Showing papers in "Materials Transactions in 2014"

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

Abstract: 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 stress­strain 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.

Development of Multilayer Steels for Improved Combinations of High Strength and High Ductility

Abstract: Multilayer steels have been developed to provide a novel route to achieving higher-performance steels by employing a high-strength steel and a high-ductility steel independently in the layer structure. In this article, a background to the development, design, fabrication, properties, and applications of multilayer steels are overviewed. Multilayer steels exhibit improved combinations of strength and ductility compared with existing monolithic steels and also excellent deformation behaviors under high-strain-rate deformation as well as good formability. Those improved performances of multilayer steel are achieved by increased interfacial toughness between the layers and decreased thickness of the brittle steel layers according to the fracture toughness of the brittle steel and the strength of the ductile steel. The concept of multilayer steels is extended by applying different combinations of components, such as Mg-steel multilayer composite. [doi:10.2320/matertrans.M2013382]

Evaluation of Surface Crack in Resistance Spot Welds of Zn-Coated Steel

Abstract: 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 meta li s Zn, which has a lower melting point than the Fe base metal on the surface, it is mainly melted in the temperature range of 400­900°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. [doi:10.2320/matertrans.M2013244]

Hall–Petch Relationship and Dislocation Model for Deformation of Ultrafine-Grained and Nanocrystalline Metals

Abstract: Models and theories to explain the Hall­Petch 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. [doi:10.2320/matertrans.MA201310]

Superior Strength in Ultrafine-Grained Materials Produced by SPD Processing

Abstract: 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 Hall­Petch 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. [doi:10.2320/matertrans.MA201325]

Hall–Petch Breakdown at Elevated Temperatures

Abstract: The Hall­Petch 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 Hall­Petch. We show that for most metals the critical Zener grain size above which Hall­Petch strengthening is more important than Orowan strengthening is lower than, and sometimes even considerably lower than 1μm, i.e., in the range of UFG metals. Breakdown of the Hall­Petch 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 Hall­Petch 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. [doi:10.2320/matertrans.MA201309]

Effect of Grain Boundary Segregation of Interstitial Elements on Hall–Petch Coefficient in Steels

Abstract: The yielding behavior of interstitial-free steels and low-carbon steels with varying amounts of C and N were investigated in connection with the Hall­Petch relation. The Hall­Petch coefficient is as small as 150MPa·µm 1/2 in interstitial-free steels but it increases to 600MPa·µm 1/2 by adding solute carbon up to 60ppm. Nitrogen does not have a significant effect on the Hall­Petch coefficient. The results of three-dimensional (3D) atom probe analysis indicated that carbon has 3­4 times greater segregation potential in comparison with nitrogen. The small effect of nitrogen on the Hall­Petch 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 Hall­Petch coefficient of iron. These results support the dislocation pile-up model of explaining yielding in polycrystalline metals. [doi:10.2320/matertrans.MA201314]

Yielding behavior and its effect on uniform elongation of fine grained if steel

Abstract: Interstitial free (IF) steel specimens with different mean grain sizes ranging from 0.4 to 12µm 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 Hall­Petch relation for coarse-grained specimens in accordance with the discontinuous yielding. The Hall­Petch 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. [doi:10.2320/matertrans.MA201317]

Application of Harmonic Structure Design to Biomedical Co–Cr–Mo Alloy for Improved Mechanical Properties

Abstract: 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 Co­Cr­Mo alloys for improved mechanical properties. In the present work, it has been demonstrated that full density Co­Cr­Mo alloy compacts with harmonic structure can be successfully prepared by controlled mechanical milling followed by spark plasma sintering of the pre-alloyed powders at 1323K 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 Co­Cr­Mo alloy also led to ¡-FCC to 3⁄4-HCP allotropic transformation. Therefore, the application of harmonic structure design leads to the new generation microstructure of biomedical Co­Cr­Mo alloys, which demonstrates outstanding mechanical properties compared to conventional materials. [doi:10.2320/matertrans.MA201303]

