Effect of Ni/Ti atom number ratio on the microstructure and mechanical properties of NiTiAlHf alloys

Scanning electron microscopy (SEM) and compressive tests at room temperature and elevated temperatures were used to investigate the effect of Ni/Ti atom number ratio on the microstructures and mechanical properties of Ni_42+xTi_50-xAl₄Hf₄ (x=0-7) alloys. It is found that Ni_42+xTi_50-xAl₄Hf₄ alloys are composed of NiTi and Ti₂Ni phases. The size and volume fraction of Ti₂Ni phase decrease rapidly and the precipitation strengthening effect weakens with increasing Ni/Ti values. At room temperature when the Ni/Ti ratio increases, the compressive yield strength (σ_0.2) and Vickers hardness (Hv) of the alloys decrease, but the ductility increases. At elevated temperatures, for the alloys of Ni/Ti<1, the precipitation strengthening of Ti₂Ni phase plays the major role, resulting in a decrease of σ_0.2 with increasing Ni/Ti values; but for the alloys of Ni/Ti>1, the solid solution strengthening effect of Hf improves, and the uniform distribution of Ti₂Ni is more favorable for the strength, leading to the increase of σ_0.2 with increasing Ni/Ti values. The high-temperature yield strength is asymmetrical with a deviation from stoichiometry (Ni/Ti=1).