Synergistic strengthening of additively manufactured NiTi shape memory alloys via amorphous/lamellar and core/shell dual structures

Abstract NiTi shape memory alloys (SMAs), fabricated by selective laser melting (SLM), demonstrate exceptional mechanical responses under both millimeter and micron‐scale compression. The micro‐pillars exhibit compressive strengths exceeding 5 GPa without localized failure, while macroscopic compression tests reveal fracture strengths above 3.2 GPa with plastic deformation exceeding 40%. Notably, the stress–strain curves show an abrupt increase following martensitic yield. To elucidate these phenomena, nanoscale observations were conducted on samples after initial plastic deformation stages (compressed to 16% train followed by unloading). Amorphization was observed, leading to the formation of alternating amorphous/B2 phases lamellae and amorphous shell/B2 core structures. Molecular dynamics (MD) simulations were employed to model ideal lamellar and core/shell structures, investigating mechanical behavior, phase transformations, and the evolution of shear bands. Results indicated that in the core/shell model, strength increased with decrease in B2 phase particle/layer thickness, while in the lamellar model, strength initially increased before decreasing, indicating a strength limit. Both models exhibited strengths surpassing 8 GPa, attributed to phase transformation strengthening, dislocation strengthening, and percolation effects of the amorphous phase. Quantitative analysis of shear transformation zones (STZs) in the amorphous phase through shear strain measurements showed that larger crystal phase sizes correlated with lower strain levels and probabilities, enabling simultaneous activation of multiple shear bands. Interestingly, dislocation density and shear strain exhibited a negative correlation. Additionally, transmission electron microscopy (TEM) and inverse FFT analyses revealed a high density of dislocations around amorphous shear bands. This study provides atomic‐scale insights into the strengthening mechanisms in amorphous/B2 phase alloys, establishing fundamental guidelines for the design of related materials.