Mechanical Properties in Small Dimensions
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Mechanical
Properties in Small Dimensions
Richard P.Vinci and Shefford P. Baker, Guest Editors Abstract This brief article describes the content of the January 2002 issue of MRS Bulletin focusing on Mechanical Properties in Small Dimensions. Articles discuss the current understanding of stress evolution during thin-film growth, elastic and anelastic behavior, dislocation-mediated plasticity, creep deformation, and fracture. Emphasis is placed on explaining the mechanisms that underlie the well-known fact that length scale can play a significant role in mechanical behavior. Keywords: mechanical properties, thin films.
Like many other properties, the mechanical properties of materials begin to deviate from bulk scaling laws when characteristic dimensions become small. Such deviations may occur when either microstructural features (e.g., grain size) or object dimensions approach the length scales of defects, defect interactions, or processes that control deformation. Unlike other physical properties, which deviate from continuum models only at atomistic length scales, mechanical properties are often found to deviate from bulk scaling behavior at surprisingly large length scales. In many cases, deviations are clearly apparent when the smallest relevant features are in the micrometer regime. A common example is the fact that micrometer-scale thin films are often found to support much higher stresses than bulk samples of the same material. This has been attributed to constraints on dislocation motion or diffusion imposed by the interfaces with the surrounding layers and to the smaller grain size that is often found in films. The combination of a tremendous technological driving force and a wealth of unfamiliar behavior has led to much interest and work in this area. However, progress has been slow. With few exceptions, mechanical behavior is determined by the ensemble behavior of defects (vacancies, grain boundaries, dislocations, cracks). A typical approach is to select a single defect
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behavior (e.g., blocking of dislocations at grain boundaries, diffusion along interfaces) that is thought to be controlling and build a model around that behavior. Some attempts have been made to combine mechanisms, but since the appropriate means for summing the effects of different mechanisms are not clear, such approaches are limited. Further complications arise due to experimental difficulties. Restricted volumes and geometries may preclude the typical approach of geometrically confining deformation to a well-defined gage section in a “dog-bone”-shaped sample in uniaxial loading. Furthermore, small strains in small volumes translate into extremely small displacements that must be imposed and measured, and controlled at boundaries. A great deal of effort has gone into developing tests that allow one to measure small samples, films, and patterned structures by a variety of means with good resolution. One promising path for the future is to combine experiments with theory and simulations that can incorporate the effects of many d
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