Ni-based superalloys have been improved during last decades using both new processing technologies and a complex alloying. Ordering is the crucial process in this group of materials, as it controls the development of characteristic two-phase microstructure and subsequently the mechanical properties. Methods of modelling phase equilibria and kinetics of phase transformations are now standard tools used in the process of new alloys design. Various approaches use either phenomenological thermodynamics or computer modelling at the atomic level. In the second group, Monte Carlo approach can be used, often in combination with other well-established methods such as molecular dynamics, static relaxation and ab initio calculations. Monte Carlo simulation is particularly suitable for modelling the details of order/disorder phase transitions. It has also the advantage of direct visualisation of atomic structure in different stages of ordering process.
In this work, ordering processes typical for Ni-base superalloys were studied in a multicomponent system Ni-Al-Cr-Co-W-Re using Monte Carlo method of computer modelling. Pair interactions up to the third coordination sphere were taken into account by means of phenomenological Lennard-Jones potentials. Alternatively, the ab initio calculated energies of tetrahedral clusters were used. Complementary calculations of thermodynamic equilibria were also run using ThermoCalc software. Together with modelling, experiments were carried out by means of analytical scanning and transmission electron microscopy after a long term annealing at high temperatures typical for application of Ni-based superalloys. The microstructure of equilibrium states was characterized as regards the phase constitution and chemical composition of phases. The comparison of results obtained by different methods is given.
The work
was supported by the