An analytical phase transformation model has been used to study the kinetics of crystallization of amorphous alloys sub- jected to either isothermal or isochronal anneals. The model has been applied to Mg82.3Cu17.7 and Pd40Cu30P20Ni10, employing iso- thermal and isochronal differential scanning calorimetry. Applying different combinations of nucleation and growth mechanisms to the same experiments, the nucleation and growth modes dominating the crystallization and the values for the corresponding kinetic parameters, including the constant activation energies for nucleation and growth, have been determined. Further, the influence of isothermal pre-annealing on subsequent isochronal crystallization kinetics, involving a gradual change of nucleation mode up to site saturation with increase of pre-annealing, can be analyzed.
Feng Liu, Changlin Yang, Wei Yang, Gencang Yang, and Yaohe Zhou State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
A solution entropy model was developed for the undercooled binary eutectic alloy systems. As an ex-tension of Taylor and Fidler et al.'s model, the present model considered the change of phase compo-sition with the increase of undercooling. Furthermore, the sub-regular solution model and the interac-tion parameter (IAB) were also introduced. In this paper, the extended model is used to calculate the solution entropy for binary eutectic phases under the undercooled condition, and the application scope of the model is also extended. Not only the growth manner of eutectic phases, but also the transition of morphologies may be predicted and explained by calculating the solution entropy of eutectic phases under the non-equilibrium condition with the developed model. Experimental results show that the developed model is valid for the undercooled Ni-Si and Ni-Sn eutectic alloy systems.
High undercooling up to 392 K was achieved in eutectic Ni70.2Si29.8 alloy melt by using glass fluxing combined with cyclic superheating. A small quantity of amorphous phase was obtained in bulk eutectic Ni70.2Si29.8 alloy when undercooling exceeds 240 K under slow cooling conditions （about 1 K/s）. The amorphous phase was confirmed by high-resolution transmission electron microscopy and differential scanning calorimetry.
Through phase transformation kinetic analysis and experimental observation,the δ/γtransformation occurring in the non-equilibrium peritectic Fe-4.33at.%Ni alloys was systematically investigated.According to JMA solid-state transformation ki-netic theory,the Time-Temperature-Transformation(TTT)curves of the δ/γtrans-formation in peritectic Fe-Ni alloy were calculated.On this basis,the physical cor-relation between the δ/γtransformation and the initial undercooling of melt(T)was elucidated.The results indicate that the change of T can alter not only the overall δ/γtransformation pathways but also the transformation fraction with re-spect to each transformation mechanism.
A large undercooling level up to 550K （0.386Te） was achieved in eutectic Ni78.6Si21.4melt by the combination of molten-glass and cyclic superheating. A microcrystaUine structure is obtained at large undercooling. Surprisingly, the morphology of ct（Ni） phase transits from the non-faceted phaseto faceted phase at large undercooling of 390K. Based on the classical nucleation theory and transient nucleation theory, the process of microstructure evolution and competitive nucleation was analyzed, and the refinement of crystal structure is determined by the high nucleation rate under large undercooling.
采用经典形核理论与负熵模型相结合,研究了多相竞争形核对竞争相的结构和过冷度的依赖性。通过对Fe70Co30合金熔体中的两个竞争相b c c相和f c c相的界面能、体积自由能差和临界形核功的计算,并结合已有的试验结果进行了分析。结果表明,存在两相或多相竞争形核的金属熔体中存在着一个临界温度Tc。当熔体温度T>Tc时,稳定相将优先形核;而当熔体温度T
