Stress-Driven Surface Topography Evolution in Nanocrystalline Al Thin Films

In their as-fabricated state, nanocrystalline (nc) materials, consisting of individual grains or crystallites with diameters less than 100 nm, are systems that exist far from equilibrium. The large volume fraction of grain boundaries (GB) present in these materials contains considerable excess free energy, which provides a large driving force for grain growth. Grain growth in nc materials has been reported to occur spontaneously and with relatively small thermal loads, but a surprisingly large number of nc materials exhibit remarkable thermal stability. The long-term stability of these materials is not fully understood and is an area that is receiving increased attention. Indeed, numerous synthesis techniques have been implemented to create nc materials that demonstrate unique optical, magnetic, electrical, and mechanical properties in comparison to their coarse-grained counterparts. Superior mechanical behavior, such as elevated strengths and improved fatigue resistance, has been reported and has fueled an intense desire to use these materials for structural applications. Of particular interest in the current study is the fact that room-temperature stress-assisted grain growth has been shown to have a dramatic and dynamic effect on the deformation behavior in certain nc metals. Although this phenomenon has not been universally reported in nc materials, studies of Al, Cu, Co, Ni, and Fe have shown changes in hardness, strength, and ductility that are due to nanostructural evolution. These experiments report post-mortem deformation observations of grain growth, but direct in situ transmission electron microscopy (TEM) observations of localized grain coalescence and agglomeration have also been reported. The profound influence of stress-assisted grain growth in nc Al is described in detail in earlier work; briefly, two general classes of deformation behavior are measured. The first is exhibited by nc metals that maintain a stable microstructure during deformation and show very strong but brittle behavior. In contrast, nominally identical specimens demonstrating microstructural evolution display intermediate strength and surprisingly large amounts of tensile ductility. Traditional driving forces for thermal grain growth (e.g., GB surface tension, surface energy minimization, inhomogeneously stored dislocations, elastic strain energy anisotropy) have been considered, but evidence to support these is limited. The inability to describe the characteristics of the grain growth in nc-Al freestanding thin films with traditional driving forces for grain growth and the observation of growth only in the highly deformed regions of the sample suggest that stress-assisted GB migration is the underlying cause of these phenomena. The level of impurities appears to be a key feature in distinguishing between nc materials that exhibit stressassisted grain growth and those that do not. In particular, increasing the impurity content, by adjusting the vacuum base pressure during deposition, increases the nanostructural stability of nc-Al thin films. This provides the potential for using processing techniques with precise dopant control as a strategy for tailoring the mechanical behavior of nc metals via nanostructural stability. The notion that the motion of low-angle boundaries is coupled to the applied shear stresses is widely accepted and usually interpreted in terms of the collective motion of the discrete dislocations that comprise the interface. Extensions of this coupling to high-angle boundaries have been elusive, due in part to the inability to directly observe and model the high-density dislocation content that defines the boundary. Nevertheless, experimental observations of stress-induced normal GB motion have been reported for Al bicrystals with both lowand high-angle tilt boundaries. More recently, a universal theory of the coupling phenomenon has been proposed, wherein ideal coupling of motion within the GB plane (shear strain) to normal motion of the boundary is described by a coupling factor, b, that depends on misorientation angle and temperature and ranges from –1 to 1. Molecular dynamics (MD) simulations have been used to illustrate the normal migration of flat low and high angle <001> symmetric tilt boundaries in Cu and <110> Al GBs, and also of grain rotation associated with the motion of curved GBs. GB migration has also recently been observed as a result of applied stress in simulations of polycrystalline systems with more general boundaries. In addition to highlighting the role of stress-assisted grain growth in nc-metals, the recent experimental observations of C O M M U N IC A IO N