Microstructure and Mechanical Properties of Cu-Ag Microcomposites for Conductor Wires in Pulsed High-Field Magnets

Abstract

The influence of heat treatment and cold deformation on microstructure and mechanical properties of Cu-Ag alloys with more than 20 wt.% Ag has been investigated. Due to the embedding of the Cu-rich dendrites by the Ag-Cu eutectic high-angle grain boundaries are absent in the as-cast state of these compositions. Hence, the continuous precipitation of Ag particles is favored in comparison with discontinuous precipitation reactions observed in Cu alloys containing less than 8 wt.% Ag. The tensile strength after heavy cold deformation is enhanced by a beneficial superposition of several effects: (i) The solid solution hardening of the Cu-rich dendrites, (ii) the age hardening of silver precipitates inside the dendrites and the high work hardening rate of the peak-aged state, (iii) the phase boundary hardening of the Ag-rich eutectic, and (iv) the increase of cooling velocity after solidification of continuously cast billets which determines the extent of the Ag supersaturation in the copper matrix. As a consequence, with a continuously cast starting billet of 12 mm diameter an optimum value for the ultimate tensile strength of 1.3 GPa is obtained after a total drawing strain of only η = 4.3.