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

MATERIALS TRANSACTIONS Vol. 55 (2014), No. 1

  • PREFACE

    pp. 1-1

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    DOI:10.2320/matertrans.MPR2013907

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  • 60 Years of Hall-Petch: Past to Present Nano-Scale Connections

    pp. 2-12

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    DOI:10.2320/matertrans.MA201302

    Pioneering research results reported in the early 1950’s by E. O. Hall and N. J. Petch on iron and steel materials have led to an expanded description of the grain size dependence of the complete stress–strain behavior of a wider range of materials and including assessments of other mechanical properties such as the ductile to brittle transition behavior and the hardness of materials, particularly, of nanocrystalline materials. The dislocation pile-up model that was presented originally for the inverse square root of grain diameter dependence of material strength has endured. Most recently, the pile-up model description has been more definitely associated with the Griffith theory of achieving a critical stress concentration at the tip of a crack; and, the Hall-Petch analysis has been connected to the macro-scale description of the fracture mechanics stress intensity parameter. These topics and other “60 years of Hall-Petch” type researches are tracked over time in the present report while giving special emphasis to current order-of-magnitude strength improvements that are reported for metals with nanopolycrystalline grain diameters.
  • Superior Strength in Ultrafine-Grained Materials Produced by SPD Processing

    pp. 13-18

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    DOI:10.2320/matertrans.MA201325

    Recent studies demonstrated that the processing of metallic alloys by severe plastic deformation (SPD) can result in not only strong grain refinement but also leads to the formation of grain boundaries (GBs) with different structures, including GB segregations and precipitations. These nanostructural features of SPD-processed alloys produce considerable influence on their mechanical properties. The paper presents experimental data demonstrating a superstrength and “positive” slope of the Hall–Petch relation when passing from micro- to nanostructured state in a number of metallic materials subjected to severe plastic deformation. The nature of the superior strength is associated with new strengthening mechanisms and the difficulty of generation of dislocations from grain boundaries with segregations.
    This new approach is used for achieving the enhanced strength in several commercial Al and Ti alloys as well as steels subjected to SPD processing.
  • Hall–Petch Relationship and Dislocation Model for Deformation of Ultrafine-Grained and Nanocrystalline Metals

    pp. 19-24

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    DOI:10.2320/matertrans.MA201310

    Models and theories to explain the Hall–Petch relationship are reviewed briefly. Then, a dislocation model to incorporate some characteristic mechanical properties of ultrafine-grained and nanocrystalline metals will be introduced and used to explain some experimental results. The model is based on the idea that dislocations emitted from grain boundaries and bow out into grain interiors during their propagation are responsible for plastic deformation and thermally-activated depinning process at grain boundaries is regarded to be rate controlling. Some implications of the model are discussed in the light of recent experimental results.
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    Readers Who Read This Article Also Read

    1. Phosphorus and Boron Segregation at Prior Austenite Grain Boundaries in Low-alloyed Steel ISIJ International Vol.42(2002), No.6
    2. 60 Years of Hall-Petch: Past to Present Nano-Scale Connections MATERIALS TRANSACTIONS Vol.55(2014), No.1
    3. Work Hardening Behavior of IF and Low Carbon Steel Sheets under Biaxial Tension Tetsu-to-Hagané Vol.90(2004), No.12
  • On the Yield Stress of Bulk Nano Grained Polycrystals

    pp. 25-27

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    DOI:10.2320/matertrans.MA201308

    In this paper we review previous analyses of the reasons for the departure from the Hall–Petch law observed in nano grained metallic polycrystals. We remind at first that the -D - 1/2 scaling of the flow stress in coarse grained metallic polycrystals is not associated with planar slip and pile ups, but that it is a consequence of the building of a 3-dimensional microstructure. We show afterwards that the main reason for the specific behaviour of nano grained metallic polycrystals lies in the difference of the extent of the route followed by the material from elastic to plastic deformation, itself a consequence of the small extent of the dislocations mean free path in these structures.
  • Effect of Grain Boundary Segregation of Interstitial Elements on Hall–Petch Coefficient in Steels

    pp. 28-34

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    DOI:10.2320/matertrans.MA201314

    The yielding behavior of interstitial-free steels and low-carbon steels with varying amounts of C and N were investigated in connection with the Hall–Petch relation. The Hall–Petch coefficient is as small as 150 MPa·µm1/2 in interstitial-free steels but it increases to 600 MPa·µm1/2 by adding solute carbon up to 60 ppm. Nitrogen does not have a significant effect on the Hall–Petch coefficient. The results of three-dimensional (3D) atom probe analysis indicated that carbon has 3–4 times greater segregation potential in comparison with nitrogen. The small effect of nitrogen on the Hall–Petch coefficient in steel is probably due to the small segregation potential of nitrogen. It was also confirmed that discontinuous yielding occurs when the difference between the yield stress and friction stress is increased by grain-refinement strengthening and that yielding occurs by dislocation emission from grain boundaries where primary dislocations have piled up. Carbon atoms segregated at grain boundaries seem to play a role in stabilizing dislocation emission sites at the grain boundaries, which enhances the Hall–Petch coefficient of iron. These results support the dislocation pile-up model of explaining yielding in polycrystalline metals.
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    Readers Who Read This Article Also Read

