bonding, as reflected in markedly-worsened elongation. The effects of silicon oxide particles on deformation and fracture behavior of bimodal structure compacts are investigated. To obtain enhanced mechanical properties, the surface silicon oxide must be reduced.
  • Changes in Mechanical Characteristics of Pre-Annealed Wires of Cu–Sn Alloy Manufactured by Continuous Draw Bending

    pp. 116-122

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    DOI:10.2320/matertrans.MD201114

    The changes in mechanical properties and microstructures of fine Cu–Sn alloy wires manufactured by deep wire drawing and heat treatment, and of the wires deformed by draw bending were systematically investigated. During draw bending, the wires were subjected to the compressive/tensile or tensile/compressive strain in the longitudinal direction. As a result, it was confirmed that the softening caused by plastic deformation was induced when easing up the wire drawing strain by heat treatment were few, such as the decreasing strength and increasing elongation. Work hardening was induced when easing up the wire drawing strain by heat treatment were large, such as the increasing strength and decreasing elongation. With decreasing strength of softening induced by plastic deformation, increase in grain size is smaller than the strength deterioration. Additionally, it was confirmed that the occupancy probability of high-angle grain boundaries increases. This can be understood by considering that the many dislocations formed inside the grains are piled up in one direction during draw bending. Additionally, it is considered that ductility enhancement is caused by the increase in work hardening rate induced by the piled up dislocations.
  • Equal-Channel Angular Pressing and High-Pressure Torsion of Pure Copper: Evolution of Electrical Conductivity and Hardness with Strain

    pp. 123-127

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    DOI:10.2320/matertrans.MD201109

    Pure Cu (99.99%) is processed by equal-channel angular pressing (ECAP) and by high-pressure torsion (HPT). The electrical resistivity as well as the microhardness increases with an increase in the equivalent strain at an early stage of straining, but saturates to a steady state at the equivalent strains more than ∼20. At the steady state, the samples processed by ECAP and HPT show a significant increase in the hardness (∼270%) but little decrease in the electrical conductivity (∼12%) when compared to the annealed state. Transmission electron microscopy confirms that the microstructure does not change at the saturated level with further straining. Evolutions of hardness, electrical conductivity and microstructures are also investigated after post-HPT annealing.
  • Dry Sliding Wear Properties of Sub-Microcrystalline Ultra-Low Carbon Steel Produced by High-Pressure Torsion Straining

    pp. 128-132

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    DOI:10.2320/matertrans.MD201118

    This study is the first to show dry sliding wear properties of sub-microcrystalline ultra-low carbon steel produced by high-pressure torsion (HPT) straining. Effects of number of turns in HPT process and counter materials in wear tests were investigated using a ball-on-disc friction method. Wear tests were carried out in a normal laboratory atmosphere of air at a sliding speed of 0.042 m/s and an applied load of 39.6 N. When the ball material was cemented carbide (WC–Co), the wear depth of HPT-processed discs decreased with increasing the number of turns in HPT-straining. This was explained by the hardness variation, i.e. the HPT-processed discs in which large strain was introduced exhibit high anti-wear resistance due to the high hardness. On the other hand, when the ball material was high carbon-chromium bearing steel (SUJ2), the wear depth of disc specimens increased with increasing the number of turns. The unusual behavior of the wear depth variation was thought to be attributed to the intensive adhesion between the HPT-processed steel disc and the SUJ2 ball because the coefficient of friction was considerably high for the ultra fine-grained specimens.
  • Estimations of the True Stress and True Strain until Just before Fracture by the Stepwise Tensile Test and Bridgman Equation for Various Metals and Alloys

    pp. 133-139

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    DOI:10.2320/matertrans.MD201112

