Correlation of velocities of the waves сontrolling the thin-plate α-martensite formation and the modulation of the transformation twin structure

Abstract

For the case of γ-α martensitic transformation (MT) in iron alloys, the concept of transformation twins is explained. As a rule, crystals of α-martensite having the form of thin plates are characterized by a fine twin structure (TS) with interchanging orthogonal directions of main compression axes. An example is given of the real structures of transformation twins that are not strictly regular. It is shown that in dynamic theory of MT the regular TS initiation is connected with coherent propagation of long (ℓ) and short (s) displacement waves belonging to controlling wave process (СWP). An analytical approximation of the dispersion law of s-waves is carried out. The threshold conditions of deformation and the qualitative picture of the formation of a modulated ТS are discussed. On the example of Fe-30Ni alloy the correlation of velocities of the waves сontrolling the thin plate α-martensite formation is established. It is shown that at the real correlation of wave velocities the modulated structure of the transformation twins is induced. The similar structure contains fragments each of which is connected with the short-wave excited cell. The fragment size is associated with the number of Nbas layers of the main component inside the ТS fragment generated by a single spontaneously activated s-cell. Fragments’ sizes depend on the site of localization of spontaneously appearing s-cell generating the fragment in the area of the СWP front. It is shown that the Nbas value can vary within rather wide limits. Therefore, in contrast to regular TS forming, we should expect repeated s-cells spontaneous excitements for long enough twinned thin-plated crystals. Along with difference of s- and ℓ-waves velocities, consideration of waves (especially s-waves) decay is one more quite determined factor making its contribution into TS modulation. Estimation of this contribution seems to be very actual.