A THEORETICAL UPPER LIMIT TO COBLE CREEP STRAIN RESULTING FROM CONCURRENT GRAIN-GROWTH

The rate of diffusional creep varies with grain size x, either as 1/X2 or 1/X3 ; depending on whether lattice or grain boundary diffusion is dominating. Since the rate of grain growth is proportional to 1/x(p), where p greater-than-or-equal-to 1, the creep and grain growth relationships can be combined to predict the transient creep that results from the two processes operating concurrently. An important result is obtained for grain boundary diffusion creep (Coble creep), where two regimes of behaviour are predicted depending on the value of p. For normal grain growth (p = 1 ) and up to a critical value p = 2, the transient gives rise to an upper limit to the grain boundary diffusional creep strain. For p > 2, no limiting strain is predicted. The role of the limiting strain is discussed in the context of the various experimental attempts that have been made to verify the Coble mechanism.