Materials Science Forum Vol. 1016

Abstract: On the thermomechanical treatments of Cu-Ni-Si alloy, cold-rolling (CR) before solution heat treatment (SHT) is commonly conducted to eliminate defects in a casting slab. In addition, a rolling is applied to reduce/adjust the thickness of casting slab before SHT. In a heavily deformed microstructure by CR, on the other hand, grain growth during a heating in SHT is likely to occur as the result of recrystallization. In general, tensile strength and fatigue strength tend to decrease with an increase in the grain size. However, the effect of difference in grain sizes produced by with and without CR before SHT on the fatigue strength is unclear. In the present study, fatigue tests of Cu-6Ni-Si alloy smooth specimens with a grain fabricated through different thermomechanical processes were conducted. The fatigue behavior of Cu-Ni-Si alloy was discussed.

Abstract: The study aims at reviewing manufacturing schedules for hot-rolled and intercritically-annealed and thermomechanically processed medium-Mn sheet steels. Major differences between these two types of processing are indicated. They include initial microstructure (low-C martensite for hot-rolled products and deformed austenite for thermomechanically processed products), partitioning of alloying elements (depending on intercritical annealing or batch annealing conditions) and microstructure prior to cooling (ultrafine-grained lath mixture of ferrite and austenite for hot-rolled, intercritically annealed products and deformed austenite for directly cooled products). The comparison of typical microstructures (LM, SEM) and phase transformation behavior are presented. A role of Mn as an austenite stabilizer in a range used for medium-Mn steels is explained. Some results are provided for economic steels containing 3% and 5% Mn.

Abstract: This paper deals with the study of the development, structural and microstructural characterization and, selected mechanical properties of Ti-25Ta-50Zr alloy for biomedical applications. The alloy was melted in an arc furnace and various solution heat treatments were performed to analyze the influence of the temperature and time on the structure, microstructure, microhardness and elastic modulus of the samples. The structural and microstructural results, obtained by X-ray diffraction and microscopy techniques, showed that the solution heat treatment performed at high temperatures induces the formation of the β phase, while solution heat treatment performed at low temperatures induces the formation of the α” and ω metastable phases. Regarding the effect of time, samples subjected to heat treatment for 6 hours have only the β phase, indicating that lengthy treatments suppress the α” phase. Regarding the hardness and elastic modulus, the alloy with the α” and ω phases, after treatment performed at a temperature of 500 °C, has a high hardness value and elastic modulus due to the presence of the ω phase that hardens and weakens alloys. The titanium alloys developed in this study have excellent mechanical properties results for use in the orthopedic area, better than many commercial materials such as cp-Ti, stainless steel and Co-Cr alloys.

Abstract: The effects of the peening conditions on the surface characteristics and fatigue strength of stainless steel were investigated by microshot peening (MSP). In recent years, MSP technology has attracted attention. The use of MSP technology with minute media has become more widespread in consideration of the reduction of the notch effect in the material surface. However, the effect of MSP technology on stainless steel has not been much studied. In the present study, an air-type machine was used. The media used was high-carbon cast steel (490 HV) and Fe-Cr-B alloy (1130 HV), with an average diameter of 0.1 mm. The peening time was in the range of 0 - 100 s. Four types of stainless steels, SUS304, SUS304L, SUS316, and SUS316L, were tested. The workpieces were annealed at 1100 K for 1.2 ks in air. In the measurement of fatigue strength, the workpieces were machined in hour-glass shape. As the results, surface roughness of the workpieces treated by MSP was small. Work hardening was evident to the depth of approximately 0.2 mm from the surface. This depth was about twice the diameter of shot media. However, the effect of the peening time on the hardness distribution was not large. The compressive residual stress was added in the surface vicinity. It is assumed that the fatigue limit had increased because the work hardened layer was formed near the workpiece surface.

Abstract: Although metal matrix composites (MMC) for the high temperature structural material have been investigated extensively for many years, applications of MMC have been still limited. Among many combinations between the ceramic fibers and the matrix materials, combination of SiC fiber and TiAl based intermetallic compounds has been expected to be one of the best combination, since both SiC fiber and TiAl have demonstrated the capabilities of the low density heat resistant materials. SiC fiber reinforced TiAl composites have been successfully fabricated using hot press method. Optimum temperature and pressure have been determined. SiC/TiAl composite having relatively low fiber volume fraction shows nearly an ideal elastic property applying the law of mixture. Effects of interface layers on the mechanical properties of composites have been studied in detail. Micro-indentation on a single fiber was carried out to examine the pull out strength of SiC fiber quantitatively. Estimated shear stress on the interface was 145-195MPa, those values are quite reasonable since the tensile strength of TiAl matrix was 420MPa and the maximum shear stress would be the half of tensile strength according to Schmid law. Three-point bending tests have been carried out to evaluate the mechanical properties of composites. Fiber volume fraction 8.9% specimen shows ideal bending stiffness compare with the calculated values based on the low of mixture. Reaction layers and the interface between SiC fiber and TiAl have been analyzed by SEM-EDS and XRD. At least two or more reaction layers have been identified. These reaction layers can be explained based on the Si-Ti-C ternary equilibrium phase diagram at 1373K. Optimum conditions of interface structure will be discussed