Evaluation of Dislocation Density for 1100 Aluminum with Different Grain Size during Tensile Deformation by Using In-Situ X-ray Diffraction Technique

Abstract: Ultra-fine-grained (UFG) aluminum with a grain size of 260nm was fabricated by annealing a severely plastically deformed A1100 alloy. The resulting UFG aluminum exhibited a 0.2% proof stress (·0.2) that was four times larger than that predicted by the conventional Hall-Petch relation. In this study, the UFG aluminum, the fine-grained aluminum with a grain size of 960nm and the coarse-grained aluminum with a grain size of 4.47µm were prepared. The change in the dislocation density, μ was investigated during tensile deformation using in-situ X-ray diffraction measurements at SPring-8. It was found that as the strain increased, the μ changed in four distinct stages. The first stage was characterized by elastic deformation, and little change in the μ occurred. For the coarse-grained aluminum, this stage was almost absent. In the second stage, the μ rapidly increased until the stress reaches ·II in which the plastic deformation begins to occur at a constant strain rate. In the third stage, only a moderate change in the μ occurred. Finally, in the fourth stage, the μ rapidly decreased as the test pieces underwent fracture. Additionally, it was found that the ·0.2-·I was followed by the conventional Hall-Petch relation in all grain size range. [doi:10.2320/matertrans.L-M2015803]

Effect of Pre-Aging Treatment on the Microstructure and Magnetic Properties of Sm(Co,Fe,Cu,Zr)7.8 Sintered Magnets

Abstract: The effects of pre-aging treatment on the microstructure and magnetic properties of Sm(Cobal.Fe0.35Cu0.06Zr0.02)7.8 were investigated. The main phase of both solution-treated magnet and pre-aged magnet was the 1 : 7 phase, and there were no distinct differences between the X-ray diffraction profiles of these magnets. Fine Cu-rich precipitates a few tens of nanometers in size were observed in pre-aged magnet by scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy mapping, but such precipitates were not observed in solution-treated magnet. As for fully aged magnets, the cell size was smaller in pre-aged magnet than in non-pre-aged magnet. Thus, the pre-aging treatment gave a fine cellular structure. Mr and HcJ of pre-aged magnet were almost same as those of non-pre-aged magnet. Squareness of the demagnetization curve for fully aged magnet was increased by pre-aging treatment. As a result, (BH)max of magnet subjected to pre-aging treatment was greater than that of magnet not subjected to pre-aging treatment. The fine cellular structure seemed to result in higher squareness. The following magnetic properties were obtained for Sm(Cobal.Fe0.35Cu0.06Zr0.02)7.8 by pre-aging treatment: Mr = 1.24T, HcJ = 1490 kA/m, and (BH)max = 266 kJ/m3. [doi:10.2320/matertrans.MBW201325]

Nanostructure Variations and Their Effects on Mechanical Strength of Ni-17Mo-7Cr Alloy under Xenon Ion Irradiation

Abstract: A nickel-base high-temperature alloy (Ni-17Mo-7Cr) has been characterized by nanoindentation and transmission electron microscopy to determine the changes of nanoindentation hardness and microstructural evolution under ion irradiation. Ion irradiation experiments for bulk and thin-foil specimens of Ni-17Mo-7Cr alloy were carried out at room temperature, up to 6.6 dpa, by 7 MeV Xe26+ and 1 MeV Xe20+ ions, respectively. The continuous stiffness measurement (CSM) with a diamond Berkovich indent was used to measure the depth profile of hardness. Nanoindentation results for bulk specimens showed an evident ion irradiation induced hardening phenomenon, and the nanoindentation hardness increases with increasing ion dose. High number density of nano-scale black spots and linear-like defects were observed in thin-foil specimens irradiated at 0.33 and 6.6 dpa, respectively. High-resolution transmission electron microscopy images revealed that the black spots were nano-scale solute clusters and dislocation loops, while the linear-like defects were found to be Ni, Mo and Cr-enrichment regions by using the high-angle annular dark field-scanning transmission electron microscope. The ion irradiation induced defects can be responsible for the hardening of Ni-17Mo-7Cr alloys.

