Phase decomposition behavior and its impact on mechanical properties in bulk nanostructured Cu-20 at.%Fe supersaturated solid solution

Abstract

Extended solid solution of immiscible systems, achieved by extreme processing approaches, can be transformed into nanostructured composites via phase decomposition, which are drawing great research attention for their excellent thermal stability and high strength. Here we fabricated the supersaturated solid solution of Cu-20 at.%Fe, a typical immiscible alloy, in bulk state by high pressure torsion, which provides a great freedom over powders for studying the microstructure evolution and mechanical properties in the process of the phase decomposition in immiscible alloys. We found that in the Cu-Fe solid solution, spinodal decomposition of the Fe phases took place at the initial stage of annealing through volume diffusion. This process gives rise to: (1) metastable γ-Fe particles in the Cu grains with coherent Fe/Cu interface, and (2) the more stable α-Fe phase at the grain boundaries. This process was accompanied by moving boundary reaction which first proceeded in the pattern of spinodal decomposition and then changed into classical nucleation-growth mode with the depletion of Fe atoms in Cu. The resultant Cu-Fe nanocomposites were jointly strengthened by the ultrafine Cu and α-Fe grains according to the rule of mixture, including grain boundary strengthening and the hardening of nanoscale γ-Fe precipitates in the Cu grains. The effects of different strengthening mechanisms were scrutinized and their contributions to mechanical properties were quantitatively evaluated. This work sheds light onto the opportunity of designing and fabricating nanostructured composites via phase decomposition in immiscible systems.