Abstract: The hydrogen embrittlement of SK85 high-strength steel sheets was evaluated using a three-point bending test. The effect of electroplating the metal with zinc-based coatings on hydrogen embrittlement was examined by baking treatment of differently electroplated steel specimens. After electroplating, all the specimens underwent hydrogen embrittlement, promoted by hydrogen incorporation into the metal frame, owing to the reduction of hydrogen ions during electroplating. The hydrogen embrittlement of both zinc-and zinc-SiO₂-electroplated SK85 steel continued after baking for 24 hours at 473 K, but that of zinc-nickel-and zinc-nickel-SiO₂-electroplated SK85 steel ceased. Furthermore, TDA revealed that the trapped hydrogen could be released from steel at approximately 473 K. However, after baking, hydrogen embrittlement did not completely disappear, and we suggest that the formation of hydrogen vacancy clusters also accounts for this fracture phenomenon. The hydrogen incorporated into steel during electroplating led to the formation of hydrogen vacancy clusters, which allowed the formation of embrittlement. However, zinc and zinc-SiO₂ films were not permeable enough to release these voids; while the peculiar zinc–nickel and zinc-nickel-SiO₂ film structure enabled the hydrogen vacancy clusters to diffuse from the substrate.

Abstract: In order to reduce the cost of β-type Ti alloys, the use of Fe as an alloying element has been studied. However, Fe is known to have a very high diffusion coefficient in β-Ti of about 2.6×10^-12 m²/s at 1200 K, and its behavior during heat treatment is expected to be difficult to control. By contrast, Mo, which is also a β-stabilizing element, has a diffusion coefficient of only about 2.5×10^-14 m²/s at 1200 K, i.e., roughly 100 times smaller than that of Fe^{1), 2)}. In this study, the effect of the partial substitution of Fe with Mo on the aging behavior of β-Ti alloys was investigated using X-ray diffraction, electric resistivity, and Vickers hardness measurements. Ti-Mo-Fe alloys were solution-treated by holding at 1173 K for 3.6 ks and then quenching in ice water. In the X-ray diffraction patterns for the resulting samples, only peaks associated with the β phase were identified. It was found that the electrical resistivity and Vickers hardness decreased with increasing Mo content. As the Mo-to-Fe ratio increased, the decrease in electrical resistivity and the increase in Vickers hardness occurred later during the isothermal aging process. This was due to a delay in isothermal ω-phase precipitation.

Abstract: The effect of layer thickness on hardness and buckling behavior was investigated on Ni-Co-Cu/Cu multilayered films. The Ni-Co-Cu/Cu multilayered films were grown on annealed copper substrates by electrodeposition. We fabricated the multilayered films with various layer thicknesses ranging from 10 nm to 1000 nm. First, dependence of Vickers hardness on the Cu layer thickness was investigated. When the Ni-Co-Cu layer had the constant thickness of 75 nm and the Cu layer thickness was smaller than 75 nm, the hardness increased rapidly with decreasing Cu layer thickness. Subsequently, compressive tests were conducted on the multilayered films having the component layers ranging from100 nm to 1000 nm, where the hardness values did not change rapidly with layer thickness. The copper substrates coated with the multilayered films were compressed until 20% strain. From SEM surface observations after the compressive tests, formations of band-like structures having a certain thickness were recognized. Cross-sectional observation revealed that some band-like structures were formed as a result of local buckling of the multilayered film. The vertical thickness of the bank-like structures increased linearly with increasing component layer thickness.

Abstract: Processing through the application of high-pressure torsion (HPT) provides significant grain refinement in bulk metals at room temperature. These ultrafine-grained (UFG) materials after HPT generally demonstrate exceptional mechanical properties. Recent reports demonstrated the bulk-state reactions for mechanical bonding of dissimilar lightweight metal disks to synthesize hybrid alloy systems by utilizing conventional HPT processing. Accordingly, the present report provides a comprehensive summary of the recent work on processing of several UFG hybrid alloy systems including Al-Mg and Al-Cu by HPT under 6.0 GPa at room temperature and a special emphasis was placed on understanding the evolution of hardness. This study demonstrates a significant opportunity for the application of HPT for a possible contribution to current enhancements in diffusion bonding, welding and mechanical joining technologies as well as to an introduction of hybrid engineering nanomaterials.

Abstract: Lightweight porous metals are focused on as energy absorbing materials for automobiles. Open-cell porous metals were manufactured through additive manufacturing process. Their cell structures were designed based on Voronoi diagrams using a commercial 3D-CAD software. Both ordered and disordered cell structures with the same porosities were successfully designed in this study. Compression tests and explicit finite element analysis revealed heterogeneous deformation behaviors in ordered porous metals. On the other hand, the porous metals with disordered cell structure showed relatively isotropic and uniform deformation, which is suitable as energy absorbing materials. Controlling the disordered cell structure designed by 3D-Voronoi diagram enables to develop the advanced porous metals having various mechanical properties.