Mechanisms of Grain Growth in Bubble-Fence-Delineated Films
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ABSTRACT The strength of polycrystalline metallic films at high temperatures is limited by grain boundaries. Boundaries transverse to the film surfaces typically fail by decohesion, while boundaries parallel to the film surfaces are susceptible to sliding. In light of these facts, multilayer films are now being made that resemble stacks of columnar-grained thin films[l]. Nearly planar arrays of insoluble bubbles, called "bubble fences," maintain the integrity of the individual layers by pinning the interlayer boundaries. However, the transverse boundaries that delimit the columnar grains within an individual layer are able to move. Processing aims to produce a highly-overlapping microstructure that isolates the weak-link transverse boundaries and increases the material's time-to-failure. In potassium-doped tungsten this strategy is an extension of current lamp filament technology. We have studied the mechanisms of grain growth within the grain layers in order to provide a basis for improving the synthesis and processing of these materials. For this type of grain growth to occur, the boundaries between grain layers must remain pinned by the bubble fences. Theory and 3D Evolver[2] simulations of a simple model provide a possible explanation for the depinning seen in recent experiments[3]. Reducing the ratio of intra-fence bubble separation to bubble diameter is predicted to eliminate the depinning. Ongoing study of a complete 3D model will further aid in tuning the synthesis and processing to create grain architectures with superior high-temperature properties.
INTRODUCTION Taking inspiration from incandescent lamp wire technology, a method was recently developed to create high-temperature-stabilized microstructures in potassium-doped tungsten and other structural alloys[4]. These new microstructures resemble stacks of columnargrained polycrystalline films (Figure 1). High-temperature strengthening is promoted by implanted barrier layers consisting of bubbles of an insoluble gas. These "bubble fences" trap the boundaries separating grain layers and hold them in stress-free planes. Additionally, they are thought to guide the recrystallization of the microstructure to favor pancake-shaped grains, bounded above and below by bubble fences[l, 3]. Using the same analysis as applies to short aligned fiber composites[5] it can be shown that failure stress increases with increasing grain overlap-to-thickness ratio, Lo,,/h or "aspect ratio," and approaches the grain yield strength. The scale of this approach is set by the aspect ratio where the failure mode crosses over from grain boundary sliding to grain failure[1]. Increases in failure stress lead to increases in lifetime at low stress and high temperature. Thus a highly-overlapping grain structure improves the material's time-to-failure. After initial fabrication, it is hoped that grain growth processing will yield grain growth within the individual grain layers. As long as the thickness of the grain layers is fixed by the bubble fence spacing, the growth of grains within
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