A critical investigation of the factors controlling the long-life fatigue properties of aluminum alloys

Abstract

In an attempt to understand which factors control the fatigue properties of aluminum alloys, Al-4% Cu and Al-15% Ag alloys have been thermo-mechanically treated so as to produce a variety of micro-structures: (1) solution-treated material both cold-worked and annealed, (2) conventionally aged material containing metastable precipitates, (3) material containing uniform dispersions of stable precipitates, unlikely to be subject to reversion, and (4) material containing different kinds of aging inhomogeneities. The fatigue properties of these materials (the structures of which were monitored by transmission electron microscopy) were established by tension-compression testing. The annealed solution-treated alloy has fatigue properties, in the range of life investigated, virtually equivalent to those of the conventionally aged alloy. The properties of the cold-worked solution-treated alloy and that containing stable precipitates are also equivalent but about 30% better than those of the conventionally aged alloy. When the density of dislocations in the material containing stable precipitates is reduced by a longer time of anneal, the fatigue properties decrease towards those of the conventionally aged. This indicates that the reversion of precipitates has little influence on fatigue and that, instead, the total amount of solute present, rather than its distribution, if this is on a fine scale, is more important. Aging inhomogeneities were observed to play a role in decreasing fatigue lives in some circumstances, e.g. in the AlAg alloy the cellular reaction provided weak sites for crack nucleation and growth.