Qian Yu1, Zhi-Wei Shan1,2, Ju Li3, Xiaoxu Huang4, Lin Xiao1, Jun Sun1 & Evan Ma1,5

  1. Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano), State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, China
  2. Hysitron Incorporated, 10025 Valley View Road, Minneapolis, Minnesota 55344, USA
  3. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
  4. Danish-Chinese Center for Nanometals, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark
  5. Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA

Correspondence to: Ju Li3Jun Sun1 Correspondence and requests for materials should be addressed to J.S. (Email: junsun@mail.xjtu.edu.cn) or J.L. (Email: liju@seas.upenn.edu).

Deformation twinning1, 2, 3, 4, 5, 6 in crystals is a highly coherent inelastic shearing process that controls the mechanical behaviour of many materials, but its origin and spatio-temporal features are shrouded in mystery. Using micro-compression and in situ nano-compression experiments, here we find that the stress required for deformation twinning increases drastically with decreasing sample size of a titanium alloy single crystal7, 8, until the sample size is reduced to one micrometre, below which the deformation twinning is entirely replaced by less correlated, ordinary dislocation plasticity. Accompanying the transition in deformation mechanism, the maximum flow stress of the submicrometre-sized pillars was observed to saturate at a value close to titanium’s ideal strength9, 10. We develop a ‘stimulated slip’ model to explain the strong size dependence of deformation twinning. The sample size in transition is relatively large and easily accessible in experiments, making our understanding of size dependence11, 12, 13, 14, 15, 16, 17 relevant for applications.