The dislocation microstructure of aluminum Deformed to Very Large Steady-State Creep Strains

Abstract

Aluminum of 99.999 pct purity was deformed in torsion at 644 K and an equivalent uniaxial strain rate of 5.04 × 10⁻⁴ s⁻¹ to various steady-state strains up to 16.33. The subgrain size and density of dislocations not associated with subgrain boundaries remained fixed throughout the wide steady-state strain range. The subgrain boundaries, however, underwent two important changes. At the onset of steady state (ε ~0.2) all of the subgrain boundaries had relatively small misorientation angles averaging about 0.5 deg. With increased strain, however, an increasing fraction of the subgrain facets were high-angle boundaries. At strains greater than about four nearly a third of the boundaries were high-angle. In specimens with both types of boundaries, the high-angle boundaries have misorientation angles (θ) greater than 10 deg, while θ for low-angle boundaries is nearly always less than 3 deg. Only rarely do subgrain boundaries have misorientation angles between 3 deg and 10 deg. In aluminum, the increased high-angle boundary area at larger strains originates from the extension of the initial boundaries through the mechanism, recently introduced by others, of “geometric dynamic recrystallization” in aluminum. The average misorientation across low-angle boundaries initially increases during steady state but eventually reaches a maximum value of about 1.2 deg at ε ≃ 1.2. Since the flow stress stays nearly constant, the dramatic changes in the character of the subgrain boundaries that are observed during steady state suggest that the details of the boundaries arenot an important consideration in the rate-controlling process for creep.