Deformation Twinning and Dynamic Strength of Copper During High-Rate Strain

We will present the results of a study of the conditions under which microstructural changes involving the formation of complex bi-periodic twin structures occurs in copper during shock wave and high strain rate ($\dot {\varepsilon }>$10$^{7}$ s$^{-1})$ shock-less loading. The overall morphology of the observed twin structures is rather complex, consisting of what we shall refer to as ``packages,'' with each ``package'' being composed of two sets of parallel twins aligned in a quasi-herringbone pattern. It is widely accepted that deformation twinning results in increased shear strength in samples recovered after shock wave loading. We have observed in this work a significant temporal component to the effect that these complex twin structures have upon shear strength. We have observed, for example, that the formation of these bi-periodic (herringbone-type) twin structures results in an initial loss of shear strength that is significant over a time period of about 0.2 to 0.4 $\mu$s. Following the initial loss of shear strength, deformation hardening produces an increase in shear strength that can be as great as several multiples of the initial value.