PSI - Issue 60

Mantosh Mandal et al. / Procedia Structural Integrity 60 (2024) 510–516 Mantosh Mandal/ Structural Integrity Procedia 00 (2019) 000 – 000

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strain gradients around the boundary [10], which have been linked to lattice mismatch and variations in local plasticity [11]. A further linkage could be made to heterogeneities in dynamic recrystallization occurring during hot deformation; the peripheral regions of the hot-compressed cylinders comprise a few abnormally large grains surrounded by fine recrystallized grains, with (Fig. 3 (b)). Consequently, the recrystallized grains serve as local soft regions within the microstructure. This heterogeneity is combined with the tensile stresses experienced by the peripheral regions due to bulging and expansion of the compressed specimen [12]. Notable, tensile stresses are also known to be the primary cause of fir tree cracking during extrusion [7]. It is inferred that the combination of intense tensile stresses at the periphery and the presence of locally soft regions created suitable conditions for crack initiation.

(a)

(b)

Figure 3 : (a) Crack initiation at a grain boundary triple junction (encircled). Arrows indicate direction of slip bands in adjacent grains. (b) Abnormally grown grain surrounded by fine recrystallized grains, indicating heterogeneity in recrystallization.

In addition to local plasticity- linked cracking, the possible roles of elemental segregation and internal oxidation [2 5] were examined using energy-dispersive spectroscopy (EDS).

The local chemical composition at three points (marked in Fig. 4) was mapped: ‘Spectrum 5’, corresponding to a cracked region, ‘Spectrum 6’, corresponding to the interface between cracked and uncracked regions, and ‘Spectrum 7’, corresponding to an uncracked region. The measured chemical compositions are listed in Table 2. Although no evidence for elemental segregation at the cracked regions was found, it was observed that the cracked region was depleted in Mo. Although the mechanism for Mo-depletion remains to be determined, the absence of this element could be another source of weakening due to reduced solid solution strengthening.