Solid state amorphization reactions in deformed Ni-Zr multilayered composites
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I. INTRODUCTION
Single phase amorphous alloys can form at relatively low temperatures (approximately half pertinent melting temperatures) by means of interdiffusion of pure, polycrystalline elements.1"3 For instance, upon heating a multilayered, Ni/Zr diffusion couple at 600 K for 30 min, amorphous material (with a thickness of approximately 1000 A) is observed to grow at the interfaces between the two metals. Thus a Ni/Zr composite with equal layer thicknesses of less than 1000 A can be completely amorphized.4 Two requirements have been proposed for a successful solid state amorphization reaction, SSAR: (a) the two metals which form the diffusion couple must possess a large, negative heat of mixing in the amorphous phase and (b) there must be a dominant moving species; i.e., one constituent of the diffusion couple should exhibit a much greater mobility than the other.1'5'6 Such a disparity in the mobility of the atoms in the diffusion couple provides a constraint on the formation of equilibrium intermetallic compounds in a given temperature range and time frame, i.e., a kinetic constraint. The movement of both constituents is apparently required to nucleate and grow crystalline material while the mobility of only one constituent is required to grow an amorphous alloy.7 "'Current address: Physics Department, University of California at Berkeley, Berkeley, California. b)Also with (and current address) The Department of Physics, Applied Physics and Astronomy, State University of New York at Binghamton, Binghamton, New York 13901. 488
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 3, Mar 1990
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Experimental observations in the Ni/Zr system have been performed supporting both of these hypotheses. Various diffusion studies indicate that Ni is the dominant moving species in the SSAR in Ni/Zr diffusion couples.8"10 Differential scanning calorimetry (DSC) measurements have been utilized to establish that a negative difference in free energy between the pure elements and the amorphous phase (8-10 kcal/mol) drives these solid state amorphization reactions in the Ni/Zr system.4'11"13 In this present work we have studied solid state amorphization reactions in mechanically deformed Ni-Zr multilayered composites.12'14 The compositional dependence of the heat of mixing has been investigated and various aspects of the kinetic processes controlling these solid state amorphization reactions, including the competition between crystallization and amorphization processes, have also been studied. We find the thermal stability of the growing amorphous phase can be qualitatively understood in terms of the thermal stability of liquid quenched metallic glasses of similar stoichiometries. II. EXPERIMENTAL PROCEDURE
Multilayered composites of transition and/or rare earth metals can be produced in bulk form relatively easily by means of mechanical deformation, thus facilitating a systematic study of solid state reactions. High purity (99.9%) foils of Zr and Ni metals were arranged in alternating layers, pla
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