Solid State Reactions in Binary Mixtures of Nanometer-Sized Particles
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ABSTRACT Solid state reactions in mixtures of nanometer-sized Cu and Zr as well as Ni and Zr cristallites produced by inert-gas condensation followed by in situ compaction - have been investigated by x-ray diffraction and thermal analysis. The annealing behavior is compared to that of corresponding multilayer samples. The results are discussed with emphasis placed on the different parameters controlling solid state reactions.
INTRODUCTION Solid state amorphization reactions (SSAR) of binary metallic systems are governed by thermodynamic as well as by kinetic factors [1,2]. Thermodynamics answers the question whether or not excess Gibbs free energy exists that can act as a driving force for the transformation of the system into the amorphous state. For this to occur, kinetic constraints must obtain that prevent the system from transforming into the energetically more favorable equilibrium state (intermetallic compound). Such kinetic boundary conditions may result from different diffusivities of both atomic species, i. e., only one type of atom has an appreciable mobility in a given temperature window, whereas the other atoms are virtually immobile in comparison to the fast-moving species. The nucleation and growth of the amorphous phase by SSAR has previously been studied by investigating the time-temperature behavior of crystalline metallic muhtilayers (ML) [1,21. The amorphous phase was found to be nucleated at defect sites in the lattice of the slow-moving species, and its growth is determined by the diffusivity of the fast-moving species through the already-grown amorphous interlayer. Unlike ML's, nanocrystalline materials (NCM) are composed of 3-dimensional objects, i.e., nmsized crystallites, separated by highly incoherent interfaces. The specific interfacial energy of NCM can be as high as twice as that of large-angle grain boundaries in conventional polycrystalline materials [3]. Also, the diffusivity in the grain boundaries of NCM has been found to be more than four orders of magnitude larger than that in equilibrated grain boundaries [4]; hence, we expect NCM to behave differently compared to multilayered materials with respect to SSAR. The model systems chosen for studying SSAR in binary mixtures of nm-sized crystals were Cu-Zr and Ni-Zr - the former because its preparation is relatively simple, and the latter because numerous experimental data on SSAR in Ni.Zrl.x ML's are available in the literature [1,2]. Both systems fulfill the necessary conditions for SSAR to occur- nonetheless, there are differences in the driving force as well as in the diffusivities (Table I).
331 Mat. Res. Soc. Symp. Proc. Vol. 321. (-1994 Materials Research Society
Table I. Free energy of mixing for binary amorphous and intermetallic alloys of equiatomic composition. Calculation of the driving force AG was performed using the semiempirical Miedema model [5]. The diffusivities of Cu, Ni and Zr in the Zr-matrix are parameters for the factors determining the kinetics.
Cu-Zr
driving force AG [kJ/mol] [5] amorphous intermetalli
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