The rhombus diagonals flipping was regarded as an elementary act of polymorphic transformation. Some of rhombus vertexes are occupied by atoms of Fe, some are occupied by atoms of Cr. The energy threshold was calculated using the Morse pair potential. The numeric parameters of the function approximating the potential were calibrated according to the experimental values of sublimation energies and the temperature dependencies of Fe and Cr elastic constants. The value of the energy barrier for pure iron and chromium at 1193 K are equal to 150 and 195 kJ/mol respectively. In the case of occupation of the acute rhombus vertex by chromium atom the energy threshold was equal to 222 kJ/mol. But in the case of occupation the obtuse vertex the threshold was only 93 kJ/mol. If two different vertexes are occupied the threshold is equal to 181 kJ/mol. The calculation adequacy was proved by the agreement between the results of the calculations and experiments.