Materials Science Forum Vols. 539-543

Abstract: Three kinds of Type 304 stainless steels: the first one was made by the conventional wrought process (I/M), the second one by conventional powder metallurgy process (P/M) and the third one by Metal Injection Molding process (MIM), were used. The corrosion behavior of these specimens was investigated through the potentiodynamic polarization curve measurement and electrochemical impedance spectroscopy method. In addition, SCC characteristics were also studied. Both the potentiodynamic polarization curve and the Nyquist plot curve of MIM specimen were similar to those of I/M specimen, but different from those of P/M specimens. Therefore it might be suggested that MIM process is quite suitable to improve the corrosion resistance. On the other hand, time constants of P/M and MIM specimens are similar, but they differed remarkably from that of I/M specimen, which is considered to be due to the remained pores in P/M and MIM specimens. Further, it was also confirmed that the SCC was more liable to take place on I/M specimen than MIM specimen.

Abstract: In order to clarify the effect of strain rates on phase transformation behaviors of Ni-Ti alloy, a compressive test using a cylindrical specimen of polycrystalline Ni-Ti alloy of Ti-50.69 at% Ni was carried out at a high strain rate and a low strain rate. The transformation temperatures were determined by a differential scanning calorimeter (DSC) using a sample cut from a compressed specimen. The transformation temperatures of the specimens before deformation were Ms= 303 K, Mf = 287 K, As = 297 K and Af = 319 K, respectively. The compressive test was carried out using specimen heated from liquid nitrogen temperature to room temperature. A universal testing machine as a static test apparatus and a Split Hopkinson Bar apparatus for a dynamic test were used. The specimen had a reoriented martensite phase after deformation because the superelastic effect was not observed upon unloading. Two reverse transformations during heating and a forward transformation during cooling were observed by DSC measurement. The first reverse transformation corresponds to that of thermal-induced martensite by immersion in liquid nitrogen and the second reverse transformation corresponds to that of reoriented martensite with slips in a polycrystalline matrix introduced by plastic deformation. The reverse transformation of the martensite phase with a slip exhibited strong strain rate dependency. Plastic strains and strain rate had strong influence on the shape recovery. The interaction between the temperature elevation by a conversion of plastic work and slip generated by dynamic plastic deformation is a complicated problem.

Abstract: We have studied effect of magnetic field on the martensite (10M) to intermediate phase transformation temperature (As) of Ni2MnGa in order to understand the influence of magnetocrystalline anisotropy on the transformation temperature under a magnetic field. In case of the transformation from multi-variant 10M phase to the intermediate phase, As decreases with increasing magnetic field H for H < 0.8 MA/m, and then it increases on further increasing H. On the other hand, in case of the transformation from the single-variant 10M to the intermediate phase, As increases monotonically with increasing H, where the easy axis of the single variant is parallel to the field direction. The difference between the multi-variant and single variant state can be explained by considering the high magnetocrystalline anisotropy of the martensite phase.

Abstract: We are using 2.45 GHz (S-Band) microwave systems and an 83-GHz, gytrotron-based, millimeter-wave beam system in material processing and other areas. We use one 2.45 GHz system in preparation of nanophase metals, metal mixtures and metal oxides, via the patented continuous microwave polyol process, with potential for large scale, low cost production. Of interest are precious metals, mixtures of magnetic and nonmagnetic metals, and mixed metal oxides for ceramic precursors. The other S-Band systems are used to develop repair techniques for ceramic matrix composites where the repairs are heated to 200-1000°C. A portable, battery-powered system is being developed for field repairs, and promises to be much more practical than alternative approaches (e.g., heating blankets). The 83-GHz system is being used in rapid sintering of polycrystalline ceramic materials intended for use in high power solid state lasers, where the requirement if for sintering to transparency with high optical quality and good lasing efficiency. Transparent Yb-doped yttria has been produced with hybrid conventional/millimeter-wave sintering of nanophase powders, as well as theoretically dense YAG. Another application for the millimeterwave beam system is in consolidation and bonding of hard coatings to light alloys, such as SiC on titanium, where the beam system allows heating of the coating to very high temperatures without overheating the metallic substrate. Finally, the millimeter-wave system is being used in the development of millimeter-wave plasma-assisted diamond deposition, where the quasi-optical system has significant advantages over conventional microwave plasma-assisted diamond deposition. Results for these various areas will be presented and discussed.

