Effect of Structure on the Anomalous Mechanical Properties of Metallic Superlattices
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EFFECT OF STRUCTURE ON THE ANOMALOUS MECHANICAL PROPERTIES OF METALLIC SUPERLATTICES IVAN K. SCHULLER *, A. FARTASH*t, ERIC E. FULLERTONt, AND M. GRIMSDITCHt *Physics Department 0319, University of California-San Diego, La Jolla, CA 92093-0319 tMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439 ABSTRACT The mechanical properties of metallic superlattices have been shown to exhibit anomalous properties. Several of the elastic constants are found to exhibit anomalies which are correlated with structural anomalies in lattice mismatched systems which do not form solid solutions. Lattice matched systems which form solid solutions in their thermodynamics phase diagram, show much smaller elastic anomalies and no structural anomalies. Anomalous plastic behavior, on the other hand, seems to be present in both types of superlattices, indicating that the plastic behavior is possibly defect induced. Detailed quantitative structural measurements combined with comprehensive mechanical properties hold the promise of determining the physical origins of the anomalous properties of metallic superlattices. INTRODUCTION It is by now well established that metallic superlattices exhibit anomalous mechanical properties. These include the presence of anomalies in the elastic moduli of lattice mismatched superlattices, and anomalous behavior in the plastic behavior in selected superlattice systems. Simultaneous, quantitative, structural studies indicate that the elastic anomalies are correlated with microscopic structural anomalies and mostly occur in systems which are lattice mismatched and do not form solid solutions in their thermodynamic phase diagram. The anomalous plastic behavior, on the other hand, does not show any clear cut correlations with crystallographic changes or the thermodynamic phase diagram. This indicates that the plastic behavior is possibly modified by defects present at the interfaces. We present here a review of structural characterization techniques including a new method developed for the quantitative structural refinement studies using X-ray diffraction. These structural studies are correlated with the mechanical properties. The studies presented here show that different types of elastic anomalies can coexist in the same superlattice and that they are correlated with structural anomalies. The exact way in which the elastic properties are affected by changes in the structure is under investigation at the present time. STRUCTURAL CHARACTERIZATION Given the strong correlation between the structural and elastic properties of superlattices, it is of the utmost importance to quantitatively characterize their structure. The structural properties of interest include both the lattice spacing changes resulting from interface strains and the amount of structural disorder (i.e. layer thickness fluctuations, interface disorder, interdiffusion, and dislocations). The two techniques most often applied to this problem are Transmission Electron Microscopy (TEM) and X-ray diffraction. If the sample can be
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