    1. Phosphorus and Boron Segregation at Prior Austenite Grain Boundaries in Low-alloyed Steel ISIJ International Vol.42(2002), No.6
    2. 60 Years of Hall-Petch: Past to Present Nano-Scale Connections MATERIALS TRANSACTIONS Vol.55(2014), No.1
    3. Work Hardening Behavior of IF and Low Carbon Steel Sheets under Biaxial Tension Tetsu-to-Hagané Vol.90(2004), No.12
  • A Practical Meso-Scale Polycrystal Model to Predict Dislocation Densities and the Hall–Petch Effect

    pp. 35-38

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    DOI:10.2320/matertrans.MA201305

    A practical SuperDislocation Model (SDM) has been developed and implemented to predict dislocation density distributions in a plastically deforming polycrystal and thereby the Hall–Petch effect. The model is composed of two stepwise simulation scales; the first scale is a finite element model of a polycrystal using a novel single-crystal constitutive equation and the second scale redistributes the mobile part of the dislocation density within grains consistent with the plastic strain distribution, and enforces slip transmission criteria at grain boundaries that depend on local grain and boundary properties.
    In this work, deformation of Fe–3% Si tensile specimen is simulated using SDM to compare dislocation densities obtained from the high-resolution electron backscatter diffraction (HR-EBSD). The model accurately predicts the measured dislocation density at 10% deformation. In addition, size-dependent simulations show that the model qualitatively predicts Hall–Petch slope as well as the grain boundary strength of Fe–3% Si.
  • On the Roles of Grain Size Dispersion and Microscale Hall–Petch Relation on the Plastic Behavior of Polycrystalline Metals

    pp. 39-43

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    DOI:10.2320/matertrans.MA201307

    Micro-macro scale transition theories were essentially developed to model the inelastic behavior of polycrystals starting from the local behavior of the grains. The anisotropy of the plastic behavior of polycrystalline metals was mainly explained by taking into account the crystallographic textures. Issues like plastic heterogeneities due to grain size dispersion, involving the Hall–Petch relation at the microscale, were not often taken into account in the literature, because only the role of a mean grain size was investigated. Here, the coupled roles of grain orientation and size distributions on the plastic deformation state of grains are examined for a tensile deformed IF steel in the light of EBSD experimental data based on misorientation parameters and of a micromechanics-based modeling regarding grain and overall behaviors.
  • Hall–Petch Breakdown at Elevated Temperatures

    pp. 44-51

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    DOI:10.2320/matertrans.MA201309

    The Hall–Petch effect responsible for the strength of fine-grained and ultrafine-grained (UFG) metals is almost exclusively measured at room temperature. One reason for this is that at elevated temperatures grains tend to coarsen, and this negates the strengthening. The grains may, however, be stabilized by small volume fractions of fine dispersoids. These dispersoids cause direct Orowan strengthening and, by stabilizing the so-called Zener grain size, indirect strengthening due to Hall–Petch. We show that for most metals the critical Zener grain size above which Hall–Petch strengthening is more important than Orowan strengthening is lower than, and sometimes even considerably lower than 1 µm, i.e., in the range of UFG metals. Breakdown of the Hall–Petch relationship, which occurs at elevated temperatures once mechanisms weaker than Hall–Petch start to control the strength, is best studied for grain sizes well above this critical grain size. The Hall–Petch breakdown due to either Coble creep or grain size-dependent dislocation creep is modeled. We present model calculations for copper and verify our approach by comparing with experimental results for ferritic steels containing nanoscale dispersions.
  • Mechanical Responses of Copper Bicrystalline Micro Pillars with Σ3 Coherent Twin Boundaries by Uniaxial Compression Tests

    pp. 52-57

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    DOI:10.2320/matertrans.MA201312

    Two types of bicrystalline micro pillars (BCMs) involving Σ3 coherent twin boundaries along the pillar axis and single-crystalline micro pillars of two grains constituting the BCMs were fabricated in an oxygen-free copper sample by focused ion beam milling. These pillars were compressed using a nanoindenter to investigate slip transfer across grain boundary (GB). Referring their stress–strain (SS) curves, the possible slip systems were resolved from scanning electron microscope images after deformation and GB interaction criteria with the three largest Schmid factors (SFs) of each grain and the geometric relationship of intersections between the slip planes and the GB. The quite different SS behaviors were observed in the two types of BCMs, of which one was the unstable elastic-almost perfect plastic deformation due to large strain bursts and the other was work-hardening accompanying several small strain bursts. The slip transfer analyses suggest that the slip directions of both grains with the maximum SF are almost parallel to GB plane to the former behavior, and the combination of three slip systems from both grains produces the terraced hardening with repeated slipping across GB and piling-up against GB to the latter.
  • Dislocation Multiplication from the Frank–Read Source in Atomic Models

    pp. 58-63

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    DOI:10.2320/matertrans.MA201319

