An Pang Tsai

A Stable Quasicrystal in Al-Cu-Fe System

A thermodynamically stable quasicrystalline single phase with an icosahedral structure was found to be formed at an atomic composition of Al65Cu20Fe15 in a fully annealed state as well as in a conventionally solidified state. The stable quasicrystal consisted of large grains with an average size of 0.2 mm after annealing for 48 h at 1118 K (0.98 Tm).

Aluminum-Based Amorphous Alloys with Tensile Strength above 980 MPa (100 kg/mm2)

New Al-based amorphous alloys with high strength and good ductility were produced in an Al-Y-Ni system by liquid quenching. The tensile fracture strength ( σf) and Vickers hardness reach 1140 MPa and 300 DPN for Al87Y8Ni5. The specific strength defined by the ratio of σf to density is as high as 38, being much higher than that for conventional alloy steels. The high-strength Al-base amorphous alloys are expected to attract strong attention as a new type of high-strength material with low density.

A stable binary quasicrystal.

All stable quasicrystals known so far are composed of at least three metallic elements. Sixteen years after the discovery of the quasicrystal, we describe a stable binary quasicrystalline alloy in a cadmium–ytterbium (Cd–Yb) system. The structure of this alloy represents a new class of packing of 66-atom icosahedral clusters whose internal structure breaks the icosahedral symmetry. The binary quasicrystal offers a new opportunity to investigate the relation between thermodynamic stability and quasiperiodic structure, as well as providing a basis for the construction of crystallographic models.

New Amorphous Alloys with Good Ductility in Al-Y-M and Al-La-M (M=Fe, Co, Ni or Cu) Systems

New aluminum-based amorphous alloys were found to be formed in Al-Y-M and Al-La-M (M=Fe, Co, Ni or Cu) ternary systems by melt spinning. Their compositional ranges are 3 to 33 at%M and 3 to 22 at%Y or 2 to 18 at%La. The alloys containing more than about 80 at%Al have good bend ductility. The crystallization temperature Tx increases from 414 to 831 K with an increase of M and yttrium or lanthanum contents. The effect of the solute elements on Tx is interpreted to be dominated by the attractive interaction between aluminum and M, yttrium or lanthanum atoms.

Quasicrystal strengthened Mg–Zn–Y alloys by extrusion

Abstract Superior mechanical properties with high ductility at room and higher temperatures have been achieved by extruding Mg 95 Zn 4.2 Y 0.8 and Mg 92.5 Zn 6.4 Y alloys at 523 K. The strengthening phases are the icosahedral quasicrystal, finely distributed nano-sized precipitates of a ternary phase τ 1 and thin β 1 ′ phase rods several hundreds of nanometers long.

Highly ordered structure of icosahedral quasicrystals in Zn-Mg-RE (RE ≡ rare earth metals) systems

Abstract A new group of stable icosahedral phases (i-phases) in the Zn-Mg-RE system were found to have an ideal composition close to Zn50 Mg42RE8 (RE ≡ Y, Gd, Tb, Dy, Ho or Er). The new i-phases exhibit a highly ordered and nearly perfect face-centred icosahedral lattice as revealed by electron and X-ray diffraction and high-resolution electron microscopy. Powder X-ray diffraction indicates that the i-phase has long-range structural order with a correlation length over 1000 A, which is the largest found in the Frank-Kasper group. These new i-phases have the common valence concentration (about 2.08) and could be regarded as a new class of Hume-Rothery alloys.

Stability and transformation to crystalline phases of amorphous ZrAlCu alloys with significant supercooled liquid region

Abstract Crystallization from the supercooled liquid region was examined for an amorphous Zr65Al7.5Cu27.5 alloy with a significant supercooled liquid region before crystallization, with the aim of investigating the reason for the appearance of the wide supercooled liquid region. The crystallization takes place through the precipitation of a b.c.t. Zr2(Cu, Al) single phase which grows with a dendritic mode. The Avrami exponent (n value) and the activation energies for the nucleation and crystallization are 3.7, 260 and 230 kJ mol−1 respectively. Based on existing structural data as well as the present results, it is presumed that the appearance of the supercooled liquid region is due to the difficulty of growth resulting from the necessity of long-range redistribution of Al at the solid-liquid interface and to the difficulty of precipitation of Zr2(Cu, Al) caused by the large solid-liquid interfacial energy resulting from the highly dense random packed structure consisting of elements with significantly different atomic sizes and attractive bonding nature.

The Zn-Mg-Y phase diagram involving quasicrystals

Abstract Isothermal sections of the Zn—Mg-Y equilibrium phase diagram at 700 K, 773K and 873 K have been determined in the region of 30–70 at.%Zn, 20–60at.%Mg and 0–20at.%Y using scanning electron microscopy (SEM), wavelength dispersive X-ray, X-ray diffraction and transmission electron microscopy techniques. The icosahedral phase has equilibrium phase fields with three distinct phases (L, Z, H) at 873 K, four distinct phases (L, W, Z, H) at 773 K and five distinct phases (L, W, Z, H, α) at 700 K. The equilibrium range of composition for the i-phase at these temperatures was determined.

New Stable Icosahedral Al-Cu-Ru and Al-Cu-Os Alloys

A thermodynamically stable quasi-crystalline single phase with an icosahedral structure was found to be formed at atomic compositions of Al65Cu20Ru15 and Al65Cu20Os15 in conventionally solidified and fully annealed states as well as in a rapidly solidified state. The quasi-crystalline nature determined by X-ray diffractometry is the highest for the fully annealed ingot, indicating the existence of a truly equilibrium icosahedral structure. The stable quasi-crystals appear to have a growth morphology of pentagonal dodecahedron with an average size of 0.03 mm in a fully annealed state.

Stable Zn[sbnd]Mg[sbnd]rare[sbnd]earth face-centred icosahedral alloys with pentagonal dodecahedral solidification morphology

Abstract A thermodynamically stable face-centred icosahedral phase with a pentagonal dodecahedral solidification morphology has been found in Zn[sbnd]Mg[sbnd]RE (RE [tbnd] Y, Tb, Dy, Ho or Er) alloys. We report here the structure, stability and morphology in this new group of icosahedral alloys on the basis of results for the Zn[sbnd]Mg[sbnd]Y system.