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Can amorphous and nanometers achieve 1+1>2?

Time:2020/07/14 丨 source:未知 丨 visit count:

Can amorphous and nanometers achieve 1+1>2?

Amorphous alloy, also known as metallic glass, has the structural characteristics of long-range disorder, short-range order and isotropy. It is in a non-equilibrium metastable state. Although it appears as a solid, structural rearrangement and relaxation always occur inside. Its engineering application is limited due to its poor thermal stability and ductility. In order to solve these problems, a strategy recently proposed is to combine amorphous and nanocrystalline in a single alloy. In this way, this derived Amorphous nanocrystalline alloys will likely inherit the respective characteristics of amorphous and nanocrystalline, such as excellent corrosion resistance, strength, hardness, wear resistance, soft magnetism, etc., while improving thermal stability and ductility, This is difficult to achieve for amorphous or nano-alloys alone.
Figure 1 lists the development trajectory of amorphous-nano-alloy related technologies. It can be seen that after Klement discovered the first amorphous alloy in 1960, Chen et al. obtained the amorphous nano-crystalline alloy by thermodynamic annealing in 1969. . After that, in 2009, Ruan and Schuh obtained the amorphous nanocrystalline film phase by electrodeposition. The latest development is that Khalajhedayati and Rupert reported in 2015 that Cu-Zr nanocrystalline alloy segregated in the grain boundary solute atoms during the annealing process, forming an amorphous intergranular phase, which indicates that the grain boundary Amorphous nano-alloys can be obtained by crystallization [1-3].

Figure 1 Development history of amorphous alloys, amorphous nanocrystalline alloys and nanocrystalline alloys [3]

An intrinsic feature of the dynamic behavior of amorphous alloys is the appearance of Bose peaks, which corresponds to the excess vibration dynamic density that occurs in materials in the mid-to-low frequency range; another intrinsic feature is the structural inhomogeneity at the nanoscale. Liu Yanhui and Wang Weihua of the Institute of Physics of the Chinese Academy of Sciences proposed that the use of local quintic symmetry as a structural parameter can better describe the structural evolution during the glass transition through molecular dynamics simulations, and by analyzing the structural relaxation time of the alloy melt, Atomic movement capability, structural spatial correlation and thermodynamic characteristics establish a quantitative relationship between local quintic symmetry and dynamics, as shown in Figure 2.

Figure 2 (a) The evolution of local quintic symmetry during the glass transition; (b) The quantitative relationship between the symmetry parameter and the relaxation time of the structure

Control crystallization in amorphous alloys

In the past few years, a variety of methods have been developed to form nano-amorphous alloys from monolithic amorphous alloys, such as inert gas condensation (IGC), furnace (Furnace) or flash (Flash) Annealing, severe plastic deformation (SPD), electron/ion/pulse laser irradiation, ultrasonic vibration, etc. The principle of the IGC method is shown in Figure 3. Heating the master alloy to a molten state in an inert gas environment, the evaporated atoms collide with the inert gas molecules and condense into nano-scale amorphous particles, which are deposited under the influence of thermal convection. The liquid nitrogen is cooled on the column, and then the particles are scraped off and collected by a scraper and then subjected to high-pressure forming in situ. The alloys successfully prepared by this method include Au-Si, Au-La, Fe-Si, Fe-Sc, La-Si, Pd-Si, Ni-Ti, Ni-Zr, Ti-P, etc.

Figure 3 Schematic diagram of IGC preparation method

Crystallization is inseparable from the nucleation and growth of crystals. Therefore, there are basically two basic mechanisms. As shown in Figure 4, the first type needs to be able to provide a slow cooling rate if it is cooled at a relatively fast rate (> 103K/s), then a monolithic amorphous structure without quenching nuclei will be formed directly over the crystal nucleation. At this time, we can easily distinguish the glass transition from the crystallization in the corresponding amorphous alloy DSC trace, and subsequent annealing will cause nanocrystallization. The second type is that nucleation cannot be avoided but its speed is very slow. This type of amorphous alloy may trigger the further growth of quenched crystal nuclei during the reheating process, accompanied by the disappearance of the glass transition characteristics on the DSC curve [4, 5].

