Strain-annealed grain boundary engineering process investigated in Hastelloy-X

The mechanisms responsible for microstructure evolution during strain-annealed grain boundary engineering in Hastelloy-X have been investigated. GBE-quantifying parameters such as Sigma 3(n) density and fraction, triple junction density and fraction have been utilized to quantify the extent of GBE in the processed microstructures. Strain-annealed GBE process favors the formation of Sigma 3 boundaries through dissociation mechanism. When the amount of strain/cycle is high, the change in microstructure is limited and most of the Sigma 3 boundaries are remnant annealing twins from the initial microstructure. When the amount of strain/cycle decreases, more grain boundary migration takes place and majority of Sigma 3 boundaries start integrating into the GB network through dissociation mechanism. AGG is found to take place under special conditions: when (i) the dislocation density stored in the microstructure is low and (ii) the fraction of Sigma 3 boundaries that are integrated into the general grain boundary network is high. It is the preferential growth advantage of CSL boundaries that leads to abnormal grain growth.