Multi-scale shape memory effect recovery in NiTi alloys additive manufactured by selective laser melting and laser directed energy deposition

Abstract NiTi shape memory alloys (SMAs) are fabricated using powder bed fusion via selective laser melting (SLM) and laser-based directed energy deposition (LDED) additive manufacturing (AM) techniques in order to characterize the microstructure and compressive shape memory effect (SME) recovery of as-built alloys. Composition and grain structure vary spatially for LDED alloys relative to SLM. Columnar grains oriented in the build height direction and spanning multiple layers exist in SLM alloys. LDED produces equiaxed grains with dimensions that correlate with the layer thickness. LDED and SLM alloys are martensitic at room temperature with complete phase transformation between room temperature up to 110 °C. In spite of the contrasting microstructures, the elastic moduli, critical stresses, and yield stresses are equivalent. Consequences of the contrasting microstructures become evident by the differential SME responses. In excess of the critical stress, LDED NiTi alloy responses exhibit a hardening like response compared to a plateau for SLM. SME recovery for LDED alloys ensued immediately upon heating whereas a finite thermal input was required to initiate recovery for SLM alloys. The contrasts indicate the residual martensite was relatively unstable in the LDED alloy microstructure. For multiple stress-strain-temperature cycles, the critical stress and elastic moduli decrease until the values become stable. Underlying martensitic phase transformation morphologies, visualized from full-field strain measurements, evolve uniformly for the homogeneous SLM alloy microstructure and localized strain concentrations evolve for LDED. The deformation analysis confirmed complete SME recovery of approximately −2.0% macro-scale and −4.0% micro-scale/concentrated strains.