Correlation between creep behavior in Al-based solid solution alloys and powder metallurgy Al alloys

The creep characteristics of Al-based solid solution alloys and powder metallurgy (PM) Al alloys are reviewed. Consideration of available information shows that the creep behavior of PM Al alloys is different from that of Al-based solid solution alloys with regard to the stress dependence of creep rate as inferred from the value of the stress exponent, n, and the temperature dependence of creep rate as measured by the value of the activation energy, Q. For example, according to well-documented experimental evidence, the activation energies for creep in Al-based solid solution alloys, whether they behave as class I (alloy class, n=3) or class II (metal class, n=5), are close to that for self-diffusion in Al. By contrast, the apparent activation energies for creep in PM Al alloys are not only much higher than those reported for Al-based solid solution alloys but also variable. It is suggested that the aforementioned differences arise from the presence of a threshold stress for creep in PM Al alloys; that the origin of such a threshold stress is related to the interaction between moving dislocations and oxides particles which are present as a result of processing these alloys by powder metallurgy; and that by incorporating the threshold stress and its temperature dependence in the analysis, the true creep characteristics of PM Al alloys become similar to those of Al-based solid solution alloys. The consistency between this suggestion and experimental trends for two PM Al alloys, PM 6061 Al and PM 2124 Al, is examined.