(116f) Topological Transitions of Spiral Nanocomposites in Extreme Environments

Advances in high-temperature technology have raised the need for materials with superior strength, rigidity, and ductility at elevated temperatures. To meet the challenge of higher operation temperatures, composites based on intermetallics have been considered. Directional solidification of (binary) eutectics can produce regular patterns consisting of a nanoscale spiral intermetallic phase. In particular, it was recently reported that nanospiral formation follows a two-step process, wherein the first step is mediated by the low solid–liquid interfacial energy of a precursor phase, and the second step by crystallographic defects on the precursor [1]. Understanding the behavior of these spiral nanocomposites at elevated temperature is critical to their function in extreme environments. Under such conditions these spiral nanocomposites could degrade as a result of oxidation, reaction, and shape/size instabilities.

Here, we investigate the thermal stability of these spiral nanocomposites through continuous isothermal annealing near their eutectic temperature using in situ X-ray nano-tomography (nTXM), and further corroborated by ex situ crystallographic and compositional analyses. Collectively, the results identify that the structure does not evolve self-similarly, with topological breakdowns in both short- and long-term annealing due to reactive and bulk diffusion, respectively: (i) Within minutes, solid-state, reactive diffusion (a high Damkohler number event) leads to phase transition from the metastable to the stable phase, which in turn causes lamellar pinch-off of the thinner portions of the intermetallic phase. (ii) During prolonged annealing (hours), bulk diffusion due to Ostwald coarsening leads to more pinch-off events and thus the formation of a greater number of disjoint components of the intermetallic phase. By unearthing the above solid-state dynamics, this study helps to generate accurate structure-performance relationships that could guide the future application of these spiral in situ composites in extreme environments.