Investigations Of The Atomic Order And Molar Volume For The Binary Sigma Phase By DFT And CALPHAD Approaches

The sigma phase can serve as a prototype of topologically close-packed(TCP)phases,as the sigma phase bears a broad homogeneity range and there are numerous experimental data available for the sigma phase.In the present work,physical properties,including atomic order,molar volume,enthalpy of formation and bulk modulus,of the binary sigma phase were investigated by using first principles calculations and CALPHAD method combining with the experimental data from the literature.Firstly,we found that the atomic order(i.e.atomic constituent distribution or site occupancy preference on nonequivalent sites of a crystal structure)of the sigma phase is affected by the size factor and electron configuration of the constituent elements.Furthermore,we dissociated the effect of the individual influencing factor on atomic order.Secondly,the atomic order was found affecting physical properties of the sigma phase,such as enthalpy of formation,bulk modulus and molar volume.When in the ordered state at 0 K,the sigma phase bears a low enthalpy of formation and a large bulk modulus.The influence of atomic order on the molar volume of the sigma phase depends on the electron configuration of the two constituent elements.Thirdly,the molar volume database of the binary sigma phase was built up within the CALPHAD framework,which can greatly facilitates material design.Finally,we tentatively discussed the site occupancy prediction of the sigma phase by using the CALPHAD method combined with first-principles calculations.