Relationship between three-dimensional microstructure and Avrami exponent for recrystallization in pure iron

Abstract

We undertook a three-dimensional analysis of the recrystallization that occurs in pure iron from the viewpoint of consistency between the growth direction of the recrystallized ferrite grains and the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. Cold-rolled specimens were heated to 550 °C and, then, held at that temperature for a range of durations. The recrystallization behavior during the annealing was investigated both theoretically and experimentally. The formation of subgrains in non-recrystallized ferrite grains was found to occur before the start of recrystallization. Additionally, the fraction that recrystallized increased with the duration at 550 °C, and the slope became gradual after the annealing duration exceeded 60 s. The values of the Avrami exponent in the early and later stages of annealing were 1.86 and 0.37, respectively. Based on the JMAK theory, the growth direction of the recrystallized ferrite grains was assumed to be one-dimensional. On the other hand, the two- and/or three-dimensional growth of the recrystallized ferrite grains was confirmed by the three-dimensional analysis. Using a modified JMAK equation that takes the effect of recovery in recrystallization during annealing into consideration, the experimental data was found to be in good agreement with the modified JMAK plot. Thus, the inconsistency between the theoretical and experimental results could be attributed to the interaction between the recovery and recrystallization.