Investigating Hot Deformation Behavior and Shape Memory Properties of Ni50Ti40Hf10 High Temperature Shape Memory Alloy

NiTiHf high temperature shape memory alloys are highly popular for high temperature industrial applications due to their high transformation temperature, thermal stability and lower cost. However, they have weaker shape memory properties than NiTi binary alloys. The solution to this problem is to increase the strength of the alloy in order to prevent plastic deformation caused by slipping. Thermomechanical treatment is a method to improve the recovery strain. In this research, the Ni50Ti40Hf10 shape memory alloy was cast by vacuum arc melting and then the hot tensile test was performed on the samples with a strain rate of 0.01 s-1 at 800, 900, 1000 and 1100 C, also specimens were hot- rolled at 1000 C with a reduction of thicknesses by 10, 15, 20 and 30%. To determine the amount of recovery strain and recovery ratio by bending test strains of 2.6 to 6 were applied to the samples. The results of the hot tensile test showed that the temperature of 1000 C is acceptable for hot rolling due to the suitable ductility of the alloy. The maximum recovery strain in the cast sample was 5.7 with a recovery ratio of 88%. By applying 10% of hot rolling, the recovery strain and recovery ratio increased to 5.8 and 92 respectively, while after 20% of rolling, these values increased to 5.9 and 94. The formation of uniform and coaxial grains in the microstructure of the samples indicated the occurrence of dynamic recrystallization, which led to an increase in the recovery strain.