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In order to quantify the fracture toughness and reveal the failure mechanism of crystalline/amorphous nanolaminates (C/ANLs), in-situ micro-cantilever bending tests were performed on Ag/Cu–Zr and Mo/Cu–Zr C/ANLs in a scanning electron microscope over a wide range of cantilever widths from several microns to the submicron scale. The results demonstrate that the fracture behavior was strongly influenced by sample size and constituent phases, respectively. The Ag/Cu–Zr micro-cantilevers failed in a ductile manner, with fracture toughnesses higher than the Mo/Cu–Zr samples that exhibited brittle failure. Both materials also displayed different cantilever width-dependences of fracture toughness. The Ag/Cu–Zr beams showed a fracture toughness that increases with the cantilever width, mainly due to a size-dependent constraining effect on the deformation of the crystalline phase. For the Mo/Cu–Zr beams, the fracture toughness decreased gradually to a low plateau as the cantilever width exceeded ∼1500 nm, which can be rationalized by a transition in stress condition. The underlying fracture mechanism of the Ag/Cu–Zr micro-cantilevers was identified as the interconnection of microcracks initiated in the amorphous Cu–Zr layers, compared to a catastrophically penetrating crack propagation in the Mo/Cu–Zr samples. The discrepancy in size-dependent fracture behavior between the two material systems is discussed in terms of plastic energy dissipation of ductile phases, crack tip blunting, crack bridging and the effect of strain gradient in the plastic zone on crack propagation.

Nanostructured crystalline Al/amorphous AlN multilayer films with a wide layer thickness (h) range from ∼10 nm up to ∼200 nm were prepared by using magnetron sputtering. Nanotwins and 9R phase were substantially observed in the Al layers, showing a strong thickness dependence. The 9R phase predominantly penetrated through the Al layer in the II regime of h ≤ ∼20 nm, while mainly terminated within the layer interior in the I regime of h > ∼20 nm. On the contrary, the coherent nanotwins were boosted when h > ∼20 nm and the percentage of twinned Al grains was greatly increased. The formation mechanisms of 9R phase and coherent nanotwins were discussed in terms of the interfacial chemistry/physics modulated by the amorphous AlN layers, which displayed gradient characteristics and hence was sensitive to the layer thickness. A significant thickness dependence of hardness was also evident that the hardness monotonically increased with reducing h in the I regime, while reached a peak value and hold almost unchanged in the II regime. The hardness in the II regime is about 1 GPa greater than the predictions from an interfacial barrier crossing model. This discrepancy is mainly contributed by the layer-penetrating 9R phase rather than the nanotwins. This study provides a new perspective on fabricating nanotwinned Al by utilizing heterophase interfaces.

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.

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.

Metallic materials with outstanding cryogenic mechanical properties are highly demanded for cryogenic engineering applications. Here we developed a novel (NiCoCr)92Al6Ta2 medium-entropy alloy (MEA) towards superior cryogenic mechanical properties via elaborating the chemical composition designing and the thermo-mechanical processing. It appears that the twinned (NiCoCr)92Al6Ta2 MEA shows a strongly temperature-dependent mechanical behavior, i.e., when the testing temperature from 298 down to 77 K, the yield strength, ultimate strength and tensile ductility are increased from ∼600 to ∼800 MPa, from ∼1.0 to ∼1.35 GPa and from ∼52% to ∼90%, respectively. An excellent strength-ductility synergy and extraordinary strain hardening capacity were realized in this alloy, in particular the product of tensile strength and elongation is more superior to their reported counterparts. There is a strongly temperature-dependent deformation mechanism transition from the ordinary planar-slip at 298 K to the cooperative plastic mechanisms at 77 K, including stacking faults (SFs) and deformation twins. The excellent cryogenic mechanical properties of this designed MEA stems from the synergic effects of nanotwins, hierarchical SFs and Lomer-Cottrell locks, as well as their extensive interactions, rarely observed in their siblings deformed at room temperature. Furthermore, the deformation twinning acting as an additional/significant mechanism for plasticity and favoring strain hardening, along with the effect of temperature-dependent dislocation-twin interactions, collaboratively delays the onset of necking for enhanced ductility at 77 K.

