Influence of cellular subgrain feature on mechanical deformation and properties of directed energy deposited stainless steel 316 L

The fundamental deformation mechanism responsible for the exceptional combination of strength and ductility exhibited by SS 316 L material manufactured using directed energy deposition (DED) based additive manufacturing (AM) technique was studied. First, the stress-strain behaviors under uniaxial tensile testing of specimens containing different intensities of cellular subgrain feature were obtained. The stress-strain behaviors showed large differences with different intensities. Then, the microscopic deformation under in-situ tensile testing was characterized. At intragranular length scale, results revealed an extensive formation of a heterogeneous surface texture with a characteristic length scale and morphology consistent with the geometry of the cellular subgrain feature. The results also revealed that the high strain hardening rate regime of the macroscopic stress-strain behavior is primarily driven by deformation slipping, not deformation twinning. Finally, the evolution of the slip bands under uniaxial tensile testing was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results revealed a barrier effect of cellular subgrain feature on slip band evolution. For a given macroscopic strain the width, depth, and spacing of slip band formation varied with the intensity of the cellular subgrain feature. The heterogeneous texture formation and barrier effect on slip band evolution observed in this study were clear evidence of the extensive influence of cellular subgrain feature on microscopic deformation and mechanical properties. Additionally, these remarkable observations revealed that dynamic slip band refinement induced by cellular subgrain feature is the fundamental mechanism that facilitates the exceptional combination of strength and ductility of DED manufactured SS 316 L