Modeling the Pseudoelastic Design Space of NiTi Fabricated by Laser Powder Bed Fusion

This work explores the underlying origins for an experimentally-observed correlation between the cyclic pseudoelastic response of Ni_50.8Ti_49.2 and the hatch spacing used in Laser Powder Bed Fusion (LPBF). In particular, an increase in hatch spacing from 80 to 180 µm produces a transition during cycling from a more flag-shaped to a linear pseudoelastic response, thus spanning a range of potential applications. These observations are studied through the development of a microstructural model that incorporates single crystal pseudoelastic constitutive behavior, texture, plasticity, and residual stress from the LPBF process. The model predicts that an increase in <111> and other non-<001> texture components, observed experimentally at larger hatch spacing, promotes stress redistribution between texture components, leading to greater strain ratcheting for larger hatch spacing and a more linearized stress-strain response after cycling. More generally, the model predicts that the design space for pseudoelastic response – including recoverable strain, linearization, and strain ratcheting – can be systematically varied through control of texture and residual stress from the LPBF process, leading to variations in recoverable strain and ratcheting by up to 4 %. The results suggest a rich design space for architected shape memory alloy materials and structures produced by LPBF.