Al Pd Mn System Research Articles

The solidification of Al–Pd–Mn studied by high-energy X-ray diffraction from electrostatically levitated samples

Abstract We report on the results of a high-energy x-ray diffraction study of Al–Pd–Mn to investigate the solidification products obtained during free-cooling using an electrostatic levitation furnace. The primary solidification product from the melt is i-Al–Pd–Mn which coexists with a significant remaining liquid component. As the sample cools further, we find that the solidification pathway is consistent with the liquidus projection and pseudo-binary cut through the ternary phase diagram reported previously. At ambient temperature we have identified the major phase to be the ξ′-phase orthorhombic approximant, along with minor phases identified as Al and, most likely, the R-phase orthorhombic approximant. We have also observed a distinct prepeak in the liquid at high temperature, signifying the presence of extended atomic order. Interestingly, this prepeak was not observed in previous neutron diffraction measurements on the Al–Pd–Mn system. No undercooling was observed preceding the solidification of the i-Al–Pd–Mn phase from the melt which may signal the close similarity of the short-range order in the solid and liquid. However, this can not be clearly determined because of the potential for heterogenous nucleation associated with the presence of an Al2O3 impurity at the surface of the sample.

Structural themes in approximant and decagonal quasicrystalline phases in Al based alloys

The structure of two periodic phases in the Al–Pd–Mn system showing the importance of chemical ordering in the formation of decagonal quasicrystalline and approximant crystalline structures is reviewed. Different descriptions of the periodic phases in terms of clusters, dense planes and pentagonal chains of transition metals are presented. We also review a structural model for AlPdMn decagonal quasicrystals derived from the general schematic distribution of TM existing in the approximant phases. This model is discussed in relation of an unique decagonal overlapping cluster in a Penrose pentagonal tiling.

Physical properties of the complex metallic alloy phases in the Al–Pd–Mn system

The Al–Pd–Mn system of intermetallics contains complex metallic alloy (CMA) phases, whose crystal structures are based on giant unit cells comprising up to more than a thousand atoms per cell. We performed investigation of the magnetic, electrical, thermal transport and thermoelectric properties of the ξ′ phase and the related Ψ phase on single-crystalline samples.

Stability of quasicrystalline phase in Al–Cu–Fe, Al–Cu–Co and Al–Pd–Mn systems by high energy ball milling

The stability of the quasicrystalline phase present in as-cast Al 65Cu 20Fe 15, Al 65Cu 20Co 15 and Al 72Pd 19.5Mn 8.5 alloys during high energy ball milling and on subsequent annealing has been studied. The synthesis and stability of the quasicrystalline phase prepared by mechanical alloying of Al 65Cu 20Fe 15, Al 70Cu 20Fe 10 and Al 65Cu 20Co 15 alloys has also been studied. In all the cases, the quasicrystalline phase is unstable with long hours of milling and transforms to the β-phase (bcc phase), which is stable on further milling and on subsequent annealing up to 1073 K. The quasicrystalline to β-phase transformation appears to occur for grain sizes below 20 nm.

Magnetism of approximants in the Al–Mn and Al–Pd–Mn systems

Magnetic properties of approximants in the Al–Mn and Al–Pd–Mn systems have been investigated. Whereas most phases are non-magnetic, localized moments are found on a fraction of the Mn atoms in the hexagonal μ-Al4Mn, orthorhombic T-Al73.1Pd5.2Mn21.7 and cubic 1/1-Al68Pd11Mn14Si7 phases. In all these phases we could identify the crystallographic sites occupied by magnetic Mn. Localized moments are associated with a few particular sites. The moment formation is influenced by the sp–d hybridization. These results contribute to clarify the origin of magnetism in quasicrystalline phases.

Heat capacities of icosahedral and hexagonal phases of Zn–Mg–Y system

Heat capacity measurements have been performed in the temperature range 4–300K for an icosahedral phase of Zn–Mg–Y alloy (Zn55Mg35Y10) and the related crystalline phases, a hexagonal phase (Zn65.2Mg27.9Y6.9) and the Laves phase of MgZn2, to study the feature of lattice vibrations in quasicrystals. A single quasicrystal of Zn56.8Mg34.6Y8.7 was also investigated for comparison. The vibrational density of states is delineated in terms of the temperature dependence of the apparent Debye temperature (ΘD) on a reduced basis using the limiting value at high temperature (Θ∞). A deeper minimum (ΘD/Θ∞∼0.82) is observed at substantially lower temperatures (T/Θ∞∼0.055) in the quasicrystals than in the crystals, which suggests significant enhancement of the density of states at the corresponding energies. The fact is compatible to that obtained previously for Al–Cu–Ru, Al–Pd–Re and Al–Pd–Mn systems. The presence of some defects or inhomogeneity in quasicrystals might be responsible for the anharmonic lattice vibrations.