Effects of Microstructure on Discharge Behavior of AZ91 Alloy as Anode for Mg–Air Battery

Abstract: The effects of precipitate distribution in commercial Mg alloys on the discharge behavior of a Mg­air battery were investigated. Rolled Mg­9mass%Al­1mass%Zn (AZ91) alloy sheets were selected as the anode. The discharge behaviors were evaluated by constant current discharge tests, and the corrosion behaviors were estimated by salt immersion tests and potentiodynamic polarization measurements. In the discharge tests, the peak-aged specimens exhibited much shorter discharge time than the solution-treated specimen. In the corrosion tests, the aged specimens exhibited quite a low corrosion rate as compared with the solution-treated specimen. From the potentiodynamic polarization measurements, the aged specimens showed a much lower corrosion current density and higher corrosion potential than the solution-treated specimen. It is suggested that a stable corrosion barrier by densely distributed ¢-phase precipitates contributed to both the reduction of discharge time and the suppression of corrosion rate for the aged specimens. Microstructural evaluations of the aged specimens revealed that fine ¢-phase precipitates were densely distributed throughout the specimens and that there was little Al-rich ¡ phase, which would accelerate galvanic corrosion between the ¡ matrix and the ¢ phase. It was confirmed that the finely and homogenously distributed ¢ phase in the aged AZ91 specimens acted as a barrier to the dissolution of the ¡-Mg matrix because a large Al-rich ¡ phase was not distributed in the AZ91 specimens in this study. [doi:10.2320/matertrans.MC201403]

Correlation between Continuous/Discontinuous Yielding and Hall­Petch Slope in High Purity Iron

Abstract: 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 Hall­Petch slope, ky, significantly changed depending on the different yielding behaviors. [doi:10.2320/matertrans.MA201326]

Crystallization Behaviors of Copper Smelter Slag Studied Using Time-Temperature-Transformation Diagram

Abstract: In recent years, while Cu production has increased, ore quality has degraded. Consequently, copper smelting industries generate large amounts of byproducts and wastes, including slag. However, the use of these byproducts and wastes involve high costs and most of the wastes are discarded in landfills after processing. In some cases, these byproducts and wastes contain valuable components, which may be profitable to recover. In other cases, toxic or hazardous chemicals are required in the treatment of these wastes to prevent their release. Therefore, the processing of byproducts is a significant activity, which determines the profitability of copper production facilities. Magnetic separation of precipitated magnetite (Fe3O4) crystals in the copper slag is one of the most effective methods to recover iron resources. It is preferable to convert molten fayalite slag to magnetite during the cooling of the slag with oxidation. With a time-temperature-transformation (TTT) diagram, the microstructure of the slag obtained after heat treatment could be estimated through a designed cooling path according to the purpose for which the slag is to be used. Slag recycling, mainly the recovery of the precipitated magnetite crystals, will be enhanced by controlling the slag cooling conditions. In the present study, using an infrared furnace, the crystallization behavior of copper smelter slag with regard to obtaining a TTT diagram was assessed by X-ray diffraction (using an internal standard), scanning electron microscopy and energy dispersive spectroscopy. Moreover, the distribution behaviors of the heavy metals such as Cu, Zn, As, and Cr were also studied. [doi:10.2320/matertrans.M-M2014819]

