Laser directed energy deposition of NiTi shape memory alloys with herringbone grain architectures for enhanced ductility and superelasticity

Abstract

Laser directed energy deposition (L-DED) of NiTi shape memory alloys (SMAs) is hindered by poor ductility and limited superelasticity in as-built parts. Here, we overcome these challenges by engineering a herringbone grain architecture, achieving exceptional tensile ductility (34 ± 5 %) and a recoverable superelastic strain of 6.8 ± 0.3 % surpassing prior additive manufacturing (AM) benchmarks. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) reveal that herringbone grains architectures exhibit dual epitaxial growth directions of B2 grains, creating jagged boundaries that buffer crack propagation and homogenizing deformation. The weakened <0 0 1> texture (multiple of uniform density (MUD) = 2.97) enhances multiple {0 1 1}<1 0 0> slip activation. Directionally resolved tensile testing demonstrates ductility dependence on grain boundary geometry and crystallographic orientation. A process map linking laser parameters (energy density: 78–91 J/mm 3 , scanning speed: 300–400 mm/min) to herringbone formation enables microstructure-property tailoring. This work establishes a pathway for high-performance L-DED NiTi alloys in biomedical and aerospace applications without post-processing.