CREEP TRANSITIONS IN ALUMINUM SOLID-SOLUTION ALLOYS

The creep behavior of two solid-solution alloys of Al, Al-3 at% Mg and Al-10 at% Zn, have been investigated over a wide range of stresses and temperatures.;For Al-3 at% Mg, the dislocation substructures developed during the steady state in the range of normalized stresses extending from 10('-5) to 10('-3), have been studied using the etch-pit procedure and the transmission electron microscopy. Based on a detailed examination, the substructural results can be divided into three regions. In region I (high stresses), the dislocations are long, smoothly curved and uniformly distributed and there is a little tendency for development of subgrains. In region III (low stresses), very well-developed subgrains are noted and dislocations sometimes form irregularly spaced networks inside subgrains. In region II (intermediate stresses) which represents a transition region between region I and region III, long curved dislocations and subboundaries in the process of development co-exist.;For Al-10 at% Zn, the creep behavior, which is investigated over the temperature range of 573 to 800 K and under normalized stresses varying from 10('-6) to 10('-3), is divisible into four regions: region I (very low stresses), region II (low stresses), region III (intermediate stresses) and region IV (high stresses). Although the creep data is limited in region I, the creep characteristics are similar to those associated with Harper-Dorn creep. In region II and region III, the values of the stress exponent, the extent of primary creep and the nature of transients after stress reduction are in agreement with dislocation-climb controlled and viscous-glide controlled creep in regions II and III, respectively. Region IV is characterized by extensive primary creep and stress exponent close to 5.;The change in the stress exponent from 3 to 5 with decreasing stresses in both alloys and the corresponding change in the features of substructures typical of viscous glide (stress exponent = 3) to those typical of dislocation climb (stress exponent = 5), observed in the Al-3 at% Mg alloy are in conformity with the prediction of the deformation criterion of solid-solution alloys that transition from viscous-glide controlled to dislocation-climb controlled creep can be observed in a single alloy provided a favorable combination between the experimental conditions and the alloy parameters is achieved.