Overview no. 35 Dynamic recrystallization: Mechanical and microstructural considerations

Abstract

The critical strain criterion ε p = ε x for the transition from cyclic to single peak recrystallization is demonstrated to be invalid for the high temperature deformation of f.c.c. metals in tension and compression. The role of the strain and strain rate gradients present in solid torsion bars in raising the apparent torsion peak strain ε p above the ε p values obtained from homogeneous tension or compression testing is clarified. A similar, and larger, effect is shown to cause discrepancies in the torsion values of the recrystallization strain ε x . An alternative criterion for the transition is described, based on grain size considerations. The latter indicate that cyclic flow curves are associated with grain coarsening and that single peak flow curves are associated with grain refinement. The critical condition is D 0 = 2 D s , where D 0 and D s are the initial and stable grain sizes respectively. The transition in flow curve shape under strain rate change conditions is also analyzed. It appears that after an increase in strain rate, the flow curve displays a single peak, whereas, after a strain rate decrease, multiple peaks are observed. The critical condition at which the shape of the stress-strain curve changes from the multiple to the single peak type is D s1 = D s2 , where D s1 and D s2 are the stable dynamically recrystallized grain sizes before and after the change in strain rate, respectively. The results indicate that single peak behaviour is caused by the “necklace” or “cascade” recrystallization of coarse-grained materials, which produces a large spread in the nucleation strain ε c , and accordingly a highly unsynchronized form of local recrystallization. The growth process (and consequently the grain size) in this case appears to be deformation limited. By contrast, recrystallization is nearly completely synchronized in fine-grained materials, because the high density of grain nuclei leads to a small spread in the nucleation strain. The grain size under these conditions is determined by impingement, and is thus nucleation not growth controlled. Finally, it is concluded that the interpretation given to the transition in flow curve shape by the relative grain size model, expressed in terms of the spread Δε c in nucleation strain ε c , is in broad agreement with the one derived by earlier workers on the basis of computer simulations, and in the absence of grain size considerations.