Dependence of dislocation density in Harper-Dorn creep on the peierls stress

Abstract

Experimental studies of creep processes on crystalline materials have shown that power law dislocation creep at high stress transfers to Newtonian dislocation creep - Harper-Dorn (H-D) creep at low stress when the grain size is too large for diffusional creep to be rate controlling. Such transition has been reported for metals and alloys and minerals. The transition has been demonstrated to take place at the Peierls stress, [tau][sub [rho]], of a given material. It is well known that in power law creep regime, the density of free dislocations contained within subgrains, [rho], varies with the square of the applied shear stress, [tau][sup 2]. It is the objective of this paper to show that [rho] in H-D creep is determined by [tau][sub [rho]]; the higher the [tau][sub [rho]], the larger the [rho]. It will also be shown that in H-D creep, a steady dislocation density is established, probably when the stress, produced by a dislocatino array, is in equilibrium with [tau][sub [rho]]. Although H-D creep has been studied for many materials, only for a few of them has the [rho] been measured by etch-pitting (EP) or transmission electron microscopy (TEM). The available [rho] data for Al, Al-5Mg, NaCl, [alpha]-Zr, (MgFe)[sub 2]SiO[submore » 4] and SiO[sub 2] are presented.« less