Transient and steady state creep behaviour of polycrystalline MgO

Abstract

Stress change experiments during compressive creep tests at high stresses on polycrystalline MgO at 1596 K have shown that the creep rate at any instant during transient and steady state creep is predicted by the ratio,r/h, wherer is the rate of recovery (=−∂σ/t6t) andh is the coefficient of strain hardening (=∂σ/∂ε). Over most of transient and steady state creep, whenh is constant and the decrease in creep rate,$\dot \in$, is a direct result of a decrease inr, the variation of the creep strain,ε, with time,t, is accurately described as

$$ \in = \in _0 + \in _T (1 - e^{ - mt} ) + \dot \in _s t$$

whereε ₀ is the instantaneous strain on loading,ε _T the transient creep strain,m relates to the rate of exhaustion of transient creep and$\dot \in _s$ is the steady creep rate. Deviations from this equation occur during the initial 10 to 15% of the transient creep life, whenh increases rapidly.

The variations in$\dot \in$,r andh during transient and steady state creep are explained in terms of a model for creep in which the rate-determining process is the diffusion controlled growth of the three-dimensional dislocation network within subgrains to form dislocation sources allowing slip to occur.