    True stress (σ)–true strain (ε) relationships until just before fracture, i.e., the plastic deformation limit, were estimated by the stepwise tensile test and the Bridgman equation for various metals and alloys with different crystal structures. The estimated σ–ε relationships were different from the nominal stress–strain curves including the conventional tensile properties. In the relationships between the true stress (σpdl) and true strain (εpdl) at the plastic deformation limit, SUS304 and SUS329J4L indicated a better σpdl–εpdl balance. On the other hand, SUS329J4L, tempered martensite, and an ultrafine-grained steel showed superior results in the yield strength–εpdl balance. The estimated σ–ε relationship for the ultrafine-grained steel suggests that grain refinement strengthening can improve σ and ε up until the plastic deformation limit. The value of εpdl became larger with increasing the reduction in area and a decrease in the fracture stress. The products of σpdl and εpdl became larger with increasing work-hardening rate at the plastic deformation limit.
  • First-Principles Tensile Tests of Tilt and Twist Grain Boundaries in Al

    pp. 140-146

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    DOI:10.2320/matertrans.MD201130

    In order to clarify the basic mechanical behavior of grain boundaries (GBs) in Al under tensile stresses, the first-principles tensile tests of coincidence tilt and twist GBs in Al have been performed by using the projector augmented wave (PAW) method based on the density-functional theory (DFT) within the generalized gradient approximation (GGA). For the {221} Σ = 9 tilt and {001} Σ = 5 twist GBs in Al, we have obtained ideal strength and stress–strain curves, and analyzed bond-breaking behavior. Results for the Σ = 9 tilt GB are consistent with previous results using the local density approximation (LDA). The Σ = 5 twist GB reveals complex behavior of deformation and failure, different from tilt GBs. In both the interfaces, reconstructed interfacial bonds reveal high strength, inducing large deformation in the GB regions before the failure, due to the covalent bonding nature of Al.
  • Grain Boundary Structure Dependence of Extrinsic Grain Boundary Dislocation Emission Phenomena: A Molecular Dynamics Study

    pp. 147-155

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    DOI:10.2320/matertrans.MD201124

    Recently, it has been reported that extrinsic grain boundary dislocations (EGBDs) are often present in the grain boundaries of ultrafine-grained (UFG) metals produced by severe plastic deformation; therefore, the emission of EGBDs from the grain boundaries could afect the mechanical characteristics of UFG metals. In this paper, we use molecular dynamics to simulate the emission of EGBDs from grain boundaries, and we examine the grain boundary structure dependence of the emissions. Then we apply J-integral analysis to evaluate the Peach–Koehler force required for the grain boundaries to emit the EGBD. It can be confirmed that the Peach–Koehler force required to emit the EGBD is highly dependent on the relationship between the Burgers vector components of the EGBD and intrinsic grain boundary dislocations (IGBDs), which form the equilibrium grain boundaries. Comparing analyses of the linear elastic theory with atomic simulations, we confirm that nonlinear structural changes in the dislocation cores of the EGBD and IGBDs, which can only be expressed by atomic scale resolution, are responsible for such strong grain boundary structural dependence. We also verify that normal stress components perpendicular to the slip plane of EGBDs have a significant effect on the emission of EGBDs.
  • Grain Size Dependence of Creep in Nanocrystalline Copper by Molecular Dynamics

    pp. 156-160

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    DOI:10.2320/matertrans.MD201122

    The grain size dependence of creep is critical to understand the plastic deformation mechanism of nanoscale metals. Here we used molecular dynamics to study the stress-induced grain size exponent transition in creep of nanocrystalline copper. The grain size exponent was found to initially increase with increasing stress, then decrease after some critical stress. The derived grain size exponents are in agreement with experimental results for diffusional and grain boundary sliding creep at low stress. While, the founded decreasing grain size exponent with increasing stress for dislocation nucleation creep in nanocrystal is in contrast with conventional materials. We propose a constitutive equation for dislocation nucleation governed creep in nanocrystal to explain its grain size dependence transition with stress.
  • Crystal Plasticity Simulation Considering Oxidation along Grain Boundary and Effect of Grain Size on Stress Corrosion Cracking

    pp. 161-166

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    DOI:10.2320/matertrans.MD201126