A transmission electron microscope and an atom probe tomography were used to quantitatively characterize the microstructural evolution of Al-XCu alloys (X = 1.0, 1.5, and 2.5 wt%) with 0.3 wt% Sc addition. A dual solute alloying/microalloying effect on the microstructural evolution was demonstrated. On the one hand, the nucleation and coarsening of Al 3 Sc dispersoids displayed a Cu alloying effect . By increasing the Cu content, both the Al 3 Sc disperoid size and the volume fraction decreased after solution treatment. On the other hand, the precipitation of θ′ -Al 2 Cu strengthening particles during aging treatment was promoted by Sc segregation at the θ′ /matrix interfaces, showing a notable Sc microalloying effect . The strongest interfacial Sc segregation was generated in the Al-2.5 wt%Cu-Sc alloy, resulting in the most promoted θ′ precipitation. The Sc partitioning between Al 3 Sc dispersoids and Sc segregation at the θ′ /matrix interfaces, tailored by the Cu content, impacted the mechanical properties and deformation behavior at both room temperature and high temperature. The Al-2.5 wt%Cu-Sc alloy had a room temperature yield strength of approximately 2.2 times that in its Sc-free counterpart and approximately 1.8 times that in the Al-1.5 wt%Cu-Sc alloy, which is rationalized by strengthening models. In addition, the improvement in the high-temperature mechanical properties after Sc addition was discussed in terms of the Sc segregation-induced high coarsening resistance of θ′ precipitates.

The hardness, tensile ductility, and strain rate sensitivity of crystalline Cu/amorphous Cu–Zr nanolaminates (Cu/Cu–Zr C/A NLs) have been measured as a function of modulation ratio (η). With reducing η, the tensile ductility first decreased and subsequently increased, leaving a minimum value at η ∼ 1.0. However, the strain rate sensitivity (m) increased monotonically with reducing η and spanned from a negative value at η over ∼1.0 to a positive one at η below ∼1.0, indicating a tunable strain rate sensitivity in engineered C/A NLs. Careful microstructural examinations reveal that a deformation-induced devitrification (DID) in the amorphous nanolayers is the key factor responsible for the aforementioned experimental phenomena. For thinner amorphous nanolayers, the DID becomes more intense. The size-dependent DID drives the pure Cu–Zr amorphous single layer films to (i) exhibit a thickness-dependent tensile ductility opposite to that of pure Cu single layer films, and (ii) have a negative m contrary to the positive m in their pure Cu counterpart. When the two layers are engineered into C/A NLs, a competition exists between the two inverse constituent nanolayers. This competition is strongly η-dependent, resulting in a non-monotonic evolution in tensile ductility and significant change in m when η spans from 9.0 to 0.1. The fracture mode of the C/A NLs transformed from shearing at small η to opening at large η; this can be rationalized by considering the competition between the two constituent nanolayers as a microcrack initiator. In addition, the strengthening mechanisms of the C/A NLs were analyzed and the η-dependent hardness was quantitatively described using a modified mechanistic model.

Nanoindentation methodology was used to measure the hardness, strain rate sensitivity (SRS) and activation volume of Cu/Cu–Zr crystalline/amorphous nanolaminates (C/ANLs) with layer thickness ( h ) spanning from 2.5 to 150 nm before and after He ion-implantation at room temperature. It is interestingly to uncover that the ion radiation-induced devitrification (RID) occurs in the glassy Cu–Zr nanolayers, in which the nanocrystallites transit from the Cu 10 Zr 7 intermetallics at large h to the fcc Cu–Zr solid solution at small h . Compared with the as-deposited Cu/Cu–Zr C/ANLs associated with monotonic increase in hardness and SRS (or a monotonic decrease in activation volume) with reducing h , the irradiated Cu/Cu–Zr manifested enhanced hardness in the form of two hardness plateau and an unexpected non-monotonic variation in SRS (similarly in activation volume). It was clearly unveiled that the SRS of irradiated Cu/Cu–Zr firstly decreased with reducing h down to a critical size of ∼50 nm and subsequently increased with further reducing h to ∼10 nm, below which a SRS m plateau emerges (The activation volume of irradiated Cu/Cu–Zr had exactly an opposite variation). These phenomena are rationalized by considering a competition between dislocation-interface and dislocation-bubble interactions. A thermally activated model based on the depinning process of bowed-out dislocations pinned by obstacles was employed to quantitatively account for the variation of SRS with h in Cu/Cu–Zr C/ANLs before and after radiation. Our findings not only provide fundamental understanding of the effects of radiation-induced defects on plastic characteristics of C/ANLs, but also offer guidance for their microstructure sensitive design for performance optimization at extremes.

