Microstructure and mechanical properties of Ni-rich TiNiZr alloy associated with repeated cold-rolling and annealing

TiNi shape memory alloys have clear target demand in a variety of fields owing to their excellent structural and functional properties. In this paper, focusing on B2 structural austenitic Ni-rich TiNi alloys, it is proposed to modulate the microstructure of the TiNi alloys through repeated cold rolling annealing, with an expectation to enhancing the lower mechanical strength and plastic deformation ability of the alloys. The dependence of the mechanical properties and microstructure of Ti_48.9Ni_50.9Zr_0.2 and Ti_47.9Ni_51.9Zr_0.2 alloys on cold rolling deformation and recrystallization annealing was systematically investigated. The tensile mechanical property tests show that the repeated cold rolling annealing can significantly enhance the overall mechanical properties of the alloys, in which the tensile strength of Ti_48.9Ni_50.9Zr_0.2 alloy is increased from 550 MPa in the unrolled samples to 1070 MPa after 6 pass of cold rolling annealing, and the elongation after fracture has grown from 4.9% to 10.0%. The EBSD, SEM and TEM microstructural observation reveals that the deformation and recrystallization structures of the alloy change alternately after multi-pass cold rolling and annealing, and the alloy grains are significantly refined and undergo an obvious preferred grain orientation. In addition, the basal texture of the alloy is further enhanced. The Ti₂Ni and Ti₃Ni₄ precipitates are broken and refined during cold deformation, in which the nano-sized Ti₃Ni₄ precipitate exhibits a favorable matching with matrix. Furthermore, stress-induced martensite as well as a large number of high-density dislocations in the vicinity of the precipitation phases appear in the alloy. The mechanical properties of alloys are strongly related to the microstructure, and the strengthening mechanisms can be understood by grain refinement strengthening, dislocation strengthening, precipitation strengthening and texture strengthening.