Effect of magnesium content on the stress exponent and effective stress in the steady state creep of Al-Mg alloys

Abstract

The stress exponent of steady state creep,n, and the internal (σ _i) and effective stresses (σ _e) have been determined using the strain transient dip test for a series of polycrystalline Al-Mg alloys creep tested at 300° C and compared with previously published data. The internal or dislocation back stress,σ _i, varied with applied stress,σ, but was insensitive to magnesium content of the alloy, being represented by the empirical equationσ _i=1.084σ ^1.802. Such an applied stress dependence ofσ _i can be explained by using an equation forσ _i of the formσ _i α(dislocation density)^1/2 and published values for the stress dependence of dislocation density. Values of the friction stress,σ _f, derived using the equationσ _e/σ=(1−c) (1−σ _f/σ), indicate thatσ _f is not dependent on the magnesium content. A constant value ofσ _f can best be rationalized by postulating that the creep dislocation structure is relatively insensitive to the magnesium content of the alloy.