Interface Effects on the Mechanical Properties of Nanocrystalline Nanolaminates

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Interface Effects on the Mechanical Properties of Nanocrystalline Nanolaminates Alan F. Jankowski Texas Tech University, Mechanical Engineering Department, Lubbock, TX 79409-1021, U.S.A. ABSTRACT Nanocrystalline nanolaminate (ncnl) structures are widely used in the study of physical properties in order to engineer materials for a variety of industrial applications. Often, novel and interesting mechanical behaviours that are found in nanolaminate materials can be linked with two characteristic features of structure. These are the layer pair spacing and the grain size. For the case of nanolaminates synthesized by physical vapor deposition processes, the layer spacing corresponds with the repeating sequence of layer pairs and can be referred to as composition wavelength. The grain size is the average width of the tapered columnar structure along the growth direction. Since the mechanical properties of strength and hardness are known to functionally vary with the separation between dislocations in crystalline materials, both structural features can potentially contribute to the total interfacial area and the characteristic separation of interfaces that mitigate dislocation motion. In this investigation, the individual contribution of layer pair spacing and grain size to the total interfacial structure are each quantified in an assessment of strength and hardness. A model is proposed for the total interfacial area of the material volume under plastic deformation that can quantify the interfacial area contribution from the layer pairs and the grain size. It is anticipated that each structural feature can potentially dominate the plastic deformation of the nanolaminate as dependent upon the specific layer pair spacing, the grain size, and the extent of plastic deformation. INTRODUCTION Synthesis and structure Nanocrystalline nanolaminate (ncnl) structures are widely used in fundamental physical behaviour studies of materials. A variety of relevant applications are found industrially, such as optical band-pass filters for x-rays and neutrons [1-4], giant magneto-resistance [5-6] for highdensity recording media, in low-temperature stability analysis [7-9], for bonding through high energetic reactivity and ultra-hard wear-resistant surfaces [10-13]. Novel and interesting mechanical behaviours can appear when the nanolaminate structures are artificially synthesized as, for example, through physical vapor or chemical deposition methods. In general, the physical properties are dependent upon the specific nanostructural arrangements of atoms at interfaces and grain boundaries as well as within the constituent layers. Nanolaminates can have various degrees of crystallinity and are usually comprised of different elemental materials as, e.g., A and B in the A/B binary laminate system. The two primary structural features to describe the ncnl are: the characteristic layer pair spacing (hλ) which is known as the composition wavelength for a repeating sequence of A and B layers; and the grain size (hg). A schematic is show