Effect of nano-scaled precipitates on shape memory behavior of Ti-50.9at.%Ni alloy

In order to clarify the effect of low temperature aging (<600 K) on the shape memory behavior of a Ti-50.9at.%Ni alloy, the deformation behavior and microstructures were investigated using tensile tests, X-ray diffractometry and transmission electron microscopy (TEM). The specimens were solution-treated at 1073 K for 3.6 ks followed by aging at 373, 473 and 573 K for various times ranging between 1.2 and 10,800 ks. TEM observation revealed that after aging at low temperature (<600 K) the size of Ti3Ni4 precipitates hardly increases, but their density increases with increasing aging time in the early stage of formation of Ti3Ni4 precipitates. The critical stress for slip deformation abruptly increases with increasing density of fine Ti3Ni4 precipitates (<10 nm) in the specimens aged at the low temperature region. This is attributed to the increase in the number of pinning points to hinder the movement of dislocations. With increasing aging time and temperature the size of Ti3Ni4 precipitates increases, resulting in decreasing the critical stress for slip.