B. Grushko

The constitution of aluminum-cobalt alloys between Al5Co2 and Al9Co2

Abstract A part of the equilibrium AlCo phase diagram was revised. The study revealed four stable intermetallic phases in the range between the Al 5 Co 2 and Al 9 Co 2 phases. Three related phases form close to Al 13 Co 4 , two of them associated with the known monoclinic and orthorhombic Al 13 Co 4 structures. The existence of a phase close to Al 3 Co was confirmed and it was associated with the so-called τ 2 -Al 13 Co 4 structure reported by Ma and Kuo ( Metal. Trans., 23A (1992) 1121). In addition, metastable structures were observed in as-cast alloys.

Undercooling and solidification behaviour of melts of the quasicrystal-forming alloysAl–Cu–Fe and Al–Cu–Co

Abstract Al–Cu–Fe, Al–Fe and Al–Cu–Co melts of different compositions were undercooled by containerless processing in an electromagnetic levitation facility. The phase selection during solidification from the undercooled melt was determined by direct measurements of the temperature changes during recalescence. Complementarily, the phase selection and microstructure development was studied by scanning- and transmission electron microscopy (SEM, TEM) and X-ray diffraction (XRD) on the as-solidified samples with the undercooling and the alloy composition as experimental parameters. For comparison, rapidly quenched samples of the same alloys were produced by splat-cooling and investigated by TEM and XRD. The undercooling results were analyzed within the framework of classical nucleation theory. The activation threshold for the nucleation was found to be small for the icosahedral quasicrystalline phase in Al–Cu–Fe, medium for the decagonal D-phase in Al–Cu–Co and crystalline phases with polytetrahedral symmetry elements (Al13Fe4 and Al5Fe2), but large for the cubic phase of Al50(CuCo)50 with non-polytetrahedral crystalline symmetry. These results are explained assuming of an icosahedral short-range order that prevails in the undercooled melt and gives rise to an interfacial energy decreasing with increasing degree of polytetrahedral order in the solid nucleus.

Transition between Periodic and Quasiperiodic Structures in Al-Ni-Co

Abstract A series of Al–Ni–Co alloys forming stable decagonal (D-ANC) quasicrystals was studied in as-cast and annealed states. It was shown that under certain conditions periodic structures with pseudodecagonal (PD) symmetry can be produced at the same compositions as stable decagonal quasicrystals. Different variants of D-ANC and PD were observed in a compositional range of 70–72.5 at.% Al and 13–18 at.% Co. As-cast D-ANC can be transformed to single-phase PD of the same local composition. Single-phase PDs can be transformed to D-ANC of the same composition by heating to a temperature higher than the formation temperature of these PDs. The transition between PD and D-ANC was studied in more detail in Al 7 1 Ni 14.5 Co 14.5 and Al 70 Ni 1 5 Co 1 5 by electron microscopy, powder X-ray diffractometry and differential thermal analysis. The results of this study do not confirm the thermodynamic stability of this PD structure.

Plastic deformation of decagonal Al± Ni± Co quasicrystals

Plastic deformation experiments have been performed on Czochralski-grown decagonal Al-Ni-Co single-quasicrystals at temperatures between 780 and 860°C. Compression tests at a strain rate of 10-5s-1 with different orientations of the compression axis relative to the tenfold quasicrystal direction show an anisotropy of the plastic behaviour. If the compression axis is oriented parallel to the tenfold direction multiple slip and weak work hardening is observed. If the compression axis is tilted by 45°, single-slip conditions and deformation softening are found. Microstructural investigation by transmission electron microscopy indicates that plastic deformation is mediated by a dislocation mechanism. The results are interpreted in terms of a model in which the chemically ordered columnar clusters of the decagonal structure represent rate-controlling obstacles.

Solidification of Al–Cu–Fe alloys forming icosahedral phase

The solidification of a series of Al–Cu–Fe alloys containing an icosahedral phase has been studied by scanning electron microscopy, energy-dispersive x-ray spectroscopy, powder x-ray diffraction, and differential thermal analysis. The constitution and morphology of alloys solidified at different rates are presented. The overall compositional range of the icosahedral phase was determined in as-cast, slowly solidified alloys and those annealed at 800 and 600 °C. The solidification of this phase follows closely the Al3Fe–AlCu compositional direction. The first icosahedral phase solidified close to Al65.5Cu21.5Fe13; its compositional region at 800 °C lies between about Al64.5Cu23Fe12.5 and Al62Cu26.5Fe11.5, while at 600 °C it lies between Al62Cu26.5Fe11.5 and Al60.5Cu29.5Fe10. The formation of the icosahedral and related structures observed in Al–Cu–Fe alloys is discussed.

