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Published online: 27 January 2010 | doi:10.1038/nchina.2010.12

Mechanical properties: Size stress

Felix Cheung

Introduction

© (2010) Nature

There are two major mechanisms responsible for the deformation of crystalline materials: ordinary dislocation plasticity (ODP) and deformation twinning. Studies in recent years have demonstrated that ODP-mediated deformation, and hence the apparent strength of the crystal, is strongly influenced by crystal size. However, it is unknown if crystal size has any effect on deformation twinning. Jun Sun at Xi'an Jiaotong University, Ju Li at the University of Pennsylvania and co-workers¹ have now conducted compression experiments on tiny crystals of titanium alloy. They found that the stress required for deformation twinning increases dramatically with decreasing crystal size.

The researchers studied the deformation behaviour of individual titanium alloy crystals during compression. They observed obvious shearing on the surfaces and the formation of 'micro-pillars' – characteristics of deformation twinning – in 8.0 μm (see top image) and 1.0 μm (see middle image) crystals. Deformation twinning was particularly severe for the 1.0 μm crystal.

However, the researchers noticed that the features for submicrometer crystals were markedly different. They observed continuous plastic flow without any major strain burst, as well as mushroom-shaped pillars, in 0.7 μm (see bottom image) and 0.4 μm crystals. Transmission electron microscopy revealed a high density of tangled dislocations, but there were no signs of deformation twinning.

The findings demonstrate that deformation twinning increases prominently with decreasing crystal size. For submicrometer crystals, however, ODP completely overrides deformation twinning.

The authors of this work are from:
Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano), State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, China; Hysitron Incorporated, Minneapolis, Minnesota, USA; Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, USA; Danish-Chinese Center for Nanometals, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Roskilde, Denmark; Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, USA.

Reference

1. Qian, Y. et al. Strong crystal size effect on deformation twinning. Nature doi:10.1038/nature08692 (2010). | Article | OpenURL