Ni Decagonal Quasicrystals Research Articles

Chemical ordering of Co and Ni in a W-(AlCoNi) crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by atomic resolution energy-dispersive X-ray spectroscopy

A W-(AlCoNi) crystalline approximant, which is closely related to Al–Co–Ni decagonal quasicrystals, in an Al72.5Co20Ni7.5 alloy has been studied by atomic resolution energy-dispersive X-ray spectroscopy (EDS), in an instrument attached to a spherical aberration (Cs)-corrected scanning transmission electron microscope. On high-resolution EDS maps of Co and Ni elements, obtained by integrating many sets of EDS data taken from undamaged areas, chemical ordering of Co and Ni is clearly detected. In the structure of the W-(AlCoNi) phase, consisting of arrangements of transition-metal (TM) atoms located at vertices of pentagonal tilings and pentagonal arrangements of mixed sites (MSs) of TM and Al atoms, Co atoms occupy the TM atom positions with the pentagonal tiling and Ni is enriched in part of the pentagonal arrangements of MSs.

Structure of a crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy and atomic-resolution energy dispersive X-ray spectroscopy

Six types of decagonal quasicrystals (DQCs) found in Al–Co–Ni alloys with a wide compositional ratio of Co/Ni are considered to be stabilized by chemical ordering of Co and Ni, but the elucidation of this ordering has never been performed on alloys containing neighbouring elements Co and Ni. In order to examine the chemical ordering, an Al–Co–Ni crystalline phase, the PD3c phase, which is an important approximant for understanding the structures of ordered Al–Co–Ni DQCs, in an Al71.5Co16Ni12.5 alloy has been studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM) and atomic-resolution energy dispersive X-ray spectroscopy (EDXS). From the combination of observed high-angle annular dark-field and annular bright-field STEM observations, an atomic arrangement of the crystalline approximant can be directly derived. The chemical ordering of Co and Ni in the arrangement of transition-metal (TM) atoms and mixed sites (MSs) of TM and Al atoms can be clearly detected on atomic-resolution EDXS maps obtained with the special technique. Co atoms are located at the TM atom positions arranged in pentagonal tiling with a bond length of 0.76 nm, whereas Ni is enriched in the MSs located in pentagonal tiles of Co atoms. It can be concluded that the change of an area ratio of Co-rich clusters to Ni-rich MSs produces various types of DQCs with different compositional Ni/Co ratios.

Arrangements of Transition-Metal Atoms in Three Types of Al–Co–Ni Decagonal Quasicrystals Studied by Cs-Corrected HAADF-STEM

Arrangements of Transition-Metal Atoms in Three Types of Al–Co–Ni Decagonal Quasicrystals Studied by Cs-Corrected HAADF-STEM

Spontaneous formation of the B2 phase from a decagonal quasicrystal under reduced constraint

Since the discovery that vacancies can quasiperiodically order in a basic B2 structure to yield a one-dimensional quasiperiodicity, these phases were subjected to intense investigations for possible links with quasicrystal. The B2 phase was found to coexist extensively with the icosahedral quasicrystalline phase, particularly in Al–Cu–TM (TM = transition metal) systems where the two phase fields are often adjacent to each other. The two phase exhibit a well-defined orientation relationship. There exist several studies dealing with quasicrystalline to B2 transformation, particularly in decagonal quasicrystals Doblinger et al. has carefully explored different metastable states in Al–Co–Ni decagonal quasicrystals including nanodomained 1D quasicrystals and multiple twinned approximant phase. The decagonal to a cubic B2 ordered phase transformation can often be observed. The nanocrystals of the B2 phase could be observed on the surface of the decagonal quasicrystal. Zhang and Urban have studied the stability of the QC phase by employing electron irradiation in an electron microscope and observed the transformation of the decagonal quasicrystal (DQC) phase, in an Al–Cu–Co–Si alloy, first to a disordered phase [body centered cubic (BCC phase)] and then to an ordered B2 phase. Zurkirch et al. observed a decagonal to crystalline structural transformation occurring on a surface of Al–Co–Ni upon sputtering the surface. The annealing treatment was found to restores the decagonal structure. Shimoda et al. confirmed the above observation. A similar result has been reported recently by Fluckinger et al. who have reported more than one variant of the cubic crystal at the surface. These authors attributed the structural change to the change in the surface composition during sputtering. The annealing treatment allows the A1 atoms from the subsurface to diffuse back and restore the composition resulting in a reappearance of the decagonal quasicrystal. The formation of a BCC phase during ion milling has also been reported in Al–Cu–Co decagonal quasicrystals. Recently Muthy et al. has reported a quasicrystalline to $_\beta$ phase transformation in as-cast $A1_{65}Cu_{20}Fe_{15}, A1_{65}Cu_{20}Co_{15}$ and $A1_{72}Pd_{19:5}Mn_{8:5}$ alloys during high energy ball milling. They have correlated the phase change with a critical grain size of 20 nm.

