Electron microscopy of quasicrystalline phases in aluminium-

Abstract

The discovery of icosahedral quasicrystals in rapidly solidified Al-Mn alloys has generated considerable interest among solidstate scientists, since the icosahedral pointgroup symmetry (m35) is incompatible with translational periodicity. The present study aims to understand the nucleation, growth, morphology, and structure of quasicrystalline phases in Al-Mn and Al-Pd alloys. During the investigation, it was established for the first time that a new phase, called the Tphase, exhibits twodimensional quasiperiodicity combined with onedimensional translational periodicity. The pointgroup symmetry of the decagonal phase is 10/mmm, which is also incompatible with translational periodicity. Owing to its structural similarity to the icosahedral phase, similar studies on nucleation, morphology, and structure were undertaken for the decagonal phase as well. Additionally, vacancyordered phases reported in the literature were investigated, and their onedimensional quasiperiodicity was demonstrated. In general, materials exhibiting quasiperiodic spacing modulations in three, two, and one dimensions were examined. Aluminium-manganese alloys with Mn concentrations of 5 at%, 10 at%, 14 at%, and 16 at% were selected. Highpurity (4N) starting materials were used. Rapid solidification was achieved using meltspinning and twinroller quenching techniques. Al-6% Pd samples were obtained as rapidly solidified ribbons from Dr. H. A. Davies, University of Sheffield. The icosahedral phase was found to form in Al-Mn alloys with Mn concentrations as low as 5%. Both nucleation rate and the amount of icosahedral dendrites increased with increasing Mn concentration. At low Mn concentrations, nucleation appeared to be heterogeneous, likely due to the insufficient driving force for homogeneous nucleation since the ideal composition is ~20 at% Mn. For Mn concentrations 10%, homogeneous nucleation of the icosahedral phase was observed. A transition from nonfacetted to facetted dendritic morphology was observed with increasing Mn concentration. This study provided the first report of facetted quasicrystals. With cooling rate held constant, compositions above ~10 at% yielded facetted dendrites. Analysis of morphologies showed that nonfacetted icosahedral dendrites resemble a modified dodecahedron with protrusions along the twenty 3fold axes, resembling a twentypronged starshaped solid composed of prolate rhombohedra. In contrast, the facetted form exhibited a pentagonal dodecahedral morphology. In both cases, growth occurred along the 3fold axis. Beyond the electrondiffraction patterns corresponding to the 5, 3, and 2fold symmetry axes reported in earlier literature, additional patterns corresponding to the [10], [1\overline{1}110], and [11\overline{1}10] zone axes were calculated using Landaugeneration techniques and confirmed stereographically. Experimental and simulated patterns showed excellent agreement. Intensity differences in the 2fold and related patterns were attributed to electrondiffraction dynamical effects or atomic decoration variations on the quasilattice. Highresolution electronmicroscopy images revealed quasiperiodic fringe patterns. Images normal to the 5fold axis showed five interpenetrating fringe sets oriented at 72°, while images normal to the 2fold axis showed two sets corresponding to the 2fold and 5fold directions. Some fringes corresponding to the inflated directions appeared periodically spaced. The decagonal phase formed in Al-Mn alloys at Mn concentrations 14% and coexisted with the icosahedral phase. It nucleated epitaxially on preexisting icosahedral grains. At lower cooling rates, the decagonal phase coexisted with crystalline AlMn and appeared to nucleate independently from the undercooled melt. The Al-Mn decagonal phase had a platelike morphology and exhibited characteristic striated contrast in TEM images. The disappearance of contrast when viewed along the 10fold axis indicated the presence of defects aligned parallel to this axis. In Al-Pd alloys, the decagonal phase appeared as a eutectic constituent with fcc aluminium and coexisted with metastable cubic AlPd, orthorhombic AlPd, and ordered multiple twins. The Al-Pd decagonal phase exhibited striated contrast similar to its Al-Mn counterpart. Electrondiffraction patterns of the Al-Mn decagonal phase closely resembled those of the icosahedral phase. A new structural model based on a distorted edgecentred icosahedron introducing commensuration along one direction was proposed to explain the diffraction patterns. A stereogram constructed from these vectors accounted for all observed patterns. Streaks perpendicular to the 10fold axis suggested cylindrical diffracting regions aligned along the axis. Although Al-Pd decagonal phases yielded similar diffraction patterns, subtle differences suggested that a vertexcentred icosahedral model might be more appropriate. The ordering vector along the decagonal axis was divided into six equal parts in Al-Mn but into eight parts in Al-Pd. The periodicity was 1.24 nm for Al-Mn and 1.65 nm for Al-Pd. The Al-Mn phase displayed ordering consistent with the Fibonacci sequence (1,1,2,3,5,...), whereas the Al-Pd phase followed the Lucas series (3,4,7,11,18,...), implying quasiperiodic modulation of latticeplane compositions. Highresolution images normal to the 2fold axis of the Al-Mn decagonal phase showed periodically spaced fringes of 1.24 nm, consistent with diffraction data. The fringes displayed chainlike features and local discontinuities, indicating significant structural defects. In Al-6% Pd alloys, twinned cubic crystals with rhombohedral distortion were also observed. Their lattice parameter was determined to be 1.98 nm. Multiple twinning of five such crystals produced pseudotenfold diffraction symmetry. Vacancyordered phases were shown to behave as onedimensional quasicrystals, with stacking sequences along [111] following the Fibonacci sequence. Structurefactor calculations and higherdimensional projections reproduced experimental intensities and spacings, demonstrating that for repeat periods beyond thirteen layers, phases become indistinguishable from true quasiperiodic structures. This supports the hypothesis that vacancy ordering contributes to stabilizing quasiperiodicity in one, two, and three dimensions.