Controlled precipitation and recrystallization to achieve superior mechanical properties of severely deformed Inconel 718 alloy

Abstract

Severely deformed machined chips of Inconel 718 were subjected to an age-hardening treatment at 600 °C for 10 h in order to form a high fraction of γI and γII precipitates. These nano-sized precipitates help in pinning the grain boundaries and hence stabilizing the boundaries by impeding their motion at elevated temperatures. This strong pinning effect on grain boundaries is expected to result in increased strength and improved thermal stability. However, presence of a large fraction of nano-precipitates also induces precipitation hardening which results in limited ductility of the material. Therefore, in order to improve the ductility, age-hardened chips were given a subsequent heat-treatment at 700 °C, 800 °C and 900 °C for 15 min, hereafter called as post-aging heat treatment. It was anticipated that this heat-treatment at elevated temperatures would increase the dislocation mobility and result in dislocation annihilation, which can improve the ductility of aged samples. Results show that the recrystallization fraction increases with an increase in the temperature of post-aging heat treatment. Heat-treatment at 900 °C was found to result in a microstructure with highest fraction of recrystallized grains, a high fraction of coincidence site lattice (CSL) boundaries, comparatively smaller grain size and better ductility. Results also indicate that there are two counteracting effects undergoing at the same time during post-aging heat-treatment which determine the possibility of enhanced dislocation motion and associated improvement in ductility. One of these effects is related to the pinning force caused by the presence of precipitates at the boundaries. Higher pinning force by fine precipitates will lead to suppressed recrystallization by hindering the grain boundary motion and hence no improvement in ductility. Another effect is related to the driving force for recrystallization. Higher driving force, especially in the highly strained ultra-refined shear zones, will promote the reduction in overall dislocation density through recrystallization and hence leading to an improvement in ductility. Present study provides further insights into the competition between pinning and recrystallization of aged materials at elevated temperatures.