PSI - Issue 69

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

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in the length direction can vary from 100 mm/sec to < 0.001 mm/sec. If an obstacle such as an inclusion is encountered, the growth stops and then resumes after some time. In the region of the obstacle, lateral growth does not continue, and the needle remains narrow. In Fig. 1(b), the needle boundaries are classified as types I, II and III. Interference microscope measurements in combination with observations of the deflection of scratches showed that the shear shape deformation angle in the immediate region of boundary I is approximately 8.5º. The unilateral lateral growth [Klostermann (1968)] can be explained by the larger specific volume of the α-phase. This volume change means the transformation is facilitated by hydrostatic tensile stress while being inhibited by hydrostatic compression. Fig. 2(a) schematically illustrates the shape deformation as simple shear, showing both the deformation direction on the assumed habit plane and the region where tensile hydrostatic stress components occur. For the case of a very narrow needle formation, expansion occurs exclusively in the direction where shape deformation generates hydrostatic tensile stress (Fig. 2b).

Fig. 2. Scheme of one-sided widening of the trace of the SM crystal: (a) – general view; (b) – illustration of the role of hydrostatic tensile stresses (Klostermann (1968)). Figure 3(a) shows a series of observed joints of SM crystals. To decipher the joint variants, Table 1 shows the correspondences between the short notation α i (i = 0, 1, 2… 11) of habit orientations and their notation in the form {hhl}. It is evident from Figure 3(a, b) that stacks of crystals of the same orientation with layers of residual austenite between adjacent crystals are formed.

Fig. 3. Traces of SM crystals of different orientations: (a) – on the surface of the sample ( 1" 2 1) (Klostermann (1968)); (b) – results of EBSD measurements for surface martensite (Baur et al. (2017)).