Synergistically enhanced cryogenic strength and ductility in 304L stainless steel made by directed energy deposition additive manufacturing

Abstract

We leverage the rapid cooling rates and thermal cycling inherent to laser directed energy deposition (DED) additive manufacturing (AM), to synthesize 304L stainless steel with bimodal grain size distribution and intragranular cellular dislocation structures. We find that regardless of the printing strategy, DED AM 304L exhibits synergistically enhanced strength and ductility from room temperature down to cryogenic temperature (77 K); that surpasses the mechanical performance of 304L synthesized via conventional processes. We follow the evolution of statistically representative microstructure regions using electron microscopy during quasi in-situ tensile tests interrupted at different strain levels and reveal the deformation mechanisms responsible for this behavior. First, cellular dislocation structures act as emission sites for Shockley partials, leading to a dense network of deformation twins at room temperature, and martensite at 77 K. Second, martensite formation takes place at different rates in the bimodal microstructure, leading to sustained work-hardening. Using the Olsen-Cohen model, we reveal a higher formation rate of ${\alpha }^{\prime }$ martensite in comparison with previous literature on wrought 304L; as well as distinct transformation rates between the large and small grains in our DED 304L. These findings highlight the potential of DED AM as a promising synthesis method for stainless steels in cryogenic applications.