High temperature creep of alpha iron in terms of effective stress and dislocation dynamics

The dynamics of dislocations in both steady state and transient creep in alpha iron of about 99.5 pct purity was investigated in the temperature interval 773 to 923 K, and the applied stress range 24.5 to 220.5 MN m⁻². The applied stress sensitivity parameter of the steady state creep rate m∲ = (∂ In ε/∂ In σ) _T increased linearly with increasing applied stress σ from about 5 at σ = 24.5 MN m⁻² to about 12 at σ = 196 MN m⁻². The apparent activation energy of steady state creep rate was found to decrease linearly with stress from 89 K cal mol⁻¹ at σ = 98 MN m⁻² to 81 K cal mol⁻¹ at a = 147 MN m⁻². Measurements of the mean effective stress σ^* by the strain transient dip test technique led to a nonlinear relation between σ^* and σ, indicating a dependence of the ratio σ*/σ on the applied stress. The effective stress sensitivity parameter was lower than m′.However, the apparent activation energy was equal toQ. Using the stress sensitivity technique, the relation between transient creep rate and effective stress at various constant internal stresses and temperatures was obtained. The effective stress sensitivity of transient creep rate was found to be lessthan that of steady creep rate.