On the bcc, fcc, hcp, and amorphous polymorphs of Zr 3 Al
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S. Gialanella Dpto di Ingegneria dei Materiali, Universita di Trento, Mesiano 38050, Italy
T. BenAmeur LTPCM-CNRS UA29, Institut National Polytechnique de Grenoble, BP 75, Domaine Universitaire, 38402 St. Martin d'Heres, France
R.W. Cahn Department of Materials Science and Metallurgy, University of Cambridge, United Kingdom B. Bochu CMTC, Institut National Polytechnique de Grenoble, BP 75, Domaine Universitaire, 38402 St. Martin d'Heres, France (Received 13 February 1992; accepted 8 October 1992)
Rapid solidification of the Zr3Al liquid alloy allows retention of the high temperature /S-Zr solid solution with bcc structure. Mechanical grinding is shown to amorphize this metastable phase very easily. Calculations show that the retained bcc phase has a free energy above that of the amorphous phase. The density of bcc Zr3Al at room temperature is found to be 2% lower than that of its equilibrium Ll 2 -ordered fee structure as determined from their respective lattice parameters. The bcc phase thus represents a 2% volume expansion with respect to the fee structure.
Zr3Al can be rendered amorphous by techniques such as heavy ion bombardment1-2 or hydrogen absorption3 and by mechanical grinding.4 It has been thought that as the superstructure's long-range order parameter S is reduced by various techniques, the alloyfree energy is raised by energetic antisite defects and below a certain critical value of S, the alloy energy exceeds that of the metastable amorphous phase which forms instead, as in the case of CuTi intermetallics.5 However, irradiation by high energy electrons, which have been used to amorphize many other intermetallic phases, fails to amorphize Zr3Al even after reaching S = 0.6'7 Thus, Zr3Al appears to be particularly susceptible to lattice instabilities that result in amorphization but under conditions that are as yet only partially understood. Lattice expansion during ion irradiation and a shear instability (sharp drop of the elastic modulus) detected before amorphization have led to the suggestion that a critical level of lattice dilatation (corresponding to about 2% volume) is needed before crystalline order can crumble into an amorphous state.8 The amorphous phase of Zr3Al and other compositions (in atomic fractions) in the range 0.15 < CM < 0.4 were also obtained by Fecht et al.9 via mechanical alloying of pure Al and Zr powder mixtures. The hep a - Z r solid solution was maintained up to CM ~ 0.15. More recently, Ma and Atzmon10'11 measured the enthalpy release AH during return to equilibrium of 242 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 2, Feb 1993 Downloaded: 13 Mar 2015
mechanically alloyed Zr-Al powders and formation of intermetallic compounds (such as Zr2Al) using differential scanning calorimetry (DSC) and found a cusp in the AH versus concentration CM curve that followed the cusp formed at the intersection of the calculated free-energy curves of a - Z r A l and amorphous-ZrAl alloys between CM = 0.22 and 0.25, as in Fig. 1. According to the common tangent rule, if inter
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