On the microscale, metallic materials are made of homogeneous crystalline regions—grains—separated by disordered boundaries. In general, materials with smaller grains are stronger because they have more grain boundaries, which impede deformation. But researchers have now demonstrated a radical departure from this rule: With rapid deformation, such as that from an explosive impact, finer grained metals are softer, not harder [1]. This new insight, the researchers hope, could be useful for engineers developing impact-resistant alloys for armor, aerospace structures, or hypersonic vehicles.
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