    Stress corrosion cracking is a critical concern for light water reactors because it can degrade structural components over a long period. It takes the form of intergranular stress corrosion cracking (IGSCC). Many studies on IGSCC have been conducted over several decades in the past. However, the mechanism of IGSCC initiation and propagation is still not fully understood. In this study, a crystal plasticity model expressing IGSCC is proposed by considering information about the oxidation along the grain boundaries and the failure of an oxide film caused by the localization of a deformation. From a crystal plasticity finite element analysis and an oxygen reaction-diffusion finite difference analysis based on the presented model, the IGSCC is numerically reproduced, and we discuss the effect of grain size on the crack propagation behavior.
  • Microstructural Features and Erosion Wear Resistance of Friction Stir Surface Hardened Spheroidal Graphite Cast Iron

    pp. 167-172

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    DOI:10.2320/matertrans.M2011224

    A ferritic spheroidal graphite cast iron (also named as SG cast iron, ductile cast iron, ductile iron) was treated with friction stir process (FSP) to harden the surface layer owing to a unique microstructure into which the ferritic structure transforms after high temperature deformation and subsequent direct cooling. When the friction stirred surface experiences thermomechanical cycle during FSP (here named friction stir surface hardening, FSSH), a non-traditional bainite structure can be obtained through subsequent cooling process. The bainite structure primarily consists of iron carbide (Fe3C), acicular ferrite and martensite with retained austenite aggregates. It is evident that the FSSH structure caused by deformation at austenite temperature has resulted in a significant increase in the microhardness of about 1000 HV yielding a primarily martensitic accompanying bainitic phase transformation. The experimental results also show that the process has resulted in significant improvement in erosion resistance at low angle impingement than that of ferritic specimens. In addition, the maximum erosion rate of ferritic specimens occurs at 20–25°of impact while the peak of the FSSH specimen shifts to higher angle resulting from the formation of continuously cooled martensitic and bainitic structure.
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  • Effects of Short-Time Solution Treatment on Cold Workability of Ti–6Al–4V Alloy

    pp. 173-178

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    DOI:10.2320/matertrans.M2011182

    This study was conducted to investigate the effects of short-time solution treatment on the cold workability of α+β titanium alloy Ti–6Al–4V. In this treatment, the titanium alloy was heated at 1098–1223 K for 1 s and then water-quenched. The cold workability, such as deep drawability and bendability, was improved through a short-time solution treatment because the yield ratio (yield strength/tensile strength) decreased, while the ductility simultaneously increased. The results of the X-ray diffraction, TEM observation, and selected area electron diffraction suggested that the improvement in the cold workability was the result of stress-induced transformations in both the α′′ martensite phase and the metastable β phase. Furthermore, the short-time aging effect was investigated on the material solution-treated at 1148 K for 1 s and pre-deformed to a 16.7% strain. The results showed that the yield strength recovered and then exceeded the original strength level through short-time aging.
  • Strengthening of Sintered Austenitic Stainless Steels through Low-Temperature Carburization

    pp. 179-184

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    DOI:10.2320/matertrans.M2011150

    The tensile strength and hardness of pressed-and-sintered 316L and 304L are generally poor due to the low sintered density and austenitic structure. To improve these properties, low-temperature carburization (LTC) was applied to these materials. In contrast to fully dense parts, for which LTC can increase the surface hardness with only limited depth, carbon can effectively diffuse into the center of pressed-and-sintered specimens through interconnected pores and harden all pore surfaces inside the compact. For sintered 316L with a density of 6.71 × 103 kg/m3, the hardness increased from 25 to 75 HRB (from HV70 to HV137) and the tensile strength increased from 295 to 520 MPa after LTC, while the corrosion resistance remained almost the same because no chromium carbide formed. The hardness and tensile strength of sintered 304L were also improved after LTC. For sintered 304L with a density of 6.70 × 103 kg/m3, the hardness increased from 27 to 74 HRB (from HV72 to HV135), and the tensile strength increased from 291 to 519 MPa. The bulk hardness and tensile strength of the high-density part were lower than those with a low density, since less carbon could diffuse into the center and fewer carburized regions formed.
  • Determination of the Load Carrying Capacity of Damaged Pipes Using Local Approach to Fracture

    pp. 185-190

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    DOI:10.2320/matertrans.M2011210

