Improving the mechanical properties and superelasticity of NiTiFe shape memory alloys through heterogeneous structures

Abstract

This study prepared three different microstructures of NiTiFe shape memory alloys (SMAs) through cold rolling and recrystallization annealing. Among them, the heterostructure (HS) type alloy achieved a synergistic combination of strength and ductility while improving its superelastic stability. The ultimate tensile strength of the HS type alloy was 912 MPa, with a uniform elongation of 21.78 %. The residual strain after a single tensile cycle at 7 % strain was 2.66 %, and after ten tensile cycles, the residual strain was 2.69 %. The initial morphology and post-stretching deformation of the NiTiFe SMAs were captured using electron backscatter diffraction (EBSD), revealing the distribution of grain size, high-angle grain boundaries, subgrain boundaries, kernel average misorientation (KAM), and geometrically necessary dislocation (GND) density for all three microstructures. Based on the <111 >/, <100 >/{110}, and <100 >/{010} slip systems, the maximum Schmid factor for each grain slip system in NiTiFe SMAs was obtained. The HS type NiTiFe exhibited superior overall performance due to the synergistic effect of its unique recrystallized grains and non-recrystallized regions. This study provides valuable insights into improving the comprehensive performance of NiTiFe SMAs, which can be applied in a wide range of engineering applications.