Dynamic dislocation–defect analysis

Abstract

The dynamic internal variables which control plastic flow can only be assessed by dynamic materials testing at any given instance. The testing method championned by our studies has been precision strain rate sensitivity (PSRS) whereby the change in flow stress due to a set change in strain rate is taken to be an operational measure of the activation volume and its product with the flow stress gives rise to the operational activation work. Also, from the work hardening slope, a modelled parameter proportional to the mean slip distance (λ) is simultaneously determined. The deviation from the linear Cottrell–Stokes relation as determined with the Haasen plot indicates the evolution of secondary defects other than monopole dislocations. Hence PSRS can assess the theoretical predictions of the activation distance (d) and work as a function of temperature, resulting in quantitative values that are in accord with dislocation theory at temperatures below that where point defects become mobile. A method to calibrate λ using Stage II slope θ_II shows that λ/ℓ, where ℓ is the mean forest dislocation spacing, is inversely proportional to θ, the work hardening coefficient. This analysis has led to a new plot of θ_II/θ versus b ²λ/ν where b is the Burgers vector and its slope is directly proportional to d. An example using an alumina-dispersed high conductivity copper shows that geometrically necessary punched out loops are continuously generated. The role of point defect mobility is dramatically illustrated by load drops in [001] aluminium crystals with the formation of slip clusters.

Acknowledgements

The author thanks Dr. H. Mughrabi for discussion on the correlation of mean slip distance to mean obstacle spacing which led to elucidating our experimentally based approach on plastic flow. He is also grateful to the reviewer for highlighting the modelling concepts of this correlation which required clarification. The continuing financial support from the Natural Science and Engineering Research Council of Canada for these fundamental studies is gratefully acknowledged. The engineering applications of the developing techniques are supported by General Motors Canada Ltd and Novelis Inc.