Mechanical alloying by severe plastic deformation can be used to synthesize alloys with solubilities far beyond equilibrium. This processing method has already been successfully applied to different immiscible systems, but the underlying processes that realize deformation-induced supersaturation have not been entirely clarified until now.
In this work, cast and powder compacted Cu-Ag alloys with low volume fractions of Ag were severely deformed by high-pressure torsion. The as-cast structures show some inherent structural inhomogeneities, such as eutectic regions and strong variations in the structural sizes. On the other hand, for powder compacted materials, a similar particle size of Cu and Ag was used to provide more evenly distributed phase dimensions.
In general, a similar deformation behavior was observed for both cast and powder material. At low applied strains, a lamellar microstructure is formed through co-deformation of Cu and Ag. With ongoing deformation, the homogenous thinning of Ag results in the dissolution of Ag in the Cu matrix and single-phase supersaturated solid solutions are obtained. The required applied strain to reach a steady-state regime strongly depends on the initial size of the Ag regions. In the cast alloys, a bimodal initial Ag precipitate size distribution is present, with Ag existing in spherical precipitates well below 100 nm and around several µm. The small Ag grains dissolve first at relatively low applied strains (γ<30), while the larger ones require an applied strain of about 300 to fully dissolve. In the powder compacted alloys, all Ag regions have dimensions around 20-60 µm and they are continuously dissolved until at a strain of around γ~500 a homogenous single-phase solid solution is obtained. The dissolution process is also reflected in the hardening behavior of the alloys, unraveling characteristic hardening regimes for cast and powder alloys.
High resolution transmission electron microscopy studies revealed that Cu/Ag phase boundaries remain atomically sharp for lamella thicknesses down to about ~10 nm. Further thinning below this critical lamella thickness leads to a roughening of the interfaces, possibly induced by dislocation slip across the Cu/Ag phase boundaries. The present results indicate that the solid solution of Cu and Ag is energetically more favorable than the lamellar configuration, if a critical phase dimension is reached, and thereby, facilitating the dissolution process further.