A systematic study of the role of stacking fault energy (SFE) on shock-hardening in Cu and Cu-Al alloys

摘要

The role of stacking fault energy (SFE) in shock-hardening was systematically studied in Cu (SFE ～ 78 ergs/cm~2) and a series of Cu-Al solid-solution alloys (0.2, 2, 4 and 6 wt.% Al with SFE ～ 78, 25, 13, and 6 ergs/cm~2, respectively). The materials were deformed under quasi-static compression, before and after a shock-prestrain to pressures of 10 or 35 GPa. Shock hardening was found to be higher at the higher shock-pressure though the hardening decreases with a decrease in SFE at both shock pressures. In the literature, shock-hardening has been qualitatively attributed to deformation twinning and a high dislocation density. In the present work, the strengthening contribution of deformation twins was determined using a modified Hall-Petch equation (using inter-twin spacing as an 'effective' grain size). The calculated deformation twin strengthening suggests that the contribution of deformation twins to the post-shock strength increases with decreasing SFE which, in turn, implies that the dislocation contribution to the post-shock strength concurrently decreases. The dislocation density in shock-deformed materials was indirectly estimated using stored energy measurements via a differential scanning calorimeter (DSC). The DSC data indicates a lower stored dislocation density in the lower SFE alloys which could explain the reduced shock-strengthening in the lower SFE materials. This lower dislocation density in the shock-deformed low SFE materials can be attributed to twinning being the preferred mode of deformation and to the inability of the material to generate and store additional dislocation line-length due to an increased Bauschinger effect with more planar glide upon decreasing SFE.