Effect of Hydrogen Absorption in TiNi Shape-Memory Alloy Thin Wire on SSRT Strength

Abstract

Effect of hydrogen absorption on the slow strain rate test (SSRT) of a TiNi shape-memory alloy was investigated by using thin wire with a diameter of 0.7mm. Three kinds of tests were conducted: one was tension test in air after hydrogen charging, and others were SSRTs with or without initial stress under successive hydrogen absorption. For SSRT without initial stress under hydrogen absorption, fracture stress and ductility decreased with decreasing strain rate. The tensile strength for SSRT without initial stress was almost identical to that for tensile tests in air with the same hydrogen charging time. On the other hand, for SSRT with initial tensile stress larger than 400MPa, tensile strength was higher than those of tensile tests in air and SSRT without initial stress, because exfoliation of wire surface due to the coalescence of small cracks reduced the depth of cracks formed in the surface brittle layer, which brought the fracture predicted by the net-stress criterion.