Bimodal microstructure is formed in high-T c superconducting ceramics YBa 2 Cu 3 O 7−x deformed by torsion under pressure at T d = 1008°C. The appearance of second peak on the grain length distribution is associated with formation of abnormally coarse grains with length up to l = 300-500 μm. Cooling rate has almost no effect on the density of abnormal grains n and l, indicating that such grains are formed during deformation. The l value increases monotonically with increasing twist angle α, while n increases only up to α = 25°, and then drops. The value of α above which n decreases sharply corresponds to the formation of strong texture with a factor F ≥ 0.95. Formation of abnormal grains can be explained by strain-enhanced grain growth during deformation. Intergranular sliding produces damage zones, the healing of which takes place due to the local migration of grain boundaries. Apparently, abnormal grains appear in places of most intense intergranular sliding. As the texture increases the degree of freedom of movement of grains decreases, so the possibility of accommodation of sliding of neighboring grains decreases. This leads to increase in the number of damage zones and strain value in them that is accompanied by the growth of n and l. After it reaches a certain critical level of texture (F ≈ 0.95 at α ≥ 25°) the possibility of stress relaxation due to local migration of initial grains is exhausted, and dynamic recrystallization of abnormal grains begins. Therefore, when α ≥ 25° the n value is decreased sharply.
The paper reports the results of studies on the effect of the tool pin length on the microstructure and hardness of 2024 aluminum alloy sheets with a thickness of 3.0 mm during friction stir processing (FSP), as well as computer simulation data. The alloy in the initial state contains particles of phases θ (Al 2 Cu), S (Al 2 CuMg), and phases of complex composition (AlCuMnSiFe) with sizes ranging from 0.5 -3 to several tens of microns. There is also a small amount of silicon-rich particles with sizes of 2 -3 μm. The phases (AlCuMnSiFe) in their initial state often form skeletal precipitates up to 30 -40 μm in size. Such precipitates consist of three phases differing in the ratio of elements. FSP of the material led to an increase in microhardness in the center of the treated zone and the advancing side from 53 ± 4 HV to 110 ± 8 HV, while the highest microhardness values practically do not depend on the pin length. The distribution of microhardness and microstructure in the processing area is uneven and depends on the length of the tool pin. The highest hardness of the central FSP zone is provided by a pin 2.0 mm long. It has been established that during the FSP of the alloy, coarse particles of intermetallic phases are refined and the products of refining in the FSP zone are distributed. The particles of the θ and S phases are refined weakly, while the others are refined to submicron sizes. To substantiate the choice of the effective pin length, a three-dimensional finite element modeling of the FSP in the DEFORM-3D software environment was performed. Distributions of effective deformation, temperature fields, and displacement of material points in the zone of thermomechanical action are analyzed. The simulation results agree with data of the physical experiment in that the most preferable pin under the considered processing conditions is a pin with a length of 2.0 mm since it allows one to obtain the widest and most symmetrical regions of intense heating and deformation.
The plate of eutectic Al-Si alloy (AK12D) with a thickness of about 5 mm was subjected to friction stir processing (FSP). Samples for the study were cut perpendicular to the welding direction. Next, the samples were annealed for 1 h in the temperature range 150 -500°C, quenched in water, and subjected to natural aging for 3 months. Thermal stability of the microstructure in various zones after FSP and subsequent annealing was studied by scanning electron microscopy. The volume fractions of dendrites and thickness of the eutectic interlayers were calculated depending on the annealing temperature. It was found that in none of the zones did annealing up to Т = 500°C lead to a noticeable change in the microstructure of the alloy. It has been established that the temperature dependences of the microhardness of the stir zone and base material are different: for the base material it is of a "V-shaped" form with a minimum of 85 HV after annealing at T = 350°C, while for the stir zone it remains constant in the range of 95 -98 HV after annealing up to 350 -400°C, then increases and practically coincides with the values of the microhardness of the base material. The microhardness behavior of the stir zone is explained by the fact that the effect of FSP is similar to the effect of incomplete quenching.
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