Direct Observation of Pit Initiation Process on Type 304 Stainless Steel

Abstract: In situ and real-time optical microscopic observations of pit initiation process on a commercial Type 304 stainless steel with low-sulfur content (0.004mass%) were performed in 3M NaCl solution at 298K. MnS inclusions with diameters of ca. 1μm were found to act as the initiation sites of pitting, as was the case in a re-sulfurized Type 304 stainless steel (0.027mass%). The pit was initiated at the boundary between the MnS inclusion and the steel matrix, and grew with time. After a few seconds, no visible change was observed for 1 s, even though the anodic current was measured, suggesting that the steel dissolution proceeded in the depth direction. After that, a small hole suddenly appeared on the steel surface. The hole widened with time, steadily becoming a large stable pit. This pit initiation process in the low-sulfur stainless steel is much like that found in the re-sulfurized stainless steel. [doi:10.2320/matertrans.M2014028]

Effect of Micro-Alloying Elements on Deformation Behavior in Mg–Y Binary Alloys

Abstract: The effect of minor addition of yttrium element on deformation behavior was investigated using Mg­X 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 12}-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. [doi:10.2320/matertrans.M2013303]

The Influence of Fluoride Anion on the Equilibrium between Titanium Ions and Electrodeposition of Titanium in Molten Fluoride–Chloride Salt

Abstract: NaCl­KCl­TiClx was prepared via using titanium sponge to reduce titanium tetrachloride in a NaCl­KCl melt under a negative pressure at 1023K. The relationship between titanium valence states and [F]/[Ti] molar ratios was investigated with successive adding potassium fluoride in the pre-prepared NaCl­KCl­TiClx. It was found that the average valence of titanium ions tended to be stable around 3.0 when [F]/[Ti] molar ratio was greater than 1.80. The equilibrium redox potentials, ETi3þ=Ti2þ, ETi4þ=Ti3þ, ETi3þ=Ti and ETi2þ=Ti, were also calculated through the obtained concentration of equilibrium titanium ions with different molar ratios of [F]/[Ti]. Meanwhile, the influence of the fluoride anion on over-potential and characteristics of titanium electrodeposition were investigated through changing the molar ratio of [F]/[Ti]. The results showed that, with the increasing of [F]/[Ti] molar ratios, the grain size of electrodeposition products became smaller, while the over-potential was higher. [doi:10.2320/matertrans.M2014071]

The Effect of Pre-Strain on the Resistance to Hydrogen Embrittlement in 316L Austenitic Stainless Steel

Abstract: The effect of pre-strain before hydrogen charging on the resistance to hydrogen embrittlement (HE) in the 316L austenitic stainless steel was investigated through the slow strain rate tensile test (SSRT), transmission electron microscopy, and thermal desorption analysis (TDA). The pre-strain suppressed mechanical twinning during the SSRT, regardless of hydrogen charging. However, it accelerated the ¾-martensitic transformation in hydrogen-charged specimens. The TDA revealed that whereas hydrogen atoms migrated from grain boundaries and dislocations mainly to the austenite (£)/¾ interfaces in pre-strained specimens during the SSRTs, they moved to the boundaries of fresh mechanical twins, which newly formed during the SSRTs, in the annealed specimen. The elongation loss by hydrogen charging became greater with increasing the pre-strain, indicating that pre-straining deteriorated the resistance to HE. This elongation loss by pre-strain resulted from both the increase in fraction of ¾-martensite with pre-strain and the segregation of hydrogen atoms to the £/¾ interfaces.

Dislocation Multiplication from the Frank–Read Source in Atomic Models

Abstract: Dislocation multiplication from the Frank­Read 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 Frank­Read source (even at the nanometer scale). This is expressed by the function L11 lnL, which is obtained by a continuum model based on elasticity theory. The critical shear stresses for the Frank­Read source are compared with those for grain boundary dislocation sources, as well as the ideal shear strength. [doi:10.2320/matertrans.MA201319]

Quantitative Nondestructive Evaluation of Plastic Deformation in Carbon Steel Based on Electromagnetic Methods

