A thermodynamic and kinetic basis for understanding metastable phase formation during ion-beam mixing of nickel-aluminum
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Lynn Rehn Argonne National Laboratory, Materials Science and Technology Division, Argonne, Illinois 60439
(Received 1 December 1986; accepted21 March 1988) A quantitative thermodynamic explanation for the formation of metastable phases in the nickelaluminum alloy system through heavy-ion irradiation is presented. The role of kinetics in the transformation to a metastable state is also investigated. Experiments involved the irradiation of both layered nickel-aluminum samples and ordered intermetallic compounds with 500 keV krypton ions over a range of temperatures and compositions. Samples were formed by alternate evaporation of layers of nickel and aluminum. A portion of these samples was subsequently annealed to form intermetallic compounds. Irradiations were performed at both room temperature and 80 K using the 2 MV ion accelerator at Argonne National Laboratory. Phase transformations were observed during both in situ irradiations in the high-voltage electron microscope at Argonne and also in subsequent electron diffraction analyses of an array of irradiated samples. Metastable phases formed included disordered crystalline structures, an amorphous structure, and a hexagonal-close-packed structure. These phase structures were modeled using the embedded atom method to compute heats of transformation Ai7s_ms from stable to metastable states. It was found that metastable states that have moderate heats of transformation, A//S_ms a 15 %-20% of the heat of formation of the stable phase, form under irradiation. Metastable states with high heats of transformation, A.Hs_ms =s 50% of the heat of formation of the stable phase, do not form under irradiation. Kinetics also play an important role in determining the effect of temperature and initial structure on the formation of metastable phases.
I. INTRODUCTION A. Background The formation of metastable phases through ion irradiation and ion-beam mixing has received much attention in recent years (see, for example, Refs. 1 and 2.) One reason for this interest is the potential for creating materials with very useful properties, such as relatively high superconducting transition temperatures.3 The goal of much recent work has been to explain why certain metastable phases form in some alloys but not in others. The term "metastable" refers to a phase with a free energy higher than that of the stable phase under the prevailing conditions of temperature and pressure. It is natural, therefore, to seek a thermodynamic explanation for the effect of irradiation on the formation of metastable structures. After investigating the amorphization of pure silicon by heavy-ion irradiation, Swanson4 proposed that the primary effect of the irradiation was the production of point defects in the material. The energy of these defects raised the free energy of the silicon to the point where the defected crystalline structure had a higher free energy than that of amorphous silicon, and so the material transformed to the amorphous state. This con626
J. Mater. Res. 3 (4), Jul/Aug 1988
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