Deformation substructures induced by high rate deformation

The influence of increasing strain rate on the deformation substructures in metals and alloys which deform predominately by slip is very similar to that seen following quasi-static deformation at increasingly lower temperatures or due to a decrease in stacking fault energy (γ_sf). For constant strain, deformation at higher rates: 1) produces more uniform dislocation distributions for the same amount of strain, 2) hinders dislocation cell formation, 3) decreases the cell size, and 4) increases misorientation with more dislocations trapped within the cell interiors. The suppression of thermally-activated dislocation processes in this regime, added to high temperature and strain-rate sensitivity of the yield stress exhibited by many materials, can lead to stresses high enough to nucleate and grow deformation twins even in high stacking fault energy FCC metals such as copper and Al-4.8 wt.% Mg. In addition, substructures formed under high-rate conditions differ from those formed under low-rate conditions due to the suppression of dynamic recovery processes. Finally, in high-rate deformation such as shock loading, the subsonic restriction on dislocation motion leads to higher dislocation and point defect generation rates, resulting in enhanced hardening when compared to materials deformed to equivalent strains at quasi-static rates. In this paper examples of the deformation substructure evolution observed in aluminum, copper, Ni₃Al, iron, Ti-6Al-4V, and TiAl at high and shock-loading strain rates will be presented and compared to that seen following low-rate deformation paths.