    Dislocation multiplication from the Frank–Read source is investigated in aluminum by applying atomic models. To express the dislocation bow-out motion and dislocation loop formation, we introduce cylindrical holes as a strong pinning point to the dislocation-bowing segment. The critical configuration for dislocation bow-out in atomic models exhibits an oval shape, which agrees well with the results obtained by the line tension model. The critical shear stress for the dislocation bow-out in atomic models continuously increases with decreasing length L of the Frank–Read source (even at the nanometer scale). This is expressed by the function L−1 ln L, which is obtained by a continuum model based on elasticity theory. The critical shear stresses for the Frank–Read source are compared with those for grain boundary dislocation sources, as well as the ideal shear strength.
  • Atomistic Simulation of Shear Mode Deformation of Nanocrystalline Copper with Different Grain Sizes

    pp. 64-68

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    DOI:10.2320/matertrans.MA201315

    Mechanical deformation of nanocrystalline copper has been simulated by means of molecular dynamics. The embedded atom method (EAM) potential was adopted for calculating the interatomic interaction. Samples with different grain sizes, 2.7 and 4.2 nm, were prepared by sintering spherical nanocrystals of different sizes. About 38000 atoms were contained in the sample, and a heterogeneous structure composed of crystalline and amorphous regions was realized. A shear mode strain was applied to the sample by sliding its upper and lower parts. The simulation was performed under free boundary condition for the surfaces perpendicular to the shear plane, and the stress–strain relation was obtained. The determined flow stress was lager for larger grain sample, namely, the inverse Hall–Petch effect was observed. Simulations for temperature and strain-rate dependences of the flow stress were also performed.
  • Correlation between Continuous/Discontinuous Yielding and Hall–Petch Slope in High Purity Iron

    pp. 69-72

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    DOI:10.2320/matertrans.MA201326

    High purity iron specimens containing 11 ppm carbon and 8 ppm nitrogen with different grain sizes were fabricated by cold rolling and subsequent annealing. It was found that the specimens exhibited entirely different yielding behavior in tensile tests depending on different cooling processes after annealing. The water-cooled specimens exhibited continuous yielding while the air-cooled ones exhibited discontinuous yielding. It was found that the Hall–Petch slope, ky, significantly changed depending on the different yielding behaviors.
  • Yielding Behavior and Its Effect on Uniform Elongation of Fine Grained IF Steel

    pp. 73-77

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    DOI:10.2320/matertrans.MA201317

    Interstitial free (IF) steel specimens with different mean grain sizes ranging from 0.4 to 12 µm were fabricated by the accumulative roll bonding (ARB) process and subsequent annealing. Tensile tests at room temperature have revealed that by decreasing the mean grain size down to an ultra-fine range, the yielding behavior gradually changes from the continuous yielding to the discontinuous yielding, accompanying a yield drop phenomenon. It has been found that the yield stress of specimens having fine grain sizes shows extra-hardening, deviated from the original Hall–Petch relation for coarse-grained specimens in accordance with the discontinuous yielding. The Hall–Petch analysis also has indicated that the loss in the uniform elongation in the ultrafine grain size range is related to the appearance of the discontinuous yielding behavior.
  • Effect of Ferrite Grain Size on Dynamic Tensile Properties of Ultrafine Grained Low Carbon Steels with Various Chemical Compositions

    pp. 78-84

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    DOI:10.2320/matertrans.MA201323

    The effects of ferrite grain size on dynamic tensile properties of low carbon steels with various chemical compositions are shown. The strain rate dependence of flow stress, represented by the difference between flow stresses at high and low strain rates, Δσ, was the highest in the interstitial free (IF) steel having ferrite single-phase microstructure and the 0.1% C-steel having ferrite–cementite (FC) microstructure. The Δσ decreased when P and Mn were added. In all the steels used, the Δσ was almost constant or decreased only slightly when the ferrite grains were refined down to 2–4 µm, and it decreased significantly by further grain refinement down to sub-micrometer region. The FC and ferrite single-phase microstructures strengthened mainly by grain refinement of ferrite showed higher dynamic absorbed energy compared with the other steels strengthened by alloy addition.
  • EBSD Analysis of Microstructure Evolution of Pure Iron Subjected to Sliding Wear and Related Change in Vickers Microhardness

    pp. 85-92

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    DOI:10.2320/matertrans.MA201301

