Effects of Hf additions on the microstructures and mechanical properties of Ni-rich Ni₅₅Ti₄₅ alloys

The microstructural characterization and mechanical properties of the oil quenched binary Ni₅₅Ti₄₅ alloy and Hf-modified Ni₅₅Ti_45-xHf_x (x = 3, 5, 7 at.%) alloys were investigated. The results showed that the microstructure of binary alloy is composed of NiTi phase, nanoscale Ni₄Ti₃ precipitates, and a small amount of Ti₄Ni₂O and Ni₃Ti₂ phases. The Hf additions suppresses the formation of the Ti₄Ni₂O and Ni₃Ti₂ phases and promotes the HfO₂ particles. When the Hf content is less than 5 at.%, the main precipitated phase is still Ni₄Ti₃, but which is gradually refined and spheroidized with the increase of Hf content. While the Ni₄Ti₃ phases disappeared and the fine H-phases precipitated massively in the Ni₅₅Ti₃₈Hf₇ alloy. With the gaining content of Hf, the Vickers hardness increases, but the fracture toughness and tensile strength increase first and then decrease. When the addition of Hf is 3 at.%, the alloy possesses the best property with the fracture toughness of 21.02 MPa/m^1/2 and the tensile strength of 2.25 GPa.