Electron backscatter diffraction analysis of dynamically recrystallized grain structures in a Ni-Cr-Fe base alloy

Dynamic variations in grain structures and grain boundary characteristics of NiCrFe-based alloy 718 during hot uniaxial compression as well as stress relaxation after the compression were investigated in this article. An electron backscatter diffraction (EBSD) technique was used for the specimens that were compressed at temperatures of 1010 °C and 1066 °C and strain rates of 0.5 and 0.005 s⁻¹, up to a strain of 0.7. Stress relaxation was observed by keeping the upper die in position at the test temperatures as soon as the compression was completed. The variations in the CSL boundary distribution and in the misorientation angle distribution during compression and stress relaxation were thoroughly analyzed to characterize the dynamically recrystallized grain (DRX) boundaries. During deformation at a high strain rate of 0.5 s⁻¹, dynamically recrystallized grains were formed by progressive subgrain rotation. Active dynamic recovery (DRV) at 1066 °C was inferred from the similar degree of strain softening in spite of the different fraction of dynamic recrystallization, which is supported by the high frequency of low misorientation angle boundaries. Stress relaxation was caused by a coalescence of subgrains having very small misorientation angles. Directional grain growth and a redistribution of the grain boundary character caused by the grain rotation occur during the stress relaxation, resulting in reduced total boundary energy. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.