Effect of Nb content on martensitic transformation temperatures and mechanical properties of Ti-Ni-Nb shape memory alloys for pipe joint applications

Abstract

Several Ti-Ni-Nb shape memory alloys (SMAs) with various Nb contents (0-15 mol%) or Ni/Ti atomic ratios (1.0-1.1) were tested to investigate the martensitic transformation temperatures and mechanical properties. The martensitic transformation temperatures (Ms, Mf, A s and Af) of the solution-treated Ti-Ni-Nb alloys at 1173 K for 300 seconds with Ni/Ti ratio of 1.0 decrease with increasing the Nb content. These transformation temperatures also depend on the Ni/Ti ratio, heat treatment condition and cold working rate. Furthermore, in the alloy with Nb content of 6mol% and Ni/Ti atomic ratio being less than 1.05, the fracture stress and fracture strain reached about 1,000MPa and over 20%, respectively. Based on the obtained results, several processing conditions such as the chemical composition, fabrication process and heat treatment are optimized for the Ti-Ni-Nb SMAs to develop SMA pipe joints. © 2007 The Japan Institute of Metals.