Figure 4 Schematic diagram of two types: (a) No nuclei, (b) Quenched-in nuclei

Many Zr-based amorphous alloys, such as Zr41.2Ti13.8Cu12.5-Ni10Be22.5 (Vit1), are good or bulk glass formers and can be used to make the first type of nano-amorphous alloys, but Since there are some local geometric short-range ordered structures (SROs), such as icosahedral clusters, they are not compatible with the spatial topology, so in most cases it will affect the overall nanocrystallization. According to Xing, Cang et al., these icosahedral clusters can serve as sites for heterogeneous nucleation of primary crystals, thereby promoting nanocrystallization. Wang et al. found through TEM studies that icosahedral clusters improved nanocrystallization due to the "pinning" effect. However, when the nucleus encounters icosahedral clusters during the outward growth process, it will be affected by "pinning". And inhibit growth, as shown in Figure 5 [6,7]

Figure 5 Schematic diagram and TEM image of the "pinning" effect of icosahedral clusters [7]

The existence of quenching nuclei in Al-based amorphous alloys makes nucleation easier. Wang and Bokeloh et al. found that the growth of these quenching nuclei will cause the crystallization temperature of some Al-based amorphous alloys to be lower than their theoretical crystallization temperature. Figure 6b shows a dark field TEM image of an amorphous nanocrystalline alloy. The alloy contains high-density, small-size nanoparticles (approximately 16 nm). These particles are the Al-based amorphous alloy during the annealing process. Produced [8,9]

Figure 6 TEM images of Zr-based (bright field) and Al-based (dark field) amorphous nano-alloys [9]

Some Fe-based amorphous alloys, especially soft magnetic alloys that can be used to make nanocrystals, can also be used to make amorphous nanoalloys. In this type of alloy, Cu-centric clusters are usually required as the primary bcc to form -Catalytic site of Fe-based nanocrystals. Hono et al. first used 3DAPT to form nanocrystals in the FINEMENT (FeSiBNbCu) alloy. They found that high-density Cu clusters (1024m-3) must be formed before crystallization as a heterogeneous nucleation for the first crystallization Point.
Pradeep et al. found similar results, as shown in Figure 7. In the third stage, Nb atoms can be used as fixed points to prevent the coarsening of bcc-Fe nanocrystals and stabilize the nanostructures. Figures 5b~c show that Liu, Li and others found that in some alloys with high Fe content, the mechanism of action is different from that of FINEMENT alloys, and similar to Al-based alloys, the stabilization of their nanostructures originates from the nanocrystalline shielding layer. Between the soft-impingement effect [10,11].

Fig.7 Microstructure evolution of the primary crystallization of Fe-based alloy (a), APT image of Cu clusters in Fe84.75Si2B9P3C0.5Cu0.75 alloy (b) and nano-crystallization of amorphous alloy (c) [10,11]

Controlling amorphization in nano-alloys

In addition, amorphous nanocrystalline alloys can also be obtained by solid-state amorphization, that is, partial amorphization of crystals, including high-energy irradiation, hydrogen absorption, diffusion couple annealing, pressure-induced amorphization, mechanical alloying, and large mechanical deformation These amorphization processes are because the free energy caused by the accumulation of non-equilibrium solid solution or crystal defects in the crystal is higher than the amorphous state. If solid amorphization occurs at the grain boundary, an intercrystalline amorphous layer may be generated. Since 2015, the grain boundary amorphization phenomenon has been confirmed in binary, ternary, and multicomponent alloys, such as Ni-W, Cu-Zr-Hf, Ni high-entropy alloys, etc. These intercrystalline amorphous phases The thickness is usually a few nanometers, as shown in Figure 8.
In 2016, Pan, Rupert et al. used Monte Carlo and molecular dynamics to simulate the segregation-induced grain boundary phase transition process in Cu-Zr alloy. The simulation results show that when the solute concentration reaches a certain critical value, the crystal The boundary phase will evolve from an ordered state to an unordered state. In 2017, Schuler, Rupert and others proposed material selection rules to predict amorphous GB complexions, mainly based on two considerations: 1. Increase the segregation of dopants at the interface; 2. Decrease the glassy forming energy. They verified in binary Cu-based alloys Cu-Zr, Cu-Hf, Cu-Nb, Cu-Mo, and found that the type of GB complexions can be controlled by segregation entropy and mixed entropy (ΔHseg-ΔHmix) [12].