A comparative study of the plastic behaviour of icosahedral and ξ′-Al–Pd–Mn

In the Al–Pd–Mn system, large single crystals of high structural quality can be grown of the icosahedral and the ξ′-phase. The latter is an approximant of the icosahedral phase. Though the structure of ξ′-Al–Pd–Mn is periodic, it possesses a local order which is very close to that of the icosahedral phase. In this paper, we present a comparative study of the plastic behaviour of single crystals of the icosahedral and the ξ′-phase.

Quasicrystalline coatings: Thermal evolution of structure and properties

Quasicrystals (QCs) are known to exhibit unique properties as a result of their unique quasiperiodic structure. Real quasicrystalline (QC) materials, however, may exhibit complex phase structures, and as a consequence, their properties may differ from expectations. In the present work, QC coatings of the Al–Cu–Fe, Al–Cu–Fe–Cr, and Al–Pd–Mn systems were prepared by a plasma spray process, followed by heat treatments in the range 500–800 °C. The phase structure and evolution of the coatings were evaluated, and thermal diffusivity, hardness, and friction coefficient were measured. The presence of quasicrystalline and crystalline phases and their influence on these properties is systematically considered for the first time. Broadly, the coatings exhibit the properties expected of QC materials, low thermal diffusivity, high hardness, and low coefficients of friction, but it is also shown that these properties can be sensitive to the phase structure of the coatings. This suggests that phase structure may be manipulated by heat treatment to optimize the properties of QC coatings.

Quasimagnetic clusters in icosahedral Al–Pd–Mn system

We have studied the magnetization of the Al–Pd–Mn system using high-quality single crystals. Below room temperature, field-cooled magnetization exceeds zero-field-cooled magnetization. In addition, thermoremanent magnetization has disappeared at the same temperature. The results suggest the formation of quasimagnetic clusters in the Al–Pd–Mn system, although the relaxation of the thermoremanent magnetization was not observed. We discuss this anomalous magnetic phase in the Al–Pd–Mn system and refer to the origin of the behaviors.

Sliding wear of quasicrystalline coatings

The mass loss during sliding wear of several quasicrystalline (QC) coatings has been measured. QC coatings of the Al–Cu–Fe, Al–Cu–Fe–Cr and Al–Pd–Mn systems have been investigated and compared with a hardened tool steel and a WC–6% Co hardmetal. The wear rates of the coatings are in general comparable to conventional metallic materials. There is some variation in sliding wear behaviour of different QC coatings.

Local order of molten quasicrystals and approximant phases

The structure factor obtained by neutron scattering of a melt in equilibrium with the icosahedral phase of the AlPdMn system exhibits features typical of local icosahedral ordering. The same experiment on a melt in equilibrium with an approximant ξ-phase yields a less pronounced icosahedral local order superimposed on an ordering typical for a random hard sphere packing. A comparison between the present results and data previously obtained on the magnetic properties of Al-Pd-Mn alloys indicates that the local icosahedral order in liquid states depends on the Mn concentration and is reminiscent of the local order in the solid.

The atomic structure of the decagonal phase and its approximants as random coincidence networks

It is shown that the decahedral recursive model (DRM) explains at atomic level the structure of the decagonal (D) phase and associated approximant structures as random coincidence networks (RCNS), random packings of clusters satisfying DRM energy minimizing rules. RCNS are degenerate random tilings that become quasiperiodic when entropic disorder is maximized at high temperature through phason flips requiring small atomic movements. Such phason flips can change the relative ratio and geometry of tiles, making transformations between systems using different tile sets possible. Electron microscopic observations for AlCuCoSi, AlCuFeCr, AlCoCu and AlPdMn systems are reproduced, including the observed Fibonacci sequenced lines in the former.