Abstract: Plastic deformation may occur in a mechanical structure during its manufacturing and service process, and may cause serious problem in the structural integrity. Therefore, a reliable pre-service or in service quantitative non-destructive evaluation (NDE) of plastic deformation is very important especially for a structure after suffering giant load such as a large earthquake. However, there is still no satisfactory method being established for the quantitative NDE of plastic deformation in key structures such as those of a nuclear power plant. For this purpose, three electromagnetic NDE methods, i.e., Magnetic Barkhausen Noise (MBN), Magnetic Incremental Permeability (MIP) and Magnetic Flux Leakage (MFL) method are studied via experiments in this paper to investigate their feasibility for evaluation of plastic deformation in carbon steel SS400. A special testing system integrated these three electromagnetic NDE methods is established to measure the magnetic property of testpieces with different plastic deformation, which was introduced by a tensile testing machine. It is found that the measurement signals of all these three methods have clear correlation with the plastic strains and show coincident tendency, which reveals the validity of these methods for the quantitative evaluation of plastic deformation. Among these methods, the MFL signals are of higher stability and repeatability but of relative low spatial resolution. The MBN method can give better resolution but of bigger standard deviation and is also not valid to evaluate a plastic strain of large scale. On the other hand, the MIP signals are more sensitive to the liftoff of sensor and to the remanent magnetization status, i.e. of more noise. Therefore, to measure the feature parameters of these three methods at the same time with an integrated testing system and to evaluate the plastic strain through signal fusion may give a better detectability and evaluation precision. [doi:10.2320/matertrans.M2014173]

Recycling of Rare Earth Magnet Waste by Removing Rare Earth Oxide with Molten Fluoride

Abstract: Recycling of rare earth magnet scrap is required for improving resource conservation. Removal of rare earth oxide from off-specification magnet alloy scrap was investigated by remelting the scrap together with fluoride flux, LiF­50mol% NdF3 and LiF­25mol% NdF3­25mol% DyF3, at 1503K for the development of novel recycling process. As a result, separation of magnet alloy from fluoride flux after remelting was fine, and neither suspension of alloy in the flux nor suspension of flux in the alloy was observed. Oxygen concentration of alloy decreased from 5000 to 160mass-ppm by charging flux of 1.5 times of equivalent amount for complete dissolution of oxide. Substitution of neodymium in the alloy and dysprosium in the flux was observed. The investigated process should be utilized for mildly contaminated waste such as a used (endof-life) magnet because of simple process, energy saving and unlimited location. [doi:10.2320/matertrans.M-M2013836]

Prediction of the Constitutive Equation for Uniaxial Creep of a Power-Law Material through Instrumented Microindentation Testing and Modeling

Abstract: Indentation creep tests and finite element simulations were performed on a model material to show that the constitutive equation for conventional uniaxial creep can be derived using the instrumented indentation testing technique. When the indentation pressure and the indentation creep rate are maintained at constant values of ps and _inðsÞ, respectively, the contours of the equivalent stress and the equivalent plastic strain rate in the region beneath the conical indenter expand according to the increase in the displacement of the indenter while maintaining geometrical self-similarity. These findings indicate that a pseudo-steady deformation state takes place around the indenter tip. The representative point exhibiting the creep behavior within the limited region, which actually determines the indenter velocity, is defined as the location where the equivalent stress � · r equals ps=3. The equivalent plastic strain rate _ ¾r at this point is found to be _inðsÞ=3:6 in the case when the stress exponent for creep is 3. The stress exponent and the activation energy for creep extracted from the results of Al­5.3mol%Mg solidsolution alloy indentation tests are in close agreement with those of tensile creep tests reported in the literature. In addition, the values for � · r and _r agree well with the values for the applied stress and the corresponding creep rate in tensile creep tests at the same temperature. The above results show that the creep characteristics of advanced materials, which are often available in minute quantities or as small-volume specimens, can be obtained from carefully designed indentation creep tests, and furthermore the constitutive equation for tensile creep can be predicted with sufficient accuracy through indentation creep test results. [doi:10.2320/matertrans.M2013370]

