Enhanced ductility of ultra-fine grained metallic materials

The ultra-fine grained metallic materials obtained via severe plastic deformation typically show very high mechanical strength but low tensile ductility, which dramatically limits their practical utility. Significant efforts were made to improve uni-axial tensile ductility of ultra-fine grained metallic materials. The developed approaches can be divided into two main groups: the 'mechanical' and 'microstructure-based' strategies. The 'mechanical' strategies are based on idea of manipulation with testing parameters (temperature, strain rate, stress state, etc.) to delay localization of plastic deformation at macro scale. Intelligent microstructural design to suppress necking at early stages of plastic deformation is employed in 'microstructural' strategies. This work aims to provide a short overview of these approaches with an emphasis on the effects of basic deformation mechanisms, chemical composition and microstructural features. It is demonstrated that the developed strategies are able to restore the uni-axial tensile ductility of the ultra-fine grained materials to the level of their coarse-grained counterparts and even to further improve it. Special attention is paid to formability of ultra-fine grained metallic materials during deformation under complex stress state. The ultra-fine grained metallic materials can demonstrate enhanced formability in deformation under complex stress state even if their uni-axial tensile ductility is very low. This is related to activation of deformation mechanisms promoting ductility, which are not active during uni-axial tensile testing.