Coble and Harper-Dorn creep in iron at homologous temperatures [formula omitted] of 0.40-0.54

Abstract

The creep of high purity iron at low stresses (0.1–3.5 MPa) was investigated over a homologous temperature range T T m of 0.40-0.54 (where T is the temperature under consideration and T m is the temperature of the melting point) using the helicoid specimen technique; the mean intercept grain size ranged from 82 to 478 μm. The results obtained enabled the most probable dominant deformation mechanism to be identified as Coble creep for intercept grain sizes smaller than about 123 μm and Harper-Dorn creep for intercept grain sizes greater than 123 μm. Coble creep in α-Fe was found to be characterized by Bingham behaviour. The threshold stress increases exponentially with decreasing temperature but does not depend on the mean intercept grain size. Under the experimental conditions considered, Harper-Dorn creep is most probably dislocation core diffusion controlled. It was found that for Harper-Dorn creep in α-Fe there is a threshold stress which increases exponentially with decreasing temperature and does not depend on the mean intercept grain size. The transition from Coble creep to Harper-Dorn creep is discussed. Also, the higher power law (recovery) creep above the transition stress is analysed and briefly discussed. The results indicate lattice diffusion aided by dislocation core diffusion as the rate-controlling processes. Maps of the creep mechanisms are presented.