How to defeat the conflict of strength vs. toughness and achieve unprecedented levels of damage tolerance within either metallic crystalline or metallic glassy family is a great challenge for designing structural materials. The combination of glassy with crystalline nanolayers can manifest extraordinarily high toughness, i.e. superior strength in conjunction with high ductility, when the constituent layers approach a critical internal feature size. Three-point bending and uniaxial microcompression tests were performed on Cu/Cu–Zr crystalline/amorphous nanolaminates (C/ANLs) with equal layer thicknesses ∼50 nm to investigate their toughening behaviors. The dislocations absorbed by the amorphous phase not only render defect-free nanocrystals, but also create nanocrystallites in glassy nanolayers. It is revealed that the Cu/Cu–Zr C/ANLs self-toughen via the combination of the extrinsic shielding effect of crystalline/amorphous interfaces on a crack growth accommodated by an extensive shear-band sliding process and the intrinsic deformation-induced devitrification mechanism associated with the brittle-to-ductile transition of glassy nanolayers. The findings indicate that the high damage tolerance potentially accessible to glassy materials can extend beyond the benchmark ranges towards levels previously inaccessible to metallic crystalline–amorphous composites.

Both the homogeneous boundaries and the heterophase interfaces play important roles in crystalline plasticity as they often serve as obstacles for dislocation motion, as well as dislocation sources/sinks. In this work, microcompression tests were carefully performed to explicitly identify the relevant plasticity mechanisms of nanostructured micropillars with five distinct nanostructures: (i) Cu nanotwinned multicrystalline micropillars; (ii) Cu nanocrystalline multicrystalline micropillars; (iii) Cu/X (X = Cr, Zr) nanotwinned nanolayered micropillars; (iv) Cu/X nanocrystalline nanolayered micropillars; and (v) Cu/Cu–Zr crystalline/amorphous nanolayered micropillars. By characterizing their stress–strain response and evolution of strain-rate sensitivity (SRS) with strain, our findings elucidate the effects of homogeneous boundaries, heterophase interfaces and their coupling effects on the plastic yield, and reveal the fundamentally different roles these perform in the rate-limiting process of nanomaterials. In sharp contrast to the normal strain-dependent SRS in nanostructured Cu micropillars with homogeneous boundaries that monotonically decreases with increasing strain, nanolayered Cu-based micropillars with heterophase interfaces exhibit inverse strain-dependent SRS that monotonically increases with increasing strain. These expected (normal) and unexpected (inverse) SRSs are quantitatively explained by a dislocation model in terms of the strain-related dislocation mean free path. These findings provide valuable insights into our understanding of the fundamental roles that homogeneous boundaries and heterophase interfaces play in plastic deformation.

Three-dimensional atom probe analysis is employed to characterize the Sc segregation at θ ′/ α -Al interfaces in Al–2.5 wt.% Cu–0.3 wt.% Sc alloys aged at 473, 523 and 573 K, respectively. The interfacial Sc concentration is quantitatively evaluated and the change in interfacial energy caused by Sc segregation is assessed, which is in turn correlated to yield strength and ductility of the alloys. The strongest interfacial Sc segregation is generated in the 523 K-aged alloy, resulting in an interfacial Sc concentration about 10 times greater than that in the matrix and a reduction of ∼25% in interfacial energy. Experimental results show that the interfacial Sc segregation promotes θ ′ precipitation and enhances the strengthening response. A scaling relationship between the interfacial energy and precipitation strengthening increment is proposed to account for the most notable strengthening effect observed in the 523 K-aged alloy, which is ∼2.5 times that in its Sc-free counterpart and ∼1.5 times that in the 473 and 573 K-aged Al–Cu–Sc alloys. The interfacial Sc segregation, however, causes a sharp drop in the ductility when the precipitate radius is larger than ∼200 nm in the 523 K-aged alloy, indicative of a transition in fracture mechanisms. The underlying fracture mechanism for the low ductility regime, revealed by in situ transmission electron microscopy tensile testing, is that interfacial decohesion occurs at the θ ′ precipitates ahead of crack tip and favorably aids the crack propagation. A micromechanical model is developed to rationalize the precipitate size-dependent transition in fracture mechanisms by taking into account the competition between interfacial voiding and matrix Al rupture that is tailored by interfacial Sc segregation.

