PSI - Issue 41

A.M. Ignatova et al. / Procedia Structural Integrity 41 (2022) 527–534 Ignatova A.M., Vereschagin V.I., Naimark O.B./ Structural Integrity Procedia 00 (2019) 000–000

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Fig. 4. Diagrams descriptive of the macrostructure parameters of cast glass-ceramic materials of spinel-pyroxene type depending on the distance from melt/air interface: (а) maximum spinel size, (b) distribution density of spinels, © the average size of spinels. The average and maximum spinel size in the first and second modes differ by nominal values since in the case of lower cooling rate, the average spinel size amounted to 11.45 μm, and in the case of higher cooling rate – to 11.55 μm. The maximum spinel size in the milder mode reached 219.91 μm, and in the more non-equilibrium mode - only 87.88 μm. Maximum values of spinel size in both modes occur in the casting center, in the contactless solidification zone. Each zone corresponds to its own typical spinel size. Distribution density of spinels in the first and second solidification modes is closely related to zoning. On average, the distribution density of spinels in the milder mode amounted to 6.08ꞏ10-4 units/μm2, and in the more non equilibrium mode to 5.09ꞏ10-4 units/ μm2. Building Voronoi diagrams helped identify an additional characteristic feature: large cells formed by the largest spinels, respectively, always border with clusters of small cells formed by the smallest spinels. This pattern does not depend on the conditions of solidification and crystallization or the crystallization rate. The identified special feature signifies that the large spinels act as foci of the structure's microzonation. These observations confirm the statement in [8], suggesting that the process of crystallization and advance of its front starts from solid inclusions and gas pores in the melt (Fig. 5). The authors assume that in the process of crystallization, a specific critical size of spinel is achieved when the spinel acquires its own "zone of influence," that is, a particular area of the melt proportional to the size of the spinel becomes material for the formation of a shell around the spinel by way of epitaxial growth.