三元(Co_(0.5)Cu_(0.5))_(100-x)Sn_x合金的热物理性质与液固相变机理

Thermophysical properties and liquid-solid transition mechanisms of ternary(Co_(0.5)Cu_(0.5))_(100-x)Sn_x alloys

本文系统研究了三元(Co_(0.5)Cu_(0.5))_(100-x)Sn_x(x=10,20,30,40,50 at%)合金的热物理性质及其在近平衡条件下的微观凝固组织特征.采用差示扫描量热法(DSC)确定了合金的液相限、固相限温度和熔化潜热,并建立了它们与合金成分之间的函数关系.实验发现,Sn元素的引入提高了液态三元(Co_(0.5)Cu_(0.5))_(100-x)Sn_x合金的过冷能力,当Sn含量为50 at%时,合金的过冷度达到最大值68 K.基于DSC曲线和微观组织形态确定了近平衡条件下合金的液固相变过程和室温下的相组成,发现当Sn含量低于30 at%时,初生相为(Co)相,而当Sn含量超过30 at%时,Co3Sn2相成为领先形核相.在293—473 K温度范围内,实验测定了固态三元(Co_(0.5)Cu_(0.5))_(100-x)Sn_x合金的热扩散系数和比热.结合所测定的固态合金密度,导出了三元(Co_(0.5)Cu_(0.5))_(100-x)Sn_x合金在室温293 K下的热导率,发现其随Sn含量的增加呈现先增大后减小的变化规律.

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-xSnxalloys 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 1.06 × 10-5to 1.12 × 10-5m2·s-1for ternary Co（45）Cu（45）Sn（10） 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-xSnxalloys 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-xSnxalloys at 293 K are derived.With the Sn content increasing up to 40 at%,the thermal conductivities for（Co0.5Cu0.5）100-xSnxalloys monotonically decrease from 33.83 to 7.90 W·m-1·K-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 Co（30）Cu（30）Sn（40） alloy.The phase constitution investigation indicates that with the increase of the Sn content,the（Cu） solid solution phase disappears,whereas intermetallic compounds,including Cu（41）Sn（11）,Cu3Sn,and Cu6Sn5 phases successively precipitate from the alloy melts.The（Sn） solid solution phase even appears when Sn amount reaches 50 at%.