Influence of Fe addition on phase transformation temperatures and mechanical properties of TiNi shape memory alloys

Polycrystalline ingots of TiNiFe (Fe=2.5%, 3% and 3.5%) shape memory alloys are prepared by arc melting repeated for five times with high purity element (99%) under argon atmosphere. After homogenizing at 1123 K for 24 hours in a vacuum quartz ampoule, the alloys are forged into rods with 7 mm in diameter. The influence of Fe content on the grain size, phase transformation temperature and mechanical property of TiNi shape memory alloys are studied by optical microscope observation, electrical resistance measurement and mechanical tensile test, respectively. The results show that with the increase of Fe content from 2.5% to 3.5%, the grain size deceases from 49 μm to 26 μm and the martensite transformation temperature decreases from -73.5°C to -190°C, and, moreover, the yielding strength and tensile strength are remarkably improved, while the percentage elongation changes a little. The martensite transformation temperature and mechanical properties of TiNiFe alloy satisfy the requirements of pipe coupling in aviation application.