Synergy of elastic strain energy and electron wind force on thin film grain growth at room temperature

Highlights: • Grain growth in metals typically requires very high temperatures and longer times. • A room or low temperature electro-mechanical metal annealing process is proposed. • In-situ TEM shows grain growth from 5 nm to 100 nm at 25 °C in <5 min. • MD simulation suggests synergy of electron wind force and elastic strain energy. -- Abstract: Grain growth in polycrystalline materials is commonly achieved with thermal annealing. Here, we explore an alternate athermal route through a synergy of electrical current and tensile strain at or near room temperature. Our experimental approach involves passing electrical current in 100 nm thick freestanding palladium films with about 5 nm initial grain size. The resulting Joule heating increases the temperature up to 470 K (homologous temperature of 0.25) at the middle section of the specimen. The massive heatsinks at the two ends of the specimen constrain them at the room temperature. At current density of 7 × 10⁵ A/cm², in-situ transmission electron microscopy shows more than two orders of magnitude grain growth in the high temperature regions and little growth at the room temperature regions. However, application of elastic strain (about 0.1%) dramatically increased the grain size to about 100 nm in a few seconds at the room temperature regions. Our finding suggests that the synergy of elastic strain energy and electrical current density may achieve grain growth in metallic materials even at the room temperature. Molecular dynamics simulation of this phenomenon reveals that the externally applied strain is localized at the grain boundaries in nanocrystalline metals, which promotes the effects of electron wind force on the grain boundary atoms. We conclude that a synergy of two or more stimuli can achieve grain growth at room or even lower temperatures.