  Diagonal flipping of a rhombus consisting of two triangles sharing a common
edge with atoms (ions) in vertices of the triangles is considered as an
elementary act of the polymorphic transformation in metals. The estimation of
the energy threshold for the diagonal flipping has been carried out for various
combination of rhombus vertices occupation by Fe, Cr, and Mn atoms. The energy
threshold has been calculated in the framework of Morse interatomic pair
potential. Numerical coefficients for the approximation of the pair potential
function have been scaled by experimental values of the sublimation energy and
temperature dependencies of elastic constants for Fe, Cr, Mn. Values of the
energy threshold at 1193 K were estimated equal to 195, 150 and 94 kJ/mole for
pure Cr, Fe and Mn respectively, i.e. in the same sequence as values of elastic
bulk modulus for these metals. The substitution of one Fe atom by Cr or Mn atom
results in alteration of the energy threshold. This alteration is dependent on
the angle type at this vertex (an acute or obtuse angle), as well as on the
ratio of bulk modulus values of iron and alloying element.
