Anisotropic negative thermal expansion behavior of as-smelted Ti-rich Ti-Ni alloys with different Ni contents

摘要

Ti-Ni alloys have been found to exhibit unique negative thermal expansion (NTE) performance, namely, contracting upon heating, which is thought to result from the reversible martensitic phase transformation behavior. In Ti-Ni alloys, the high-temperature phase, i.e., austenite (A), has the CsCl structure, while the low-temperature phase, martensite (M), has the monoclinic structure. The parameters of austenite and martensite are as follows: ao=0.3015 nm, V_A=0.02741 nm~3 and a=0.2889 nm, b=0.4120 nm, c=0.4622 nm, β=96.8° and V_M=0.05463 nm~3 [1]. Clearly, the volume of the low-temperature phase is almost twice that of the high-temperature phase. Mavoori et al. [2] reported that Ti-Ni alloys have NTE behavior and the coefficient of thermal expansion (CTE) of uniaxially strained Ti-Ni alloys is up to -200×10~(-6) K~(-1). Uchil et al. [3] studied the thermal expansion in various phases of Ti-Ni alloys using TMA and DSC. Nevertheless, there is still a lack of deep and comprehensive understanding of the NTE behavior of Ti-Ni alloys.