Microstructure and mechanical properties of Nb- and Mo-modified NiTi–Al-based intermetallics processed by isothermal forging

A compositionally modified Ni–45Ti–5Al–2Nb–1Mo (at%) alloy was developed by simultaneously adding Nb and Mo in NiTi–Al-based alloy and processed by forging isothermally. The as-forged Ni–45Ti–5Al–2Nb–1Mo alloy consists of NiTi matrix, Ti2Ni and dispersive (Nb,Ti)SS phases. Mo addition reduces the solubility of Nb in the NiTi matrix and gives rise to the precipitation of numerous nanoscale (Nb,Ti)SS phases (bcc structure) within the grains. The (Nb,Ti)SS phase is semi-coherent to the NiTi matrix with a lattice misfit of 6.7%. The addition of Mo and Nb dramatically enhances the NiTi–Al alloy at high temperatures, which is mainly attributed to the solid solution strengthening of Nb and Mo elements as well as the precipitation strengthening of (Nb,Ti)SS phases. The Ni–45Ti–5Al–2Nb–1Mo alloy is expected to become a high-temperature structural material using at intermediate temperatures with strengths of 650 MPa at 923 K and 350 MPa at 1073 K.