Diffusion pack cementation of hafnium powder with halide activator on Ni–Ti shape memory alloy

Abstract

The effect of a Hf chloride activator on the pack cementation of Hf powder on a Ni–Ti shape memory alloy wire was investigated. For this purpose, a Ni–Ti wire with a diameter of 0.5 mm was pack cemented in a powder mixture consisting of Hf and HfCl4 powders at 1000 °C for 24 h. It was observed that Hf noticeably diffused into the Ni–Ti matrix with the aid of the HfCl4 activator. The diffusion distance significantly increased as the amount of HfCl4 activator increased. By the addition of 10 mass% HfCl4, the martensite-to-austenite phase transformation start and finish temperatures increased from 12 to 142 °C and from 28 to 200 °C, respectively. The diffusion kinetics model was established based on Fick’s first law. It is suggested that 48 h of halide-activated pack cementation with 10 wt% HfCl4 is necessary to increase the overall Hf content above 15 at.% throughout the Ni–Ti wire.