Stefano Curiotto

Liquid-liquid phase separation and remixing in the Cu-Co system

This article deals with the metastable liquid-liquid separation in the Cu-Co system. Several samples with different compositions were investigated by differential scanning calorimetry. High undercooling with respect to the liquidus was reached by means of the glass fluxing technique. The alloys were cycled with several heating and cooling runs in order to determine the temperature of the liquid-liquid separation and of the remixing. For each composition, demixing and remixing temperatures were found to be equal. Nucleation rate calculations of the liquid phase separation were carried out to explain the experimental results. The liquid-liquid separation in the Cu-Co system was found to be a nucleation process occurring with no detectable undercooling below the binodal line.

Undercooling and demixing of copper-based alloys

Since the beginning of materials science research under microgravity conditions immiscible alloys have been an interesting subject. New possibilities to investigate such systems are offered by containerless processing techniques. Of particular interest is the ternary system Cu-Fe-Co, and its limiting binaries, Cu-Co and Cu-Fe. They all show a metastable miscibility gap in the regime of the undercooled melt. Within the ESA-MAP project “CoolCop”, different aspects of this alloy have been investigated; results obtained so far are reported here.

The Solidification in the Presence of a Metastable Miscibility Gap: The Case of Co-Cu and Co-Cu-X Alloys

Alloys displaying positive enthalpy of mixing demix below a critical temperature. In Co-Cu and related ternaries the miscibility gap is metastable, i.e. it occurs at temperatures lower than the liquidus. In order to study the liquid phase separation high melt undercooling is necessary. This was obtained via rapid solidification techniques using melt spinning and casting in moulding devices, as well as high temperature DSC experiments with samples embedded in a flux. Results are given for Co-Cu, Co-Cu-Fe and Co-Cu-Ni systems. Phase diagrams were optimised using the DSC data. The mechanism of phase separation was investigated by comparing samples produced under different cooling conditions. The hierarchy of microstructures obtained was interpreted accounting for the processing technique and the phase diagram. They constitute a database useful for the interpretation of the thermal history of samples processed in microgravity.