Superplasticity and Newtonian-viscous Flow in Fine-grained Class I Solid Solution Alloys

Abstract

Elevated temperature (1 023 to 1 193 K) tensile properties are described for a fine (6 μm) and coarse (100 μm) grained ultrahigh carbon (1.25 %) steel containing 10 % aluminum. The coarse-grained steel exhibits Class I solid solution alloy behavior wherein a stress exponent of three is noted. The fine-grained steel exhibits exceptionally low stress exponents, where a stress exponent of two is observed at low strain rates and decreases to as low as 1.35 with an increase in strain rate. A creep model is developed to explain these rersults based on grain boundary sliding accommodated by two different dislocation processes. When n=2, the accommodation process is attributed to dislocation climb and when n is less than 2, dislocation solute-dragged glide becomes the important accommodation process. The model predicts ideal Newtonian-viscous flow (n=1) will occur in Class I solid solution alloys at sufficiently high temperatures and at fine-grain sizes. It is shown that the superplastic alloys Al-Mg-Cu, Ti-6Al-4V and Mg-33Al exhibit the predicted fine-grained Class I solid solution alloy behavior, whereas the superplastic alloys Ni-39Cr-10Fe-7.5Ti-1Al, Cu-39.4Zn, Fe-26Cr-6.5Ni-0.05Al, Fe-1.6C and Ag-28Cu exhibit the predicted fine-grained Class II solid solution behavior.