Correlation of velocities of the waves controlling 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 developed. 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 of the real structures of transformation twins that are not strictly regular is given. It is shown that in dynamic theory of MT the regular TS initiation is associated with coherent propagation of long (ℓ) and short (s) displacement waves belonging to controlling wave process (CWP). An analytical approximation of the dispersion law of s-waves is obtained. The threshold conditions of deformation and the qualitative picture of modulated TS formation are discussed. The correlation of velocities of the waves controlling thin-plate α-martensite crystals formation is established by the example of Fe-30Ni alloy. It is shown that at the real correlation of wave velocities a modulated structure of transformation twins is induced. Such structure contains fragments each of which is connected with the short-wave excited cell. The fragment size is associated with Nbas — the number of layers of the main component inside the TS 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 CWP 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. © 2017, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved.