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 Hfmodified Ni55Ti45-xHfx (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/m1/2 and the tensile strength of 2.25 GPa.

Automated summary

The microstructural and mechanical properties of Ni55Ti45 alloy and Hf-modified Ni55Ti45-xHfx alloys were studied. Hf addition suppressed certain phases and promoted the formation of HfO2 particles. The alloy with 3% Hf content exhibited the best combination of fracture toughness and tensile strength.