Strain hardening behaviors and strain rate sensitivity of gradient-grained Fe under compression over a wide range of strain rates

In the present work, gradient-grained Fe was synthesized by means of surface mechanical grinding treatment, and then the compression behaviors of the coarse-grained Fe and the gradient-grained Fe were investigated under both quasi-static and dynamic loading conditions over a wide range of strain rates (from 5 x 10(-4) to 10(4) s(-1)). After surface mechanical grinding treatment, equiaxed ultrafine grains, elongated lamellar ultrafine grains, full-developed sub-grains with dense dislocations walls, non-fully-developed dislocation cells, and deformed coarse grains are sequentially observed along the depth from the treated surface. The grain/cell size increases while the measured micro-hardness decreases along the depth for the gradient-grained Fe. The gradient-grained structure shows apparent strain hardening behaviors at all strain rates up to 10(4) s(-1) although the strain hardening exponent (n) for the gradient-grained Fe is smaller than that of the coarse-grained Fe at the same strain rate. This apparent hardening behavior is attributed to the hardening from both the coarse-grained center and the surface gradient layers when the strain localization trend for the ultrafine-grained surface layers is suppressed by the coarse-grained center. The extra hardening might be due to the back stress hardening associated with the constraint and mechanical incompatibility between different layers in the gradient-grained structure. The dynamic strain rate sensitivity of the gradient-grained Fe is observed to be slightly larger than that of the coarse-grained Fe, which is controversial to the general observation that strain rate sensitivity should decrease with reduction of grain size for BCC metals. The geometrically necessary dislocations associated with the back stress hardening and the grain size gradient result in additional increase in dislocation density, which may be the reason for the enhanced dynamic strain rate sensitivity in the gradient-grained Fe even it has smaller average grain size compared to the coarse-grained Fe. The present results should provide insights for the applications of gradient-grained structure under dynamic conditions. (C) 2016 Elsevier Ltd. All rights reserved