High strain rate superplasticity and threshold stresses

The superplastic behavior of a PM processed Al-5Mg-1.5Cr alloy over the temperature range of 370 to 550 degrees C is described. This material was processed by the hot compaction of powders, followed by additional thermal-mechanical processing, resulting in a fine-grained (2 mu m) two-phase material. The strain-rate sensitivity exponent was found to be high, with maximum m values exceeding 0.5. High elongations were observed at high strain rates, and additionally, superplastic behavior is predicted at strain rates up to 10 per second. The material was identified as a fine-grained Class I solid solution alloy; grain boundary sliding was found to be the principal deformation mechanism at low strain rates, and a dislocation-solute drag slip process was found to be the principal deformation mechanism at high strain rates. Threshold stresses were observed, the values of which decreased with increasing temperature. These threshold stress results have been compared with those observed in MMC and MA aluminum alloys that also exhibit high strain rate superplasticity. Speculations on the physical meaning of these threshold stresses are presented and their values are compared with threshold stresses that are associated with diffusion-controlled dislocation creep processes.