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Register a new userElectron microscopy study of scratch-induced surface microstructures in an Al-Cu-Fe icosahedral quasicrystal
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Microstructure modifications induced by sliding a WC-Co indenter in scratch tests on the surface of a single phase AlCuFe icosahedral quasicrystal (IQC) was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The scratch track was shown tocomprise many smaller tracks. Dislocations were discovered to emerge from the edges of the smaller scratch tracks. Along a small track where shear stress is concentrated, a phase transition from IQC to a body-centered cubic (b.c.c.) phase with lattice parameter a=0.29 nm was pointed out. A modulated quasicrystal state as well as a deformation twin of IQC were determined in the region beneath the scratch.
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Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations of sintered Al-Cu-Fe icosahedral quasicrystal (IQC) have been carried out to understand the origin of some ductility previously noticed within tracks produced by standard tribological scratch tests. Transformation of the icosahedral phase to a modulated structure is shown and a transformation of the IQC to a bcc phase has been found beneath the tracks. Twins and dislocations have also been observed.
Selective laser melting (SLM) is rapidly evolving to become a mainstream technology. However, the fundamental mechanisms of solidification and microstructure development inherent to the non-equilibrium conditions of this additive manufacturing method, which differ largely from those typical of conventional processing techniques, remain widely unknown. In this work, an in-depth characterization of the microstructure of Al7075 SLM processed samples, built from powder mixtures containing ZrH2 microparticles, demonstrates the occurrence of icosahedral quasicrystal-enhanced nucleation during laser fabrication. This solidification mechanism, only observed to date in cast Al-Zn and yellow gold alloys containing minute additions of Cr (Kurtuldu et al., 2013) or Ti (Chen et al. 2018), and Ir (Kurtuldu et al., 2014), is evidenced by the presence of an abnormally high fraction of twin boundaries and of five-fold orientation symmetry between twinned nearest neighbors lying within a matrix of equiaxed, randomly textured, ultrafine grains. This research attests to the wide range of possibilities offered by additive manufacturing methods for the investigation of novel physical metallurgy phenomena as well as for the design of advanced metals.
We report the discovery of a new binary icosahedral phase in a Sc-Zn alloy obtained through solution-growth, producing millimeter-sized, facetted, single grain, quasicrystals that exhibit different growth morphologies, pentagonal dodecahedra and rhombic triacontahedra, under only marginally different growth conditions. These two morphologies manifest different degrees of quasicrystalline order, or phason strain. The discovery of i-Sc$_12$Zn$_88$ suggests that a reexamination of binary phase diagrams at compositions close to crystalline approximant structures may reveal other, new binary quasicrystalline phases.
Understanding the mechanisms which relate properties of liquid and solid phases is crucial for fabricating new advanced solid materials, such as glasses, quasicrystals and high-entropy alloys. Here we address this issue for quasicrystal-forming Al-Cu-Fe alloys which can serve as a model for studying microscopic mechanisms of quasicrystal formation. We study experimentally two structural-sensitive properties of the liquid -- viscosity and undercoolability -- and compare results with textit{ab initio} investigations of short-range order (SRO). We observe that SRO in Al-Cu-Fe melts is polytetrahedral and mainly presented by distorted Kasper polyhedra. However, topologically perfect icosahedra are almost absent an even stoichiometry of icosahedral quasicrystal phase that suggests the topological structure of local polyhedra does not survive upon melting. It is shown that the main features of interatomic interaction in Al-Cu-Fe system, extracted from radial distribution function and bong-angle distribution function, are the same for both liquid and solid states. In particular, the system demonstrates pronounced repulsion between Fe and Cu as well as strong chemical interaction between Fe and Al, which are almost concentration-independent. We argue that SRO and structural-sensitive properties of a melt may serve as useful indicators of solid phase formation. In particular, in the concentration region corresponding to the composition of the icosahedral phase, a change in the chemical short-range order is observed, which leads to minima on the viscosity and udercoolability isotherms and has a noticeable effect on the initial stage of solidification.
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.
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