Influence of Mo and W on High Temperature Hardness of M7C3 Carbide in High Chromium White Cast Iron

Abstract: The influence of Mo and W addition on the high temperature hardness of M7C3 carbide was investigated using unidirectionally solidified hypereutectic cast iron with 25mass%Cr. Concentrations of alloying elements in primary M7C3 carbide were measured by EDS. As Mo or W content of the cast iron increases, the concentration of Mo and W in the carbide increased and that of Fe decreased. However, the Cr content was almost the same as about 60mass% in spite of increasing the Mo and W contents. It was found from the XRD results that the lattice constant of M7C3 carbide changed and its attendant volume of a unit cell increased as the Mo or W content was increased. From this point of view, it can be considered that Fe atoms in the M7C3 carbide were substituted by Mo or W which has larger atomic radius than Fe. In all the specimens, the hardness of M7C3 carbides are about 1600HV0.3 at the room temperature, and it decreases gradually with a rise of the test temperature. The decreasing ratio of carbide hardness becomes smaller at high temperature as the Mo or W concentration in M7C3 carbide increases. Thus, the dissolution of Mo or W atom into M7C3 carbide is very effective to maintaining the high temperature hardness of carbide. However, the increment of the hardness became smaller at higher Mo and W contents, so that an excess addition of both elements gave less effect than expected. The fracture toughness of M7C3 carbide at the room temperature was measured using an indentation fracture method, and the values were very similar among the carbides regardless of Mo and W concentration in the M7C3 carbide. [doi:10.2320/matertrans.F-M2014801]

Effects of Natural Aging on Bake Hardening Behavior of Al–Mg–Si Alloys with Multi-Step Aging Process

Abstract: In the present paper, the effects of natural aging on the bake hardening behavior of four Al­Mg­Si alloys, i.e., Alloy A (Al­0.6Mg­0.6Si) (mass%), Alloy B (Al­0.6Mg­1.0Si), Alloy C (Al­1.0Mg­0.6Si) and Alloy D (Al­1.0Mg­1.0Si), were investigated by means of Vickers hardness test, tensile test, differential scanning calorimetry analysis (DSC) and transmission electron microscopy (TEM). Two kinds of nanoclusters, i.e., Cluster(1) and Cluster(2) were controlled with the multi-step aging process. As the results, it was found that Cluster(1) formed during natural aging caused the decreased bake hardening response even though the pre-aging was conducted before natural aging. The decrease of the bake hardening response with increasing the natural aging time was markedly higher in the later stage of bake hardening than in the early stage. Exothermic peaks of Peak 2 and Peak 2A were observed in all of four alloys pre-aged at 343 and 363K. Peak 2A became larger with the natural aging time. The size distribution of the ¢′′ precipitates became wider with the natural aging time for Alloy A heated up to the temperature of Peak 2A. This is well understood by the following model. The transition from Cluster(2) formed during pre-aging to ¢′′ occurs preferentially at the early stage of bake hardening or during heating up to the temperature of Peak 2. Then the growth of ¢′′ is inhibited by the presence of Cluster(1) at the later stage of bake hardening. The secondary nucleation of ¢′′ occurs just after the dissolution of Cluster(1) into the matrix during heating up to the temperature of Peak 2A. The combined formation of Cluster(1) and Cluster(2) by the multi-step aging essentially affects the BH response and the ¢′′ precipitates in Al­Mg­Si alloys. [doi:10.2320/matertrans.L-M2014827]

Electronic and Local Crystal Structures of the ZrNiSn Half-Heusler Thermoelectric Material

