Grain Growth Kinetic Of Co Nanocrystalline Prepared By High-Energy Ball Milling

It was widely recognized in the fields of physics, chemistry, and material science that metal nanocrystalline materials have advanced properties as compared with conventional polycrystalline materials. The nanograin growth and its control have been hot issues in the field of nanoscience and nanotechnology in the recent twenty years. It is scientifically significant to study the thermal stability and grain growth behavior of nanocrystalline materials. In this paper, we use the Co nanocrystalline powders prepared by the high energy ball-milling as an example to investigate the kinetic characteristics of nanograin growth at different temperature for same time, and different holding time at a certain temperature. Also we study the nanograin growth behaviour containing the second-phase particles.First of all, we established the stable processing of high energy ball-milling for preparing the nanocrystalline Co powders. The nanocrystalline Co powders with mean grain size of 20nm and homogeneous grain size distribution as well as high purity can be prepared through adjusting the processing parameters, such as the ratio of ball to powders, the rotation speed, the milling time, etc. under Ar gas protection.Then the pure Co nanocrystalline powders prepared by high energy ball-milling were held for 1h at different temperatures to investigate the kinetic characteristics of nanograin growth. The results show that there has a sharp increase in nanograin size around the critical temperature of 550℃. By the DSC analysis for the energy state and the HRTEM microstructures at different temperatures, we proposed that the nanograin growth is accelerated by the remaining stored energy as a supplied driving force and through a particular dominant mechanism of nanograin rotations.The pure Co nanocrystalline powders were held at 550℃for different times to investigate the isothermal kinetic characteristics of nanograin growth. It was found that the grain growth rate varies during isothermal holding, which is faster at the beginning period and slows down with increase of holding time. When characterized by the equation of the grain growth kinetics, it was found that the grain growth exponent changes from 1.2 at the early stage to 4.2 at the later stage of the isothermal process. It was proposed that the faster nanograin growth at the early stage is a result of enhanced grain growth promoted by part of the stored energy as an additional driving force, based on which rapid grain growth occurs through a particular dominant...