Micromechanical response quantification using high-energy X-rays during phase transformations in additively manufactured 17-4 stainless steel

Recent studies of additively manufactured (AM) 17-4 stainless steel produced via laser powder bed fusion of nitrogen atomized powders have been found to contain large volume fractions of austenite (gamma) compared with the fully martensitic (alpha') microstructure of wrought 17-4. These AM 17.4 stainless steels have metastable microstructures that transform from a mixed phase composition to predominantly martensitic during deformation. This transformation process, combined with strong preferred crystallographic orientation (texture) that arises during building, produces complex micromechanical interactions that dictate the macroscopic response. Here, high-energy X-ray diffraction performed at a synchrotron light source is used to quantify the volume fraction of austenite and martensite, texture, and the complete orientation dependence of lattice strain (strain pole figures) at various macroscopic strains levels in-situ during uniaxial tension of AM 17-4. Results from wrought 17-4 are also shown for comparison. Initial martensite volume fraction of the stress-relieved AM specimen was measured to be 0.46 and increased to 0.88 after the application of a macroscopic strain of 0.03. During the transformation process, minimal crystallographic texture evolution was observed in either the gamma austenite or alpha' martensite. The distribution of strains in the specimen is found to be heavily influenced by both the transformation process and the initial texture. Phase transformation is found to generate tensile strains perpendicular to the applied load in untransformed gamma austenite, while texture is found to produce high heterogeneity of lattice strains within lattice plane families.