Investigation of the Al–Cu–Rh phase diagram in the vicinity of the decagonal phase

Abstract Phase equilibria were studied in Al-rich Al–Rh and Al–Cu–Rh alloys. We present a partial Al–Rh phase diagram. In addition to previously reported Al–Rh phases, two complicated orthorhombic compounds were found close to the Al 3 Rh composition. Partial 900 and 800°C isothermal sections of the Al–Cu–Rh phase diagram were studied in the vicinity of the decagonal phase. At these temperatures the stable decagonal phase was found to be the only ternary compound. It is in equilibrium with the ternary extensions of three binary Al–Rh phases and at 900°C also with the liquid.

Isothermal sections of the Al–Pd–Co alloy system for 50–100 at.% Al

Abstract The Al–Pd–Co alloy system was studied in the range of 50 to 100 at.% Al. Isothermal sections at 1050, 1000, 940 and 790 °C were determined. The isostructural AlCo and AlPd phases form a continuous range of solid solutions. The Al–Pd e-phase(s) region extends up to ∼16.1 at.% Co, Al 3 Pd 2 up to 7.1 at.% Co, M-Al 13 Co 4 , Al 5 Co 2 , Al 9 Co 2 and the Al–Co Z-phase exhibit moderate extensions into the ternary compositional range of the phase diagram. Six stable ternary compounds designated W, Y 2 , U, V, F and C 2 were revealed and characterized.

Decagonal quasicrystals in AlCo and ternary alloys containing Cu and Ni

Abstract Investigation of a series of AlCo(Cu,Ni) alloys of composition adjacent to Al3Co is reported. The metastable nature of the AlCo decagonal phase produced by rapid solidification was confirmed. This phase was found to form preferentially at compositions of about 72–74 at.% Al. At lower cooling rates, achievable by conventional solidification, decagonal-like metastable structures can be formed in the same compositional range, corresponding to the binary Al5Co2Z range of the equilibrium phase diagram (Grushko et al., J. Alloys Comp., 233 (1996) 279). The relation between these binary phases and the stable decagonal phases in AlCoCu and AlCoNi is discussed.

Synthesis of the cubic Z-phase in the TiAlO system by a powder metallurgical method

Abstract The formation of protective or non-protective oxide scales on γ-TiAl alloys is closely related to the composition of the subsurface depletion layer. Recently it was shown that in the depletion layer a cubic Z-phase is formed, which was until then not known in the TiAlO phase diagram. In the present study a method was developed to synthesise this cubic phase by a powder metallurgical method. It was found that the Z-phase can be produced by a reaction between the α-TiOx solid solution and aluminium using self-propagating high temperature synthesis (SHS). The phase analysis and microstructural characterisation of the Ti-O-Al sintering products show that at 900–950 °C the Z-phase exists in equilibrium with γ-TiAl(O) and α-Al2O3, as well as with Ti2O and α2-Ti3Al(O) ordered solid solutions. According to X-ray and electron diffraction data the Z-phase has a primitive cubic structure of one of the space groups P 213 or P 4132 and has a composition close to Ti5Al3O2. The lattice parameter of the cubic phase was determined to be a = 0.691 ± 0.001 nm.

Orthorhombic ε-phases and transitional structures in Al-Pd-(Fe)

Abstract The results of an investigation of the structures formed in the range of about Al73Pd27 to Al76Pd14Fe10 by electron diffraction and high-resolution transmission microscopy are reported. The orthorhombic ε6, ε16, ε22 and ε28 phases were observed in specific compositional sub-ranges. Their lattice parameters a ≈ 2.34 and b ≈ 1.62 nm are essentially the same while the c parameters are ≈1.23, 3.24, 4.49 and 5.70 nm, respectively. Apart from these regular structures, also structures aperiodic along the c-direction are revealed at intermediate compositions. Thus, inside the wide ε-phase range only slight continuous variations of the orthorhombic a and b cell parameters are accompanied by complicated modulations of the c cell parameter.