Shape of transient creep curves following stress change during steady state creep

Abstract

The shape of the strain transient following a sudden stress change (drop or increment) during steady state creep has been investigated for AI, AI–Mg alloys, AI–Li alloys, Cu, and Zr−1 wt-%Nb over a range of temperatures. The strain transients were classified as N type, for which the initial creep rate is high but decreases with time eventually reaching a constant value, or I type, for which the creep rate gradually increases to a constant value. The shape after a stress increment can be used as a guide to the rate controlling process of creep with pure metals and Class 2 alloys in which recovery is rate controlling showing an N type transient, and Class 1 alloys in which viscous glide is rate controlling showing I type transients. The shape of the strain transient after a sudden stress drop gives no clear indication of the rate controlling mechanism since, in general, pure metals and Class 1 and Class 2 alloys each show I type transients.