Alloy Design For NCAXB-type Ferrous Superelastic Alloys

Due to low cost and excellent cold workability, ferrous superelastic alloys are considered to be a promising candidate for replacing Nitinol. In order to achieve superelasticity in these alloys, a certain combination of matrix solid-solution strengthening, precipitation hardening, and microstructural texturing is required. CALPHAD methods are employed here to examine compositions in NCAXB-type ferrous alloys. In Fe-Ni-Co-Al-X-B (X= Ta, Cr, Nb, V, or W) superelastic alloys, the gamma prime ((Fe, Co, Ni)₃(Al, X)) phase is available for precipitation strengthening, in addition to matrix solid-solution strengthening; both of which are tracked in the CALPHAD modeling. The Ta segregation problem in the NCATB alloy after melting is solved through heat-treatment. The hardness of Fe-Ni-Co-Al-X-B alloys was measured after aging at different temperatures and times. The precipitation of gamma prime phase is confirmed by both hardness test and XRD results. Tensile tests show that the strength of the gamma matrix affects the stress-induced martensitic (SIM) transformation. Differences in the behavior and properties of the Ta and Cr containing alloys will be compared, with advantages and disadvantages of each discussed.