Direct observation of grain rotations during coarsening of a semisolid Al–Cu alloy

Abstract

Sintering is a key technology for processing ceramic and metallic powders into solid objects of complex geometry, particularly in the burgeoning field of energy storage materials. The modeling of sintering processes, however, has not kept pace with applications. Conventional models, which assume ideal arrangements of constituent powders while ignoring their underlying crystallinity, achieve at best a qualitative description of the rearrangement, densification, and coarsening of powder compacts during thermal processing. Treating a semisolid Al-Cu alloy as a model system for late-stage sintering-during which densification plays a subordinate role to coarsening-we have used 3D X-ray diffraction microscopy to track the changes in sample microstructure induced by annealing. The results establish the occurrence of significant particle rotations, driven in part by the dependence of boundary energy on crystallographic misorientation. Evidently, a comprehensive model for sintering must incorporate crystallographic parameters into the thermodynamic driving forces governing microstructural evolution.3D microstructural evolution | sintering | Ostwald ripening | grain rotation | x-ray imaging W hether we realize it or not, our daily lives are marked by encounters with sintering, ranging from natural rock formations and glaciers to artificial products like the porcelain from which we eat or the amalgam fillings in many teeth. Sintering is an efficient method for combining loose granular precursors into solid objects of predetermined shape at relatively low temperature, circumventing the processing route of melting, casting, and machining. The applications of sintering are widespread. For example, it is the predominant method for producing bulk technical ceramics (1), and, thanks to advances in powder metallurgy, sintering is of growing importance in the fabrication of metals, as well (2). The focus in recent years on nanostructured materials-specifically, on materials for energy storage and conversion-has intensified the scientific investigation and modeling of sintering processes. The latter find application in the production of fuel cells (3) and battery electrodes (4). In other applications, such as catalysis (5), sintering can be highly undesirable, as it reduces the connectivity of pores and the free surface of nanoparticles. In all of these cases, we need a firm understanding of sintering fundamentals to achieve and retain desired materials properties during thermal processing.The reduction in free energy that accompanies the removal of a powder compact's surface area is the driving force behind sintering (2, 6). When two particles come into contact, two free surfaces are replaced by a single solid/solid boundary. If the particles are crystalline, the new interface is a grain boundary, the (excess) energy of which is generally on the order of one-third of that of a (single) free surface of equal area (6); consequently, the sintering process results in a net release of energy. As free surface area decreases, the powder ...

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