Evolution of Grain Size During Recrystallisation

G. Hopkin and H. K. D. H. Bhadeshia

A discussion of recrystallisation can be found in Lecture 7 of Metals and Alloys.

The following three samples are of an annealed austenitic stainless steel. The images are produced from flat samples using scanning electron microscopy, with electron channelling contrast. The samples were rolled to different reductions: 83%, 88% and 93% leading to final thicknesses of 1.1mm, 0.95mm and 0.8mm respectively. They were then given an identical anisothermal heat treatment for 180 s, reaching a maximum temperature of 885oC.

The differences in stored energy due to the variations in the degree of rolling deformation alter the recrystallisation rate. Thus, the recrystallised grain size decreases as the rolling reduction increases. At the same time, the overall rate of recrystallisation increases. Therefore, an identical heat treatment has recrystallised a greater proportion of the 93% reduction sample when compared with the 83% reduction sample.

grain size

This graph illustrates how the recrystallised grain size should vary with the degree of deformation. Deformation puts energy into the material, which drives recrystallisation. The nucleation and growth rates are higher at higher deformations. The greater the nucleation rate, the finer the ultimate grain size. There is a critical level of deformation below which there will be no recrystallisation at all since recovery removes the effects of deformation before recrystallisation can occur. In fact, a critical strain anneal can lead to a single crystal on recrystallisation.

Compare the micrographs presented below and you should see that the grains become finer as deformation given to the sample before the recrystallisation heat treatment increases. The initial deformations are 83%, 88% and 93% for the top, middle and bottom micrographs respectively. The fuzzy, irregular regions are unrecrystallised whereas the clean, equiaxed grains are recrysatllised.


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