Internal Stress and Dislocation Structure during Sigmoidal Transient Creep of a Copper–16 at.-% Aluminium Alloy

Abstract

Creep tests on pure copper and single-phase copper-aluminium alloys (5.8, 11.5, and 16.0 at.-% Al) have been made at 450, 550, and 650° C (723, 823, and 923K) and 19.6, 39.2 and 58.8 MN/m² (2.0, 4.0, and 6.0 kgf/mm²). Both normal and sigmoidal transient-creep curves were observed, according to the creep-stress and temperature conditions. Dislocation structures in Cu-16 at.-% Al were examinedby transmission electron microscopy, to clarify the origin of the sigmoidal transient creep, in which an inverse transient creep at an early stage was followed by a normal transient creep after the creep rate had passed through a maximum. Average internal stress during creep was also determined by the so-called stress dip test. The main results obtained were as follows: <list list-type="order"> <list-item>

Normal transient creep behaviour changed to sigmoidal with decreasing stress and temperature and with increasing aluminium content of the alloys. In pure copper, however, only normal transient creep was observed under all the creep conditions examined.</list-item> <list-item>

During the inverse transient stage of sigmoidal creep, the density of isolated dislocations increased rapidly with no cells being formed, but the average internal stress level, σ _i /σ (σ_i = internal stress, σ = external stress), did not change noticeably. During the normal transient stage σ _i /σ increased rapidly, cell formation proceeding in parallel with creep deformation.</list-item> </list> From these results it is concluded that the average internal stress arises mainly from long-range stress fields of cell boundaries consisting of dislocation tangles.