In this research, the peculiarities of the magnetic state of austenite of carbon steels were studied. The effect of an external magnetic field on the process of martensitic transformation is discussed. It is shown that there are nanoscale regions with ferromagnetic order in austenite. These regions are possible locations for the nucleation of martensite. In the case of magnetic field influence, the ferromagnetic clusters are able to perceive the energy of the external magnetic field by magnetostrictive stresses (which reduce the energy of martensite formation). Ferromagnetically ordered clusters have a certain critical size, below which the single-domain region becomes unstable. The dependence of the critical size of the ferromagnetic cluster on the intensity of the magnetic field was observed. This dependence shows that the higher the intensity of the field, the smaller the size of the clusters that can remain stable. Thus, a strong magnetic field increases the amount of ferromagnetic nanoclusters (consequently, the number of the nucleation centers of martensite increases). The results of the research allow estimating more accurately the effect of a magnetic field on decreasing the formation energy of a nucleation center.
The relationship between the interaction of ferromagnetically ordered clusters in austenite with dislocations, twinning and nucleation of the martensite phase is considered. It is shown that the regions with short-range order existing in austenite affect the dislocation structure. In turn, dislocations are involved in the formation of twins and martensite nuclei. The imposition of an external magnetic field enhances the magnetic inhomogeneity of austenite and the effects of magnetoelastic interaction between clusters and dislocations.
The thermodynamic conditions corresponding to the instability of the crystal lattice before the martensitic transformation are analyzed. The instability of the crystal lattice before the martensitic transformation can be crucial for the occurrence of such a phenomenon as superplasticity of the martensitic transformation. Phenomena arising in an instability state are considered, a theoretical analysis of possible structural states detected experimentally is carried out. The phenomenon of superplasticity of the martensitic transformation under tension in the Ms
– Md
interval and the effect exerted by the magnetic field in this interval are described. As a result of exposure to an external magnetic field in this temperature range, the resulting magnetostrictive stresses in ferromagnetically ordered austenite nanoclusters under superplastic conditions initiate the formation of martensite.
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