Effect of cold rolling process on recovery strain and recovery ratio of Ni50Ti40Hf10 shape memory alloy

Among NiTi base high temperature shape memory alloys, NiTiHf alloys seem to be more attention, because of their high transformation temperatures, good thermal stability and lower price in comparison with other shape memory alloys. Recovery strain and recovery ratio of this alloys in comparison with binary NiTi alloys is less. One of methods for improvement of recovery strain is thermomechanical treatment. In this research, the Ni50Ti40Hf10 shape memory alloy was casted by vacuum arc melting and after homogenization, specimens were cold rolled and annealed. By bending test, applied strain from 2.6 to 5.4 carried out on specimens and then amount of recovery strain and recovery ratio were measured. The result showed that with increasing applied strain, the amount of recovery strain increased. In all of cold rolled samples until 3% applied strain, recovery is complete 100% but in applied strain above this amount due to occurrence of slip deformation, recovery is less than 100%. In cold rolled specimens, due to increasing of hardness and also strength, recovery strain in comparison with casting specimen increased. Maximum recovery strain in casting specimen, was amount of 5 with 87% recovery ratio and with applying 10% and 20% rolling, recovery strain to 5.25 and 5.28 and also recovery ratio to 92% and 93.6% increased.