Effect of Nb addition on the microstructure and martensitic transformation of NiTiHfScNb high temperature shape memory alloys

In the present work, the effects of Nb addition and aging on the microstructure evolution and martensitic transformation of (Ni50.3Ti28.7Hf20.0Sc1.0)100-xNbx (x = 0, 5, 10, 15 at.%) high temperature shape memory alloys were investigated. The addition of Nb results in the formation of the eutectic β-Nb phase along with martensite and Sc2O3 at room temperature. Aging at 550 °C for 3 h leads to the formation of nanoscale H-phase. The surface area fraction of the H-phase decreases with increasing Nb content to 10 at.%. As the Nb content increases, the transformation temperatures of the solution-treated NiTiHfScNb alloy increase and then decrease due to the variation of Ni content in the matrix. The aged samples show a smaller transformation hysteresis in comparison with the solution-treated samples. The difference of thermal hysteresis between the solution-treated and the aged samples increases with increasing Nb content because of the H-phase precipitation and Nb solution in matrix.