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Nanograined materials possess ultrahigh strength, while their processing and technological applications are constrained by inherent thermal and mechanical instability. Existing experiments show that the stability of Cu nanograins can be enhanced by performing a rapid heating pretreatment that reduces the grain boundary (GB) energy by changing the GB structure. The variation in the GB structure inevitably affects the migration mechanism of GBs. However, the effect of the pretreatment-induced variation in migration mechanisms on stability remains unclear. Here, the shear deformation of a series of ⟨ 100 ⟩ symmetrical tilt GBs after rapid heating pretreatment is systematically investigated by molecular dynamics simulations. The simulations of unheated GBs are also included for comparison. Our results show that the rapid heating pretreatment does not improve the mechanical stability of GBs with tilt angles larger than 36.87° but rather enhances the mechanical stability of those with tilt angles less than 36.87° by the transformation of migration behavior from the normal ⟨ 110 ⟩ mode to (i) a ⟨ 100 ⟩ mode; (ii) an inhomogeneous mixed one that reconciles the ⟨ 110 ⟩ and ⟨ 100 ⟩ modes; and (iii) an inhomogeneous ⟨ 110 ⟩ mode. The former leads to an increase in the critical shear stress that is required to initiate the migration, whereas the latter two result in a decrease in the migration distance. The variation in the GB migration mechanism is attributed to the change in the GB structure from an ordered kite structure to a disordered one. The research gives an atomic insight into the stabilizing mechanism of nanocrystalline materials with rapid heating pretreatment. [ABSTRACT FROM AUTHOR]

Experiments have shown that the ultrahigh strength of nanolayered metallic composites originates from their high-density interfaces of special characteristics. Hence, the modulation of interface structures becomes an effective route to enhance the mechanical performance of the nanolayered composites. One of the general ways to tune the interfacial feature is to introduce interlayers of several nanometers among constituent layers, such as amorphous (disordered) and crystalline (ordered) interlayers. Here, the deformation of a Cu/Ni layered composite with Ag interlayers of different thicknesses was simulated by molecular dynamics simulations. Our simulations show that the yield stress of 25 nm Cu/25 nm Ni nanolayered composites with Ag interlayers can be significantly enhanced, i.e., it can be 56.4% higher than that of their counterparts without interlayers. We also found that the yield strength of the new composites can be maximized by selecting an appropriate thickness for the Ag interlayer. The optimum interlayer thickness is 2.1 nm in tension and 4.2 nm for compression. It is revealed that the extra strength results from the alleviation of stress concentration by stimulating abundant interfacial dislocations at the Cu–Ag and Ag–Ni interfaces. These findings show that the introduction of additional interlayers is a new route to design stronger nanolayered metallic composites. [ABSTRACT FROM AUTHOR]

Nanolayered metallic composites have attracted intensive scientific interests due to their ultrahigh strength. However, the deformation incompatibility among the component layers with high mechanical contrast leads to extremely low tensile ductility in the nanolayered composites, which is a great setback for their engineering applications. Here, by molecular dynamics simulations, we show that a heterogeneous nanolayered design by combining 2.5 nm and 24 nm Cu/Ni bilayers in a composite in an appropriate way can promote the dislocation activity of the hard phase, i.e., the Ni layers. In the new heterogeneous structure, each 24 nm Cu or Ni layer is coated on both surfaces by one 2.5 nm Cu/Ni bilayer. The simulations show that the dislocations in the 24 nm Ni layers can nucleate and glide almost synchronously with those in the 24 nm Cu layers. The enhanced dislocation activities are attributed to the presence of the 2.5 nm Cu layer that can promote the dislocation nucleation and motion in the 24 nm Ni layer by forming more nodes in the dislocation network of the interface. [ABSTRACT FROM AUTHOR]