Monoclinic Al13Co4 Research Articles

Room temperature plasticity in m-Al13Co4 studied by microcompression and high resolution scanning transmission electron microscopy

In several complex crystals, like Al13Co4, metadislocations have been shown to facilitate plastic flow at high temperature. Consistent deformation has been revealed in small scale testing down to room temperature, but until now, no high resolution studies of the underlying dislocation core structures have been conducted to confirm the presence of metadislocations where diffusion slows down towards room temperature. Here, we used microcompression and high resolution TEM to reveal that room temperature plasticity in the monoclinic Al13Co4 phase does indeed occur by motion of metadislocations and that these are closely related to those reported previously in the orthorhombic Al13Co4 phase.

Structural, mechanical and thermal characterization of an Al-Co-Fe-Cr alloy for wear and thermal barrier coating applications

A structural, mechanical and thermal characterization of an Al-Co-Fe-Cr coating alloy was performed. An atomized powder with atomic composition of Al71Co13Fe8Cr8 was thermally sprayed by high velocity oxygen fuel (HVOF) on a steel substrate. X-ray diffraction, scanning and transmission electron microscopy and differential scanning calorimetry were used to characterize the atomized powder and the sprayed coating. Vickers microhardness and pin-on- plate wear test were carried out for mechanical and tribological characterization and an infrared camera was used to evaluate the insulation capacity of the coating material. The results show that both the atomized powder and the coating material were composed predominantly by a quaternary extension of the hexagonal Al5Co2 phase and by the monoclinic Al13Co4. Both phases are quasicrystalline approximants of a decagonal Al-Co quasicrystal. The coating samples presented high values of micro-hardness, close to 500HV and substantially low friction coefficient values, around 0.05. The coatings were good thermal insulators, decreasing by 30% the surface temperature of a sample exposed to a hot plate in comparison with the carbon steel substrate.

Assessment of phase constitution on the Al-rich region of rapidly solidified Al-Co-Fe-Cr alloys

The formation of quasicrystalline approximants in rapidly solidified Al-Co-Fe-Cr alloys was investigated. Alloys of atomic composition Al71Co13Fe8Cr8, Al77Co11Fe6Cr6 and Al76Co19Fe4Cr1 were produced using melt spinning and arc melting methods and their microstructural characterization was carried out by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Up to the present there is no consensus in the literature regarding the formation of quasicrystalline phase or quasicrystalline approximants in the Al71Co13Fe8Cr8 alloy. This work presents, for the first time, a detailed structural characterization of selected alloys in the Al-Co-Fe-Cr system close to the atomic composition Al71Co13Fe8Cr8. The results indicated the samples to be composed, mostly, by two intermetallic phases, which are quaternary extensions of Al5Co2 and Al13Co4 and are quasicrystalline approximants. Although the Al5Co2 phase has already been reported in the Al71Co13Fe8Cr8 alloy, the presence of the monoclinic Al13Co4 is now identified for the first time in the as cast state. In the binary Al-Co system a quasicrystalline phase is known to form in a rapidly solidified alloy with composition close to the monoclinic and orthorhombic Al13Co4 phases. This binary quasicrystalline phase presents an average valence electron per atom (e/a) between 1.7 and 1.9; thus, in addition to the Al71Co13Fe8Cr8 alloy, the compositions Al77Co11Fe6Cr6 and Al76Co19Fe4Cr1 were chosen to be within the region of formation of the quaternary extension of the Al13Co4 phase and also within the (e/a) of 1.7 to 1.9. However, no quasicrystalline phase is present in any of the studied alloys. The Al-Co-Fe-Cr system, around the compositions studied, is composed of quaternary extensions of Al-Co intermetallic phases, which present solubility of Fe and Cr at Co atomic sites.

Formation of a decagonal quasicrystalline film by reactive diffusion in Al/Co multilayers

The formation of a decagonal Al-Co quasicrystalline film by reactive diffusion in a multilayer material and its thermal stability are studied. From experimental results obtained by differential scanning calorimetry, in situ ramped resistance measurements, X-ray diffraction and transmission electron microscopy, it is shown that Al/Co multilayers corresponding to an Al/Co ratio of 13/4 transform through heat treatment into a single-phase decagonal structure. Two reaction steps, which correspond to the transformation of the film into a monoclinic structure of the Al9 Co2 type, precede the formation of the quasicrystalline state observed at about 500oC. Once formed, the quasicrystalline film is stable upon annealing from room temperature up to 710oC. Above this temperature, it transforms into periodic structures of the monoclinic Al13 Co4 and orthorhombic Al3 Co (or Al11 Co4 ) types.

