Precipitation behavior and microstructural evolution during thermo-mechanical processing of precipitation hardened Cu-Hf based alloys

A recently found precipitation hardened Cu-Hf alloy is a promising candidate for the fabrication of novel high strength and high electrical conductivity materials. However, the precipitation behavior and microstructural evolution during thermo-mechanical processing remain poorly recognized. The present paper focuses on the crystallography, thermodynamics and kinetics of precipitation process during isothermal aging and the inter-action between nano-precipitates and nano-twins during liquid nitrogen temperature (LNT) rolling in Cu-Hf alloys. The isothermal aging precipitation procedure of Cu-Hf alloys was identified as supersaturated FCC-Cu -> coherent Cu5Hf phase -> incoherent Cu51Hf14 phase. The coherent Cu5Hf is oriented with respect to the Cu matrix: (1 1 1)Cu//(3 1 1)Cu5Hf, [0 -1 1]Cu//[0 -1 1]Cu5Hf. According to thermodynamic assessment and kinetic analysis, the formation of intermediate Cu5Hf phase is due to the lower accommodation energy and interfacial free energy of coherent nano-precipitates, and the change of the growth mode results in shape change in nano-precipitates. Based on molecular dynamics simulations, the significant improvement in the twinning ability of Cu-Hf alloys by the pre-existing nano-precipitates leads to the formation of plenty of nano-twin bundles during LNT-rolling. Finally, a new type of Cu-Hf based alloys with an excellent combination of high strength, high electrical conductivity, as well as high heat-resistance were fabricated by routine process.