    Sliding wear tests were carried on pure iron to investigate evolution of microstructure below worn surface. After the wear tests, grain boundary formation and lattice rotation were analyzed with electron backscatter diffraction (EBSD) method. In the vicinity of the worn surface, submicron grains separated by high-angle grain boundaries were generated. Below the submicron grain region, dominant microstructures were two kinds of low-angle grain boundaries which were horizontal and inclined to the worn surface, respectively. At deeper area from the worn surface, continuous lattice rotation was detected. To correlate the microstructure and strength, Vickers microhardness was measured over a cross section of the wear-affected zone. In the submicron grain and the low-angle grain boundary regions, the microhardness was proportional to the reciprocal square root of boundary spacing. In the lattice rotation region, we calculated geometrically-necessary (GN) dislocation density from gradient of lattice rotation. The microhardness value in the lattice rotation region showed good correlation with the square root of the GN dislocation density.
  • Microstructure and Mechanical Properties in Progressively Drawn Pearlitic Steel

    pp. 93-98

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    DOI:10.2320/matertrans.MA201316

    This paper studies the relationship between the microstructural changes caused by the cold drawing process in pearlitic steel wires (axial orientation of the pearlitic lamellae together with decrease of the average interlamellar spacing) and the improvement of their mechanical properties. The strength is related to plastic strain by means of the Embury-Fisher equation, and also by a new Hall–Petch expression, where to calculate the distance between barriers against dislocational movement one must consider, apart from the average interlamellar spacing, the average orientation angle. A modelling of the evolution of pearlitic lamellae with cold drawing was made, assuming that initially all angles appear with the same probability, that lamellae change their geometry along the longitudinal section of the wire similarly to the specimens and that the projection of the average interlamellar spacing on the cross section of the wire is proportional to the specimen diameter. The results obtained with this modelling show a good correspondence with experimental data.
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    2. Size Dependence of Delamination of High-carbon Steel Wire ISIJ International Vol.41(2001), No.2
  • Application of Harmonic Structure Design to Biomedical Co–Cr–Mo Alloy for Improved Mechanical Properties

    pp. 99-105

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    DOI:10.2320/matertrans.MA201303

    Harmonic structure is a recently introduced concept for material microstructure design. It is essentially a bimodal microstructure in which deliberately introduced structural heterogeneity has a specific order: interconnected network of ultra-fine grained (UFG) regions, called “shell area”, and coarse-grained regions called “core area”. Such microstructural features dictate a unique set of properties to the Harmonic-structured materials. The present paper deals with the application of harmonic structure design to biomedical Co–Cr–Mo alloys for improved mechanical properties. In the present work, it has been demonstrated that full density Co–Cr–Mo alloy compacts with harmonic structure can be successfully prepared by controlled mechanical milling followed by spark plasma sintering of the pre-alloyed powders at 1323 K for 3.6 ks. Sintered compacts exhibited an excellent combination of strength and ductility. Moreover, it has been also shown that the mechanical properties depend strongly on the volume fraction of the inter-connected three-dimensional network of fine-grained regions, i.e., shell volume fraction. In addition, the plastic deformation of harmonic structure Co–Cr–Mo alloy also led to α-FCC to ε-HCP allotropic transformation. Therefore, the application of harmonic structure design leads to the new generation microstructure of biomedical Co–Cr–Mo alloys, which demonstrates outstanding mechanical properties compared to conventional materials.
  • Mechanical Properties of Bulk Ultrafine Grained Aluminum Fabricated by Torsion Deformation at Various Temperatures and Strain Rates

    pp. 106-113

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    DOI:10.2320/matertrans.MA201321

    A commercial purity aluminum was heavily deformed up to an equivalent strain of 4 at various temperatures and strain rates by torsion deformation to produce specimens with various ultrafine grained (UFG) microstructures. The microstructures were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The microstructural observation revealed that the torsion deformed specimens had various mean grain sizes ranging from 0.38 to 8.6 µm. The grain size and dislocation density in the microstructures depended on the deformation conditions organized by Zener–Hollomon parameter. The mechanical properties of the torsion deformed specimens were investigated by tensile test at room temperature. It was found that the ultrafine grained specimens showed high strength which reached a value almost three times higher than that of the starting material. The strength of the UFG aluminum was higher than the level expected from the Hall–Petch relationship for conventionally coarse grained aluminum. The strengthening mechanisms in the UFG aluminum were discussed in terms of substructures introduced during torsion deformation.
  • The Effect of Microstructure on Mechanical Properties of Forged 6061 Aluminum Alloy

    pp. 114-119

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    DOI:10.2320/matertrans.MA201324

