Severe plastic deformation-produced gradient nanostructured copper with a strengthening-softening transition

Bo Wu, Hui Fu, Xiaoye Zhou, Lei Qian, Jiasi Luo, Jiaming Zhu, Wing Bun Lee, Xu Sheng Yang

Research output: Journal article publicationJournal articleAcademic researchpeer-review

Abstract

Low-excess energy twin boundary can effectively stabilize the conventional grain boundary. It has been reported that deformation-activated nanotwins in nanograined metals produced by severe plastic deformation techniques can significantly enhance mechanical-thermal stability. However, fabrication, structural evolution, and the effect of grain size and twin thickness on the mechanical stability of nanograined-nanotwinned metals, where both the grain size and twin thickness reach the nanometer scale (especially grain size is lower than 40 nm), remain unclear. In this study, a gradient nanostructured layer containing a nanograined-nanotwinned sub-layer region and an extremely refined twin-free nanograined top surface layer with grain size as small as ~10 nm is achieved on copper by using an ultrahigh-strain rate single point diamond turning technique. High-resolution transmission electron microscope observations, atomistic molecular dynamic simulations, and nanoindetation tests were performed to reveal the size-dependent mechanisms of grain refinement and hardness along the gradient direction. The propensity of deformation multifold twinning is increased firstly in large-size nanograins and then decreased once grain size is below ~48 nm, finally replaced by detwinning to form extremely fine twin-free nanograins at the topmost surface layer. In other words, both the zero-macrostrain-induced deformation multifold twinning and symmetry-breaking-based detwinning processes can continuously refine nanograins along the gradient direction. Critical grain sizes for deformation multifold twinning and detwinning are discussed. Interestingly, a Hall-Petch strengthening-softening transition is discovered at a critical grain size of ~30 nm in the gradient nanostructured layer. The softening mechanisms are elucidated to be attributed to the twin thickness effect on deformation mode in nanograined-nanotwinned structures and the pure grain boundary-mediated plasticity in extremely fine twin-free nanograins. A series of critical twin thicknesses for softening in nanograins with different grain sizes are discussed; that is, the smaller the grain size is, the smaller the critical twin thickness will be. This study offers the potential for understanding and developing stable nanostructured metals.

Original languageEnglish
Article number141495
JournalMaterials Science and Engineering A
Volume819
DOIs
Publication statusPublished - 5 Jul 2021

Keywords

  • High-resolution transmission electron microscopy
  • Multifold twinning
  • Nanograined-nanotwinned Cu
  • Strengthening-softening transition
  • Ultra-precision machining technique

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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