Decagonal quasicrystal in the Al–Pd–Mn system

Abstract A stable decagonal quasicrystal in Al70Pd30−xMnx alloys (x = 10–20) was examined by electron diffraction and high-resolution electron microscopy. The decagonal quasicrystalline grains are formed with definite crystallographic relationships to adjacent icosahedral and Al3Mn crystalline grains. The structure of the decagonal phase, which is formed as the main phase at near Al70Pd10Mn20 composition, is a mixture of decagonal quasicrystalline regions with some linear phason strain and microcrystalline regions. The structures of both regions may be interpreted in terms of quasiperiodic and periodic tilings, constructed with two types of bond lengths, S (about 2 nm) and L (= τ · S, where τ is the Golden ratio), of the same atom cluster with decagonal symmetry.

Dynamical properties of stable icosahedral alloys

Results from theoretical investigations of the vibrational properties of quasicrystals are compared with those obtained so far for the atomic dynamics of stable icosahedral alloys by neutron inelastic scattering, especially using as an example the most recently studied icosahedral AlPdMn system. It is concluded that although many of the dynamical properties to be expected on the basis of the theoretical investigations of three-dimensional quasicrystals have been found in experiments done with stable icosahedral alloys, some very important questions concerning properties specific for quasiperiodic lattices like the existence of gaps or localization still await a final answer.

Quasicrystals and approximants in the AlCu(Fe,Ru) and AlPdMn systems

The formation and thermal stability of a quasicrystal and its approximant have been investigated in the ternary AlCu(Fe,Ru) and AlPdMn systems. In the AlCuFe system, the rhombohedral approximant is observed with microcrystalline or a coarse-grained structure, depending on the annealing temperature. Also the intermediate state with τ ≈1.75 is observed in Al 62Cu 26Fe 12. In the AlCuRu system, the icosahedral phase is observed in two different compositional regions, which satisfy the condition e / a = 1.75. The 1/0 cubic approximant is found at the composition of Al 71Cu 7Ru 22. In the AlPdMn system, the icosahedral and decagonal phases as well as their approximants are observed.

Chemically driven structural change in quasicrystalline AlPdMn alloys

Quasicrystals, including face-centered icosahedral (FCI), simple icosahedral (SI) and decagonal (D) phases with periodicities of 1.6 nm and 1.2 nm along periodic axis, are found to form over a wide composition range in the AlPdMn system. A very strong composition dependence of the chemical order is seen in icosahedral Al 90− x Pd x Mn 10 alloys alloys. A discontinuous change in the lattice parameter, a R, with the Pd content of the i-phases suggests that the replacement of Al by Pd is selective. The predominant occupation site for the Pd atoms is icosahedral structure varies between the FCI and SI lattices. It is proposed that the FCI phase can be constructed as a two-cell structure, originating from the AlPd and AlMn binary systems.

An icosahedral quasicrystal and its cubic approximant in the Al—Pd—Mn system

Abstract Structural changes after heat treatment of an icosahedral phase of melt-spun Al—Pd—Mn alloys have been studied by X-ray powder diffraction and transmission electron microscopy. Around the compositions Al70Pd20Mn10 and Al70Pd23Mn7, no phase transition occurs on annealing, and the icosahedral phase is stable up to the melting point. However, for the Al70Pd26Mn4 sample, the icosahedral phase transforms into a 2/1 cubic approximant crystalline phase at 750°C. A decagonal quasicrystal is formed for a melt-spun Al75Pd20Mn5 alloy.

The structure of the icosahedral AlPdMn quasicrystal

Abstract Neutron diffraction data were obtained from quasicrystals of the AlPdMn system and the isomorphic modifications AlPd(FeCr). From contrast variation effects, partial structure factors were determined. In the high-dimensional periodic description, the structure can be viewed as a six-dimensional cubic lattice belonging to the Pm space group, with a superstructure due to a chemical order which somewhat induces an F-type lattice.

New icosahedral alloys with superlattice order in the Al–Pd–Mn system prepared by rapid solidification

Abstract An icosahedral quasicrystalline phase was found to be formed in rapidly solidified Al–Pd–Mn alloys. The quasicrystalline structure was confirmed to have a superlattice structure of the simple icosahedral quasicrystal, similar to the face-centred icosahedral structure for Al–Cu–Fe quasicrystals. The intensity of the superlattice reflections increases successively with increasing Pd content, and hence Pd atoms seem to occupy predominantly the superlattice site rather than the fundamental lattice in the icosahedral structure.