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 twinning^{1, 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 crystal^{7, 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 strength^{9, 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 dependence^{11, 12, 13, 14, 15, 16, 17} relevant for applications.