Threshold stress in high strain rate superplasticity of powder metallurgy aluminum alloys and composites

Abstract

Many dispersion strengthened aluminum alloys and discontinuously reinforced composites exhibit high strain rate superplasticity when they are processed into ultra-fine grain sizes by powder metallurgy using rapidly solidified or mechanically alloyed powders. Grain boundary sliding is considered to be the mechanism for the superplasticity by incorporating the concept of threshold stress. The threshold stress introduced to fit the stress vs. strain-rate data decreases the apparent now stress, and is strongly temperature dependent. The temperature dependence of threshold stress appeared in literatures has been reviewed, and analyzed by a thermally activated process. The modulus-compensated threshold stress data fit well with an Arrhenius-type plot, and two modes of thermally activated processes an be visualized. At lower temperatures, the activation energy for the threshold stress is about 50kJ/mole or less, which is similar to the that interpreting the creep of similarly processes aluminum alloys. At higher temperatures near an incipient melting condition, the activation energy is much higher, and it's origin cannot be explained by various models used in the literature, Based on the serrated now curves and microstructural observation in the Al-10wt%Ti alloy, the solute drag against gliding dislocations has been suggested as a possible source for the threshold stress at very high temperatures.