Why does the martensitic transformation temperature strongly depend on composition?

The reason for the strong composition and heat-treatment dependence of the martensitic transformation temperature was investigated by a simple Landau-type model. Assuming the anharmonic and coupling coefficients are insensitive to composition, we obtained an important result: martensitic transformation occurs at a critical elastic constant c′ and a critical TA₂ phonon energy ω-η$/², which are independent of alloy composition. This result gained support from a large body of experimental data of Cu-based alloys. Since c′ and phonon energy are strongly dependent on composition, the constancy of c′ at Ms demands that the (transformation) temperature must exhibit an opposite effect to compensate the composition effect. Therefore, the lower the c′, the higher the Ms is. Because the temperature dependence of c′ is weak (due to the 1^st order nature of the transformation), the big c′ change by a slight composition change must be compensated by a large change in temperature. Thus Ms has strong composition dependence. The effect of quench is to increase point defects, being equivalent to a composition change, thus has a strong effect on Ms. From the present study, we can conclude that the strong composition dependence of Ms is mainly a harmonic effect.