    The subject of this study was the application of local approach to ductile fracture in order to estimate the integrity of damaged seam casing pipes for oil and gas drilling rigs. The experimental testing included tensile testing of specimens and a pressure test of a pipe with different levels of damage simulated by machined notches. In exploitation, such structures (i.e., pipes with local thin areas) can fail by the ductile fracture mechanism or by plastic collapse in the ligament. However, the majority of the procedures for determining their integrity are based on limit loads, i.e., plastic collapse criteria. In this work, a pipe subjected to internal pressure was modelled using the finite element method and local approach to fracture (the Complete Gurson Model - CGM), with the aim of determining damage development in the material (i.e., at the bottom of a machined defect) and of establishing the criteria for the maximum pressure that a damaged pipe can withstand. The results obtained using the micromechanical model are discussed and compared with several often used limit load expressions from the literature and a stress-based finite element criterion. It is shown that local approach can give appropriate results and represent failure criterion for pipes with local thin areas.
  • The Novel Wetting Behavior of Periodic TixSn1−xO2 Nanostructures

    pp. 191-194

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    DOI:10.2320/matertrans.M2011269

    Regular arrays of TixSn1−xO2 nanostructures were produced through glancing angle sputter deposition onto self-assembled close-packed arrays of 200-nm-diameter and 1-µm-diameter polystyrene microspheres respectively. The anisotropic nanoflakes grown on 200-nm-diameter polystyrene microspheres exhibited macroscopic-wetting anisotropy, with the apparent contact angle observed parallel to the direction of the nanoflakes larger than the apparent contact angle in the perpendicular direction. This anisotropic wettability is ascribed to the difference of the three-phase contact line structure for the parallel and perpendicular direction, resulting from the anisotropic topography. Compared with the nanoflakes, the “spheric shells” were obtained using the 1-µm-diameter polystyrene microsphere templates, on which the apparent contact angle of the drop was nearly uniform along the contact line. After annealing at 823 K for 3 h, both the films were crystallized to TiO2/SnO2 composite and maintained good thermal stability in the morphology. It was shown that the “spheric shells” can be reversibly switched between hydrophobicity and superhydrophilicity by alternating visible light illumination and dark storage. This novel visible light-responsive behavior was explained by the enhanced separation efficiency of photogenerated electron–hole pairs in the TiO2/SnO2 composite.
  • Growth Behavior of Surface Oxide Layer on SUS316L Stainless Steel at the Early Stage of Exposure to 288°C Water

    pp. 195-200

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    DOI:10.2320/matertrans.M2011200

    Surface oxide layer on SUS316L stainless steels exposed to 288°C pure water with 2 ppm dissolved oxygen (DO) for 1–100 h were analyzed using Focused Ion Beam (FIB) and Scanning Transmission Electron Microscope (STEM) technique to understand the early stage of surface oxide layer formation. In order to analyze the multi layered surface oxide, the interfaces between the outer and the inner oxide layers and that between the inner oxide layer and SUS316L substrate were determined from Energy Dispersive X-ray Spectroscopy (EDX) line profiles. At 1 h exposure, double oxide layer which is composed of compact inner oxide layer and outer oxide layer with Fe-rich and Ni-rich oxide particles was formed. At the outermost region of the SUS316L substrate, Ni and Cr were enriched. At 100 h exposure, growth of the inner oxide layer was suppressed and the Ni and Cr enriched region at the alloy substrate was preserved underneath the Ni-rich outer oxide particles. At 1 h exposure, most of the outer oxide particles were composed of Fe-rich ones, at 10 h exposure, another Ni-rich outer oxide particles were nucleated and grew faster than Fe-rich ones. Consequently, a part of pre-formed Fe-rich outer oxide particles were covered with Ni-rich ones.
  • Simultaneous Determination of Interfacial Friction and Flow Curves of Temperature and Rate Dependent Materials for Bulk Metal Forming Processes

    pp. 201-208

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    DOI:10.2320/matertrans.M2011246

