Deformation-Controlled Design of Metallic Nanocomposites

: Achieving the theoretical strength of a metallic alloy material is a demanding task that usually requires utilizing one or more of the well-established routes: i. decreasing the grain size to stop or slow down the dislocation mobility, ii. adding external barriers to dislocation pathways, iii. altering the crystal structure, or iv. combining two of the previous discrete strategies, i.e., implementing crystal seeds into an amorphous matrix. Each of the outlined methods has clear limitations, hence further improvements are required. We present a unique approach that envelops all the different strength-building strategies together with a new phenomenon – phase transition. We simulated the plastic deformation of a Zr-Nb nanolayered alloy using molecular dynamics and ab initio methods, and observed the transition of Zr from HCP to FCC, and then to BCC during compression. The alloy, which was prepared by magnetron sputtering, exhibited near-theoretical hardness (10.8 GPa) and the predicted transition of the Zr structure was confirmed. Therefore, we have identified a new route for improving the hardness of metallic alloys.

procedures were devised in order to approach the theoretical limits. After gaining a clearer understanding of dislocation phenomena that underpin plasticity, subsequent attempts focused on the most natural strategy: stopping or slowing down the dislocation mobility. Initial design attempts focused on adding more barriers to dislocation glide pathways, an effect commonly known as Hall-Petch strengthening. This cemented the inverse proportionality between the average grain size and the resulting strength of a polycrystalline material 1 . This was followed by the addition of extensive inner-boundaries resulting in twin boundary strengthening. [2][3][4][5][6][7][8][9] Finally, a modern design for boundary-dominated strengthening has become prominent, achieved by stacking series of binary systems with defined periodicity yielding multilayered nano-composites (MNCs) [10][11][12][13][14] . All these efforts were limited to a minimum crystallite size threshold of approximately 10 nm 15 due to the inverse Hall-Petch effect. In connection to Hall-Petch strengthening, the second design approach is known as precipitation or solute-hardening.
Metallic glasses are promising candidates among strength-inducing microstructures due to their unique structural capabilities. In contrast to the crystalline counterparts, metallic glasses have neither grain boundaries nor crystal structure; thus, the deformation can take place predominantly via the formation of shear bands. Thanks to the transformation from dislocation based plasticity to amorphous deformation, the strength of these metals doubles, and even higher values are reached in modern alloys combining dual-phase strengthening behaviour.
However, this comes with an important drawback: the loss of ductility. Beyond 2% of plastic strain, metallic glasses show drastic failure. A recent study 19 presented a promising way to cover the size and side effects of the metallic and amorphous alloys by sputtering two phases together, although the fabrication of such an alloy as a bulk material is an enormous challenge.

