Effect of residual and pre-existing solute clusters on dynamic strain ageing in dilute solid solutions

A theoretical study is presented on the effect of residual and pre-existing solute clusters/organization on the strain rate sensitivity of non-heat treatable dilute solid solutions. Pre-existing clusters are solute structures that exist in the specimen before deformation. During deformation, additional solute concentration fluctuation takes place as solute diffuses towards dislocations, which themselves form an evolving population. Cluster growth takes place at the same rate with cluster dissolution, which produces residual clusters, i.e. solute clusters that are not located on a dislocation at given time and are dissolving slowly. A conceptual model is used to study the effect of both types of solute structures on the strain rate sensitivity. The model predicts that residual clusters lead to the shrinkage of the negative rate sensitivity domain in the temperature-rate plane, the effect being more pronounced in the region corresponding to low temperatures/high rates. The effect of pre-existing clusters is to shift the domain to lower temperatures and/or higher rates. This is in agreement with recent experimental observations made on Al-Mg alloys.