K. H. Kuo

Decagonal quasicrystals with different periodicities along the tenfold axis in rapidly solidified Al65Cu20M15 (M = Mn, Fe, Co or Ni)

Presentation des resultats par microscopie electronique en transmission et diffraction electronique a aire selectionnee ou en faisceau convergent

A NEW ICOSAHEDRAL PHASE WITH M35 SYMMETRY

Abstract A metastable phase with the point group symmetry m35 but no ordinary translation symmetry has been found in rapidly quenched (Ti 1−xVx)2Ni alloys with × = 00-0-3 by electron diffraction and high-resolution electron microscopy.

A new icosahedral quasicrystal in rapidly solidified FeTi2

DALIAN INST TECHNOL,DEPT MAT ENGN,DALIAN,PEOPLES R CHINA.;DONG, C (reprint author), ACAD SINICA,INST MET RES,ATOM IMAGING SOLIDS LAB,SHENYANG 110015,PEOPLES R CHINA

The formation of η carbides

η carbides exist in two forms, η1 with the ideal general formula A3B3C, and η2 with the ideal general formula A2B4C. Here A represents a transition metal of period IV and B a metal of period V or VI in the periodic system. It has been proved that metals A and B have diagonal positions in the periodic table, the connecting line from A to B running downwards to the left, as shown by the η carbides found in this investigation: V3Zr3C, Cr3Nb3C, Mn3Mo3C, Mn3W3C, Fe2Mo4C, Co2Mo4C, Ni3Mo3C, and Ni2Mo4C. Ta does not form a double η carbide with any transition metal of period IV, but many triple carbides containing Ta as B atom are found to exist; e.g., (V,Fe)3Ta3C, (V,Co)3Ta3C, (V,Ni)3Ta3C, (Cr,Fe)3Ta3C, (Cr,Co)3Ta3C, (Cr,Ni)3Ta3C, and (Cr,Cu)3Ta3C. Nb also forms a triple carbide (V,Ni)3Nb3C. The formation of this family of carbides is discussed from the point of view of the relative strength of the transition metal-carbon bond, of the atomic size factor, and of the possibility of their being electron compounds.

Decagonal quasicrystals with different periodicities along the tenfold axis in rapidly solidified Al-Ni alloys

Abstract A decagonal quasicrystal with a periodicity of 1·6 nm along the tenfold axis, similar to that in Al─Pd and Al─Co, has been found in a rapidly solidified Al6Ni alloy containing a few per cent of silicon. In addition, a new decagonal phase has been found in Al4Ni which has a periodicity of 0·4 nm along the tenfold axis. This decagonal phase is different from those reported before in the aluminium–transition metal alloys. A new metastable crystalline phase Al9Ni2 having the structure of Al9Co2 has been found.

Decagonal quasicrystal and related crystalline phases in slowly solidified Al-Co alloys

A two-dimensional decagonal quasicrystal (DQC) has been found in binary Al-Co alloys in the composition range Al11Co4 to Al10Co4 after slow solidification from the melt. The crystalline phase found most frequently coexisting with this DQC is two new structure variants, oneC-centered and the other primitive, of the monoclinic layer compound Al13Co4. Theira andc parameters are roughly τ2 times (τ = (1 + √5)/2 = 1.6180…) larger than the corresponding ones in Al13Co4. Moreover, a new orthorhombic phase Al3Co or Al11Co4 (Pnmn, a = 1.25 nm,b = 0.81 nm, andc = 1.46 nm) has also been found. As shown by the characteristic electron diffraction patterns (EDPs), these crystalline phases can be considered as approximate structures of the DQC. The lattice relationship between these phases has been discussed.

Stable Al-Cu-Co decagonal quasicrystals with decaprismatic solidification morphology

Abstract A decaprismatic growth morphology has been found for the first time for the decagonal quasicrystal in the Al65Cu20Co15 alloy slowly cooled from the melt. Substituting a little si for Al in this alloy, for example, Al62Si3Cu20Co15, results in quasicrystal decaprisms of 0·5 mm in diameter and several millimetres in length. These decaprisms showed possibly a 10/mmm morphology which was proved later by electron and X-ray single crystal diffraction analyses. Moreover, the sharp selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) patterns show clearly a high degree of perfection and the X-ray precession photographs prove the single crystalline nature across a few millimetres of this decagonal quasicrystal.

Ternary laves and sigma-phases of transition metals

Abstract The B metal of the Laves phase AB 2 can be replaced by a mixture of B′ and B″, thus forming a ternary phase A(B′, B″) 2 . The metals B′, B, and B″, belong to the same period of the Periodic Table and B lies between B′ and B″. This kind of substitution produces many ternary Laves phases of transition metals. This kind of substitution can also be applied to the formation of the sigma-phase A a B b . However, in this case the ratio a b changes, e.g., σ-MoCr 2 Ni 2 and σ-WCr 8 Ni 5 as compared with σ-MoFe and σ-WFe.

Five-dimensional structure refinement of decagonal Al65 Cu20 Co15

Abstract A single-crystal X-ray structure analysis of decagonal Al65 Cu20Co15 has been performed using the n-dimensional approach. The structure has been solved by Patterson analysis and subsequent least-squares refinement, both in the five-dimensional description. A final value of wR=0.097 for 259 independent reflections was achieved with I≥2σ(1) and 12 variables. There are three atoms in the asymmetric unit. The calculated density of the refined, structure model is ρc = 4.8 M gm −3. The three-dimensional decagonal structure consists of two planar quasiperiodic layers, related by the tenfold screw axis, stacked periodically one upon the other. Characteristic large elements of the crystalline Al13 Fe4 structure type can be found in new combinations in the quasiperiodic layers. It is remarkable that pentagonal channels appear which are empty or occupied statistically.

ORIENTATION RELATIONSHIP BETWEEN THE ICOSAHEDRAL AND CRYSTALLINE PHASES IN (TI1-XVX)2NI ALLOYS

Abstract By heating the icosahedral phase (i) of a (Ti1−xVx)2Ni in situ in a Philips EM430 electron microscope, the crystalline NiTi2-type phase (c) was formed with the following orientation relationship: i5//c[110], etc., i3//c[111], etc., and i2//c[112], etc.