科学家发现晶体形变的机制

当晶体发生变形时，有两个主要机制在发挥作用：普通脱位弹性和形变孪晶。虽然前者已知依赖于晶体尺寸、因而在纳米尺度影响样本强度，但后者的尺寸依赖性迄今尚未被研究过。

现在，Ju Li及其同事利用微压缩和纳米压痕实验发现，形变孪晶在尺寸小于一微米的晶体中被完全抑制，从而使得普通脱位弹性成为惟一形变模式。这也许是因为形变孪晶是一个集体现象，对于小尺寸晶体不能发生。这一发现为在微观尺度操纵材料机械性质的新方法铺平了道路。

Nature 463, 335-338 (21 January 2010) | doi:10.1038/nature08692

Strong crystal size effect on deformation twinning

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, Ris? 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

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.