Theoretical Prediction of Phase Diagrams for Al–Co–Ni Decagonal Quasicrystals

Atomic structures of Al–Co–Ni decagonal quasicrystals are studied theoretically by Monte-Carlo type of calculation in a wide range of the atomic composition. It is found that two different tiling structures are obtained depending on the composition. A ternary phase diagram is obtained. It is discussed that the tiling structures are selected to avoid the non-ideal tile decoration introduced by a change of the Al concentration. A decagonal cluster with TM and Al pentagons at the center is found as a characteristic structural unit in the Co-rich samples and it is shown that the Al-transition metal interaction favors the atomic arrangement of the decagonal cluster.

Nanomechanical characterization of Al–Co–Ni decagonal quasicrystals

This paper addresses some issues related to the nanomechanical responses of decagonal quasicrystals using nanoindentation techniques. A Hysitron Triboscope with a Berkovich indenter was used to carry out the nanoindentation tests on single crystals of Al–Co–Ni decagonal phase. The anisotropy in terms of nanohardness at higher load was observed, whereas it was not possible to establish the same at lower loads. However, the Young's modulus was found to be indistinguishable in all these cases. Discontinuity in load displacement, which is known as a ‘pop-in’ effect, was observed frequently at various load regime. It was found that this discontinuity was not due to cracking or a phase transformation effect, but to a plastic yielding phenomena and nanodeformation of the material. Significant differences between the nanohardness and microhardness were noticed and are discussed based on various experimental parameters and the consequent mechanical response of materials.

Structure of type I and type II modifications of Al–Co–Ni decagonal quasicrystals studied by HAADF-STEM technique

The structure of type I and type II modifications of Al–Co–Ni decagonal quasicrystals was studied by using the high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) technique. It was found that the quasi-unit-cell should be selected as columnar decagon of 3.2 nm in diameter and 0.82 nm in height, and its center consists of a bright circle with 0.76 nm of the outer diameter, indicating that atomic distribution in the quasi-unit-cell is disordered. Lattices obtained by connecting the centers of the atom clusters in HAADF images reveal that the lattice of the type I modification is rhombic Penrose tiling and the lattice of the type II is a mixture of the rhombic and pentagonal tilings. Namely, the lattice of the type II modification is a mixture of the lattices of the type I and S1 modifications.

Reversible phase changes of Ni-rich Al?Co?Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al–Co–Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al 70 Co 12 Ni 18 alloy, which is stable at 900 °C, transforms into the Type-I phase by annealing at 650 °C and that the Ni-rich basic phase formed in the Al 71.5 Co 9 Ni 19.5 alloy, which is stable at 900 °C, transforms into the S1-state phase by annealing at 650 °C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 °C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy.

Dissolution patterns caused by chemical etching of Al–Co–Cu and Al–Co–Ni decagonal quasicrystals with a HF–HNO3–H2O solution

Dissolution patterns essential for Al–Co–Cu and Al–Co–Ni decagonal quasicrystals (d-QCs) have been investigated in detail by chemical etching using a HF–HNO3–H2O solution followed by scanning electron microscopy (SEM) observations. The chemical etching of definite surface areas of the samples, which are either normal or parallel to the tenfold axes, using a solution with HF–HNO3–H2O (1 : 5 : 4 in volume ratio; 0°C; 5–10 min), produces a large number of microfacet pits of decagonal prismatic shape. In addition, the same etching test in combination with SEM observations was carried out on a deformed sample of the Al–Co–Ni d-QC, which had been subjected to concentrated mechanical stress at an elevated temperature of 850°C by means of the Vickers indentation technique. The morphological features of the resulting micropits and their possible origins are discussed on the basis of results obtained by SEM observations.

Effect of Cu substitution in Al–Co–Ni decagonal quasicrystals

A stable decagonal phase has been observed in both as-cast and rapidly solidified Al–Co–Ni and Al–Co–Cu alloys, which form in the composition range of 15–20 at.% Co and 10–15 at.% Ni or 15–20 at.% Cu. In the present investigation, copper was substituted for Co and Ni in Al 70Co 15Ni 15. Compositions corresponding to Al 70Co 15− x Cu x Ni 15 and Al 70Co 15Cu x Ni 15− x were investigated. It is found that when the copper concentration reaches a value of 5 at.%, the disorder marker, i.e. the streaking along the 111,113 rows becomes predominant. For Al 70Co 15− x Cu x Ni 15, when all of the Co is replaced by Cu, the decagonal phase disappears and the only phase observed is the vacancy ordered phase. In Al 70Co 15Cu x Ni 15− x when the Cu concentration reaches a value of 10 at.%, the decagonal and crystalline phases coexist.

