Stress-induced heterogeneous transformation and recoverable behavior of laser powder bed fused Ni-rich Ni50.6Ti49.4 alloys without post treatment

Abstract

In comparison with the conventional processes, laser-based additive manufacturing can induce a different mechanical response due to the formation of highly non-equilibrium microstructure. In this paper, the heterogeneous phase transformation and recoverable behavior of Ni50.6Ti49.4 samples fabricated by laser powder bed fusion (LPBF) during the compression were reported. Based on the indirect in-situ X-ray diffraction method, the residual lattice strain and relative volume fraction of B2 and B19′ phases were evaluated and analyzed. It was found that the change trend of residual lattice strain of B19′ phase with the applied strain was basically consistent with that of the macro-scale residual strain. But the evolution of residual lattice strain of B2 phase demonstrated a complicated case. Besides, the evolution curves of relative volume fraction of B2 and B19′ phases showed a typical ‘X’ shape with an apparent plateau stage, which indicated considerable reversible transformations still occurred even at a severely deformed condition. The maximum recoverable strain could reach 0.079 at room temperature. A different strain stage division was therefore proposed, including B2-phase dominated stage, reversion stage, balance stage and B19′-phase dominated stage.