PSI - Issue 69

Nadezhda M. Kashchenko et al. / Procedia Structural Integrity 69 (2025) 89–96

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Table 1 Correspondence between the notations α i and {hhl} α 0 α 1 α 2 α 3 α 4 α 5 α 11 (121) ( 2" 11) ( 2" 11) (12 1" ) ( 1"1" 2) ( 1" 12) ( 1" 2 1" ) ( 1" 1 2" ) ( 2" 1 1" ) (211) (112) ( 1" 21) Previous studies [Kurdyumov et al. (1977); Georgieva et al. (1970)] have reported that surface martensite (SM) crystals in polycrystalline samples exhibit several distinctive characteristics. Firstly, there is an orientation of habit planes that is close, according to [Kurdyumov et al. (1977)], to {12 14 17}. Secondly, there is a tendency to form a "roof-shaped" relief from pairs of articulated SM crystals (see Fig. 4(a)). Such pairs most often arise along the boundaries of annealing twins. α 6 α 7 α 8 α 9 α 10

Fig. 4. Surface martensite (polycrystal): (a) – “roof-shaped” joint in Fe-29.3 Ni-0.02 C alloy [Georgieva et al. (1970)]; (b) – electron microscope image of SM crystal; M 1 , M 2 , M 3 – parallel crystals [Georgieva et al. (1970)]. Thirdly, when observed in an optical microscope, the thickening of the crystal (after a long holding period of 2-3 months) could be interpreted as the result of an exceptionally slow continuous process. However, electron microscopic studies (see Fig. 4(b)) demonstrate the actual state of affairs: the presence of thin parallel plates separated by layers of distorted austenite. Adjacent martensite crystals, as a rule, have a mutual twin orientation. Fourthly, curved bands are found in SM crystals, which are a dense accumulation of dislocations (Fig. 13). Usually, these bands run across the crystals and are apparently responsible for the transverse microrelief on the needles of surface martensite. Another feature is irregularly shaped microtwins (decoherent twins). Diffraction analysis showed that the twins belong to the {112} <111> system, which is typical for twins in the bcc lattice. This dislocation and twin structure of surface martensite crystals differs from the internal structure of the well-studied {259} martensite, in which coherent twins form a midrib and screw dislocations are grouped into regular systems. As emphasized in [Klostermann (1968)], the observed features of the formation of SM crystals with habits {112} do not fit into the framework of the crystallogeometric approach [Wechsler et al. (1953)]. Crystals with habits {112} do not satisfy the postulate of the habit as a macroscopic invariant plane. We note immediately that such questions are naturally resolved within the framework of the dynamic theory of MT. The aim of this work is to show that the main noted features of the formation of surface martensite receive a natural interpretation within the framework of the dynamic theory of martensitic transformations.