Sequential hardening mechanisms in additive-manufactured NiTi shape memory alloys that can significantly delay compressive failure

Abstract

Most material design strategies devoted to the strength-ductility synergy in tension are based on the delay of the necking instability. However, the counterpart investigations under compression remain elusive. Recent developments in simulating the failure process reveal that microscopic voids or cavities under compression/shear-dominated loadings will evolve sequentially as flattened to a micro-crack, rotating and elongating into kinks, and then interacting and coalescing with neighboring micro-cracks. This entire process can be significantly slowed down if the matrix material sustains a high work hardening exponent. Motivated by this novel micromechanical understanding, this work successfully realized hierarchically heterostructured microstructures in NiTi shape memory alloys (SMA) by using the selective laser melting (SLM) additive manufacturing technique. It is the sequential or progressive transition from the martensitic phase transformation, to heterogeneous deformation induced hardening, and then to amorphous lamellae hardening at later stage that sustains a high work hardening rate which results into an exceptionally high failure strain of the as-fabricated (without post-treatment) SMA under compression. The occurrence of amorphization at high strains is attributed to the high dislocation density and to the oxygen-assisted energy barrier reduction. This work thus offers an unprecedent design approach for the development of high performance SMAs.