Creep-Induced Elemental Redistribution at Grain Boundaries of 304L Stainless Steel – An Experimental Evidence for Diffusional Creep Mechanisms

Abstract

Seventy-five years ago, Nabarro and Herring—and subsequently, Coble—theorized Newtonian diffusional creep mechanisms in crystalline materials. They postulated that the deformation rate is governed by the flow of atoms along the direction of applied stress via a vacancy-atom exchange mechanism under diffusional creep conditions. These theories render a compelling picture for materials scientists to design microstructures with improved creep resistance. However, they remain vigorously debated due to a dearth of direct experimental evidence and poor predictions of the observed creep rates in certain materials and conditions. In this paper, we tested the hypothesis that, in alloys undergoing diffusional creep, elemental redistribution would manifest at nearby grain boundaries if the diffusivities of the alloying elements were significantly different. Furthermore, the extent of elemental redistribution would vary depending on the orientation of the grain boundary with respect to the tensile loading axis. To this end, 304L stainless steel alloy tensile specimens were crept under relatively low applied stresses (2.5-15 MPa) at 750°C. Chemical composition at the grain boundaries in the deformed and undeformed sections of the specimen were then compared and the trends were rationalized using thermodynamic and kinetic modeling. In support of our hypothesis, the results revealed a notable change in the nature and extent of creep-induced elemental redistribution (CIER). Our experimental observations of CIER, complemented with modeling results, suggested the occurrence of Newtonian diffusional creep. Further mechanistic understanding of diffusional creep via characterization and modeling of CIER is expected to make the approach more viable and enable new pathways for designing advanced creep-resistant materials for extreme environments.