Abstract: The pitting corrosion behavior of sintered SUS 444 stainless steels (P/M specimens) with different kinds of porosity ratios of 13.3, 9.11 and 6.88 % was studied in an aqueous solution containing MgCl2 compared with a conventionally processed SUS 444 stainless steel(I/M specimen). The pitting corrosion potential decreased almost linearly as porosity ratio increased, and the I/M specimen showed the best resistance against the initiation of pitting corrosion. The pitting corrosion morphology by SEM observation showed that more small pits were observed on the specimen with larger porosity ratio while less number but larger diameter pits were observed on the specimen with smaller porosity ratio, and it was also confirmed that the I/M specimen had the least number but the largest and deepest pits.

Abstract: The near-equiatomic TiNi alloy has been demonstrated to possess high wear resistance, which largely benefits from its pseudoelasticity (PE). However, the PE occurs only in a small temperature range, which makes the wear resistance of this alloy unstable as temperature changes, caused by environmental instability or frictional heating. Therefore, enlarging the working temperature of PE could considerably improve this alloy as a novel wear-resistant material. One possible approach is to develop a self-built temperature-dependent internal stress field by taking the advance of the difference in thermal expansion between the pseudoelastic matrix and a reinforcing phase. Such a T-dependent internal stress could adjust the martensitic transformation temperature to respond changes in environmental temperature so that the temperature range of PE could be enlarged, thus leading to a wide temperature range in which the minimum wear loss is retained. Research was conducted to investigate effects of an added second phase having a negative thermal expansion (NTE) coefficient on the wear resistance of a near-equiatomic TiNi alloy. It was demonstrated that the temperature range of this modified material in which the wear loss dropped was enlarged. In addition, the wear resistance of such a TiNi-matrix composite was on one order of magnitude higher than that of unmodified TiNi alloy.

Abstract: Magnetic field effect on a Heusler-type Ni2MnGa off-stoichiometric alloy having a martensitic transformation temperature around room temperature which is coincident with a Curie temperature has been investigated. The process of martensitic transformation of a single crystal was investigated by neutron diffraction under the magnetic field up to 8 [T]. It was found that the magnetic field, which is applied at a temperature near the transformation temperature, causes the martensitic transformation. The process of the transformation caused by the increase in magnetic field is quite similar to the process caused by the decrease in temperature.

Abstract: Shape recovery and superelasticity of Ti-50at%Pt and Ti-50at%(Pt, Ir), whose martensitic transformation temperature are above 1273 K, were investigated by thermal expansion measurement in dilatometer and loading-unloading compression test. The shape recovery was found in all compounds in at least one of the testing methods. The highest shape recovery, about 4% was found in Ti-25Pt-25Ir using loading-unloading compression test. On the other hand, superelasticity was found in only ternary compounds. Larger superelasticity was observed in ternary compounds with higher Ir contents. Potential of Ti-50Pt and Ti-50(Pt, Ir) as high-temperature shape memory alloys is discussed.

Abstract: NiTi shape memory alloy fiber-embedded denture-base-resin matrix smart composites were developed as a new denture base material for a “smart denture”, whose shape could be recovered simply by heating after fracture. Three types of fiber surface treatment were applied for the composites and their properties were evaluated by the fiber-pull-out test and bending test, and shape change after repair was examined. A high interface debonding strength increased the fracture strain but did not affect bending strength, and a low interface sliding strength minimized shape change after repair. These results indicate that the fiber-matrix interface with a strong bonding but easy sliding after debonding could improve the preciseness of “smart repair”.