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The present study further advances material strengthening mechanisms through the implementation of the kinetic control of microstructures, which is achieved by manipulating the activation of position-dependent deformation mechanisms within sputtered Zr/Nb multilayers.
Selective stacking of amorphous and crystalline layers may provide a combination of all major strengthening mechanisms described above, together with a new component of localized phase transformation.
Using classical MD we can explore various effects in the deformation mechanism of a multilayered system, where layers differ in crystal structure (BCC-Nb or HCP-Zr) as well as in crystallization state, from amorphous to perfect crystalline for both elements.  To support our classical MD simulations and reveal the underlying mechanism pivotal for our design strategy, we performed a comprehensive ab initio study regarding the Zr phase transition under pressure and the effect of the Nb interface. Firstly, HCP-Zr was compressed along the c 5 | P a g e axis (normal to the (0001) plane) by decreasing the value of the lattice constant in this direction.
The phase transition is observed at about 40% compression at 0 K. Figure 2 shows the energy profile for this compression process, setting the total energy in a fully relaxed state as a reference for both the 0 K and 300 K case. The results indicate that there is a 0.3 eV/atom energy barrier for the HCP-BCC transition and the energy difference between these phases is about 0.13 eV/atom at 0 K 24 . At 300 K these values drop to 0.11 eV/atom and 0.05 eV/atom, respectively, in agreement with previous studies 25  Therefore, the Nb interface is vital to stabilize BCC-Zr formed during plastic deformation. 6 | P a g e Depositing the selected amorphous/crystalline interfaces in the form of a nanolayered coating is a challenging endeavour and requires rigorous control of incident atom energy. During the sputtering process, due to the momentum transfer from one atom to another, the exhausted heat is transferred upwards (Figure 3c). Although the temperature difference between two separated layers is small, it is possible to create, gradually, crystal nuclei on top of each other. The results from our MD simulations show that the consumed kinetic energy leads to the formation of a heat-based crystal gradient between sputtered layers during the growth sequence, i.e., the activation of crystal seeds in the amorphous matrix is possible. This phenomenon is highlighted in Figure 3a, which shows the mechanism of thermal assistance in creating a gradual 7 | P a g e crystallization process. As the number of layers increases, the stored thermal energy overcomes the activation potential of the crystal phase. Within the initial layers, due to the lack of thermal energy, both Zr and Nb layers will be amorphous. After reaching a critical thickness and gaining sufficient thermal energy, nucleation will be attained firstly in the Nb layer (due to its higher symmetry), and then gradient crystallization will be observed in the following layers. Once the Nb layers become fully crystalline, Zr follows a similar trend, i.e., changing from completely amorphous to fully crystalline (see Figure 3a). In agreement with previous experimental and theoretical studies, our simulations indicate that the use of highly energetic atoms during thinfilm deposition promotes layer-by-layer growth (usually referred to as Frank-van-der-Merve growth 26 ) resulting in an amorphous metallic phase. At low incident-atom energies, threedimensional islands form on the surface and subsequently coalesce into a polycrystalline film (Volmer-Weber growth) 27 . Figure 3b shows the difference between the pair distribution functions (g(r)) of Zr and Nb in pure crystalline and amorphous phases. Colour code: blue = BCC (Nb), red = HCP (Zr), white = disordered atoms. In our samples, the thickness of every layer is 6 nm. b) Pair distribution functions, g(r), of Zr and Nb in pure crystalline and amorphous phases. c) Kinetic energy per atom of both Nb (blue) and Zr (red) during the deposition simulation.
Following the atomistic design perspectives, we realize our hypothesis with a multilayered Zr-   Finally, we carried out multiple load nanoindentation tests to investigate the mechanical response of our deformation-controlled-design (DCD). The measured hardness was 10.8 GPa, significantly higher than that of a fully crystalline Zr/Nb multilayer (4.8 GPa) 28 and almost reaching the theoretical threshold value of the Zr-Nb nanocomposite system.

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Detailed investigation of loading curves is a suitable tool to study the activation of slip and twinning systems 2-5 as a function of nanolayer composition 29,30 , and/or as a function of crystalline/amorphous layer combination [31][32][33] . A simple approach to study the activation of different slip systems in each nanolayer is through small variations, known as 'pop-in', recorded along the loading curve of a nanoindentation test ( Figure S2). From the load derivative curve An important indication of the possible strengthening mechanism is a large difference between the geometrically necessary dislocation density, gnd, which is generated in a small and confined plastic deformation volume, and the stored dislocation density (see SI).
The generation of dislocations and their evolution in this system is obviously very complex and further analysis by molecular dynamics is presented in the supporting information. Zr undergoes a complicated HCP to BCC phase transition through a metastable FCC (See Movie 3), which cannot be followed experimentally with ex-situ observation.
The effects of the plastic deformation on the microstructure of the multilayer are shown in

Conclusions
We designed a multilayered-metallic nano-composite material in which multiple deformation modes are activated separately by the local arrangement of crystal structure during single step magnetron sputtering. The major goal here is adding another milestone to the historical material strengthening race by controlling dominant deformation mechanisms within the confined 13 | P a g e volume of Zr-Nb nanocomposites. The key idea is to consume additional deformation energy by confined space phase transformation and glass-like phase crystallisation.
The starting hypothesis is supported by advanced experimental and nanoscale computational studies, and results in a significant strengthening of the Zr-Nb multilayer with a hardness of 10.8 GPa, which is approximately 6 GPa higher than its homogenous crystalline counterparts.
Moreover, there is plenty of room for improvement by optimizing layer thicknesses and/or a combination of amorphous and crystalline layers. The concept applies to various combinations of metals and represents a promising way to produce strength-tunable alloys for extreme engineering applications.