Performance optimization of ternary Ti−Nb−Cu shape memory alloys based on d electron theory

The phase constituents of Ti−Nb−Cu shape memory alloys were designed based on d electron theory, in order to optimize their performances. The XRD analysis and TEM observation revealed that Cu addition led to the reduction of the bond order () and metal d-orbital energy level (), which caused the evolution of phase constituents. With increasing Cu content, the variation of phase constituents can be concluded as follows: β+α″→β+ω→β+α″+ω→β. The yield strength, ultimate tensile strength and elongation of Ti−Nb−Cu shape memory alloys firstly increased and then decreased with increasing Cu contents. By optimizing the content of Cu alloying element, Ti−Nb−Cu shape memory alloys possess superior mechanical properties with yield strength of 528 MPa and ultimate tensile strength of 742 MPa, which can mainly be attributed to the comprehensive effect of solution strengthening and grain refinement as well as precipitation strengthening.