Spontaneous formation of sub-4 nm nanocrystalline alloy via polymorphic phase transformation

A new phase-transformation-induced path to spontaneous formation of extreme nanograin structure is reported. In-situ-heating-mode-microscopy exhibited a substantial grain-growth of Cu₆Sn₅. During cooling, the grain-growth continued, but it spontaneously switched to grain-refinement mode on phase transformation through ∼180 °C from η-Cu₆Sn₅ to η’-Cu₆Sn₅, ending up with an extremely small nanograin size of ∼2.5 nm. The cooling cycling always restores the nanograin size regardless of thermal exposure history, making this to be the first demonstration to stabilize the nanograin with its own spontaneous behavior. The Young’s Modulus was significantly reduced by ∼×3, and the elongation was remarkably increased by ∼×8 to ∼9%.