The effect of grain boundary structure on chromium carbide precipitation in alloy 600

Abstract

Abstract The aim of the present work is to study the effect of grain boundary structure on the precipitation of chromium carbides in Alloy 600. The material was processed by thermo-mechanical treatment consisting of hot rolling (at 1000 °C) and a short-time annealing treatment (for 10 min) to generate a high fraction of twins and disrupted random boundaries network. These samples were then given sensitization heat treatment at 704 °C for various times in the range 0.5-100 h, in order to introduce varying degrees of carbide precipitation at the interface regions, depending on the structure of grain boundaries and triple junctions. The characteristics of chromium carbides (identified as Cr23C6) precipitation on different kinds of grain boundaries and triple junctions were investigated using field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) based orientation imaging microscopy (OIM) techniques. The propensity for chromium carbides precipitation increased with increasing aging time. However, it was largely dependent on the structure of grain boundaries. Low angle grain boundaries (LAGBs) and low-Σ coincidence site lattice (CSL) boundaries were observed to retard the precipitation of carbides. In contrast, on random high angle grain boundaries (HAGBs), coarse and discrete carbides were observed. Amongst the low-Σ CSL boundaries, the deviation in misorientation was observed to have a profound effect on the precipitation of chromium carbides. Low-deviation twin (Σ3) boundaries (having misorientation deviation within 1°) were found to be strongly resistant against carbide precipitation. However, on high deviation twin and Σ9 boundaries, needle-like carbides evolved. Based on the results, it can be concluded that among the special (i.e., low-Σ CSL) boundaries, only low-deviation twin boundaries have sufficiently low interfacial free energy to completely subdue the precipitation of chromium carbides.