Patrick R. Cantwell

Segregation-induced ordered superstructures at general grain boundaries in a nickel-bismuth alloy

Giving grain boundaries more structure The properties of metals change depending on the composition and structure of grain boundaries in polycrystalline materials. Yu et al. discovered a surprising grain boundary superstructure in a nickel-bismuth alloy. Previously, the structure was only known to exist in a specific type of uncommon grain boundary, and experiments had focused on bicrystals. Unexpectedly, this alloy has grain boundary superstructures across a wide range of boundaries in polycrystalline samples. This likely also occurs in other alloys, which opens an avenue for grain boundary engineering to tune the physical properties of metals and ceramics. Science, this issue p. 97 High-resolution atomic transmission electron microscopy reveals new order across a range of grain boundaries in a Ni-Bi alloy. The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains. This discovery shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.

A Grain Boundary “TTT” – “Tribute to Thomas” !

I had the good fortune to study electron microscopy at UC Berkeley during my graduate studies under the expert guidance of Gareth Thomas. He was a true master of the electron microscope and had an incredible gift for applying electron microscopy to solve critical problems. My year-long fellowship under Gareth was a pivotal experience in my life and had a profound influence on my conception of how materials science research should be conducted. I am deeply grateful to Gareth for his influence on my development as a materials scientist and I truly believe that he deserves recognition for his positive impact on an entire generation of materials scientists such as myself. I would like to honour Gareth by highlighting a few of my research endeavours in which electron microscopy has played a crucial role.