The microcompression method was used to investigate the compressive plastic flow behavior of nanolayered crystalline/amorphous (C/A) Cu/Cu–Zr micropillars within wide ranges of intrinsic layer thicknesses (h ∼ 5–150 nm) and extrinsic sample sizes (350–1425 nm) with the goal of revealing the intrinsic size effect, extrinsic size effect and their interplay on the plastic deformation behavior. The nanolayered C/A micropillars exhibited deformation behaviors of strain-hardening followed by strain-softening that were dependent on the thickness of the layers. At h ⩽ 10 nm, the strain-softening is related to shear deformation that is caused by fractures in the amorphous layers. At h > 10 nm, however, the strain-softening is related to the reduction in dislocation density caused by dislocation absorption. Correspondingly, the deformation mode of the C/A micropillars transitioned from homogeneous-like to shear band type as h decreased to the critical value of ∼10 nm, which is indicative of a significant intrinsic size effect. The extrinsic size effect on the plastic deformation also became remarkable when h was less than ∼10 nm, and the interplay between the intrinsic and extrinsic size effects leads to an ultrahigh strength of ∼4.8 GPa in the C/A micropillars, which is close to the ideal strength of Cu and considerably greater than the ideal strength of the amorphous phase. The underlying strengthening mechanism was discussed, and the transition in deformation mode was quantitatively described by considering the strength discrepancy between the two constituent crystalline and amorphous layers at different length scales.

By using microcompression methodology, deformation of nanolayered Cu/Zr micropillars was systematically investigated within wide ranges of intrinsic layer thickness (5–100 nm) and extrinsic sample size (300–1200 nm). The intrinsic size effect, extrinsic size effect and their interplay were respectively revealed. Competition between the intrinsic and extrinsic size effects leads to experimental observation of a critical layer thickness of ∼20 nm, above which the deformation is predominantly intrinsic-size-related and insensitive to sample size, while below which the two size effects are comparable. The underlying deformation mechanisms were proposed to transform from bulk-like to small-volume materials behavior. Deformation mode is correspondingly transited from homogeneous extrusion/barreling to inhomogeneous shear banding, but the two competing modes coexist in the layer thickness range from ∼50 to 20 nm. In the regime of shear deformation, the extrinsic size dependence is displayed in that the deformation was controlled by shear bands nucleation in larger pillars while controlled by shear bands propagation in smaller pillars. A deformation mode map is developed to clearly elucidate the coupling intrinsic and extrinsic size effects on the deformation mode of nanolayered pillars.

Hardness, activation volume and strain-rate sensitivity of Cu/Cr (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films have been systematically measured as a function of modulation period (L) and modulation ratio (η), respectively. Significant size effects were found for all the three plastic deformation characteristics, i.e. enhanced hardness and activation volume but reduced strain-rate sensitivity with decreasing the dimension length L. Microstructure evolution was statistically examined to rationalize these size dependences. It was crucially observed that abundant nanotwins existed in the Cu grains, though nanotwin formation was depressed with smaller L. This inverse size-dependent nanotwin formation is responsible for the reduction in strain-rate sensitivity, because the negative effect induced by the decreased nanotwins predominates over the positive effect coming from the raised interfaces/boundaries. A mechanistic model is modified to account for the interface effect as well as the nanotwin effect, which yields calculations of strain-rate sensitivity in broad agreement with the experimental results when L is larger than about 20 nm. Below this critical length size, there are discrepancies between the calculations and the experimental results, due to the change in deformation mechanism from dislocation nucleation/slip in confined layers to dislocation crossing interfaces. A confined layer slip model is also modified by considering the nanotwin strengthening to quantitatively describe the L-dependent hardness. In addition, the effects of constituent phases and their relative content on the activation volume and strain-rate sensitivity of NMFs are discussed with regard to variation in η.

The plastic deformation and fracture behavior of two different types of Cu/ X ( X = Nb, Zr) nanostructured multilayered films (NMFs) were systematically investigated over wide ranges of modulation period ( λ ) and modulation ratio ( η , the ratio of X layer thickness to Cu layer thickness). It was found that both the ductility and fracture mode of the NMFs were predominantly related to the constraining effect of ductile Cu layers on microcrack-initiating X layers, which showed a significant length-scale dependence on λ and η . Experimental observations and theoretical analyses also revealed a transition in strengthening mechanism, from single dislocation slip in confined layers to a load-bearing effect, when the Cu layer thickness was reduced to below ∼15 nm by either decreasing λ or increasing η . This is due to the intense suppression of dislocation activities in the thin Cu layers, which causes a remarkable reduction in the deformability of the Cu layers. Concomitantly, the constraining effect of Cu layers on microcrack propagation is weakened, which can be used to explain the experimentally observed λ and η -dependent fracture mode transition from shear mode to an opening mode. Furthermore, the fracture toughness of the NMFs is also found to be sensitive to both λ and η . A fracture mechanism-based micromechanical model is developed to quantitatively assess the length-scale-dependent fracture toughness, and these calculations are in good agreement with experimental findings.