Low stress and superplastic creep behavior of Zn-22 Pct Al eutectoid alloy

Abstract

The temperature, grain size, and stress dependence of steady-state strain-rates were investigated for Zn-22 pct Al eutectoid alloy using double shear type specimens. Tests were performed on specimens of grain size from 1.3 m to 3.7 μm over a range of temperature from 450 to 525 K. The applied stresses were in the range between 10−1 and 4.0 x 101 MPa with the resulting true strain-rates ranging from 10−7 to 10−2s−1. The alloy exhibited two distinct regions of constant stress dependence with stress exponents 1 and 2.2. The transition stresses between these two regions were between 3 × 10−1 and 7 x 10−1 MPa. The true activation energies associated with these regions were found to be 95.9 ±2.1 and 69.9 ±2.1 kJ/mol, respectively. The grains remained equiaxed following large deformation, although there is evidence of excessive grain growth and “grain clustering.” Specimens deformed in both regions showed few or no dislocations within the grains. There was no evidence of subgrain formation or dislocation pile-up at the grain boundaries. It is possible that dislocations found during deformation were annihilated when stress was withdrawn or lost during thin foil preparation. There was clear evidence of primary stage in both regions. However, it is suggested that whereas the primary stage in Region II is due to dislocation multiplication process, at low stresses the primary stage in Region I may be due to elastic bowing of the existing dislocations in the structure. At low stresses Nabarro-Herring diffusional creep was found to be the rate controlling mechanism. At intermediate stresses, superplastic creep was found to be the dominant mechanism. The transition between these two mechanisms, as well as Coble creep and the ranges of manifestation for superplastic creep, are analyzed and the importance of the preexponential term A, in the dimensionless relation:γκT/DGb = A(b/d) p(Τ/G)n and the role of the concurrent grain growth are emphasized.