Thermodynamic Study And Computer Simulation Of Thermal Stability Of Nanocrystalline Alloys

Nanocrystalline alloys have a series of excellent properties comparing withconventional polycrystalline systems. However, the coarsening of the microstructuremay weaken their properties or even leads to the loss of the good performance.Therefore, it’s of great significance to study the thermal stability of nanostructures,which can help to reserve the nanostructures and the advanced properties of thenanocrystalline alloys. In this paper, the thermal stability of the nanocrystalline alloy,as well as its evolution characteristics is studied systematically using thenanothermodynamic calculations and computer simulations.Firstly, the precise quantitative relationship between the excess volume （ΔV） atthe grain boundaries and the grain size d in nanocrystalline alloys was deduced. Basedon the ΔV-d relationship, the nanothermodynamic models for the nanocrystallinealloys were developed. Based on the nanothermodynamic models, variousthermodynamic properties and fundamental thermodynamic functions ofnanocrystalline alloys can be quantified as a function of nanograin size andtemperature. The thermodynamic features of the nanograin boundaries described inour nanothermodynamic models were introduced into the Cellular Automaton （CA）algorithm, thus the two-dimensional nanothermodynamic/CA hybird model wasestablished, by which the quantitative and visual simulation of nanograin growth innanocrystalline alloys can be performed to study the thermal stability ofnanostructures.Applying the nanothermodynamic models, the physical properties parametersand the fundamental thermodynamic functions with variables as nanograin size andtemperature of the nanograin boundaries in single-phase nanocrystalline SmCo₇alloyand SmCo_9.8alloy were calculated. Combining the nanothermodynamic calculationsand the two-dimensional nanothermodynamic/CA hybird model, the thermal stabilityand its evolution characteristics of the single-phase nanocrystalline SmCo₇alloy andSmCo_9.8alloy were studied. It was found that the nanostructure of SmCo₇andSmCo_9.8alloy with grain size below a critical value can have higher thermal stabilitythan the corresponding coarser nanograin structure, and the nanograin growthbehaviors of Sm-Co alloys described by the two approaches agree well with eachother.However when the extra energy is provided, the nanocrystalline alloy maydestabilize, and accompanying with the discontinuous grain growth.The model for the coexisting nanocrystlline and amorphous phases innanocrystalline alloys was established. Using the SmCo₇alloy and SmCo_9.8alloy asexamples, the effect of the amorphous phase on the thermal stability ofnanocrystalline alloys was studied by the nanothermodynamic calculations and the two-dimensional nanothermodynamic/CA hybrid model. Both results show that theexistence of the amorphous phase can significantly influence the nanograin growthbehavior. At a constant temperature, for the nanostructure with grain size below acritical value, amorphous phase can hinder the nanograin growth while for thenanostructure with grain size larger than the critical value, the rapid nanograin growthmaybe initiated by the amorphous phase.In the present paper, the thermodynamic models developed for nanocrystallinealloys can on one hand characterize the thermodynamic properties as a function ofnanograin size and temperature, on the other hand can predict the thermal stability ofnanocrystalline alloys quantitatively. The hybrid nanothermodynamic/CA modelproposed here can not only show the evolution of nanocrystalline structure visuallybut also give reliable predictions on the thermal stability of nanocrystalline alloys.The research on the thermal stability of nanocrystalline alloys by thenanothermodynamic calculations combining with the two-dimensionalnanothermodynamic/CA hybrid model is of great significance to predict and improvethe properties of nanocrystalline materials and develop new-type nanocrystalline alloymaterials.