Abstract: We investigated the electronic and local crystal structures of the sintered half-Heusler ZrNiSn alloy by synchrotron radiation photoemission spectroscopy (SR-PES), synchrotron radiation X-ray powder diffraction (SR-XRD) measurements, and electronic band structure calculations to clarify mechanisms leading to improvements in the thermoelectric properties of materials. In contrast to the predicted semiconductor-like electronic structure, the SR-PES results show a pseudo-gap at the Fermi level, and the SR-XRD analysis reveals an interstitial Ni disorder in the half-Heusler structure. An improvement in the thermoelectric properties can be achieved by material design based on the pseudo-gap electronic structure of half-Heusler ZrNiSn-based alloys. [doi:10.2320/matertrans.E-M2014803]

Influences of Fine-Particle Bombarding and Conventional Shot Peening on Surface Properties of Steel

Abstract: This study was conducted to investigate the influences of fine-particle bombarding (FPB, maximum diameter of collision particles: 34 μm) and conventional shot peening (510μm) on the surface properties of steel. The influence of injection pressure in FPB was also examined. The microstructures near surfaces were nano-crystallized by FPB and shot peening. Grain size of nano-crystals was more markedly decreased by FPB than shot peening, and further reduced by the use of high injection pressure in FPB. Surface hardness increased as the grain size of nano-crystals decreased. The hardened layers of the FPBed materials became thicker by the use of high injection pressure; however, they were still shallower than the hardened layers of the shot-peened materials. Compressive residual stress measured on surfaces was higher in FPBed materials than in shot-peened materials. The use of high injection pressure in FPB was not effective to further increase this stress. [doi:10.2320/matertrans.M2013322]

The Effect of Microstructure on Mechanical Properties of Forged 6061 Aluminum Alloy

Abstract: 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 Zener­Hollomon 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, Acrss (the value equivalent to critical resolved shear stress in the slip direction ACRSS 1 ACRSS0 = kAdA1mA, ACRSS = s··02, s: the averaged Schmidt factor in the tensile direction dA: grain sizes in the slip direction) was less dependent on dA In consideration of the texture, the yield strength of 6061-T6 is essentially less dependent on the grain size as reported previously [doi:102320/matertransMA201324]

Defect Detection in Thick Weld Structure Using Welding In-Process Laser Ultrasonic Testing System

Abstract: A new approach of non-destructive testing for thick welded structural materials based on laser-ultrasonic technique is investigated. In this study, weld part of structural materials, which should be conventionally inspected after welding, is inspected during welding process in order to save time and cost of manufacturing. The laser-ultrasonic is a method to generate and detect ultrasonic signals by laser beams and has potential to be applied to remote inspection/monitoring of materials under welding at elevated temperature. Bulk longitudinal acoustic wave generated by a Q-switched Nd:YAG laser irradiation and detected as surface vibration by laser interferometer coupled with a long pulse detection laser is used to detect defects around the weld. To overcome the lack of sensitivity of laser-ultrasonic testing on thick welded part having a thickness of more than 100mm at higher temperature, we have originally developed a modified synthesis aperture focus signal processing technique (m-SAFT). The in-process testing with actual piping weld having a thickness of 150mm with high temperature more than 200 degrees C. was demonstrated. By using m-SAFT, an actual weld defect of 1.5mm in diameter at 106mm depth in the specimen was clearly observed. The measurement result well agreed with the result of conventional ultrasonic testing conducted after weld process and also the cross-sectional observation of the specimen. [doi:10.2320/matertrans.I-M2014809]

Alloy Designs of High-Entropy Crystalline and Bulk Glassy Alloys by Evaluating Mixing Enthalpy and Delta Parameter for Quinary to Decimal Equi-Atomic Alloys

Abstract: The values of mixing enthalpy (¦Hmix) and Delta parameter (¤) 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 ¦Hmix­¤ 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 (30.19% in 73C5), whereas those in zones B1 and B2 for conventional and Cuand Mg-based BMGs, respectively, were 1036385 and 21518 (36.90 and 30.14%), respectively. This kind of statistical approach using ¦Hmix­¤ diagram will lead to finding out unprecedented H-E alloys and HE-BMGs. [doi:10.2320/matertrans.M2013352]