E.A. Starke

Microstructural and mechanical characterization of a superplastic 6xxx aluminum alloy

Abstract A new thermomechanical process has been developed which produced a fine grain structure in an Al–Mg–Si–Cu alloy. The alloy under investigation falls within the composition limits of both 6013 and 6111. The refined microstructure has an average grain diameter of approximately 10 μm and an average aspect ratio near 1.6. Superplasticity was investigated using ambient pressure, uniaxial tensile tests and cone tests with backpressure. The refined material exhibits superplasticity above 500°C. Uniaxial tests indicated a strain rate sensitivity of 0.5 at 540°C, where the elongation reached 375% for a flow stress of 680 psi (4.7 MPa). Cone tests revealed excellent overall formability, and the suppression of cavitation with backpressure. The effects of strain on grain size and porosity were determined.

Effects of preferentially aligned precipitates on plastic anisotropy in Al-Cu-Mg-Ag and Al-Cu alloys

Abstract The influence of a preferential alignment of plate shaped precipitates on the yield strength anisotropy in aluminum alloys was investigated. Stress-aging in tension, i.e. externally applied tensile stresses during aging, was utilized to produce preferential nucleation of precipitates on those 100 and 111 habit plane variants that formed the smallest angle with the load. In-plane yield anisotropy was investigated in tension for various heat treatment conditions. The data was evaluated using the Taylor/ Bishop-Hill model for texture-induced anisotropy as well as the plastic and elastic inclusion models proposed by W.F. Hosford, R.H. Zeisloft, Metall. Trans 3 (1972) 113–121 and P. Bate, W.T. Roberts, D.V. Wilson, Acta Metall. 29 (1981) 1797–1262; 30 (1982) 725–737, which incorporate anisotropic particle strengthening. In a cube textured Al-Cu alloy containing θ′ on only two out of the three possible 100 variants the maximum deviations in yield strength reached 14% when compared to conventionally aged material. In an Al-Cu-Mg-Ag alloy containing the Ω phase on 111 and having a strong brass type deformation texture, less pronounced effects were found after partial removal of one of the Ω variants. Qualitative predictions of the plastic and elastic inclusion models were in good agreement with the findings for anisotropic particle strengthening by randomly distributed precipitates. Effects of aligned precipitates coincided somewhat better with predicted trends by the elastic inclusion model, however, additional verification for either model is required. Stress-aging provides a tool to control anisotropy in high strength aluminum alloys.

Particle-stimulated nucleation of recrystallization for grain-size control and superplasticity in an Al-Mg-Si-Cu alloy

Abstract Grain refinement of 6xxx aluminum alloys for superplasticity through particle-stimulated nucleation of recrystallization (PSN) has been a difficult task in the past due to the inhomogeneous nature of the precipitate distributions produced by traditional overaging heat treatment methods. Stretching prior to aging does not alleviate the problem. A new approach has been developed, wherein the interfaces of deformation bands caused by severe rolling were exploited as heterogeneous nucleation sites for precipitates in an Al–Mg–Si–Cu alloy. (US and International patents are pending.) This approach, combined with a two-step low–high heat treatment resulted in a homogeneous distribution of globular precipitates near 1 μm in diameter. In contrast to the globular shape, the plate-shape morphology was observed in the absence of pre-age deformation. Subsequent rolling and recrystallization resulted in a fine, uniform, equiaxed grain structure with an average grain diameter of 10 μm. The grain structure was weakly textured, statically stable, and superplastic above 500°C. A maximum strain rate sensitivity of 0.5 was achieved, with a corresponding maximum elongation of 375%.

Predicting slip behavior in alloys containing shearable and strong particles

Deformation behaviour is predicted by combining slip intensity calculations for shearable precipitates (δ′) with critical particle size (for dislocation shearing-to-looping transition) estimates for strong precipitates (T1, S′). The deformation behavior of the microstructures correlates fairly well with the predictions. Calculations imply that significant volume fractions of shearable phases affect the shearability of strong precipitates. Although specific examples are drawn from AlLiCuMg alloys, the general theory is applicable to most precipitation-hardened alloys. The predictions of the mechanical behavior made for the various microstructures correlate well with the available data.

Predicting plane strain fracture toughness of AlLiCuMg alloys

Abstract The microstructural features that determine fracture, toughness, and the mathematical models that relate those features to the plane strain fracture toughness, K IC , for aluminium alloys are described. Particular attention is paid to the new class of AlLiCuMg alloys. Models that relate fracture toughness to strain localization in the matrix and precipitation-free zones (PFZs) are evaluated based on experimental results of 8090 fracture behavior as a function of aging treatment. Good correlation was observed between experimental fracture toughness measurements and values predicted by the models. When verified, these models aid in identifying microstructural features that control fracture toughness and help in developing methods to improve this mechanical property.