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INTRODUCTION

POWDER metallurgy (PM) technology allows for economical production of net-shaped products and minimizes the need for machining and plastic deformation operations. Thus, the use of PM for fabricating TiNi shape memory alloy (SMA) components has become an important focus of research. However, PM TiNi has its own problems. First, high-sintered densities are difficult to obtain.[1–5] When solid-state sintering is used, excessive sintering time is required to achieve high density because of the slow diffusion rates of Ti and Ni. Moreover, the unbalanced interdiffusion rate between Ni and Ti creates numerous Kirkendall pores on the Ni side. When liquid-phase sintering is used, either by increasing the sintering temperature or by increasing the Ti/Ni ratio to form liquid phase, some large pores could be left behind at the original Ti powder sites because the Ti powder melts above the Ti-Ti2Ni eutectic temperature of 1215 K (942 C).[1–5] In addition, the increase in the Ti/Ni ratio increases the amount of Ti2Ni and Ti4Ni2X (X = O, C), which have no shape memory properties. To resolve these problems, previous studies have used methods such as optimized compacting pressures, isostatic pressing, slow heating rates, fine Ti and Ni powders, and ball milling.[1–8] With these efforts, the sintered density of press-and-sinter or

FU-CHENG YEN, Graduate Student, and KUEN-SHYANG HWANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, R.O.C. Contact e-mail: [email protected] Manuscript submitted January 13, 2011. Article published online September 21, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

metal-injection-molded TiNi compacts can now reach a density greater than 95 pct of theoretical density.[6–8] An additional drawback in fabricating PM TiNi is that the shape memory characteristics and tensile properties are modest compared with those of wrought materials. The reasons for these developments are, in addition to the porosity, that Ti2Ni and Ti4Ni2O stripes are frequently found at grain boundaries, in particular in TiNi alloys with a Ti/Ni ratio greater than 1.[6,7,9] These brittle compounds, which are crack initiation sites and crack propagation paths,[8,10] impair mechanical properties. The elongation normally found in previous reports for as-sintered compacts is below 15 pct.[6–8,10] For wrought Ti-rich TiNi alloys, the amount of Ti2Ni phase formed during solidification can be minimized using homogenization treatment, and its shape can be modified into fine dispersoids through repeated thermal-mechanical treatment. With grain refinement and uniformly distributed fine Ti2Ni dispersoids, the tensile strength and elongation of wrought TiNi can exceed 800 MPa and 20 pct, respectively.[11] However, the PM TiNi does not allow secondary thermal-mechanical treatments because most PM parts are net-shaped and have complex geometries. With changes to the morphology of the Ti2Ni phase in PM TiNi alloys, it has been demonstrated that, with hot isostatic pressing