    This paper is concerned with application of the ring test to temperature and rate dependent materials. With this regard, the stress–strain response of AZ80 magnesium alloy was obtained for various temperatures, strains and strain rates by means of this experiment. Using finite element (FE) simulations, numerical sigmoid curves and calibration curves of the ring compression test were also determined. Moreover, the effects of temperature and deformation rate on the geometry of the numerical sigmoid curves were evaluated. By comparing the load-displacement curves obtained from the FE analyses with those of the experiments, it was found that the numerical sigmoid curves, compared with analytical ones, provided more accurate flow curves. Furthermore, the influences of the temperature and strain rate on the shape of the sigmoid curves were more perceptible at higher levels of friction.
  • Formation and Growth of Tin Whiskers on Aluminum–Tin Alloys

    pp. 209-216

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    DOI:10.2320/matertrans.L-M2011830

    In order to clarify the mechanism of formation of nodules and whiskers on the conducting wires used in aluminum electrolytic capacitors, aluminum–tin binary alloys were subjected to an investigation as the model alloys for the joints in the conducting wires. The concentration of tin in the binary alloys was 1, 5 or 10 at%. The 10 at% tin alloy showed the highest number of nodules or whiskers on its polished surface after storing under ambient conditions for 7.8 Ms. Many whiskers whose length were greater than several tens of micrometers were observed for 5 at% tin alloy. The 1 at% tin alloy showed few nodules or whiskers. Growth of the nodules and whiskers is caused by diffusion of the tin atoms from the strained or high-energy areas into the low-energy ones or the root grains. The extrusion toward the surface at the root grains then develops nodules and whiskers. As a preventive measure of whisker formation, selective etching of the aluminum phase using a solution of sodium hydroxide was confirmed to be successful. Thus the aluminum phase was thought to form a non-uniform distribution of strain in the tin phase. This acts as the driving force for diffusion of the tin atoms.
  • Superelastic Deformation of TiNi Shape Memory Alloy Subjected to Various Subloop Loadings

    pp. 217-223

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    DOI:10.2320/matertrans.M2011288

    This paper investigates the superelastic deformation behaviors of a TiNi shape-memory alloy (SMA) tape subjected to various subloop loadings in relation to local temperature variations and observed surface changes during a tension test. The results obtained are: (1) Upper and lower stress plateaus appear during loading and unloading accompanying the spreading and shrinking of the stress-induced martensitic transformation (SIMT) bands. In the case of unloading from the upper stress plateau under low stress rate, strain increases due to the spreading of the SIMT bands at the start of the unloading. (2) If stress at the upper stress plateau is held constant, creep deformation appears with the spread of the SIMT bands. The volume fraction in the martensitic phase increases in proportion to the increase in strain. (3) Where the strain is made to vary at the stress plateaus during loading or unloading, a return point memory effect can be seen in the reloading stress–strain curve. The spreading or shrinking of the SIMT bands starts from the boundary of the previous SIMT bands remaining from the preceding process. (4) The inclination angle of the SIMT band boundaries to the tensile axis of the tape is 33° for an aspect ratio of 5. The inclination angle is 42° in the center of the tape and 37° in the vicinity of the end secured by the grip, for an aspect ratio of 10.
  • Effect of Coat Permeability on Temperature Drop of Molten Aluminum Alloy during Mold Filling in Expendable Pattern Casting Process

    pp. 224-228

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    DOI:10.2320/matertrans.M2011242

    In the expendable pattern casting (EPC) process for the aluminum alloy casting, the thermal decomposition rate of the expendable pattern is smaller than the cast iron, therefore the misrun by the temperature drop at the melt surface occurs easily. This study takes account of the control of the heat release to the mold to investigate numerically and experimentally the effect of the coat permeability on the molten aluminum alloy temperature during mold filling in the EPC process. An aluminum alloy plate was cast by the EPC process, and the temperature change of the melt surface to the melt flow direction was measured. The use of high permeability coat led to a higher melt velocity and a smaller temperature drop at the melt surface than the case of the normal permeability coat. The temperature of molten metal filling into the cavity was numerically simulated, and the obtained temperature drop at the melt surface agreed relatively well with the experimental values.
  • Effect of Electric Current on Microstructural Evolution in a Cold-Rolled 3% Si Steel

    pp. 229-233

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    DOI:10.2320/matertrans.M2011272

