OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 a b c d Fig. 4. Microstructure of sheets after cold rolling with 90 % deformation and subsequent annealing of alloys 1570 (left) and 1570-0.5Hf (right): a) after cold rolling; b) after annealing at 470 °C for 3 hours; c) after annealing at 500 °C for 3 hours; d) after annealing at 530°C for 3 hours It should also be emphasized that recrystallization in hafnium-containing alloys should not be interpreted as an entirely negative process. The occurrence of a super-fi ne as-cast structure and the possibility of modifi cation during recrystallization can create conditions for obtaining grain with suffi cient sizes for superplastic fl ow. This is facilitated by the fact that, according to [32], hafnium additives contribute to the modifi cation of the cast structure in 1570 alloy. In the case of recrystallization after annealing at 530 °C for 3 hours, the grain size is 25 μm. Therefore, by increasing the overall degree of cold rolling in an alloy containing hafnium and introducing several intermediate recrystallization annealing, each of which will cause a refi nement of the structure, it is possible to achieve an average grain size of up to 8 μm. This grain size is suffi cient for superplastic fl ow in aluminum alloys with high magnesium content [38]. Figure 5 depicts the mechanical properties of the alloys after undergoing diff erent treatment modes, as shown in fi gure 4. The yield strength values of the studied alloys are generally similar (fi gure 5, а). In both cases, the yield strength of the alloys dropped from 460 MPa in the cold-rolled state to around 150 MPa after being annealed at the highest temperature of 530 °С and soaked for 3 hours. This decrease is due to the recovery and dislocation annihilation that occur during the low-temperature thermal treatment. Consequently, the strength parameters after annealing at 530 °С and 3 hour soaking are almost the same as the parameters observed in the as-cast state [33]. As the temperature and soaking time increase, the ultimate strength of both alloys changes in a manner similar to the yield strength values (fi gure 5, b). The plasticity of the alloys increases, which is related to a decrease in the number of linear defects and hardening degree (fi gure 5, c). In general, the 15700.5Hf alloy has lower plasticity than the original alloy. This is due to the formation of coarse primary Al3Sc intermetallic compounds caused by the hafnium content in 1570-0.5Hf [32]. Thus, the hafnium content does not have a signifi cant eff ect on either the number of Al3Sc nanoparticles or the increase in strength properties caused by it. Separately, it is worth saying that the past recrystallization does not have a signifi cant eff ect on the strength properties. This is explained by the fact that in 1570 alloy the grain remains deformed even during
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