Phase evolution in driven alloys: An overview on compositional patterning

Irradiation of alloys with energetic particles leads to the forced chemical mixing of atoms and the creation of point defects. At elevated temperatures, the point defects are mobile and enhance diffusion. Since forced mixing occurs in energetic displacement processes, alloy constituents tend to flow down gradients in their concentration, while radiation enhanced diffusion is thermally activated and alloy components respond to gradients in their chemical potential. Phase evolution in irradiated alloys thus depends on the competition between these two dynamics. A similar situation occurs during severe plastic deformation, dislocation glide results in forced chemical mixing, while dislocation reactions lead to the creation of vacancies and enhanced thermally activated diffusion. One possible consequence of these competing dynamics is that alloys self-organize into compositional patterns, i.e., concentration variations adopt a fixed length scale determined by the nature of the driving forces and the internal dynamics of the alloy. The current perspective illustrates how the details of driving forces relate to the length scale of the compositional patterns and their morphology. A surprising finding is that irradiation and severe plastic deformation can both lead to compositional patterning even at low temperatures, i.e., in absence of thermally activated diffusion. While the compositional patterning again derives from competing dynamical processes, the characteristic length scales are determined by quite different aspects of the forced mixing. For irradiation, the ballistic recoil distance limits the maximum length scale for patterning at high temperatures, while it is the extent of the thermal spike at low temperatures. For severe plastic deformation, the glide distance of dislocations controls the largest length scale for patterning at high temperatures, whereas kinetic roughening of precipitates controls the maximum length at low temperatures.