Surface precipitation of chromium in rapidly solidified Cu-Cr alloys

Rapidly solidified ribbons of Cu-Cr alloys with 2.27 and 4.20 at.% of chromium were produced using the melt-spinning method. Alloys were analyzed by electron microscopy for complete solubility of Cr in copper matrix. To avoid disturbing effects of Cr phase particles, the kinetics and the sequence of microstructural transformations during heating were analyzed only the sample with 2.27 at.% of chromium with complete Cr solubility in the copper matrix. We then investigated the precipitation process for this alloy that was subsequently heated at a constant rate. The increased solid solubility obtained allowed the extensive precipitation of a Cr-rich phase. The kinetics and the sequence of microstructural changes that occurred during the heating were analyzed using an in situ measurement of the electrical resistance. The quenched microstructure was analyzed at transition points using scanning and transmission electron microscopy. X-ray photoelectron spectroscopy, as a very surface-sensitive method, was applied to study the changes in the chemical composition of the surface for the Cu-Cr alloy ribbons in the temperature range 400-700 °C during an in situ heat treatment in an ultra-high vacuum. The results show a relatively rapid precipitation of chromium to the surface, which starts at 400 °C and is correlated with a change in the microstructure and the electrical resistance. The Cr-precipitation is faster at higher temperatures and follows the parabolic law. The resistivity results for the supersaturated binary alloy were analyzed using the Ozawa method to give an activation energy for the precipitation of 196 ± 10 kJ mol−1.