Thermodynamic Study of Liquid, Crystalline and Quasi-Crystalline Al-Mn Phases
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Section I: Basic and Applied Research
Thermodynamic Study of Liquid, Crystalline and Quasi-Crystalline Al-Mn Phases A.I. Zaitsev, N.E. Zaitseva, N.A. Arutyunyan, and B.M. Mogutnov
(Submitted February 6, 2007; in revised form September 24, 2007) Thermodynamic properties of liquid, intermetallic, and quasi-crystalline phases of the Al-Mn system have been studied using two experimental techniques: integral effusion method and Knudsen-cell mass spectrometry. The functions of the liquid phase followed the laws of associated solution and indicated that three type complexes, AlMn, Al2Mn, and Al5Mn, existed in the melt. Phase equilibria calculated using this model and the intermetallic compounds thermodynamic characteristics were in close agreement with phase diagram. The decagonal phase was found to be more stable than the icosahedral quasi-crystals. The calculated enthalpies of the quasi-crystalline phase transformation to the equilibrium crystals were close to the available literature information. The difference between the Gibbs energies of both type quasi-crystals and equilibrium crystalline compositions increases with decreasing temperature. This evidence favors the conclusion that the stabilization of quasi-crystals is entropic in nature and that the quasi-crystalline state, similar to the glassy state in metallic alloys, is only intermediate between liquid and crystals. The conditions of the quasi-crystals formation were found to be closely related to a certain type of the chemical short-range order in liquid, namely to significant mole fraction of the Al5Mn associative complexes.
Keywords
Al-Mn system, aluminum alloys, mass spectrometry, quasi-crystal, phase diagram, thermodynamic properties, thermodynamic stability
1. Introduction Considerable numbers of investigations have been undertaken since the discovery of quasi-crystalline phases, but the question about their thermodynamic stability has not been answered. It is quite clear that only thermodynamic information about quasi-crystals can be the basis for reaching any conclusions on their nature and state (stable, metastable, or labile) as well as for understanding whether quasi-crystals can represent the ground stable state of alloys. However, the available data on the thermodynamic properties and data concerning the conditions for formation of quasi-crystals are scarce. In the only work[1] the heat capacities of the Al86Mn14 alloy in the quasi-crystalline and equilibrium crystalline states were measured over a wide temperature range (310 to 730 K). The heat capacity of quasi-crystals was shown to exceed systematically that of crystals by 2 to 3 J/(mol Æ K). This result may be considered an indication that the thermodynamic stability of the quasi-
A.I. Zaitsev, N.E. Zaitseva, and B.M. Mogutnov, I.P. Bardin Central Research Institute for the Ferrous Metallurgy, Vtoraya Baumanskaya ul. 9/23, Moscow 105005, Russia; N.A. Arutyunyan, Chemistry Department, Moscow State University, Leninskie gory, Moscow 119992, Russia. Contact e-mail: [email protected].
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