A large compressive recoverable strain induced by heterogeneous microstructure in a Ni50.6Ti49.4 shape memory alloy via laser powder bed fusion and subsequent aging treatment

Abstract

Laser induced highly metastable microstructure provides the possibility for the tunable precipitation behavior and mechanical/functional properties via direct aging treatment. In this paper, Ni-rich NiTi-based samples were fabricated by laser powder bed fusion (LPBF) and then were directly aged for different aging times that spanning three time-scales (10 min-100 h). The results showed that through the aging treatment, a heterogeneous microstructure consisting of Ti 4 Ni 2 O x nanoparticles, nanoscale Ni-rich precipitates, dislocation structures, and martensitic twin variants formed in the matrix. Due to the change in the elastic strain field, the short time aging treatment tended to induce (001) compound twins, while the long time aging assisted in the formation of< 011 > type II twins. The temperature-induced and deformation-induced phase transformation behavior were further studied. With the progress of precipitates, it was found that a rapid evolution from a single-stage phase transformation to a multi-stage one occurred. During the cyclic compression, the sample aged for 1 h showed the most excellent superelasticity with initial recoverable strain of 0.089 and steady recoverable strain of 0.087, as well as good cyclic stability. • Microstructure evolution of LPBF-ed Ni-rich NiTi SMA with the aging time was disclosed. • Effects of aged microstructure on transformation behavior and pseudoelasticity were elaborated. • A large compressive recoverable strain and its good cyclic stability were achieved via the proper aging treatment.