Microstructure evolutions of Ni-Ti-Nb-Al alloys with different Al addition.

Microstructure evolutions of Ni-Ti-Nb-Al alloys with different Al addition subjected to casting and solution-treatment have been investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD). The alloys were prepared by vacuum arc melting, and then solution-treated at 1273 K for 20 h. The alloys with 3 at.% Al addition and Al-free show a microstructure consisting by B2-NiTi matrix phase and soft β-Nb precipitate. With increasing Al addition, Al 3 Ti phase firstly forms, and Ni 3 Al only can be found in the 7 at.% Al alloy. The addition of Al promotes the diffusion of Nb during casting and solution-treatment, resulting in the refinement of β-Nb precipitate. For the as-cast alloys, the macro-hardness values increase with the increase in Al content and slightly increase after solution-treatment. [ABSTRACT FROM AUTHOR]