The role of critical dimensions of meso-level polycrystals grains during deformation in a weakly stable state of metals and alloys

The method of transmission diffraction electron microscopy showed that in the weakly stable state during deformation at the transition from one stage of strain hardening to another, the grain size of polycrystals can act as a characteristic with critical values. For this purpose, dislocation and dislocation-disclination substructures in tensile-deformed polycrystals of FCC alloys of Cu-Al and Cu-Mn solid solutions with grain sizes of 20-240 mm were studied. The interdependencies characterizing parameters of a defect substructure and the average grain size are established. A comparison is made of these dependences with the structural phase state of the alloys. The analysis indicates that when d ≈ 100 μm is reached, the main role in the accumulation of defects during deformation is played by intragranular processes, and the role of grain boundaries weakens. In fact, there is a change in the mechanisms of deformation, which reflects just the unstable state of the system with an average grain size < d > ≈ 100 μm. In the range of < d > ≈ 100 μm, both mechanisms are realized, and when the grain size exceeds the value of < d > ≈ 100 μm, a significant change in the mechanisms occurs. The behavior of the mechanical characteristics is consistent with this: a kink is observed in the dependence of the strain hardening coefficient on the grain size, which indicates significant changes in the deformation mechanisms of the alloys under consideration. Based on the analysis, the existence of a critical grain size in the mesoscale region d ≈ 100 μm was established. At sizes over 100 μm, intragranular processes play the main role in the accumulation of defects during deformation. The behavior of mechanical characteristics is consistent with this. The strain hardening coefficient in stage II (θ_II) for d > 100 μm is almost independent of grain size. For d < 100 μm, θ_II increases sharply with decreasing grain size.

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