PSI - Issue 69

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

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restructuring of the initial structure (austenite, g -phase) from face-centered cubic (FCC) lattice into the final (martensite, a -phase) with a body-centered cubic (BCC) or tetragonal (BCT) lattice. The transformation has clearly expressed signs of a first-order phase transition (change in specific volume δ~1%, temperature hysteresis up to hundreds of degrees). Moreover, despite significant supercooling below the phase equilibrium temperature T 0 , austenite is metastably stable at the temperature M s of the onset of transformation during cooling of austenite. Martensite crystals are characterized by a set of interrelated macroscopic morphological features (orientations of habit planes, interphase orientation relationships, magnitude and direction of macroshear). It is appropriate to note that iron-nickel alloys demonstrate the presence of several morphotypes with a change in nickel concentration, and M s decreases with an increase in nickel content. At a Ni content of up to 29%, packet martensite with habits close to the {557} family is observed. At 30-31% nickel, lenticular martensite is observed, the central part of which (midrib) has the shape of a plate defining habit planes close to {3 10 15}. And finally, at 32 - 35% Ni, thin-plate martensite is observed, which, like the midrib of lenticular crystals, is completely twinned; less often - "single-crystal", that is, instead of the second component of the twin structure, it contains transformation dislocations [Kashchenko et al. (2020)]. It should be emphasized that subsequent growth of the formed crystals is not typical for α-martensite in iron alloys. The increment of the martensite phase during cooling occurs due to newly formed crystals. Note that lateral growth is not typical for α-martensite crystals. In particular, spontaneous lateral growth is not observed for such crystals. However, along with the transformation in the volumes of large grains or single crystals, the formation of α-martensite crystals is detected in the near-surface regions. As a rule, surface martensite (SM) is formed in alloys with a composition close to Fe-30%-Ni, spontaneously after mechanical polishing with sandpaper and subsequent electropolishing. Moreover, the M s temperature for SM crystals is 30 - 50º higher than the MT start temperature in the grain volume. It is typical [Klostermann (1968)] that in the surface layers of single crystals of iron-nickel alloys, the orientation of the SM crystal habits is close to {112} g . It is possible that the orientation recorded in [Thome et al. (2022, 2023)] is due to the formation of SM crystals. The cross-section of SM crystals by the surface has the form of "needles", therefore, in metallographic descriptions, the term "needle" is often used. Referring to the original description of the morphology of surface martensite crystals, we, following [Klostermann (1968)], use this term without quotation marks. Figure 1(a) shows a surface martensite needle grown on a sample with the {100} orientation. Careful examination of this microstructural image reveals several noteworthy morphological characteristics.

Fig. 1. Forms of traces of SM crystals: (a) – real traces on the surface (100); (b) – trace diagrams [Klostermann (1968)]. Figure 1(b) shows that the growth of the length of the crystal trace on the sample surface occurs in one direction. There is also a broadening of the trace (in the direction perpendicular to the length of the trace). According to [Klostermann (1968)], the growth in length occurs much faster than the growth in the transverse direction. The growth