Micrometer-sized quasicrystals in the Al 85 Ni 5 Y 6 Co 2 Fe 2 metallic glass: A TEM study and a brief discussion on the
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J.Q. Wang Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
C. Kong Electron Microscopy Unit, University of New South Wales, Sydney, NSW 2052, Australia
G.B. Schaffer and M. Qianb) The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, Centre for Advanced Materials Processing and Manufacturing, Brisbane, QLD 4072, Australia (Received 16 February 2012; accepted 18 April 2012)
Large quasicrystals up to ;10 lm in size with a volume fraction of ;30% have been identified in a nitrogen gas-atomized marginal glass-forming alloy Al85Ni5Y6Co2Fe2 by detailed transmission electron microscopy. The formation of the large quasicrystal (Q) phase is discussed through the configuration of the valence electrons of its constituent elements, and the thermodynamic and kinetic factors associated with the solidification of this marginal glass-forming alloy during gas atomization. The finding leads to an important inference that marginal glass-forming alloys could be ideal systems for the formation of bulk quasicrystals under appropriate kinetic conditions. The Q phase is stable up to ;500 °C and decomposes thereafter. The activation energy for the decomposition of the Q phase is similar to the self-diffusion of Al. Two new intermetallic phases associated with the formation and decomposition of the Q phase have also been identified and characterized. I. INTRODUCTION
Quasicrystals are a unique form of solid state between the crystalline and amorphous states. They show orientational symmetry but lack translational symmetry; their discovery greatly challenged conventional crystallographic concepts.1–3 Quasicrystals normally show high hardness and/or high electrical resistivity because of their special atomic arrangement.4 They have the potential as reinforcement phases for composites5,6 and/or as thermal barrier materials.7 In addition, some quasicrystal materials show unique hydrogen storage features. A typical example is the ternary Ti45Zr38Ni17 quasicrystalline alloy, which shows a hydrogen-to-metal ratio of . 1.60.8,9 The underlying reason is that the Q phase contains a large proportion of interstices, which can accommodate interstitial hydrogen.8,9 In contrast to quasicrystals, metallic glasses (MGs) have no long-range order and possess a variety of unique
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2012.140 J. Mater. Res., Vol. 27, No. 16, Aug 28, 2012
properties such as high fracture strength, exceptional superplasticity in the supercooled liquid region and good soft magnetism.10,11 Quasicrystals do not often occur in MG-forming alloys but where they do they usually form as nanoscale quasicrystals (Table I).12–15 Their formation in MG-forming alloys can be induced by the introduction of elements such as Ag (up to 5 at.%),12 Pd (up to 35 at.%)12 or O (up to 0.82 at.%).13,14 Their nanoscale size hinders a tho
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