Intensive Plastic Deformation Influence on Phase Relations of Cobalt Nanocrystals

The choice of cobalt as an object of study is due to the fact that it is characterized by a low temperature of polymorphic transformation. This makes it possible to use it as a model material for studying the effect of crystallite sizes on the nature of these transitions and phase composition. Nanostructured samples of cobalt were obtained by the method of intense plastic deformation by torsion. High quasi-hydrostatic pressure in the working area up to 8 GPa was created on a Bridgman anvil-type installation. Anvils were made of tungsten carbide. The method allows to obtain samples of high purity without pores and impurities. X-ray diffraction studies were carried out on a DRON-7 diffractometer with cobalt radiation. X-ray analysis was performed according to the Bragg-Brentano method. To conduct phase analysis, Kα lines were used at a scan step of 0.04 degrees, and when analyzing the profile of diffraction lines and determining their width – at 0.01 degrees. The absolute error in measuring the angular positions of diffraction maxima did not exceed ± 0.020. The size of cobalt nanocrystallites reached about 25-50 nm. It is established that during low-temperature annealing recovery occurs in the structure of cobalt. Annealing above 300 °C leads to the recrystallization of its structure. The nanocrystalline cobalt obtained by intense plastic torsion deformation after heating above the phase transition temperature and cooling below this temperature retains the high-temperature fcc structure. It is shown that the nature of the fcc-hcp transition delay can be associated with changes in the size and strained state of crystallites forming nanocrystalline cobalt. All of the above confirms the relevance of our research.