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

Abstract

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 Ti2Ni and Hf-rich phases.