Origin of strengthening-softening trade-off in gradient nanostructured body-centred cubic alloys

L Li and QH Fang and J Li and H Wu, JOURNAL OF ALLOYS AND COMPOUNDS, 775, 270-280 (2019).

DOI: 10.1016/j.jallcom.2018.10.137

The gradient nanograined metals have the unexpected combination of high strength and high ductility. However, the detailed dynamic and continuous process of deformation mechanism at nanoscale need to be further understood. Herein, we report the deformation behavior and microstructure evolution of gradient-nanograined body-centred cubic alloys using molecular dynamics simulations. An analytical model is also established to predict the strength for nanograined alloys. The results show that the alloying can soften the gradient-grained and random- nanograined Fe-based alloys. With the increasing Ni concentration, dislocation density decreases, due to the fact that the Ni solute increases stacking fault energy and the deformation twinning acts as a strong obstacle to suppress dislocation motion. The gradient distributions of strain and stress in gradient-nanograined metals occur due to the plastic incompatibilities, resulting in the inhomogeneous deformation of gradient-nanograined Fe and alloys. In addition, the inverse gradient-nanograined distribution, including large grain in the top surface and small grain in the central region, generates in random- nanograined alloys. It is owing to the grain growth in surface layer, resulting in the gradient strain and stress. Interestingly, the nucleated dislocation network and the deformation twinning considered as a reduction of dislocation mean free path decline rapidly with the increasing grain size for gradient-nanograined structure. The theoretical modeling determines the total strength derived from the individual contributions of different strengthening mechanisms, revealing that the grain boundaries dominate the yield strength and the back stress contributes the most to the straining hardening in nanograined materials. (C) 2018 Elsevier B.V. All rights reserved.

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