Amorphization and metastable polymorphs of ordered intermetallics Zr 3 Al and Ni 3 Al
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Observations of various metastable crystalline and amorphous polymorphs of Ll 2 -type intermetallics Zr3Al and Ni3Al are summarized. The difference in the tendency of these two alloys to become amorphous is obvious. Regardless of the nonequilibrium routes employed for synthesis and processing, amorphous Zr3Al is obtained easily, while amorphization of Ni3Al is rare. An analysis of thermodynamic and kinetic factors responsible for the observed polymorph selection and amorphization behavior is presented. The inadequacy of a simplified model proposed earlier is also discussed.
I. INTRODUCTION Disordering, amorphization, and polymorphous phase transformations of ordered intermetallics have been topics of considerable scientific and technological interest. Recently, BenAmeur and Yavari have compared the amorphization tendency of Ll 2 (Cu 3 Au)-type intermetallics, Zr3Al and Ni3Al, upon mechanical attrition.1 In a later, related paper,2 Yavari et al. also discussed several metastable polymorphs of Zr3Al (disordered fee, bec, and hep solutions). An obvious difference in amorphization tendency upon mechanical attrition was observed: while Zr3Al amorphized completely within 5 h, the disordered fee Ni3Al phase dominated and persisted even after over 200 h. A simple model was proposed to explain this contrasting amorphization behavior in terms of the shape and position of the free energy versus composition curves (hereafter referred to as the G curve) of the disordered fee phase.1 It was argued that the positive free energy change (lattice stability term) corresponding to the hep to fee transformation of the Zr component (reference state is equilibrium hep Zr), which is absent for the disordered fee Ni-Al since fee is the equilibrium reference state for both Ni and Al, raises the G curve of the disordered Zr-Al fee solid solution toward that of the amorphous phase. After constructing a common tangent to the G curves of the fee and the amorphous phase, the contact points would therefore correspond to lower Al concentrations in the Zr-Al case compared with those in the Ni-Al case (see Fig. 10 of Ref. 1). According to the authors of Ref. 1, the relative amounts of fee phase and amorphous phase formed during mechanical attrition of Zr3Al and Ni3Al are determined by such common tangent construction and the lever rule. It was believed that the 25 at. %-Al composition is close to the common tangent contact point on the G curve of amorphous phase in the Zr—Al system, thus giving rise to mainly 592
J. Mater. Res., Vol. 9, No. 3, Mar 1994
amorphization, whereas it is close to the contact point on the G curve of the fee phase in the Ni-Al case, hence leading to little amorphization. The above reasoning, intended also to explain amorphization trends in systems other than Zr-Al and Ni-Al (e.g., Ni-Ge), 1 relies on assumptions including: (i) A metastable two-phase equilibrium between the disordered fee phase and the amorphous phase is established during attrition, such that a common tangent construction is warranted; and (ii) The presen
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