The solidification of aluminum-manganese powders
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The solidification of aluminum-manganese powders B. A. Mueller Engineered Materials Development, Battelle Columbus Division, Columbus, Ohio 43201
R. J. Schaefer Metallurgy Division, National Bureau of Standards, Gaithersburg, Maryland 20899
J. H. Perepezko Department of Metallurgical and Mineral Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received 29 April 1987; accepted 20 July 1987)
The solidification behavior of Al-Mn alloy powders was studied as a function of coojing rate and manganese content. The phases present in the powders differed from those expected at equilibrium due to competetive nucleation and growth kinetics. The equilibrium Al6Mn phase was absent due to its relatively sluggish nucleation kinetics, and in the more concentrated and rapidly cooled alloy powders, the metastable quasicrystalline phases were present. Nucleation temperatures measured in alloys cooled at 25 °C/s are interpreted to represent formation of the icosahedral phase, which subsequently transforms to the decagonal phase and provides a lower bound estimate of 870 °C for the melting point of the icosahedral phase in an Al-30 wt. % Mn alloy.
I. INTRODUCTION The Al-Mn system has received considerable interest since the identification of phases that possess noncrystallographic icosahedral1 and decagonal symmetry.2 These metastable phases exhibit sharp diffraction patterns. However, their respective five- and tenfold rotational symmetries are inconsistent with lattice translation, and as such these phases are termed quasicrystalline. The formation of the metastable icosahedral and decagonal phases in rapidly solidified Al-Mn alloys prepared by melt spinning and atomization is well documented.3"6 In addition to rapid solidification techniques, the quasicrystalline phases have been formed by the ion-beam mixing and solid-state interdiffusion of thin Al/Mn films7'8 and by a rapid pressurization treatment.9 While the formation of quasicrystals from the melt is known to be a first-order reaction, the undercooling/cooling rate conditions prevailing during rapid solidification processing are poorly known. All that can be said with certainty is that the quasicrystals nucleate below the equilibrium alloy liquidus, but above the melting point of aluminum.10 Although the formation of metastable phases is often thought to require very high liquid cooling rates, they may also be formed in slowly cooled fine powders (10-150 fim), where the solidification conditions may be measured directly.11 Utilizing slowly cooled powders, undercoolings ranging from 1-175 °C have been measured for aluminum.12 While there is currently uncertainty as to the equilibrium Al-Mn phase diagram, it is clear that it includes a eutectic reaction Al + Al 6 Mn/L, and that a series of intermetallic compounds form by peritectic or peritectoid reactions.13 J. Mater. Res. 2 (6), Nov/Dec 1987
These reactions, as well as the formation of the metastable quasicrystalline phases, allow for a wide variety of possible phase reactions. The objective of this
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