Wetting phase transitions in peritectic copper–cobalt alloys

Alloys of the Cu–Co peritectic system attract the interest of researchers due to their magnetic properties (giant magnetoresistance), their applicability as catalysts in synthesis of higher alcohols, and extraordinary mechanical properties. Therefore, in recent years, the Cu–Co phase diagram has been extensively studied and improved.

The grain boundary wetting phase transitions in the peritectic system Cu–Co before and after high pressure torsion (HPT) has been studied. These alloys with peritectic phase diagrams differ from previously studied systems with eutectic transformation by the fact that the melt layer separating grains from each other is not enriched, but is depleted by the second component (cobalt in this case). The fraction of completely wetted grain boundaries increases with temperature in Cu–2.2 wt.% Co alloy before high pressure torsion, as in eutectic systems, from zero at a temperature of 1098°C to ~80% at 1096°C. For symmetric twin boundaries, the temperature dependence of the contact angle with melt drops is constructed. As in the eutectic systems, the contact angle decreases with increasing temperature (although not to zero due to the extremely low energy of symmetric twin boundaries).

The melting process of HPT-treated Cu–2.2 wt.% Co and Cu–4.9 wt.% Co alloys was studied by the differential scanning calorimetry. The position of differential scanning calorimetry peaks showed that melting of ultrafine-grained Cu–Co alloys starts 10–20 °C below bulk solidus line. The thermal effects of melting below bulk solidus line are explained by the grain boundary premelting and permitted to construct the grain boundary solidus line.