Self-organized, size-selection of precipitates during severe plastic deformation of dilute Cu-Nb alloys at low temperatures

High pressure torsion (HPT) experiments have been carried out at very low temperatures (-78 degrees C) on dilute Cu-Nb alloys. The samples were prepared by magnetron sputtering to create solid solutions containing nominally between 1 at% and 10 at.% Nb. Some samples were pre-annealed.to 700 degrees C to form large Nb precipitates, similar to 75 nm in radius, prior to HPT. For the unannealed alloys, HPT at low temperatures led to alloy decomposition for alloys with concentrations greater than similar to 2 at.% Nb, with the steady state concentration of Nb increasing from similar to 1.5 to similar to 3 at.% as the initial concentration of Nb was increased. It was also observed that BCC Nb precipitates formed during low-temperature HPT, with a steady state precipitate size similar to 10 nm in radius, which was insensitive to alloy concentration. HPT of pre-annealed Cu-10 at.% alloy led to the same steady state microstructure as the unannealed sample, indicating that the steady state during the low temperature HPT process is independent of the initial state for these Cu-Nb alloys. A model for phase evolution in strongly immiscible alloys during severe, low-temperature deformation is offered and implemented' through kinetic Monte Carlo simulations. Good agreement with experiment is obtained. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.