Thermodynamic properties and phase transitions of ternary Co-Cu-Si alloys with equiatomic Co/Cu ratio

Different amounts of Si element were introduced into binary Co₅₀Cu₅₀ alloy to investigate the thermodynamic properties and phase transitions of ternary Co_50-x/2Cu_50-x/2Si_x (x = 10, 20, 30, 40 and 50 at%) alloys. Their liquidus and solidus temperatures versus Si content were determined by the differential scanning calorimetry (DSC) method. It was found that the addition of Si element depressed both the liquidus and solidus temperatures as compared with binary Co₅₀Cu₅₀ alloy. In particular, the additions of 10 and 20 at% Si remarkably reduced the critical undercooling for liquid demixing to only 3 and 1 K, whereas no liquid phase separation took place in other Co_50-x/2Cu_50-x/2Si_x alloys. The relationship between the enthalpy of fusion and alloy composition was also established by a polynomial function on the basis of the measured data. The solidification microstructures of the DSC samples were investigated corresponding to the calorimetric signals, based on which the solidification pathway for each Co_50-x/2Cu_50-x/2Si_x alloy was elucidated. The Si element displays stronger affinity with the Co element than the Cu element. As Si content rises, the pseudobinary eutectic (Co + Co₂Si), (Co₂Si + CoSi), (CoSi + CoSi₂) and (Cu₃Si + Si) structures were successively formed, and there were no ternary intermetallic compounds in these alloys. The thermal diffusivity of solid ternary Co_50-x/2Cu_50-x/2Si_x alloys was determined by a laser flash method in a wide temperature range from 300 to 1180 K, which showed a decreasing tendency with the increase of Si content.