Experimental investigation on the deformation behavior of an isotropic 304L austenitic steel manufactured by laser powder bed fusion with hot isostatic pressing

Post-heat treatment, such as hot isostatic pressing (HIP), is increasingly employed in laser powder bed fused (LPBF) metallic materials to release residual stress, alleviate microstructural anisotropy, and tailor mechanical properties. Here, we systematically investigate the initial and deformed microstructures of as-received and HIPed L-PBF 304L austenitic steel using various microstructural characterizations. The results indicate that the HIP procedure can decrease the porosity and generate the columnar-to-equiaxed transition (CET), resulting in an isotropic microstructural morphology better suitable for practice applications. Additionally, the HIP samples have exceptional ductility when compared to as-received samples, which is associated with the sustained strain hardening rate arising from the notable deformation twinning behavior. The majority of deformation twins are preferentially emitted from the straight annealing twin boundaries and then grow within the < 111 > - oriented annealing twins. The principle twinning mechanism involved is the twinning partial emission from the annealing twin boundaries or high-angle grain boundaries (HAGBs). Moreover, the inclusions distributed near/on the annealing twin boundaries can emit partials with multiple Burgers vectors during straining, resulting in the extra random activation of partials (RAP) twinning mechanism that sustains the deformation process. These findings give helpful suggestions for HIP process designs of L-PBF austenitic steels, as well as a solid foundation for future theoretical modeling research.