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铁道科学与工程学报

JOURNAL OF RAILWAY SCIENCE AND ENGINEERING

Vol. 19    Special 2    September 2009

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Effects of specimen size on flow stress of micro rod specimen
WANG Chun-ju(王春举)1, GUO Bin(郭 斌)1, SHAN De-bin(单德彬)1, SUN Li-ning(孙立宁)2

1. School of Materials Science and Engineering, Harbin Institute of technology, Harbin 150001, China;
2. Robot Institute of Technology, Harbin Institute of Technology, Harbin 150001, China

Abstract:With the miniaturization of parts, size effects occur in the micro-forming processes. To investigate the effects of the specimen size on the flow stress, a series of upsetting deformation experiments were carried out at room temperature for specimens with different diameters. And the grain size of billets was changed by anneal processes to analyze the grain size effects on the size dependence of flow stress. The deviation of stress was observed. The results show that the flow stress decreases with decreasing billet dimensions. As the dislocation accumulation in free surface layer is slight, the reduction degree of flow stress becomes larger when the plastic deformation goes on. The flow stress is enlarged by grain size, which can be analyzed by the grain boundary length per area. The deviation increases with decreasing specimen size. This can be explained by the effects of grain orientation stochastic distribution according to the Schmid law. As a result, the micro-forming process must be considered from the viewpoint of polycrystalline structure, and the single grains of micro-billet dominate the deformation.

 

Key words: micro-forming; size effects; free surface effects; deviation; flow stress

ISSN 1672-7029
CN 43-1423/U

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