Microstructure, transformation behavior and superelasticity of an aged Ti-40Ni-12Cu (at%) shape memory alloy

Abstract

In this study, microstructure, aging behavior and mechanical properties of a (Ni,Cu)-rich Ti–40Ni–12Cu (at%) shape memory alloy were investigated by means of scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), micro Vickers hardness tests, transmission electron microscopy (TEM) and tensile tests. Ti(Ni,Cu)2 phase remained after solution treatment at 1123 K, while it was almost dissolved into matrix after solution treatment at 1373 K. Aging at 723 K increased hardness of the 1373 K solution treated specimen, which was ascribed to the formation of the C11b-type precipitate that has tetragonality of 1.05 and coherent interface with matrix. The micro Vickers hardness increased from Hv 269 to the maximum of Hv 366 as aging time increased from 0 to 2.5 h and then decreased with further prolonging aging time. Transformation temperatures of aged specimens were dependent on aging time, displaying an increase from 274 K to 285 K with increasing aging time from 1.5 h to 2 h and did not change significantly with prolonging aging time to 2.5 h and then again increased with further increasing aging, which was explained by composition effect, strain field around the precipitates and mean free path between precipitates. Compared with the 1373 K solution-treated specimen, the specimen aged at 723 K for 2.5 h showed more excellent superelasticity with a recovery ratio of 98.3%, higher yield stress of 445 MPa and smaller stress hysteresis of 36 MPa, which was ascribed to precipitation hardening caused by metastable C11b-type precipitates. Also, small transformation hysteresis of about 8 K was obtained in the 723 K aged specimens. Aging at 873 K for 45 h of the 1373 K solution-treated specimen induced the precipitation of stable Ti(Ni,Cu)2 phase with tetragonality of 2.55, thus the C11b-type precipitate was considered to be transient metastable precipitate.