    The relationship between the microstructure and the yield strength after T6 tempering was investigated using 6061 aluminum alloy manufactured at various levels of temperature and strain rate during hot forging. Non-recrystallized structures (continuous recrystallization structure) were formed by hot forging at low Zener–Hollomon parameter (Z parameter) conditions, which consisted of fine grains surrounded by high angle grain boundaries and contained low angle grain boundaries inside. Increasing the Z parameter formed fine-grained structures, resulting in increased yield strength. Increasing further the Z parameter formed recrystallization, having coarse recrystallized structures (discontinuous recrystallization structure) of hundreds of micrometers in diameter with high angle grain boundaries, resulting in significantly reduced yield strength. The yield strength of the material with recrystallized grain structures was less dependent on the grain size. On the other hand, the yield strength of the material with non-recrystallized structures was severely dependent on the grain size, roughly in accordance with the previous data. Subgrain strengthening appeared to be more effective than recrystallized grain strengthening. In consideration of the effect of the texture on the yield strength using Schmidt factor, τ′crss (the value equivalent to critical resolved shear stress in the slip direction. τ′CRSS − τ′CRSS0 = kdm, τ′CRSS = s·σ0.2, s: the averaged Schmidt factor in the tensile direction. d′: grain sizes in the slip direction) was less dependent on d′. In consideration of the texture, the yield strength of 6061-T6 is essentially less dependent on the grain size as reported previously.
  • Influence of Grain Refinement and Texture Evolution on the Yield Strength of Mg Alloy Processed by Cyclic Extrusion and Compression

    pp. 120-122

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    DOI:10.2320/matertrans.MA201306

    The microstructure, texture and room temperature tensile property of an extruded GW102K alloy processed by cyclic extrusion and compression (CEC) at 350°C were investigated. Results show that the microstructure was effectively refined and the initial fiber texture changed to a new texture. The strength and ductility were simultaneously increased obviously. In particular, yield strength increases with decreasing grain sizes, and exhibiting clear grain size dependency according to Hall–Petch relation. The grain refines strengthening is the largest contributor to the yield strength and the texture play relatively minor roles on yield strength in CEC-processed GW102K alloy.
  • Evolution of Texture in High Permeability Grain Oriented Silicon Steel Produced by Casting Thin Slab and Rolling Process

    pp. 123-127

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    DOI:10.2320/matertrans.M2013280

    The texture of high permeability grain-oriented silicon steel produced by Thin Slab Casting and Rolling process through adopting low slab reheat temperature and AlN as the main inhibitor has was studied. The results show that the microstructure of the hot rolled strip is nonuniform through the thickness, and the Goss texture with a maximum intensity mainly concentrated on the surface. The microstructure of the normalized strip is fully recrystallized, and the Goss texture is in both the surface and the subsurface layer, with a maximum intensity in subsurface layer. The microstructure of the cold rolled strip is fibrous tissues, the γ-fibre texture is the main texture in the surface, and the {223}<110> is the dominant texture in both the subsurface layer and center layer. The microstructure of the decarburization annealed sheet is fully primarily recrystallized, and the γ-fibre texture is the main texture. The Goss grains abnormally grow after high temperature annealing, it is the single Goss texture.
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    1. Texture Control in the Production of Grain Oriented Silicon Steels ISIJ International Vol.29(1989), No.10
  • Direct Observation of Grain Boundary Migration during Recrystallization within the Bulk of a Moderately Deformed Aluminium Single Crystal

    pp. 128-136

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    DOI:10.2320/matertrans.M2013227

    A single grain growing in the bulk of a mildly deformed (30% thickness reduction through cold rolling) aluminium single crystal with an {001}<100> orientation (Cube orientation), is monitored during recrystallization with synchrotron radiation using topo-tomography. The formation and migration of planar boundary segments (facets) are analyzed using a method that determines the displacements of local boundary segments along parallel lines perpendicular to the facet plane. Facets are observed to form after a certain annealing time. They migrate at a constant rate for extended periods of time and remain planar during their migration. A change in the migration rate for one facet has been observed which is not related to changes in the experimental conditions and is most likely to be driven by the changes in grain orientation and/or the local deformation microstructure. The crystallography of the analyzed facets is not closely related to any crystallographic {111} plane of neither the growing grain nor the disappearing deformed matrix.
  • Microstructure and Mechanical Properties of Friction Stir Welded Q345 Steel

    pp. 137-140

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    DOI:10.2320/matertrans.M2013332

    The Q345 steel sheet of 2.2 mm thickness was welded by friction stir welding, and the microstructure and mechanical properties of the joints were analyzed. The experimental results demonstrate that a perfect joint, without internal defects, can be obtained using a stir-pin rotation speed of 800 rpm, a weld speed of 50 mm·min−1, and water-sprayed cooling. The macroscopic morphology of the cross-section of the weld seam is bowl-like. Phase transition occurred in part of the heat-affected zone, and austenite grains turned to fine ferrite and pearlite grains during cooling: these co-existed with the non-phase transition base metal. The strongest dynamic recrystallisation occurred in the stir zone, where proeutectoidferrite exhibited slightly overheated characteristics and the matrix was an acicular distribution of sorbite. The hardness of the weld zone was evenly distributed and 12% greater than the base material. The tensile strength of the welded joints was higher than that of the base material. The enhanced mechanical properties were mainly due to the fine grain size in the heat-affected zone and the sorbite matrix in its stir zone.
  • Effect of Aging Treatment on Microstructure and Mechanical Properties of Ti27Nb2Ta3Zr β Titanium Alloy for Implant Applications

    pp. 141-146

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    DOI:10.2320/matertrans.M2013187

