PSI - Issue 40

Oleg N. Komarov et al. / Procedia Structural Integrity 40 (2022) 231–238 Oleg N. Komarov at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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a c Fig. 2. The microstructures of the samples (magnification ×1,000) obtained from the thermites with the following content of reduct ant / scheelite: a — 22% of the reductant and 10% of scheelite concentrate; b — 23% of the reductant and no scheelite concentrate; c — 23% of the reductant and 20% of scheelite concentrate. The composition of the thermites, as well as temperature conditions of crystallization of the melt and cooling of the metal determine the mechanical properties of the latter in many ways. The experiments have shown the instability of the ultimate tensile strength and percentage elongation values due to high internal stresses in the metal. The relevant dependence can be predicted for alloys produced by aluminothermic remelting of thermites with the reductant content of 22 – 23%. The results of the mechanical testing of the samples obtained from these alloys are provided in Fig. 2. Analysis of Fig. 3 shows that in case of the samples obtained from the thermites with the reductant content of 22%, the highest ultimate tensile strength value σ w = 839 MPa is observed in the samples corresponding to the scheel ite concentrate content of 10%. The average percentage elongation value for such samples is δ =3.59%. Low ductility of the material of such samples is definitely due to the presence of tungsten in the amount of 8.33% in them. Further increase in the scheelite concentrate content in the thermites leads to a significant increase in their brittleness, which is confirmed by the low values of σ w and δ. In this regard, the practical use of such materials does not seem appropriate. The pattern of the decrease in the ultimate tensile strength values has been identified for the samples obtained from the thermites with the reductant content of 23% (Fig. 3, b). It can be seen that the samples obtained from the thermite mixtures containing no scheelite filler have the highest strength σ w = 877 MPa at the average percentage elongation values δ = 5.34%, and, accordingly, the samples corresponding to the scheelite con centrate content of 20% feature the minimum strength σ w = 268 MPa. Combination of a number of the aluminothermic process parameters (temperature, chemical composition of the components to be reacted, and interaction between the phases formed) has resulted in a sharp increase in the percentage elongation values of up to δ =4.98%; such values are characteristic of the samples obtained from the thermites with the scheelite concentrate content of 15%. The low ductility of the study sample material is definitely due to the high internal stresses in the material structure which is determined by the temperature gradient occurring when the melt is cast into the mold. This result is largely due to the high cooling rate of the melt produced by alumothermy, the temperature of which can reach 2500 - 2700°C. b