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Updated: Jan 28, 2026

Determining the Mechanical Strength of Ultra-Fine-Grained Metals
Published on: November 22, 2021
Smaller Grains Are Not Stronger: Microcrystalline Metals at Ultrahigh Strain Rates
Laura Wu1, Yuan Yao2, Luyan Li2
1Cornell University, Department of Materials Science and Engineering, Ithaca, New York 14853, USA.
Abstract:
The strength of metals and alloys increases as grain size decreases due to grain boundary strengthening. This "smaller is stronger" effect has been observed consistently in the microcrystalline, ultrafine-grained, and even nanocrystalline metals with the exception of extremely small grains on the order of ∼10 nm, where grain boundary strengthening breaks down. Here we report a reversed trend in the microcrystalline regime. When subjected to extremely high strain rates, metals with smaller grain sizes become softer. The grain sizes studied here (1-100 μm) are several orders of magnitude larger than those associated with the breakdown of the grain boundary strengthening. We attribute this softening behavior to the suppression of ballistic dislocation transport, which, consequently, limits dislocation-phonon drag in smaller grains.
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