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Grain boundary (GB) engineering via alloying opens a pathway to design the interfaces in alloys for tuning their mechanical properties, thus it is quite critical to determine the optimum addition of alloying element. Here, we prepared immiscible Cu-Cr alloyed thin films by non-equilibrium magnetron sputtering deposition to study Cr alloying effects on the microstructure and mechanical properties of Cu thin films. It is found that Cr doping can notably refine the grains and promote the formation of fully nanotwinned columnar grains at low Cr volume concentrations (≤∼3.7 at.%) associated with the average GB Cr concentrations ≤ ∼15 at.%, beyond which the formation of nanotwins is significantly suppressed. The role of Cr atoms/clusters played in the twinning process is rationalized in terms of dislocation rebound-promotion mechanism. Importantly, a maximum hardness is discovered at the optimum Cr addition of ∼2.0 at.%. The Cr concentration-dependent hardness of Cu-Cr alloyed films was quantified by a combined strengthening model. The achievement of the maximum hardness was related to the greatest GB solute segregation-induced strength contribution. It is further uncovered that the Cu-Cr system exhibits strain rate sensitivity (SRS, m ) monotonically reduced with increasing the Cr concentration, ranging from a positive m of 0.0314 (for pure Cu) to a negative m of −0.0245, which is attributed to a competition between the dislocation-boundary interactions (increasing m ) and the dislocation-solute atoms interactions (decreasing m ). These findings provide valuable insights into tuning the composition of alloyed thin films to achieve optimized mechanical performance. [ABSTRACT FROM AUTHOR]

Tuning the microstructure to optimize the mechanical performance of nanocrystalline Cu thin films via the alloying strategy is quite important for their application in microdevices. In this work, we prepared nanocrystalline miscible Cu-Ti and immiscible Cu-Mo alloyed thin films to investigate alloying effects on the microstructure and mechanical properties of Cu thin films in terms of mixing enthalpies. It is found that the dopants of both Ti and Mo can notably refine the grains, and in particular promote the formation of nanotwins below a critical content of solute, beyond which the formation of nanotwins is notably suppressed. The nonmonotonic solute concentration-dependent twinning behavior observed in Cu-Ti and Cu-Mo alloyed thin films is explained by the coupling effects between grain size and grain boundary segregated dopants that affects the stimulated slip process of partials. The increased hardness of both Cu-Ti and Cu-Mo systems with increasing the solute contents are quantitatively explained by combining several strengthening mechanisms, including solid solution strengthening, grain/twin boundary (GB/TB) strengthening, solute segregation-induced strengthening. It unexpectedly appears that with increasing the solute contents, the Cu-Ti system exhibits monotonically reduced positive strain rate sensitivity (SRS, m ), whereas the Cu-Mo system manifests almost constant negative SRS. The fundamental difference in SRS m between Cu-Ti and Cu-Mo is rationalized in terms of the interactions between solute atomic clusters and dislocations based on the cross-core diffusion mechanism. [ABSTRACT FROM AUTHOR]

Magnetron sputtering technique was employed to synthesize crystalline/amorphous high-entropy alloy (AHEA) X/AHEA (X = Cu and Ni, AHEA = Fe 20 Co 20 Cr 20 Ni 20 B 10 Si 10) nanostructured multilayers (CANMs) with equal layer thickness h ranging from 5 to 150 nm, aiming at systematically investigating their plastic deformation and microstructural evolution. Unlike traditional crystalline/crystalline multilayers often with shear banding at small h , the present X/AHEA CANMs exhibit h -independent homogeneous deformation via the thinning of both constituent layers, while associated with h -dependent deformation-induced devitrification in AHEA layers with thickness below 25 nm. This homogeneous plasticity of AHEA layers with the large incubation size for mature shear bands can be ascribed to the coupling between dislocations in X layers and shear transformation zones in AHEA layers. Meanwhile, the absorption of abundant dislocations emitted from X layers by AHEA layers with small thickness facilitates the occurrence of devitrification behavior, implying the microstructural instability under strong constraint conditions. Moreover, both of Cu/AHEA and Ni/AHEA CANMs manifest size-dependent hardness with the fashion that "smaller is stronger", which is captured well by the rule-of-mixture, based on the size-dependent strength of X nanolayers and size-independent homogenous flow of AHEA nanolayers. [Display omitted] [ABSTRACT FROM AUTHOR]

The hardness, tensile ductility and fatigue lifetime of nanostructured (NC) Cu-Zr alloyed thin films have been systematically measured at different Zr additions (0, 0.5, 2.0, 4.0, 8.0at.%). Experimental results showed that the Cu-0.5at.% Zr film exhibited the highest hardness, largest ductility and longest fatigue lifetime, which are increased by 120%, 80%, and above an order of magnitude, respectively, in comparison with its pure Cu counterpart. The simultaneous improvements of mechanical properties are rationalized with respect to microstructural evolutions related to Zr segregation at grain boundaries (GBs). Besides refinement in NC grains and enhancement in nanotwins, grain orientation was apparently modified by the GB Zr segregation. (110) grains were promoted and layer-like microstructure was formed with coexistence of three orientation of grains at the upper layer, facilitating Cu grain growth at room temperature. GB doping strengthening worked as an additional strengthening mechanism and mechanically driven grain growth became the predominant deformation mechanism and fatigue mechanism, which accounted for the optimal combination of mechanical properties achieved in the Cu-0.5at.% Zr film. When the Zr content was >2.0at.%, however, amorphous phases were formed at the GBs due to locally increased Zr concentration. The GB doping strengthening effect was concomitantly weakened and strength was gradually decreased. In addition, stress/strain localization occurred readily at the amorphous phase regions, triggering intergranular fracture and reducing ductility. During cyclic testing in the low stress range, GB amorphous phases strengthened rather impaired the GBs, which gave the Cu-Zr films a fatigue lifetime greater than that of the pure NC Cu film. The present results clearly indicate that, once solute content is suitably explored, the solute segregation at GBs in NC metals can not only retain NC grains in loading-free conditions, but also impart enhanced mechanical properties when exposed to applied stress. [ABSTRACT FROM AUTHOR]