Figure 8 High-resolution TEM image of the intercrystalline amorphous layer detected in the Cu-Zr sample [12]: (a) 2.6nm, (b) 0.8nm, (c) 4.1nm, (d) 2.9nm

Nanostructured amorphous alloy PVD forming control

At present, there are a variety of PVD methods used to manufacture metal thin films, such as thermal evaporation, magnetron sputtering, pulsed laser deposition, and molecular beam epitaxy. Among them, magnetron sputtering is the most widely used, typical magnetron sputtering process The gas-phase particles need to be condensed into a solid state at a cooling rate higher than 1012K/s. Therefore, even for marginal glass formers, the overall amorphous or crystalline-amorphous two-phase microstructure can be obtained by this process. The basic principle of magnetron sputtering (MS) is shown in Figure 9. Under the action of an electric field, a plasma is generated at high speed to bombard the target surface to cause sputtering, and the sputtered target atoms or molecules are deposited on the substrate Form a film.

Figure 9 Schematic diagram of the preparation method of magnetron sputtering

Figure 10 is high-resolution TEM and XRD images of Al-Mo alloys with different Mo contents. It can be seen that as the Mo content increases (16~50 at.%), the amorphous region also continues to expand at 32 at. The image shows the overall amorphous structure at %, and when the content is 50 at.%, it becomes the bcc crystal structure [13]

Figure 10 TEM and XRD images of Al-Mo alloy with different Mo contents [13]

Figure 11 shows a typical amorphous nanostructure. Increasing the sputtering power and pressure will promote the non-uniformity of atoms, thereby forming nano-glass, but prolonging the sputtering time will cause the coarsening of the crystal grains. In addition to the chemical composition of the target and GFA (glass-forming ability), Chen et al. found that the manufacturing process of the target also affects the formation of nanoglass [14,15].

Figure 11 SEM image of Au46Ag6Pd2Cu27Si14Al5 alloy (a) and TEM image of Au40Cu28Pd5Ag7Si20 alloy (b~c) [14,15]

High-entropy amorphous alloy

The high-entropy amorphous alloy is a new type of material that has both the structural characteristics of the traditional amorphous alloy and the compositional characteristics of the high-entropy alloy discovered after the concept of the high-entropy alloy was proposed in 2004. It generally consists of five or more elements with nearly equal atoms Than prepared.

The first discovery of the high-entropy amorphous alloy can be traced back to the preparation by the Inoue research group in 2002 in TiZrHfCuNi, TiZrHfCuFe and TiZrHfCuCo systems. In 2011, the Bai research group of the Institute of Physics of the Chinese Academy of Sciences prepared the Ca20Mg20Sr20Yb20Zn20 high-entropy alloy, and found that it has excellent mechanical properties, corrosion resistance, and the ability to produce bone cells to reproduce and differentiate. In the same year, the Takeuchi research team prepared the first high-entropy amorphous alloy Cu20Ni20P20Pb20Pt20 containing non-metallic elements. The width of the supercooled liquid region reached 65K, the reduced glass transition temperature was 0.71, and the amorphous forming ability exceeded 10 mm. In 2015, the Yao research group of Tsinghua University reported that Ti20Zr20Hf20Be20Cu20 and Ti20Zr20Hf20Be20 (Ni7.5 Cu12.5) pseudo-quinary high-entropy amorphous alloys with strong amorphous forming ability, and the maximum size of the latter can reach 30mm, and the fracture strength exceeds 2000Mpa. In 2019, the Chang research group of Ningbo Institute of Materials Science and Technology of the Chinese Academy of Sciences developed a (Fe1/3Co1/3Ni1/3)80(P1/2B1/2)20 high-entropy amorphous alloy with a critical size of 2mm, a maximum fracture strength of 3000Mpa, and compressive plasticity 4%, saturation magnetization can reach 0.9T.

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