Materials Science Forum Vols. 706-709

Abstract: Ti-Sn-Cr shape memory alloys were fabricated by the powder metallurgy (PM) process. The mixed powders in composition of Ti-3at%Sn-7at%Cr were prepared using a V-blender. The mixed powders were filled into a graphite die, and then sintered at a temperature between 1023 K and 1223 K using a pulse current sintering equipment. The sintered alloy was solution-treated and then aged. The phase structure and the tensile properties of the alloy obtained were investigated. The relative density of the as-sintered alloy increased with elevating sintering temperature, and the maximum relative density was 99.5 % at a sintering temperature of 1223 K. Although the as-sintered alloy was high density, the microstructure of the alloy was inhomogeneous. Thus it was found that the solution treatment was needed to obtain the homogeneous microstructure of the alloy. The alloy which is mainly consisted of the β phase was obtained by performing solution treatment. The plateau region due to the slip deformation was observed in the stress-strain curve of the solution treated alloy, but that due to the stress induced martensite transformation could not be done. The plateau region due to the stress induced martensite transformation, however, was appeared in the stress-strain curve of the aged alloy. This means that the aged alloy has shape memory properties. It was also found from the result of bending test that the alloy fabricated by the PM process has obviously shape recovery properties.

Abstract: Micro powder injection molding has been accepted as a potential forming technology for large-scale production of miniature components in industries. However, mold filling capability is often poor due to the blind hole structure of the die cavity, which restricts the widespread application of this technique. In this paper, ZrO₂ micro gears with addendum circle ranging from 900μm to 200μm were studied. Instead of using traditional PW-HDPE-SA binder as in powder injection molding, thermoplastic binder system is optimized here for better replication of micro gear. It is found that the dimensional accuracy and demold after injection molding can be improved by utilizing proper binder system. ZrO₂ feedstock with a powder loading of 46vol.% and the binder system consisting of BW, HDPE and SA was successfully injection molded and demolded, which indicates good mold filling capability and sufficient demolding strength. The binder system was successfully removed by debinding process in two steps, solvent debinding followed by thermal debinding. Debound components free of defects were obtained. Keywords: Micro powder injection molding, ZrO₂ feedstock, Binder system, Molding filling capability, Demolding strength

Abstract: A new method of solidifying metal powder by dynamic molding under compression stress was developed. In the solidified specimen, recrystallization is suppressed, so that nanoscale crystal grains remain. The hardness of the specimen prepared by this process is increased relative to materials prepared by other methods. The material properties and preferred orientation of an aluminum plate with crystal grains 100−200 nm in diameter prepared by the new molding process are reported. The correlation between the crystal structure and properties of crystallization are clarified.

Abstract: Ti-10V-2Fe-3Al is one of super elastic Ti alloys, which is the b type Ti alloy and shows the excellent properties such as high specific strength and corrosion resistance. By the combination of Cold working and heat treatment, high strength as same as the steels are obtained. Therefore, Ti-10V-2Fe-3Al is hoped to be a next generation’s spring material. In this study, Metal Injection Molding of super elastic Ti alloy materials are investigated by changing the process parameters such as powder type (mixed elemental or alloy), debinding and sintering conditions, and heat treatment conditions to obtain the excellent super elastic properties. Tensile strength of injection molded Ti-10V-2Fe-3Al compacts sintered at 1250 with mixed elemental powders showed approximately 85% of wrought materials. On the other hand, alloy powder compacts showed about 80% strength of wrought materials. Their super elastic behavior depended strongly on the heat treatment conditions.

Abstract: Diamond-particle-dispersed-aluminum (Al) matrix composites were fabricated in continuous solid-liquid co-existent state by spark plasma sintering (SPS) process from the mixture of diamond powders, pure Al powders and Al-5mass%Si alloy powders. The microstructures and thermal conductivities of the composites fabricated were examined. These composites were well consolidated by heating at a temperature range between 798K and 876K for 1.56ks during SPS process. No reaction at the interface between the diamond particle and the Al matrix was observed by scanning electron microscopy for the composites fabricated under the sintering conditions employed in the present study. The relative packing density of the diamond-Al composite fabricated was 99% or higher in a volume fraction range of diamond between 45% and 50%. Thermal conductivity of the diamond-Al composite containing 50 vol.% diamond reached 552W/mK, approximately 95% the theoretical thermal conductivity estimated using Maxwell-Eucken’s equation.

