The properties of two-phase Ni-Al-Fe shape memory alloys in the virgin and shape-memory-cycled states

Five polycrystalline Ni-Al-Fe alloys with 14-26 (at.)% Al and 16-26% Fe were made and heat-treated to have β and γ dual phases with from 87% to less than 1% of β phase and to exhibit M_s temperatures at about 300 K. Depending on the β phase fraction and prestraining temperature, the alloys showed pseudoelastic strain recovery ratio from 1-2 up to 35% and shape memory strain recovery ratio from over 10-90% after prestraining by up to 1%. In the alloy with 87% β phase, the latter ratio reached 100% against 0.8% prestrain after a few cycles of shape memory training. Observed results were analyzed based on a model that assumed a simple linear mixture of the properties of the β and γ phases. The results of the analysis suggest that the γ phase after the training in certain conditions is almost elastic and thus acts as a bias spring.