Microstructure and tensile properties of isothermally forged Ni-43Ti-4Al-2Nb-2Hf alloy

NiTi-Al-based alloys are promising high-temperature structural materials for aerospace and astronautics applications. A new NiTi-Al-based alloy Ni-43Ti-4Al-2Nb-2Hf (at%) was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope (SEM), X-ray diffraction (XRD), and tensile tests. Results show that the microstructure of as-forged Ni-43Ti-4Al-2Nb-2Hf alloy consists of NiTi matrix, Ti₂Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni-43Ti-4Al-2Nb-2Hf alloy are dramatically increased compared with the ternary Ni-46Ti-4Al alloy. At room temperature and 650 C, the yield stress of Ni-43Ti-4Al-2Nb-2Hf alloy reaches 1,070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni-46Ti-4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti ₂Ni and Hf-rich phases.