THE HIGH-STRAIN RATE SUPERPLASTIC DEFORMATION MECHANISMS OF MECHANICALLY ALLOYED ALUMINUM-IN90211

The tensile behavior of mechanically alloyed (dispersion strengthened) IN90211 was investigated at strain rates between 0.0001 and 340 s-1 at temperatures between 425 and 475 °C. At strain rates above 0.1 s-1, superplastic elongations were obtained (maximum elongation of 525% at 475 °C and 2.5 s-1). Superplastic elongations were found to result from grain boundary sliding. The data were analyzed assuming that a threshold stress resists dislocation motion. The threshold stresses were obtained assuming n = 2 (grain boundary sliding) or n = 3 (solute drag) for the stress exponent. Both assumptions provided equally credible values for a temperature-dependent threshold stress between 1% and 20% of the Orowan looping stress. The n = 3 threshold stresses agreed with load relaxation data, but the n = 2 values corresponded to the lower limit of the superplastic deformation regime, as indicated by creep tests at lower strain rates. Based upon the threshold stress theories in the literature, the threshold stresses are suggested to arise from a combination of local and general climb of lattice dislocations over particles. Consideration of the activation energies, details of flow behavior, and stress relaxation experiments provided strong evidence for the n = 3 solute drag mechanism to be the rate-limiting process in the superplastic deformation regime. © 1990.