Deformation mechanisms of multiphase microstructures in laser powder bed fusion processed stainless steels

Abstract

Additive manufacturing (AM) of Fe-Cr-Ni alloys via powder bed fusion - laser beam (PBF-LB) was performed on three different powder mixtures with ratios of 30:70, 50:50, and 70:30 (weight %) austenitic stainless steel (SS) 316L and duplex stainless steel (DSS) 2507. Extraordinary room temperature tensile behavior in the as-built state is correlated with the complex microstructures achieved in the mixed powder deposits that are not observed with only SS 316L or DSS 2507 powders. Polycrystalline ferritic microstructures in the 50(316L):50(DSS2507) powder mixture build containing a low volume fraction of ultra-fine scale austenite at grain boundaries has exhibited tensile strength exceeding 1 GPa with elongation to failure of ≈28 % due to enhanced Hall-Petch strengthening coefficient. Additionally, the 70:30 sample showed similar ductility to the 100 % 316L sample (∼34 % and ∼35 % elongation to failure, respectively) despite having ∼65 % ferrite in its microstructure. The retention of ductility in spite of the significant increase in tensile strength, from 410 MPa for 100 % 316L to 764 MPa for the 70 (316L):30(DSS2507) powder mixture build sample is attributed to a twin induced plasticity (TWIP) type effect in the needle-like austenite distributed within the ferrite grains. The strength, strain hardening, and ductility of the different microstructures are analyzed using dislocation theory, based on transmission electron microscopy characterization of deformation mechanisms.