Electric field strengthening during superplastic creep of Zn-5 wt% Al: A negative electroplastic effect

The influence of a non-contacting, dc electric field of 3.2 kV/cm on the superplastic deformation of fine-grained Zn-5% Al was determined in creep tests at temperatures below, approximately at and above the eutectoid temperature (275°C). The following is a summary of the results obtained and the conclusions derived therefrom. 1. The field reduced the creep rate at all three temperatures. The maximum effect occurred at 270°C and a strain rate ∈̇ ≈ 3 × 10-4 s-1. The behavior had characteristics similar to dynamic strain aging. 2. The average value of the stress exponent n = ∂ ℓn ∈̇/∂̈ ℓn σ was 2.2 without the field and 2.5 with the field; the average value of the apparent activation energy Q was 90 kJ/mole without the field and 110 kJ/mole with. These values suggest that grain boundary sliding is the rate-controlling mechanism. 3. It is proposed that the influence of the field on the creep rate results from its interaction with charged impurity atom-vacancy complexes existing at or in the vicinity of grain boundaries, which in turn affects grain boundary sliding. 4. The reduction in creep rate by the field in the Zn-5% Al alloy is opposite to the decrease in flow stress with field previously found for the 7475 Al alloy.