Structural models of τ 2-inflated monoclinic and orthorhombic Al-Co phases

It is generally believed that quasicrystals and their approximants have almost the same local structures, arranged aperiodically in quasicrystals and periodically in approximants. In the Al-Co binary system, besides monoclinic Al13Co4[1] and orthorhombic Al13Co4[2] (also called orthorhombic Al3Co[3]), phases determined by X-ray single crystal diffraction earlier, a series of τ-inflated monoclinic and orthorhombic Al-Co decagonal approximants, such as τ 2-, τ 3-, τ 4-A113Co4 and τ 2-,τ 3-,τ 4-Al3Co4, were found recently[4]. These phases all have a composition around Al3Co, but the lattice parameters a and c enlarged by about τ 2, τ 3,and τ 4 times than those of monoclinic A113Co4 and orthorhombic Al3Co, respectively. Their electron diffraction patterns (EDP), at least the strong spots, resemble the corresponding EDPs of the Al-Co decagonal quasicrystal (DQC). In theory, if such a τ-inflation continues infinitely, the unit cell will become infinitely large. A DQC with a layer structure of quasiperiodically arranged pentagons will thus form. In other words, the τ-inflated Al-Co approximants phases show more resemblance to a DQC.

Multiple twins of τ2-Al13Co4 showing fivefold symmetry

The presence of a monoclinic Τ2-A113Co4 phase, whosea andc are about Τ2 = 2.618 (Τ = $$(1 + \sqrt 5 )/2)$$ times greater than the respective parameters of the monoclinic Al13Co4, and its (100) and (001) twins has been reported earlier. Like its prototype structure, the monoclinic Al13Fe4/Al13Co4, this Τ2-A113Co4 also shows a tendency to form multiple twins displaying five-fold symmetry. A transmission electron microscopic study of the twin mechanisms has been carried out. In addition to the well-known (001) and (100) glide twins, (20-1) glide twins have also been found. Alternate (100)-(20-l) or (100)-(001)-(100) twinning will yield twins display-ing fivefold symmetry. The distribution of Co atoms in the pentagonal layer in Τ2-A113Co4 and the tessellation of pentagons in the (100) and (20-1) glide twins have also been studied by high-resolution electron microscopy.

Mechanical alloying of aluminium and Al13Co4 to an amorphous phase

An amorphous phase with blend composition of Al85Co15 was fabricated by mechanical alloying (MA) from the mixture of pure Al and monoclinic Al13Co4, which cannot be obtained by MA for the Al and Co elemental powders. Use of the MA from the compound and element is a result of the enhancement of efficiency during milling by leaving out the welding and supressing the agglomeration of the powders in the process. Mechanism of the amorphization is discussed together with the mechanical grinding of the compound. We assert that the MA between the Al and compound is an available process to produce amorphous phases in Al-enriched (Al > 85 at.%) alloys.

Crystallographic characteristics of the Al-Co decagonal quasicrystal and its monoclinic approximant τ2-AI13Co4

The binary Al-Co decagonal quasicrystal (DQC) is stable between 800 °C and 900 °C and transforms to a primitive monoclinic phase (a = 3.984 nm,b = 0.8148 nm,c = 3.223 nm, and β = 107.97 deg) above 950 °C or below 750 °C. The composition of this crystalline approximant is similar to theC-centered monoclinic Al13Co4 but witha andc parameters τ2 (τ = (1 + √5)/2 = 1.61803. . .) times larger. This τ2-A113Co4 occurs frequently as (001) and (100) twins. The transformation of the Al-Co DQC to (100) twins of τ2-A113Co4 has been followed by selected-area electron diffraction. With reference to the main zonal electron diffraction patterns and the X-ray diffractogram of the Al-Co DQC, the X-ray powder diffractogram of τ2-A113Co4 with a giant unit cell can be successfully indexed.