In-situ synchrotron diffraction study on the anisotropic deformation and phase transformation behaviors in NiTi shape memory alloy fabricated by laser powder bed fusion

The stress-induced martensitic transformation (SIMT) and plastic deformation are the crucial factors governing the functional and mechanical properties of polycrystalline NiTi shape memory alloys. This study investigated and compared the SIMT and deformation behaviors along the building direction (BD) and horizontal direction (HD) of NiTi components fabricated by laser powder bed fusion (LPBF), using electron backscatter diffraction (EBSD) and in-situ synchrotron-based X-ray diffraction during uniaxial tension. The experimental results revealed that loading along the HD resulted in both a higher SIMT rate and increased dislocation density compared to the BD of the printed block. Additionally, both HD and BD loadings demonstrated multiple lattice correspondences from the B2-austenite to B19'-martensite phase. The BD sample, with its more complex grain boundary network, densely distributed localized stress and strain, as well as, smaller grain size, contributed to a lower SIMT rate and dislocation density. These findings underscore the impact of crystallographic orientation and microstructural characteristics on the mechanical responses and SIMTs of LPBF-fabricated NiTi alloys.