Kinetics of ordering and metastable phase of alloys

Jun Ni, Department of Physics, Tsinghua University, China

 

Nonequilibrium kinetics of materials plays an important role in materials growth. In recent years, new experimental methods such as rapid quenching, laser processing, ion beam bombardment and various epitaxial processes have been used to prepare materials that are in nonequilibrium state thermodynamically. In the present work, we study the kinetics of ordering in alloys during growth. Epitaxial growth processes for SiC polytypes in which SiC substrate is employed are studied using a layered growth model. The corresponding phase diagrams of epitaxial growth processes are given. The first-principles calculations are used to determine the parameters in the layered growth model. The layered growth phase diagrams show that when the rearrangement of atoms in one surface Si-C bilayer is allowed, hat the 3C-SiC phase would grow epitaxially in low temperature and 4H-SiC would grow epitaxially in intermediate temperature and 6H-SiC or 15R-SiC would grow epitaxially in higher temperature. For the fcc alloys during the growth of a (001) monolayer, there is a transient ordered state and its occurrence depends on the ratio of the atomic exchange rate to the evaporation rate. For the fcc multilayer alloys during the epitaxial growth, we have found that there is a kinetically induced oscillatory ordered phase in addition to the equilibrium ordered phase in the kinetic phase diagrams. We have calculated the epitaxial growth of CoPt alloy films on the Pt buffer layer. We show the ordered structure of CoPt films changes from the L1₀ [001] variant to the L1₀ [100] variant with the increase of growth rate. This ordering orientation transition also occurs with the decrease of temperature at adequate growth rate. Based on this mechanism, we propose a simple method to synthesize the ordering orientation surperlattices

Keywords: Kinetics, Alloys, Ordering, Epitaxial Growth