Synergism of in-situ reinforcement nanostructures in directionally solidified Ni-Mn-Ga shape memory alloys: Nanotwins, stacking faults, 9R phase and stress-induced twins

Abstract

The synergistic optimization for recoverable strain and mechanical properties of Ni–Mn–Ga shape memory alloys is of urgently demanded. A directionally solidified Ni–Mn–Ga shape memory alloy has been demonstrated that successfully reconciles high stress tolerance (σ = 821 MPa at a strain of 10 %) and shape memory strain (ε_sme = 6.52 %). The martensitic structure obtained by directional solidification is composed of many colonies with preferred orientations along the growth direction, which effectively increases the coordination and consistency of the alloy during deformation. More crucially, the atypical pathways of stacking faults (SFs) and twin boundary transformation were revealed in martensitic de- / twinning transformation processes of Ni–Mn–Ga alloy. The formation site and strengthening mechanism of in-situ nano-reinforcement phases have been elaborated. The micro-slip bands and SFs of single-phase Ni55 alloy aggregate near the inter-plate boundaries during the martensitic detwinning, and the micro-slip bands expand to form minor variants or stress-induced twins. After shape recovery, the 9R long-period stacking ordered phase generated by SFs and the dissociation of twin boundaries can further pin and hinder the migration of dislocations, storing substantial elastic strain energy. The strengthening strategy of introducing nanoscale reinforcing phases coupled twinning induced plasticity effect has enhanced the strength of the alloy from multiple aspects, which even surpass the dual-phase alloy containing γ phase.