Superelasticity and transformation hysteresis of an aged (Ni,Cu)-rich Ti-40Ni-12Cu (at.%) alloy

초록

In this study, superelasticity and transformation hysteresis of a Ti-40Ni-12Cu (at.%) alloy were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and tensile testing. In the specimen solution-treated at 1123 K, the Ti(Ni,Cu)2 phase coexisted with the B2 matrix. Upon increasing the solution treatment temperature to 1373 K, this phase was mostly dissolved into the matrix, resulting in Ni supersaturation. Subsequent aging of the 1373 K solution-treated specimen led to the precipitation of a C11b-type phase. The average length, thickness, and inter-particle spacing of precipitates were 17.2 nm, 1.8 nm, and 28 nm, respectively, in the specimen aged at 723 K for 2.5 h; 21.9 nm, 2.1 nm, and 31 nm at 723 K for 3.5 h; 90 nm, 22 nm, and 122 nm at 873 K for 1 h. The aged specimens exhibited excellent superelasticity, which was likely associated with precipitation hardening by the C11b-type phase. Stress hysteresis decreased with decreasing aging temperature and time, probably due to reduced inter-particle spacing that increased the number of martensite nucleation sites.

키워드