    The effect of electric current pulse (ECP) treatment on grain orientation in a cold-rolled Fe–3%Si steel was investigated in this study. Results showed that the recrystallized nuclei preferred to form along the current direction in the primary period of recrystallization. The theoretical analysis revealed that the anisotropic nucleation orientation was ascribed to the different dislocation mobility derived from the electron wind force during the passing of electric current. Hence, the ECP treatment should be a special and effective method to control the nucleation orientation, and the present work is of great technological and physical importance.
  • Effect of Alloying Elements on High Temperature Mechanical Properties for Piston Alloy

    pp. 234-239

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    DOI:10.2320/matertrans.M2011259

    Recent legislative and environmental pressures on the automotive industry to produce light-weight fuel-efficient vehicles with lower emissions have led to a requirement for more efficient engines. Therefore, combustion pressures of diesel engines have increased up to 20 MPa and the more durable alloys for pistons are thus necessary to increase the thermal and fatigue resistance. The demand for more efficient engines is resulting in components operating under severe stress and temperature conditions. During start/stop of engine cycles, LCF (low cycle fatigue) phenomena is generated due to the thermal transient, also HCF (high cycle fatigue) and creep deformation occur under the steady-state engine temperature during operation of engine. A quantitative study of the effect of alloying elements on mechanical behavior of Al–12 mass%Si casting alloys for piston has been conducted. In the condition of minimizing casting defects, the influence of compounds features on the high temperature mechanical performance became more pronounced. Depending on Ni and Cu content affecting the strength of the matrix, the tensile strength was increased with Ni and Cu content, whereas the elongation was increased in the reverse case. Also, creep resistance was drastically increased with Ni and Cu contents mainly due to prevention of deformation owing to the increased eutectic and precipitation particles. LCF lives were decreased with alloy contents in Coffin–Manson relation because of the smaller elongation, but the analysis of fatigue lives with hysteresis loop energy which consists of both strength and elongation showed that the fatigue lives were normalized regardless of chemical compositions and test temperature.
  • Influence of Addition of Ge on the Microstructure and Corrosion Properties of Magnesium

    pp. 240-243

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    DOI:10.2320/matertrans.M2011240

    The corrosion properties of Mg–xGe (x = 0, 0.5, 1.0, 1.5 and 2.0 mass%) alloys were investigated. Potentiodynamic polarization and electrochemical impedance spectroscopy tests were carried out in a 3.5% NaCl solution at pH 7.2 to measure the corrosion properties of Mg–xGe (x = 0, 0.5, 1.0, 1.5 and 2.0 mass%) alloys. Microstructural analysis showed that a Mg2Ge phase formed mainly in the interdendritic areas. The volume fraction of the Mg2Ge phase was increased with increasing Ge content. The corrosion resistance of the Mg–xGe alloys was improved by Ge addition. In particular, the Mg–1.5 mass%Ge alloy showed the superior corrosion resistance of the alloys examined.
  • Synthesis of Anatase Phase Titanium Dioxide Using High-Power Nd:YAG Laser Focused on Titanium Wire in Water

    pp. 244-247

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    DOI:10.2320/matertrans.M2011183

    An experimental method to synthesize titanium dioxide (TiO2) using high-power laser in water was performed. A high-power Nd:YAG pulsed laser was used for the synthesis, with the laser energy fixed at 1 J/pulse. This laser was focused on a titanium wire set in water. This investigation recovered nano-sized anatase phase titanium dioxide with well crystallized structure. Pulsed bubbles generated in the water were confirmed by optical measurement, and their collapse may have induced high pressures and temperatures. The bubbles generated was approximately spherical in shape, with an estimated maximum size of 3.7 mm generated about 200 µs later after focusing the laser. Recovered powders were confirmed as single anatase phase titanium dioxide by XRD analysis. Effects of bubbles on synthesis and crystallization are also suggested.

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