| Recrystallized grains have been observed to occur within the adiabatic shear bands of a number of materials. In order to determine the parameters affecting recrystallization under the high-strain, high-strain-rate conditions observed in adiabatic shear bands, four materials including shock-prestrained copper, dilute aluminum-lithium alloys, polycrystalline tungsten, and upset-forged tantalum were examined. Recrystallized grains of approximately 0.2 {dollar}mu{dollar}m in diameter were observed in the copper and the Al-Li alloys. However, only subgrains were formed in Ta, and no recovered substructure was observed in W. Comparisons of the deformed microstructure of each material with existing mechanisms for dynamic recrystallization suggested that diffusion-based recrystallization models were inadequate to describe the recrystallized grain size in the time/temperature window for shear band deformation and subsequent cooling. A new mechanism was developed that utilized the lattice spin component of the deformation to calculate the level of misorientation which develops between subgrains in the shear band microstructure within the deformation time frame. A computer program was utilized to test this mechanism with parameters for the shock-prestrained copper, and the results showed that misorientations {dollar}>{dollar}25{dollar}spcirc{dollar} formed between subgrains.; However, in tantalum shear bands, subgrains were observed with very high misorientations, but which were clearly not recrystallized. The computer program also predicted high misorientations in BCC materials. This suggested that there is a second component to recrystallization that involves the recovery of dislocations at the subgrain boundaries. Comparisons were made of the dislocation annihilation kinetics in the subgrain boundaries and the cooling time for the shear band, and it was found that the amount of recrystallization within the shear bands was proportional to the rate of dislocation annihilation in the subgrain boundaries.; A new mechanism is proposed called progressive subgrain misorientation (PriSM) recrystallization in which the subgrain structure rotates to large misorientations during deformation and the boundaries refine during subsequent cooling to form recrystallized grains. This model is consistent with the results seen within the shear bands of the four materials investigated, and may be utilized to predict the occurrence of recrystallized grains in the shear bands of a number of other ductile materials. |
