Thermophysical properties and liquid-solid transition mechanisms of ternary (Co0.5Cu0.5)100-xSnx alloys

Abstract

The thermophysical properties and liquid-solid phase transition characteristics of ternary (Co0.5Cu0.5)100-xSnx(x=10, 20, 30, 40 and 50 at%) alloys are systematically investigated. The liquidus temperature and latent heat of fusion, as well as the undercooling are determined by differential scanning calorimetry (DSC) method. Based on the measured data, their relationships with Sn content are fitted by polynomial functions. The liquidus temperature shows a decreasing tendency with the increase of Sn content. The undercooling of liquid (Co0.5Cu0.5)100-xSnx alloys significantly increases with increasing Sn amount, indicating that the addition of Sn element enhances the undercoolability. By using the laser-flash and DSC methods, the thermal diffusion coefficients and specific heats of solid ternary (Co0.5Cu0.5)100-xSnx alloys are respectively measured in a temperature range from 293 to 473 K. The thermal diffusion coefficients increase linearly as temperature rises. The thermal diffusion coefficient varies from 1.0610-5 to 1.1210-5 m2s-1 for ternary Co45Cu45Sn10 alloy, which is close to that of Co element but much lower than those of Cu and Sn elements in the same temperature range. However, the thermal diffusion coefficients of other (Co0.5Cu0.5)100-xSnx alloys are far less than that of ternary Co45Cu45Sn10 alloy. The specific heat shows an increasing trend with temperature, and drops apparently with increasing Sn amount. From the measured thermal diffusion coefficients, specific heats and densities, the thermal conductivities of ternary (Co0.5Cu0.5)100-xSnx alloys at 293 K are derived. With the Sn content increasing up to 40 at%, the thermal conductivities for (Co0.5Cu0.5)100-xSnx alloys monotonically decrease from 33.83 to 7.90 Wm-1K-1, and subsequently increases slightly when the Sn content further increases up to 50 at%. In addition, on the basis of the DSC curves and solidification microstructures, the liquid-solid phase transitions are also explored. When the Sn content is less than 30 at%, the primary (Co) phase appears as coarse dendrites, whose volume fraction decreases as Sn content increases. Once Sn content exceeds 30 at%, the Co3Sn2 phase preferentially nucleates and grows during solidification, which occupies about 89% volume in the solidified Co30Cu30Sn40 alloy. The phase constitution investigation indicates that with the increase of the Sn content, the (Cu) solid solution phase disappears, whereas intermetallic compounds, including Cu41Sn11, Cu3Sn, and Cu6Sn5 phases successively precipitate from the alloy melts. The (Sn) solid solution phase even appears when Sn amount reaches 50 at%.