Orientation-Dependent Thermal Morphological Evolution of α-Fe Nanopillars

One-dimensional nanostructures are typically single-crystalline, yet the mechanisms by which crystallographic orientation governs their thermal morphological evolution, a process that critically dictates their structural integrity and functional performance in high-temperature applications, remain poorly understood. Here, by observing the shape evolution of single-crystalline α-Fe nanopillars near 0.48 of the melting temperature, we show that increasing axial index results in stronger spheroidization and faster shortening. This behavior originates from a coupled thermodynamic-kinetic effect. On low-index axial pillars, strong effective surface-energy anisotropy stabilizes {100} and {110} facets, while limited adatom generation on these facets slows surface diffusion and shortening. In contrast, geometric constraints on high-index axial pillars promote the exposure of high-index facets, reducing effective surface-energy anisotropy and facilitating adatom formation, thereby accelerating spheroidization and shortening. These results identify axial orientation as a key parameter governing the thermal evolution pathway of one-dimensional nanostructures and provide design principles for engineering thermally robust nanoscale systems.