    This work aims to clarify the influence of aging temperature and time on the microstructure and mechanical properties of the Ti–Nb alloy for biomedical applications. The ingot of Ti–27Nb–2Ta–3Zr alloy was subjected to the arc melting, hot-forging and heat treatment respectively. Microstructure characterization was investigated by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile tests showed an upward trend of the volume fraction of ω phase and α phase with the increase of aging time. When the aging temperature was higher than 673 K, the amount of ellipsoid-shaped ω phase precipitation decreased and needle-like α phase appeared, which contributed to higher tensile strength, lower Young’s modulus and better plasticity.
  • Solvothermal Synthesis of Shape-Controlled Perovskite MTiO3 (M = Ba, Sr, and Ca) Particles in H2O/Polyols Mixed Solutions

    pp. 147-153

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    DOI:10.2320/matertrans.M2013350

    Perovskite oxides MTiO3 (M = Ba, Sr and Ca) with various phase, size, and shape were systematically prepared by solvothermal method in H2O/glycols mixed solutions. Shape control of the BaTiO3 and SrTiO3 particles was achieved by choice of glycols, and anisotropic rod-shaped fine BaTiO3 particles with a tetragonal crystal system were selectively synthesized by use of ethylene glycol. On the other hand, cubic- and spherical-shaped BaTiO3 particles were also obtained in H2O and H2O/diethylene glycol systems. These BaTiO3 particles had a cubic crystal system. Meanwhile, concave cubic-shaped CaTiO3 particles with an orthorhombic phase were synthesized. In our method, triethanolamine–titanium complex was used as the titanium source of MTiO3. Both of the specific adsorption of triethanolamine and solvent effect of glycols is the competitive factor for the control of final particle size and shape.
  • Experimental Verification and Accuracy Improvement of Gas Entrapment and Shrinkage Porosity Simulation in High Pressure Die Casting Process

    pp. 154-160

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    DOI:10.2320/matertrans.F-M2013835

    In recent years, computer simulation, that is molten metal flow and solidification analysis, has become very important tool for optimal casting and casting plan design. The analysis accuracy was, however, insufficient because of complicate phenomena and difficulty of experiments especially in high pressure die casting process. In this study, effects of various analysis conditions in simple aluminum sand casting were investigated by existing experiments. And subsequently, in actual high pressure die casting experiments for measurement of molten metal pressure, air-pressure, solidification and casting defects especially gas entrapment and shrinkage porosity, computer simulation results of air-pressure, solidification, and those defects were verified.
  • Four-Dimensional Morphological Evolution of an Aluminum Silicon Alloy Using Propagation-Based Phase Contrast X-ray Tomographic Microscopy

    pp. 161-164

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    DOI:10.2320/matertrans.M2013225

    Four-dimensional propagation-based phase contrast X-ray tomographic microscopy experiments were performed on an Aluminum-29.9 mass% Silicon alloy during coarsening. Using propagation-based phase contrast, changes in the three-dimensional morphology of primary silicon particles were captured and the resulting evolution of the microstructure is discussed. While morphologies at earlier times are complex, faceted and highly interconnected, the morphologies at later times are less faceted but remain quite complex.
  • Alloy Designs of High-Entropy Crystalline and Bulk Glassy Alloys by Evaluating Mixing Enthalpy and Delta Parameter for Quinary to Decimal Equi-Atomic Alloys

    pp. 165-170

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    DOI:10.2320/matertrans.M2013352

    The values of mixing enthalpy (ΔHmix) and Delta parameter (δ) were calculated with 73 elements from Miedema’s model for multicomponent equi-atomic alloys to investigate the possibilities of the alloys to be formed into high-entropy (H-E) alloys or high-entropy bulk metallic glasses (HE-BMGs). The equi-atomic alloys from about 15 million (73C5) quinary to 621 billion (73C10) decimal systems were evaluated by referring to a ΔHmix–δ diagram for zones S and B’s for H-E alloys with disordered solid solutions and BMGs, respectively, reported by Zhang et al. The results revealed that the number of quinary equi-atomic alloys plotted in zone S is 28405 (∼0.19% in 73C5), whereas those in zones B1 and B2 for conventional and Cu- and Mg-based BMGs, respectively, were 1036385 and 21518 (∼6.90 and ∼0.14%), respectively. This kind of statistical approach using ΔHmix–δ diagram will lead to finding out unprecedented H-E alloys and HE-BMGs.
  • Evaluation of Surface Crack in Resistance Spot Welds of Zn-Coated Steel

    pp. 171-175

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    DOI:10.2320/matertrans.M2013244

