Transformation behavior in NiTi-20Hf shape memory alloys - Transformation temperatures and hardness

A series of NiTi-20Hf high-temperature shape memory alloys with varying nickel content from 50 to 51% were subjected to solution and aging heat treatments, thermal cycling, and hardness testing. Transformation temperatures for all conditions were measured and found to decrease as a function of increasing Ni content up to 50.5%. There was a gradual increase or consistent temperature for Ni contents above 50.5%, depending on the aged or homogenized condition. A similar trend was observed in the hardness values. The alloys aged at 550 degrees C for 3 h exhibited the highest hardness with an even greater increase at Ni contents above 50.5%. Published by Elsevier Ltd on behalf of Acta Materialia Inc.