Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K
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I. INTRODUCTION
IT is known that the shape memory characteristics of near-equiatomic Ti-Ni alloys are affected by the following treatments: annealing following cold working,[1–4] aging treatment in Ni-rich Ti-Ni alloys,[1,2,5] thermal cycling,[6–9] stress cycling,[7] and the addition of third elements.[10,11,12] Among these treatments, the aging treatment for Ni-rich Ti-Ni alloys is an effective method to improve shape memory and mechanical properties due to the formation of Ti3Ni4 precipitates, which act as effective obstacles such as pinning points against the movement of dislocations, with the consequence that the critical stress for slip can be improved. Commercial Ni-rich Ti-Ni alloys are usually aged above 600 K for relatively short times for optimum pseudoelasticity. Thus, much work has been done to investigate the effect of aging above 600 K on the shape memory properties of thin films fabricated by radio frequency magnetron sputtering[13,14] and of rapidly solidified ribbons[15] as well as bulk alloys.[1,2,16,17] However, the effect of aging at low temperatures (600 K) on mechanical properties such as maximum recovery strain and critical stress for slip of Ni-rich Ti-Ni alloys has not yet been systematically investigated by varying the aging condition over a wide range. Even though the research on aging below 600 K is not important practically, it is very important academically, because (1) it allows us to investigate the shape memory characteristics and formation of Ti3Ni4 precipitates at early stages of aging, and (2) it is expected that lowtemperature aging (600 K) for a long time allows Ti3Ni4 JAE IL KIM, Postdoctoral Researcher, is with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123. SHUICHI MIYAZAKI, Professor, is with the Institute of Materials Science, Tsukuba University, Ibaraki 305-8503, Japan. Contact e-mail: miyazaki@ ims.tsukuba.ac.jp Manuscript submitted April 1, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitates to form densely and finely, with an expectation that low-temperature aging can improve shape memory characteristics. The specimens aged above 600 K usually exhibit a transformation behavior involving B2 : R : M on cooling and M : B2 or M : R : B2 on heating. In recent years, however, a complex transformation behavior has been reported in the literature: a multistage martensitic transformation (MSMT) appears by aging above 600 K,[18–24] and a two-stage R-phase transformation appears after aging below 600 K in Ni-rich Ti-Ni alloys.[25] It is suggested that these phenomena are closely related to the presence of Ti3Ni4 precipitates formed in the early stage of aging. Whereas there is a common acceptance that aging-induced multiple-stage transformation behavior is related to the formation of Ni-rich precipitates, no convincing explanations for the mechanisms have been established. The purpose of this study is to compare the transformation and deformation behavior in the Ni-rich Ti-50.9 at. pct Ni alloy aged at a low temperatur
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