Breakdown of the Hall-Petch relationship in extremely fine nanograined body-centered cubic Mo alloys

F. H. Duan, Y. Naunheim, C. A. Schuh*, Y. Li

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

33 Scopus citations


Mechanical properties and deformation behavior have been studied in face-centered cubic (fcc) metals with extremely fine grain sizes, even below about 20 nm, but this range is rarely studied in the body-centered cubic (bcc) metals. Here, we study the hardness and deformation behavior of bcc Mo(O) alloys with grain sizes from 120 to 4 nm, covering the range of classical Hall-Petch behavior as well as the regime where that scaling law breaks down, between about 11 and 4 nm. A hardness as high as 17.3 GPa was achieved at the strongest grain size of 11 nm, and the breakdown at smaller grain sizes is associated with a number of changes in behavior: an inflection in activation volume (associated with a change in rate dependence of deformation) and increasing shear localization. At coarser grain sizes this is associated with shear offsets at grain boundaries, pointing to an increasing prevalence of intergranular deformation (by sliding or grain rotation). At finer grain sizes localization is associated with the formation of large shear bands at scales far greater than the grain size. These behaviors are suggestive of a crossover from crystal-like to glass-like deformation behaviors at the finest grain sizes, consistent with the increasing fraction of disordered regions in the structure.

Original languageEnglish (US)
Article number116950
JournalActa Materialia
StatePublished - Jul 2021


  • Deformation behavior
  • Grain boundary
  • Hall-Petch breakdown
  • Nanocrystalline materials
  • Refractory bcc alloys

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys


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