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Register a new userInfluence of microstructural and crystallographic inhomogeneity on tensile anisotropy in thick-section Al-Li-Cu-Mg plates
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Thick-section plates made from a recently developed Al-Cu-Mg-Li alloy have been evaluated to understand the influence of microstructure on the anisotropy of tensile strengths after natural and artificial ageing treatment. Pancake-shaped grains with a coarse substructure and strong crystallographic texture with a beta-fibre orientation at the mid-thickness position are observed. In addition, an inhomogeneous distribution of T1 precipitates through the plate thickness has been revealed with the volume fraction of intragranular precipitates ~ 40% higher at the plate centre than the thickness position. Altogether these microstructural features contribute to the in-plane anisotropy of tensile strengths that is ~ 5% higher at the mid-thickness position than the 1/4 thickness position. The variation of ageing-induced T1 precipitates through the plate thickness further contributes to the through-thickness anisotropy that is ~ 3% higher in T8 temper as compared to T3 temper.
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Intermetallic phases in a recently developed Al-Li-Cu-Mg alloy have been investigated to understand their roles in the initiation and propagation processes of intergranular corrosion. Corrosion initiation involves trenching formation in the Al matrix adjacent to the large particles of Al7Cu2(Fe, Mn) phases. These phases containing Li are electrochemically active and susceptible to self-dissolution via a de-alloying mechanism during corrosion process. The subsurface particles of Al7Cu2(Fe, Mn) and Al20Cu2Mn3 phases act as the internal cathodes for continuous corrosion propagation along the particle-matrix interface and the associated grain boundaries. Corrosion propagation along the particle-matrix interface was facilitated by the anodic dissolution of the surrounding Al matrix due to the micro-galvanic interaction with the cathodic intermetallic phases. In addition, T1 (Al2CuLi) precipitates and the isolated particles of Al7Cu2(Fe, Mn) and Al20Cu2Mn3 phases were dissolved along the path of corrosion propagation. The dissolved metal ions were redeposited through the network of crevice.
In the Al-Co-Cu alloy system, both the decagonal quasicrystal with the space group of $Poverline{10}m2$ and its approximant Al$_{13}$Co$_4$ phase with monoclinic $Cm$ symmetry are present around 20 at.% Co-10 at.% Cu. In this study, we examined the crystallographic features of prepared Al-(30-x) at.% Co-x at.% Cu samples mainly by transmission electron microscopy in order to make clear the crystallographic relation between the decagonal quasicrystal and the monoclinic Al$_{13}$Co$_4$ structure. The results revealed a coexistence state consisting of decagonal quasicrystal and approximant Al$_{13}$Co$_4$ regions in Al-20 at.% Co-10 at.% Cu alloy samples. With the help of the coexistence state, the orientation relationship was established between the monoclinic Al$_{13}$Co$_4$ structure and the decagonal quasicrystal. In the determined relationship, the crystallographic axis in the quasicrystal was found to be parallel to the normal direction of the (010)$_{rm m}$ plane in the Al$_{13}$Co$_4$ structure, where the subscript m denotes the monoclinic system. Based on data obtained experimentally, the state stability of the decagonal quasicrystal was also examined in terms of the Hume-Rothery (HR) mechanism on the basis of the nearly-free-electron approximation. It was found that a model based on the HR mechanism could explain the crystallographic features such as electron diffraction patterns and atomic arrangements found in the decagonal quasicrystal. In other words, the HR mechanism is most likely appropriate for the stability of the decagonal quasicrystal in the Al-Co-Cu alloy system.
An in-depth analysis of Zn/Al doped nickel ferrites grown by reactive magnetron sputtering is relevant due to their promising characteristics for applications in spintronics. The material is insulating and ferromagnetic at room temperature with an additional low magnetic damping. By studying the complex interplay between strain and cation distribution their impact on the magnetic properties, i.e. anisotropy, damping and g-factor is unravelled. In particular, a strong influence of the lattice site occupation of Ni$^{2+}_{text{Td}}$ and cation coordination of Fe$^{2+}_{text{Oh}}$ on the intrinsic damping is found. Furthermore, the critical role of the incorporation of Zn$^{2+}$ and Al$^{3+}$ is evidenced by comparison with a sample of altered composition. Especially, the dopant Zn$^{2+}$ is evidenced as a tuning factor for Ni$^{2+}_{text{Td}}$ and therefore unquenched orbital moments directly controlling the g-factor. A strain-independent reduction of the magnetic anisotropy and damping by adapting the cation distribution is demonstrated.
In this paper, we study the elastic properties of the entropy-stabilized oxide (Mg, Co, Ni, Cu, Zn)O, using experimental and first principles techniques. Our measurements of the indentation modulus on grains with a wide range of crystallographic orientations of the entropy-stabilized oxide revealed a high degree of elastic isotropy at ambient conditions. First principles calculations predict mild elastic anisotropy for the paramagnetic structure, which decreases when the system is considered to be non-magnetic. When the antiferromagnetic state of CoO, CuO and NiO is accounted for in the calculations, a slight increase in the elastic anisotropy is observed, suggesting a coupling between magnetic ordering and the orientation dependent elastic properties. Furthermore, an examination of the local structure reveals that the isotropy is favored through local ionic distortions of Cu and Zn - due to their tendency to form tenorite and wurtzite phases. The relationships between the elastic properties of the multicomponent oxide and those of its constituent binary oxides are reviewed. These insights open up new avenues for controlling isotropy for technological applications through tuning composition and structure in the entropy-stabilized oxide or the high entropy compounds in general.
Dislocation velocities and mobilities are studied by Molecular Dynamics simulations for edge and screw dislocations in pure aluminum and nickel, and edge dislocations in Al-2.5%Mg and Al-5.0%Mg random substitutional alloys using EAM potentials. In the pure materials, the velocities of all dislocations are close to linear with the ratio of (applied stress)/(temperature) at low velocities, consistent with phonon drag models and quantitative agreement with experiment is obtained for the mobility in Al. At higher velocities, different behavior is observed. The edge dislocation velocity remains dependent solely on (applied stress)/(temperature) up to approximately 1.0 MPa/K, and approaches a plateau velocity that is lower than the smallest forbidden speed predicted by continuum models. In contrast, above a velocity around half of the smallest continuum wave speed, the screw dislocation damping has a contribution dependent solely on stress with a functional form close to that predicted by a radiation damping model of Eshelby. At the highest applied stresses, there are several regimes of nearly constant (transonic or supersonic) velocity separated by velocity gaps in the vicinity of forbidden velocities; various modes of dislocation disintegration and destabilization were also encountered in this regime. In the alloy systems, there is a temperature- and concentration-dependent pinning regime where the velocity drops sharply below the pure metal velocity. Above the pinning regime but at moderate stresses, the velocity is again linear in (applied stress)/(temperature) but with a lower mobility than in the pure metal.
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