A theoretical model has been developed, which reveals the underlying correlation between the strong extra strain hardening achieved in the nano-grained layers of the grain size gradient structure and the non-uniform deformation of the lateral surface in surface nano-crystallized materials, based on some existing experimental observations and the concept of geometrically necessary dislocations. The proposed model led to the establishment of a simple physical law that can be expressed as H* = A*, where H* and A* are two dimensionless parameters. The former represents the extra strain hardening, while the latter characterizes the non-uniform deformation of the lateral surface. The values of these two parameters can be measured through experiments.

This work was supported by the National Natural Science Foundation of China (NSFC) (11402203), the Fundamental Research Funds for the Central Universities (3102014JCQ01039) and the Start-up Funds for the Newly-recruited High-level Talents from Northwestern Polytechnical University, China. S.C. thanks the support from NSFC through Grants #11125211, #11372317 and the 973 Nano-project (2012CB937500). A.K. Soh acknowledges the support of the Advanced Engineering Programme and School of Engineering, Monash University Malaysia, as well as the eScience Grant (Project no.: 06-02-10-SF0195) provided by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia.

Li JJ,Chen SH,Wu XL,et al. A physical model revealing strong strain hardening in nano-grained metals induced by grain size gradient structure[J]. Materials Science & Engineering A,2015-01-03,620:16-21.