Mechanism of High Temperature Creep of Aluminum-Magnesium Solid Solution Alloys

In order to develop and elaborate the Weertman’s theory of creep of solid solution alloys, the high temperature creep test was carried out with Al-Mg alloys. The results are summarized as follows: (1) In alloys containing more than 3 at%Mg, the dependence of the steady-state creep rate on stress and temperature is described by the equation, εs=Aσnexp(−Qc⁄RT), where A is a numerical constant depending on the solute concentration, n≅3 and Qc=33.5±1.5 kcal/mol. (2) Transmission electron microscopy showed that the dislocations were distributed uniformly without forming cell structures. An alternating array of oppositely signed edge dislocations is considered as the most possible distribution. In such a case, the maximum internal stress is estimated to be less than several percent of the applied stress and so is negligible compared with the latter. (3) The measured dislocation density was proportional approximately to the square of the stress and was independent of solute concentration. (4) The experimental relation between the dislocation density and the applied stress is explained theoretically by a model based on the balance between multiplication and annihilation of dislocations. The climb velocity which is estimated from comparison between the calculated and measured creep curves, is in good agreement with the theoretical value. (5) The high temperature creep of alloys is controlled by the viscous motion of dislocations and is proportional to the cube of the stress, provided that the following conditions are satisfied; a large atomic size factor, a rather high solute concentration, a high stacking fault energy and a uniform distribution of dislocations. It is therefore concluded that the high temperature creep of solid solution alloys is not always controlled by the viscous motion of dislocations.