Study On Prepartion, Thermodynamic Properties And Phase Stability Of Metal Nanocrystalline Materials

Nanocrystalline materials, as a rapidly developed new-type materials, have a broad field of potential applications in the aspects of space navigation, electronic, metallurgy, chemical, biological and iatrology fields, etc. Due to the ultrafine grains, there are a large number of interfaces which consist of grain boundaries or phase boundaries in nanocrystalline materials. Compared to the conventional coarse-grained polycrystals, nanocrystalline materials have unusually advanced physical and mechanical properties. In my research work firstly, a calculation model of the thermodynamic functions for the whole nanocrystals was developed and the fundamental thermodynamic functions of the metal nanocrystals based on the "dilated crystal model"and universal equation of state (EOS) were derived. It was confirmed that the formation temperature of stable phases depends on the grain size of nanocrystals. The thermodynamic and critical size conditions of phase transformation were predicted. Secondly, Co nanocrystalline powders by high energy ball-milling have been obtained. The Co nanocrystalline powders with the mean grain size of 40nm were prepared with the processing of 10 to 1 ball to powder weight ratio and dry-milling 30-34h in Ar gas. The shape, sub-structure and grains size of nanocrystals were observed and analysised. It is proved that Co nanocrystalline is obtained through the observation by XRD, SEM and TEM, etc. Finally, the Co nanocrystalline powders with the mean grain size of 39nm were annealed at different temperatures. The phases content and grain sizes were measured. It is identified that the predicted phase transformation tendency and the range of phase transformation temperature by the model agree with the experimental results. In addition, based on the measured thermal capacity Cp, the functions of enthalpy, entropy and the Gibbs free energy of the Co nanocrystalline powders are calculated. The changing tendency and the order of magnitude of thermodynamic parameters agree with the thermodynamic function curves calculated by the present model. Therefore, it is further proved that our model is reasonable. In the present paper, the model developed for thermodynamic functions of the...