The strength and hardness of polycrystalline metals greatly increases when reducing their grain size, commonly understood as the classical Hall–Petch strengthening due to dislocation pileup on grain boundary obstacles. As grain size becomes smaller, the increase of grain boundary density enhances blocking of dislocation glide, therefore strengthening the materials. However, an inverse Hall–Petch effect, a reduction in strength with decreasing grain size, arises in nanocrystalline metals with grain size of a dozen manometers. The variation of strength with grain size suggests a characteristic size *d*~c~, at which the predominate deformation mechanism changes from strengthening to softening, owing to grain boundary-mediated processes such as sliding and migration. In view of grain size dependent strength in noncrystalline metals, the “strongest size” *d*~c~ emerges, suggesting the strengthening effects reach the maximum at this critical length. For conventional homogeneous nanograined (NG) metals, the *d*~c~ has been found to occur at grain size of about 50 to 10 nm.

Recently, an otherwise material with nonhomogeneous or heterogeneous nanostructures has drawn increasing research interests because of the unprecedented mechanical properties that unfolded as well as the advances in processing and manufacturing routes for producing heterogeneous materials. For example, a variety of gradient nanostructured metals and alloys with a spatial gradient in grain size achieve a noteworthy combination of high strength and high ductility. This improved strength–ductility synergy can be attributed to production and storage of geometrically necessary dislocations for compatible deformation in heterogeneous structures, that brings about extra hardening during deformation and, hence, achieves higher ductility as compared with homogeneous noncrystalline systems. Because of the structural heterogeneity in gradient nanograined (GNG) metals and possibly new deformation mechanisms, it would be interesting to ask whether the aforementioned strongest size *d*~c~ varies with an imparted gradient in grain size and what associated mechanisms are responsible for a change of *d*~c~ if at all. Answering these questions is not only of fundamental interest but will also shed light on the perspective of tailoring mechanical behaviors of materials by tuning nanostructural heterogeneity.

In this study, massively parallel atomistic simulations were performed in GNG and homogeneous NG samples of Cu, a material which has been extensively studied in experiments and simulations. The results reveal that the size *d*~c~ of GNG structures becomes smaller as compared with their homogeneous counterparts, and increasing the gradient in grain size will drive the strongest size to a smaller value. By analyzing the deformation trajectories, it is found that the softening processes of grain boundary sliding and migration in the GNG are greatly mitigated, which are accompanied by enhanced plastic deformations mediated by activating multiple dislocation slip systems, deformation nanotwins, and stacking faults. The boundary stabilization and enhanced plasticity resulting from the grain size gradient suggest the fundamental deformation mechanisms underlying extraordinary work hardening and high ductility of many heterogeneous structured metals.

*Reference*: Cao, Penghui. "The Strongest Size in Gradient Nanograined Metals." *Nano Letters* 20.2 (2020): 1440-1446.


Controlling Strength and Plasticity in Metals by Tailoring Grain Size Gradient

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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