The effect of deformation texture on the thermal stability of UFG HSLA steel

Abstract

The microstructures of ultrafine grained (UFG) metals processed by severe plastic deformation are far from the thermodynamic equilibrium thus being prone to undergo coarsening processes. Theoretical and experimental investigations revealed that the stability against discontinuous grain growth in UFG metals with high stacking fault energy strongly depends on the fraction of high angle grain boundaries (HAGBs). This means that discontinuous grain growth does not occur if the fraction of HAGBs exceeds a certain level. The present work focuses on the impact of strong deformation textures on the thermal stability of UFG microstructures in a ferritic steel processed by linear flow splitting. It shows that the expected correlation between thermal stability and fraction of HAGBs is valid up to moderate texture intensities, whereas a strong deformation texture promotes discontinuous grain growth in spite of a high fraction of HAGBs. EBSD measurements reveal that this behavior is attributed to a strain-induced grain boundary migration causing a progressive orientation pinning effect with ongoing grain growth. Thereby, a large fraction of HAGBs is transformed into low angle grain boundaries (LAGBs) with low mobility. Consequently, a microstructure with a majority of LAGBs evolves being unstable against discontinuous grain growth.