ON THE INTERPRETATION OF THE INTERNAL-STRESS DETERMINED FROM DIP TESTS DURING CREEP OF AL-5AT PERCENT-MG

Stress change tests have been performed during creep of Al-5at. %Mg at sigma equals 30 MPa and 573 K. Transmission electron microscopy observations after creep showed that a subgrain structure develops for a strain of 0. 3 although the creep behavior is characteristic of the 'alloy' class of materials (class A). The instantaneous strain response to a change in stress can be characterized by an elastic modulus E//s which is equal to the dynamic elastic modulus. This indicates that there is no anelastic back flow during unloading. A (net) non-elastic back strain is observed only for stress reductions above 40%. The reduced stress sigma //i for which a non-elastic back strain of 10** minus **5 is observed is 0. 52 of the creep stress. It does not vary significantly with strain in spite of the change in dislocation structure. The physical meaning of sigma //i is discussed on the basis of the anelastic and plastic strain contributions from free dislocations and subgrain boundaries. It is shown that sigma - sigma //i can be explained as the average of a wide distribution of effective stresses acting on the dislocations in the subgrain interior. This explanation is shown to be an alternative to the 'loop model' of M. J. Mills et al.