    The development of the automotive industry is now focused not only on improving basic vehicle performance but also on reducing weight and enhancing safety and durability. Various automotive high-strength steels are being developed, and Zn-coated steels are being manufactured to prevent corrosion of the external white vehicle body. The most commonly used welding method in the car body assembly process is resistance spot welding (RSW), which has been extensively studied worldwide. In this process, the work piece is basically heated according to the contact resistivity of the interfacial between the electrode and the material as well as the bulk resistivity of the material itself. At this point, if the metal is Zn, which has a lower melting point than the Fe base metal on the surface, it is mainly melted in the temperature range of 400–900°C. It becomes easy to penetrate the grain boundary of the HAZ during welding. Also, the tensile stress in such a state decreases the ductility of the grain boundary and causes liquid metal embrittlement (LME).
    Cu5Zn8, an intermetallic compound, can be formed from the reaction of the alloy with the Cu material electrode in the expulsion current range at a high temperature. Its formation is likely to be facilitated by LME or a surface crack.
    In this study, the fatigue characteristics of a tensile shear specimen during spot welding was investigated with the welding parameters that occur in the surface crack of welds on Zn-coated steel. Finally, a controlled spot welding condition was suggested to prevent surface cracks.
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    3. Improvement of Fatigue Properties of Resistance Spot Welded Joints in High Strength Steel Sheets by Shot Blast Processing ISIJ International Vol.56(2016), No.7
  • Effects of Fine Particle Peening on Oxidation Behavior of Nickel–Titanium Shape Memory Alloy

    pp. 176-181

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    DOI:10.2320/matertrans.M2013336

    The effects of fine particle peening (FPP) on oxidation behavior of nickel–titanium shape memory alloy (Ni–Ti alloy) were evaluated. After FPP treatment, oxidation process was performed at 300, 400 and 500°C for 30 min in N2–20 vol%O2 atmosphere. Surface microstructures of oxidized specimens pre-treated with FPP were characterized using scanning electron microscope (SEM), glow discharge optical emission spectroscopy (GD-OES), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction analysis (XRD). Oxide layers formed on the specimens pre-treated with FPP were thinner than those on the un-peened ones. Amorphous titanium oxides (TiO and TiO2) were formed on the un-peened specimens by oxidation process, whereas a nickel oxide (NiO) was created on the FPP-treated surface oxidized at temperature greater than 400°C as well as titanium oxides. This was because an amorphous structure created by FPP accelerated the outward growth of nickel oxide during the subsequent oxidation process. Moreover, the effects of surface oxide layers on the biocompatibility of Ni–Ti alloy were investigated. Due to the formation of a thin oxide layer which contains nickel element, the oxidized specimen pre-treated with FPP showed higher amount of nickel ion elution than that of the oxidized one during the cell culture tests. These results suggest that microstructural feature of surface oxide layer strongly affects the biocompatibility of Ni–Ti alloy.
  • Effect of Micro-Alloying Elements on Deformation Behavior in Mg–Y Binary Alloys

    pp. 182-187

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    DOI:10.2320/matertrans.M2013303

    The effect of minor addition of yttrium element on deformation behavior was investigated using Mg–X at%Y (X = 0.01, 0.02, 0.03, 0.04 and 0.05) dilute alloys and pure magnesium with an average grain size of about 50 µm. The stress and strain curves in all the alloys showed a sigmoidal shape in the compression tests, which suggested the formation of {10\bar{1}2}-type twinning due to the lack of slip system. On the other hand, yttrium atom addition of more than 0.03 at% was effective to affect the deformation behavior: a large compressive strain of 0.5 was possible to obtain, and the sub-grained and fine-grained structures were formed even at room temperature in three kinds of alloys. The dominant deformation mechanism in these alloys was the twinning at the beginning of the state and the dislocation slip with further imposed strain.
  • Assessment of the Thermal Conductivity of Yttria-Stabilized Zirconia Coating

    pp. 188-193

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    DOI:10.2320/matertrans.M2013172

    The approach is based on an experimental set-up in which coated surface of the specimen was exposed to high temperature environment and inner metal surface was cooled by flowing air, simulating the actual gas turbine applications. The 8% yttria-stabilized zirconia coating was made by plasma spray method on a cylindrical specimen. This arrangement could be set-up in a laboratory. By measuring the overall thermal resistances of specimens, it has been shown that the thermal conductivity of the coating segment can be determined by the differences in thermal resistances of two specimens with and without coating. The objective of the present work is to extract the thermal conductivity of the thermal barrier coating under the condition that is nearly the same as the actual application. The effective thermal conductivities of coatings found to be 0.11 to 0.16 W/mK, while those measured by laser flash method in literature varied from 1 to 1.4 W/mK.
  • A Slip Enhanced Superhydrophobic Surface Based on Reticulated ZnO Nanostructure

    pp. 194-197

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    DOI:10.2320/matertrans.M2013312

