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Smaller Grains Are Not Stronger: Microcrystalline Metals at Ultrahigh Strain Rates

Laura Wu1, Yuan Yao2, Luyan Li2, Qi Tang2, and Mostafa Hassani1,2,*

  • 1Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
  • 2Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853, USA

  • *Contact author: hassani@cornell.edu
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Phys. Rev. Lett. 136, 016104 – Published 9 January, 2026

DOI: https://doi.org/10.1103/yp9h-sr2m

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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Physics Subject Headings (PhySH)

Focus

Material Strength Doesn’t Follow the Rules

Published 9 January, 2026

A textbook rule for the relationship between the structure and strength of a material breaks down for high-speed deformations, like those caused by strong impacts.

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Supplemental Material

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Phys. Rev. Lett. 136, 016104– Published 9 January, 2026
  • Received 8 May 2025
  • Revised 1 October 2025
  • Accepted 17 November 2025
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