Abstract: We irradiated the surface of a Co-Cr-Mo cast alloy plate with an electron beam in order to increase the surface hardness. An accelerating voltage of 40 kV and a current of 1~10 mA of the irradiated electron beam were used. The irradiation time for a single dot was 0.1 ms and the distance between dots (dot pitch) was 0.2 mm, corresponding to a scanning speed of 200 mm/s. A fine cellular structure was obtained by irradiating a low current electron beam. When a high current electron beam (more than 8 mA) was used for the irradiation, fine granular carbides were synthesized. The Vickers hardness of the base metal of 410 HV was increased to about 480~530 HV by the electron beam irradiation.

Abstract: In this article, a new technique for controlling crack position and its propagation direction in solder-bonding using Al/Ni exothermic reaction is described. Sputtered Al/Ni multilayer film is able to produce heat instantly by its self-propagating exothermic reaction, and the reactive film can be used as heat source for solder-bonding. During the reaction, however, volume reduction by approximately 12% occurs due to crystal structural change from fcc to bcc and lattice-spacing reduction. Consequently, cracks are produced in the reacted NiAl structure. The cracks negatively affect the strength of the bonded system. We have found a new technique for controlling crack position and its propagation direction. Multiple ignitions for reaction demonstrated that cracks in reacted NiAl film can be controlled. When applying the flash heating technique to wafer-level bonding, cracks are probably produced. If cracks can be fabricated on dicing cut lines by using the simultaneous multiple reactions technique, crack-less solder-bonded Si hermetic packages would be realized.

Abstract: Some ceramic based hybrid composites with metallic particles have excellent mechanical properties. In particular, metal/ceramic nanocomposites has high mechanical strength and improved fracture toughness. Magnetized hybrid composites can be produced by dispersing Fe, Co or Ni nanoparticle in the structural ceramics. Crack healing of metal/ceramic hybrid composites was investigated for structural applications. Cracks of nanoNi/Al₂O₃ introduced by the Vickers indentation of 49 N could be repaired completely, for example, by air oxidation at 1200°C for 6 h. Even larger cracks introduced by 490 N indentation was healed up to a comparable level of mechanical strength by oxidation at 1200°C for 24 h in air. Outward diffusion of cations during high-temperature oxidation would cause crack repair by filling with an oxidation product. Faster crack disappearance, 900°C for 1 h, was realized on Mo/Al₂O₃. Surface crack healing effectiveness promoted by outward diffusion of cations is potentially applied into various Al₂O₃ based hybrid material systems, as well as excellent mechanical properties.

Abstract: The effect of Co addition on the properties of Ni_8-xMn₄Ga₄Co_x (x=0, 0.5, 1, 1.5 and 2) ferromagnetic shape memory alloys are systematically investigated by means of the first–principles calculations within the framework of density functional theory (DFT) using the Vienna ab initio software package (VASP). The formation energy results indicate that the added Co preferentially occupies the Ni sites in Ni₂MnGa alloy. With the increase of the Co content, the optimized lattice parameters of the parent phase decreases regularly, whereas the lattice parameter a of the martensite increases and c decreases, thus leading to a decrease of the c/a ratio. The effects of Co addition on phase stability of the paramagnetic and ferromagnetic austenite are displayed. The difference in the magnetic properties in the investigated series can be explained from the electronic density of states analysis. The aim of this paper is to provide theoretical guidance for the development of new promising ferromagnetic shape memory alloys with optimized properties.

Abstract: Difference of the phase transformation behavior at deformation-induced layer depending on surface treatment methods was investigated using Fe-33mass%Ni alloy. As specimens, two kinds of specimens were prepared. One specimen has austenite (γ) single structure, and the other specimen consists of both martensite (α’) and γ phases. Using these specimens, shot-peening tests were performed, and then phase transformation behavior induced by the shot-peening was compared with that by sliding wear reported in previous literature. The deformation-induced layer induced by shot-peening has very fine microstructure consisted of both α’ and γ phases. The microstructure in the deformation-induced layer formed by shot-peening becomes similar with increasing duration of shot-peening regardless of initial microstucture. On the other hand, the deformation-induced layer induced by sliding wear is reported to have γ single structure regardless of initial microstructure. This microstructural difference between shot-peening and sliding wear comes from the difference of heat generation during the surface treatment. Therefore, it is concluded that phase transformation behavior in deformation-induced layer depends on surface treatment method.