The microstructure and anisotropy of compressive behavior in NiTiCu alloy fabricated by laser powder bed fusion processing

Abstract

The microstructure and anisotropy of compressive behavior in Ni_49.6Ti_49.4 Cu₁ alloy prepared by fusing NiTi, pure Ti, and pure Cu powders using laser powder bed fusion (LPBF) were investigated. Three distinct microstructure features were identified, namely, weakly textured columnar austenitic grains containing martensite, a gradient microstructure varying from the equiaxed grains containing martensite in the center to columnar austenitic grains at the edge of the build, and strongly textured columnar austenitic grains under different LPBF parameters. The equiaxed grains in the center of the sample built at the highest power of 400 W were primarily caused by a large thermal activation upon multiple thermal cycles during LPBF. The anisotropy in the recoverable strain is primarily caused by preferred grain orientation, while the distribution of martensite in the loading direction also contributes to the anisotropy for a gradient microstructure, especially at low strain level. The critical stress for martensite formation (σ_c) and recovery strain due to shape-memory effect (SME) exhibited no anisotropy after cyclic compression and heating in boiling water, and were primarily affected by the martensitic transformation temperatures.