Static Recrystallization of austenite after initial deformation of a Ni-30Fe alloy

Abstract

The Static Recrystallization (SRX) phenomenon of austenite in C-Mn steels have a large impact on the final mechanical properties of the steel. However, the austenite recrystallization kinetics are difficult to investigate experimentally, due to solid-state phase transformations during cooling to room temperature. The best model alloys to examine SRX are those austenitic alloys such as, Ni-30Fe alloy, that do not experience undesirable phase transformation and that have similar stacking fault energy (SFE) as austenite in C-Mn steels. The hot-working of the Ni-30Fe alloy has been conducted at high deformation temperatures, followed by an uniaxial compression test at a low strain condition (ε = 0.2) with a strain rate of 1s-1, and an annealing temperature of 900°C (above recrystallization start temperature). Thereafter, the alloy is experimentally investigated by Electron Backscatter Diffraction (EBSD), to characterize the austenite microstructure and to measure the crystallographic orientations. The experimental measurements from the EBSD technique are subsequently analyzed using MTEX software to study the SRX phenomenon that occurs during hot rolling of steels.The technique to separate recrystallized and deformed grains in this research has adopted the Kernal Average Misorientation (KAM) and Grain Average Misorientation (GAM) technique, rather than the previously utilized Grain Oriented Spread (GOS) technique which incidentally produced slower and inaccurate recrystallization kinetics. The initial and final grain size calculations have been conducted by consideration of the annealing twins, and the reported results are in close agreement with earlier conducted work of Sellars model. The EBSD scan which captures the Geometrically Necessary Dislocations (GND) have been experimentally measured after deformation to be approximately 4.5x1014m-2, by utilizing the approach of KAM, curvature tensor and the Burgers vector with an estimated 13% error. Additionally, the effect of recovery has been captured with the continuous decrease in stored energy as a function of time, where the GND density is estimated to be 7.3x1012m-2 after the annealing treatment.The GND density present in the deformed microstructure is significant to provide the net driving force necessary for nucleation, based on the validated strain-induced-boundary-migration (SIBM) mechanism. The initial nucleation site is observed at triple junctions, and the selected nucleation criterion factor for the captured GND densities, estimates the driving force for nucleation to favour single sub-grain SIBM. The minimum critical sub-grain size required for bulging is calculated to be 3.40 ± 0.44 μm after deformation, which is an increasing parameter with time, as a result of the recovery process. The number of recrystallized grains captured at different time intervals using the validated GAM technique assists in experimental nucleation rate calculations, that have been subsequently converted for a 3D microstructure, in order to compare with the nucleation framework of Rehman's model. The recrystallization nucleation process is experimentally observed to obtain peak values at 2s and continues until all the GNDs are annealed out of the specimen. There is a large discrepancy in the nucleation rate predicted by the model, which is much higher with respect to the experimental observations reported. Finally, the experimental recrystallization kinetics is observed to be in close agreement with the JMAK model during the initial phase of the annealing treatment, but deviates in the later phase of the recrystallization process.