Structural changes of Ni-rich Al–Co–Ni decagonal quasicrystals

The phase relationship among three Al–Co–Ni decagonal quasicrystals in a Ni-rich decagonal phase, and their phase changes, have been studied by electron diffraction and high-resolution transmission electron microscopy, as well as thermal analysis. At 1173 K, the so-called Ni-rich basic decagonal quasicrystal exists in a Ni-rich region from Al 72Co 8Ni 20 to Al 70Co 10Ni 20, and transforms into the S1-type ordered decagonal quasicrystal in Al 71Co 10Ni 19. The Ni-rich basic decagonal quasicrystal at 1173 K in Al 72Co 8Ni 20 transforms into the S1-type decagonal quasicrystal by annealing at 923 K, whereas the S1-type quasicrystal at 1173 K in Al 71Co 10Ni 19 alloy transforms into the type-I ordered decagonal quasicrystal by annealing at 923 K.

Reversible phase changes of Ni-rich Al–Co–Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al–Co–Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al 70Co 12Ni 18 alloy, which is stable at 900 °C, transforms into the Type-I phase by annealing at 650 °C and that the Ni-rich basic phase formed in the Al 71.5Co 9Ni 19.5 alloy, which is stable at 900 °C, transforms into the S1-state phase by annealing at 650 °C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 °C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy.

Structure of a W–(AlCoNi) crystalline phase related to Al–Co–Ni decagonal quasicrystals, studied by single crystal X-ray diffraction

The crystal structure of a W–(AlCoNi) phase, which is closely related to Al–Co–Ni decagonal quasicrystals, has been studied by the single crystal X-ray diffraction. The W–(AlCoNi) phase was obtained from an Al 72.5Co 20Ni 7.5 alloy sample annealed at 950 °C and its structure indicates the space group Cm (No. 8) with lattice parameters a=39.668(3) Å, b=8.158(1) Å, c=23.392(1) Å and β=90.05(1)°. A structural model with R( F) less than 0.087 for the observed 16 200 reflections can reproduce contrasts of atomic scale images obtained by high-angle annular detector dark-field scanning transmission electron microscopy together with high-resolution electron microscopy. The structure of the W–(AlCoNi) phase provides a fundamental atomic arrangement for the columnar atom cluster observed commonly in the Al–Co–Ni decagonal quasicrystals, and makes it possible to derive a structural model for the structures of the Al–Co–Ni decagonal quasicrystals.

The structural characteristics of Al–Co–Ni decagonal quasicrystals and crystalline approximants

The structural characteristics of four modulations of Al–Co–Ni decagonal quasicrystals, called S1-type and type-I superstructures, Co-rich basic structure and 5-fold superstructure, and crystalline approximants, W-(AlCoNi) and PD 3c phases, all of which are found in and around an Al–Co–Ni decagonal phase with a wide compositional range, are systematically discussed on the basis of results obtained from high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) observations. All the structures of the modulations of decagonal quasicrystals and crystalline approximants are described as arrangements of columnar atom clusters, which have a decagonal section of 2.0 nm in diameter and pentagonal symmetry. The structures of the S1-type and type-I superstructures can be characterized as ordered arrangements of two types of the atom clusters with two orientations of pentagonal symmetry in pentagonal and rhombic quasiperiodic lattices, respectively. On the other hand, the structures of the Co-rich basic state and 5-fold superstructure are described as arrangements of the atom clusters with one orientation of pentagonal symmetry. The PD 3c and W-(AlCoNi) crystalline phases can be concluded to be approximants of the type-I and pentagonal superstructure, respectively.

Experimental observation and computer simulation of high-order Laue zone line patterns of Al—Co—Ni decagonal quasicrystals

Abstract High-order Laue zone (HOLZ) line patterns of an Al70Co15Ni15 quasicrystalline decagonal phase with a periodicity of 0·4 nm along the tenfold axis have been obtained experimentally with a large angular range by connecting a series of large-angle convergent-beam electron diffraction patterns. The computer-simulated HOLZ line patterns, which were calculated using the cut-and-projection method and a simple quasi-lattice model, show good agreement with the experimental observations.