    We achieved a hierarchical reticulated ZnO nanostructure by designing a hydrothermal method combined with a low temperature dehumidification step. Compared with the traditional vertically aligned nanostructures, the obtained surface exhibits better drag reduction effect owing to the increased pitching size and reduced solid–liquid fraction. This result is meaningful to researchers in designing a suitable superhydrophobic surface to increase the slip performance in the real applications.
  • Size, Shape, Composition and Separation Analysis of Products from Waste Refrigerator Recycling Plants in South Korea

    pp. 198-206

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    DOI:10.2320/matertrans.M2013306

    With the rapid increase in the amount of waste household appliances, there is increasing awareness about finding more efficient ways to reutilize used appliances. There are several plants for recycling household appliances in Korea. The process of recycling involves several stages of shredding, followed by material separation. Given that these plants were built with the technology available at the time of construction, they may not operate at maximum efficiency even as appliances advance with technological improvements in manufacturing. Accordingly, samples were collected from a recycling plant in Korea, and plant performance was evaluated in this study for developing a more efficient recycling process.
  • Impact Response of Circular Aluminum Tube Filled with Polyurethane Foam

    pp. 207-215

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    DOI:10.2320/matertrans.M2013293

    The behavior of foam-filled thin-walled aluminum tube under impact was investigated focusing on different crashworthiness parameter. The diameter to thickness of tube ranging between 26.5 and 200 were filled with polyurethane foam density from 100–250 kg·m−3. The specimens were crushed by a drop hammer. The result from finite element simulation and experiment was compared and good agreement was achieved. The simulation was also used to conduct further investigation on tube with higher diameter to thickness ratio. It was found that by filling foam, the tubes change their collapse modes from asymmetric mode to axisymmetric mode. Also, the energy absorption can be enhanced by filling tube with higher foam density. The impact energy was found to be managed more efficient in foam filled tube as the load efficiency is higher in higher density foam-filled tube. However, the specific energy absorption of foam-filled tube is getting lower in higher density foam. This paper provides experimental and numerical data as well as discussion in various aspects of crashworthiness.
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  • Wear and Friction Properties of Mg–Zn–Y Alloy with Dispersion of Quasi-Crystalline Phase

    pp. 216-219

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    DOI:10.2320/matertrans.M2013301

    The wear and friction properties of the Mg–Zn–Y alloys with a dispersion of the quasi-crystalline phase were investigated using the pin-on-disk configuration. The wear loss rates and the friction coefficient were obtained to be 1.95 × 10−5 mm3/mm and 0.26 in the casted alloy, 1.75 × 10−5 mm3/mm and 0.24 in the heat-treated alloy, 2.40 × 10−5 mm3/mm and 0.24 in the extruded alloy, respectively, under the dry wear condition. An effective method to enhance the wear properties in the magnesium alloys was determined to be a homogenous distribution of the quasi-crystalline phase in the coarse-grained matrix. Meanwhile, the grain refinement was not suitable due to grain boundary sliding, i.e., materials softening.
  • Pronounced Structural Rejuvenation in Zr50Cu40Al10 Metallic Glass Strained by Torsional Straining at Elevated Temperature

    pp. 220-222

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    DOI:10.2320/matertrans.M2013354

    Zr50Cu40Al10 metallic glass was deformed by high pressure torsion at different temperatures. Thermal analysis reveals that the structural relaxation peak is intensified not only by increasing rotation revolutions, but also raising deformation temperature. The relaxation enthalpy in the sample deformed at 373 K for 10 revolutions shows similar value to the one deformed at room temperature for 50 revolutions, suggesting the analogous structural rejuvenation. Extensive atomic movement leads to the pronounced structural rejuvenation when deformation was performed at elevated temperature but lower than the starting temperature of structural relaxation.
  • Formation of Fully Annealed Nanocrystalline Austenite in Fe–Ni–C Alloy

    pp. 223-226

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    DOI:10.2320/matertrans.M2013324

    Nanocrystalline austenite having fully annealed and equiaxed morphology, grain sizes varying from 400 nm to 1.7 µm, and a large fraction of high-angle grain boundaries was prepared in Fe–24Ni–0.3C (mass%) alloy by high-pressure torsion and subsequent heat treatment. The nanostructures were formed through reverse transformation of deformation-induced martensite and recovery/recrystallization of retained austenite. The austenitic grain size decreased with increasing shear strain below the strain of 50, while it increased above the shear strain of 50 due to decreased austenite-transformation starting temperature.

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