The influence of a second-phase interface on the work hardening behavior of an aluminum — 5 atomic per cent silver alloy

Abstract

The role of a second phase interface upon the development of substructure and the flow stress behavior in the aluminum-silver alloy system was experimentally determined. Mechanical deformation studies were combined with transmission electron microscopy and selected area diffraction techniques to determine the deformation modes in both the single- and two-phase alloy. Alloy specimens were prepared by quenching to obtain a solid solution and by aging to produce the γ′ precipitate phase. Specimens in both structural conditions were then deformed by rolling. Observations of the resultant deformation structures were made in the electron microscope, and this information, was correlated with flow stress determinations made on the deformed specimens. The single phase alloy was observed to deform according to a model by Weertman. Dislocation mobility was reduced, apparently by interactions with solute clusters. The flow stress behavior corroborated the interpretation of the transmission observations. The two-phase alloy possessed three distinctly different deformation modes, all controlled by the second phase interface. At low deformations, dislocation motion appeared to be contained within the individual matrix volumes which are bounded by the γ′ precipitate phase. Here, no appreciable substructure build-up occurred since the precipitatematrix interface operated as a dislocation sink. For these deformations, no increase in flow stress was observed. At intermediate deformation levels, dislocation motion was no longer confined to these individual matrix volumes, but instead extended across the precipitate particles. Here, strain hardening, as revealed by the flow stress, increased very rapidly. Structurally, an attendant build-up of dislocation density in the matrix and particle shear were observed. At high deformation, a substructural relaxation occurred. A rearrangement of the dislocation arrays coupled with dislocation-dislocation annihilation led to a partial recovery of the substructure. These observations were interpreted, and the influence of the precipitate interface on the deformation process was evaluated.