Quantitative And Visual Simulations And Experiment Studies On Thermal Stability Of Nanocrystalline Metals

Nanocrystalline materials have a series of special features of structures and properties. However, once the nanostructure becomes unstable and the nanograins grow into the sub-micron or micron scale, the nanocrystalline materials will lose their advanced properties. Therefore, the researches on the stability of nanostructures and on the nanograin growth behavior are great significant to reserve the nano-structure and the advanced properties.In this paper, the evolution of nanocrystalline structure and the grain growth kinetics of nanocrystalline metals were investigated using computer simulations and experimental methods.Based on the structure of nanocrystalline materials, atoms of nanocrystalline bulk at the nanograin boundaries and in the grain interior were matched to a great number of atomic-scale cells of Cellular Automaton (CA), by which the simulation area was constructed, and the visual simulation of the nanocrystalline materials was realized. The thermodynamic features of the nanograin boundaries described in our thermodynamic model were introduced into the CA algorithm, thus different cells were given certain thermodynamic energy states. With the hybrid thermodynamic/CA model, the quantitative and visual simulation of nanograin growth was carried out, to study the thermal stability and the microstructure evolution of the nanocrystalline materials.Using the hybrid thermodynamic/CA model, both the isothermal and the temperature-varying nanograin growth behaviors of nanocrystalline Co were simulated. The nanocrystalline microstructure evolution, especially the migration of the nanograin boundary, was directly shown, and the relationship between the atomic fraction of the grain boundaries and the nanograin size was obtained. The simulation results show that the nanograin growth kinetics is different from that of the normal grain growth in conventional polycrystalline materials. The nanograin growth exponent, n, is not a constant as in the polycrystalline metals which equals 2, but varies with the growing process. It was found from the temperature-varying simulations that there is a big difference of grain size increase in the low- and high-temperature regions, and the transition between the two stages is discontinuous. In the low-temperature region, the grain size does not increase significantly. Then, a prominently rapid grain growth follows in the intermediate temperature range. At higher temperature, the grains grow with a relatively slow rate as compared with that in the intermediate temperature range.The Cu nanocrystalline bulk, prepared by the high-energy ball-milling and spark plasma sintering (SPS) technology, was used as an example to investigate the kinetic characteristics of nanograin growth at different temperatures. Based on analyzing the influence of the processing parameters, the optimized parameters of high-energy ball-milling for preparing the nanocrystalline Cu powders with the mean grain size of 30nm was established. Compact Cu nanocrystalline bulk was prepared by SPS technology using the nanocrystalline Cu powders, with the sintering temperature of 350℃and pressure of 157KN (500MPa). It was found from a series of annealing experiments in a wide range from room temperature to 800℃for the Cu nanocrystalline bulk that there is a sharp increase of the grain size in a narrow intermediate temperature range between 300℃and 500℃. As confirmed by the analysis of XRD, SEM and TEM, Tc=400℃was contained as the critical temperature for the destabilization of the Cu nanograin structure. Based on our analytical model that describes the thermodynamic functions of nanograin boundaries, and the analysis on the influence of excess free energy on the thermal stability of nanocrystalline structure, it was confirmed that initial nanograin size and temperature are the dominant factors for the discontinuous nanograin growth.The hybrid thermodynamic/CA model proposed in this paper, can not only show the evolution of nanocrystalline microstructure visually, but also give reliable predictions of the nanograin growth kinetic characteristics. This hybrid model finds a new effective way to study the thermal stability of nanocrystalline materials and nanograin growth. The kinetic characteristics of both the isothermal and the temperature-varying nanograin growth found in this paper have significant meanings for preserving the thermal stability and the advanced properties of the nanocrystalline materials.