Observations of common microstructural issues associated with dynamic deformation phenomena: Twins, microbands, grain size effects, shear bands, and dynamic recrystallization

Plane-wave shock deformation has been shown to produce deformation twins or twin-faults in essentially all metal and alloys. In FCC metals and alloys twinning depends upon stacking-fault free energy (SFE) and a critical twinning pressure; which increases with increasing SFE. For impact cratering where the shock wave is spherical and a prominent deviatoric (shear) stress is involved, metals and alloys with high SFE form microbands coincident with {111} plane traces while low SFE metals and alloys either form mixtures of twins and microbands or microtwins. Oblique shock loading of copper also produces mixtures of twins and microbands. Both microtwins and microbands increase in volume fraction with increasing grain size. BCC iron is observed to twin in both shock loading and as a result of impact cratering. Impact craters, shaped charges, and other examples of extreme deformation and flow at high strain rates exhibit various regimes of shear bands and dynamic recrystallization as a mechanism for solid-state flow. Deformation twins and microbands are also often precursors to this process as well. Examples of these phenomena in FCC materials such as Al, Ni, Cu, stainless steel and brass, and BCC materials such as Fe, W, Mo, W-Ta, and Ta are presented; with emphasis on optical metallography and transmission electron microscopy. (C) 2004 Kluwer Academic Publishers.