液态Ni－Fe合金的超过冷与热力学性质

HYPERCOOLING AND THERMODYNAMIC PROPERTIES OF LIQUID Ni-Fe ALLOYS

采用熔融玻璃净化法使液态Ｎｉ－１０％Ｆｅ和Ｎｉ－３５％Ｆｅ合金过冷度分别达到３７１（０．９０△Ｔ＿ｈ）和３４３Ｋ（０．９１△Ｔ＿ｈ）．实验测得其超过冷临界过冷度△Ｔｈ分别为４１３和３７６Ｋ．根据超过冷的定义导出一种测定深过冷熔体平均比热容的方法，从而得到两种深过冷合金的比热容为４１．０和４０．７Ｊ／（ｍｏｌ·Ｋ）．据此对快速凝固过程中的焓变△Ｈ＿（ＬＳ），熵变△Ｓ＿（ＬＳ），相变驱动力△Ｇ＿（ＬＳ）以及晶体形核率Ⅰ进行了理论计算．发现尽管过冷度超过了０．２Ｔ＿Ｌ，液态合金仍然发生异质形核．

Ni-10% Fe and Ni-35% Fe alloy melts were undercooled with glass fluxing technique by 371 (0. 90 △T_h) and 343 K (0. 91 △T_h) .Their hypercooling limits △T_h were determined to be 413 and 376 K respectively. A new method to measure the integral mean specific heat C_(pL) of undercooled metallic melts was developed according to the definition of hypercooling. That is, one can take C_(pL) as the ratio of latent heat of fusion △H_m to hypercooling limit △T_h, provided that the latter two physical parameters have been obtained through experiments. The C_(pL) values of the above Ni-Fe alloys were found to be 41. 0 and 40. 7 J/ (mol . K). Theoretical calculations were also carried out to reveal the relationships of enthalpy change △H_(LS), entropy change △S_(LS), Gibbs free energy change △G_(LS) and crystal nucleation rate I during rapid solidification vs undercooling. Although the maximum undercoolings achieved by both Ni-Fe alloys have exceeded the critical level of 0-2 T_L, the calculated results indicate that heterogeneous nucleation must have taken place in preference to homogeneous nucleation. Those heterogeneous nucleants with a wetting angle range of 53°< θ< 80°played the dominating role to prevent Ni-Fe alloys from further approaching their hypercooling limits.