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Multi-principal element alloys

As a new class of metallic crystalline materials, multi-principal element alloys (MPEAs), also named medium entropy alloys (MEAs) for ternary systems and high entropy alloys (HEAs) for quaternary, quinary, or senary systems, have been attracting much attention in the structural metals research community.

As the numbers of medium- to high-entropy alloys being studied and impressive structural properties they exhibit increase rapidly, questions regarding the role played by their complex chemical fluctuations rise concomitantly. Here, using a combination of large-scale molecular dynamics (MD), a hybrid MD and Monte-Carlo simulation method, and crystal defect analysis, we investigate the role lattice distortion (LD) and chemical short-range order (CSRO) play in the nucleation and evolution of dislocations and nanotwins with straining in single crystal and nanocrystalline CoCrNi, a medium entropy alloy (MEA). LD and CSRO effects are elucidated by comparisons with responses from a hypothetical pure A-atom alloy, which bears the same bulk properties of the nominal MEA but no LD and no CSRO. The analysis reveals that yield strengths are determined by the strain to nucleate Shockley partial dislocations, and LD lowers this strain, while higher degrees of CSRO increase it. We show that while these partials prefer to nucleate in the CoCr clusters, regardless of their size, they find it increasingly difficult to propagate away from these sites as the level of CSRO increases. After yield, nanotwin nucleation occurs via reactions of mobile Shockley partials and is promoted in MEAs, due to the enhanced glide resistance resulting from LD and CSRO. (Acta Materialia 199, (2020) 352-369)

Metallic glasses and their composites

Bulk metallic glasses (BMGs) are fabricated by cooling high-temperature alloy melts rapidly and retaining their disordered atomic arrangement. Due to their amorphous atomic structure, BMGs have some unique mechanical and physical properties, such as high strength, high elastic limit and the absence of dislocation.

We investigate short- and medium-range orders in Cu46Zr54 metallic glasses, as represented by icosahedra and icosahedron networks, respectively, under shock compression with molecular dynam- ics simulations. Complementary isothermal compression and isobaric heating simulations reveal that compression below 60 GPa gives rise to increased coordination and thus high-coordination-number Voronoi polyhedra, such as icosahedra; however, pressure-induced collapse or thermal dis- integration of icosahedra (and subsequently, icosahedron networks) occurs at pressures above 60GPa or at melting, accompanied by free volume increase. The evolutions of the short- and medium-range orders upon shock loading are the effects of compression combined with shock-induced melting. The structural changes are partially reversible for weak shocks without melting (below 60 GPa) and irreversible for strong shocks. Crystallization does not occur under isotherma or shock compression at molecular dynamics scales. (Journal of Applied Physics 118, 1 (2015) 015901)
We investigate tensile deformation of metallic glass/crystalline interpenetrating phase nanocomposites as regards the effects of specific area of amorphous/crystalline phase interfaces, and grain boundaries. As an illustrative case, large-scale molecular dynamics simulations are performed on Cu64Zr36 metallic glass/Cu nanocomposites with different specific interface areas and grain boundary characteristics. Plastic deformation is achieved via shear bands, shear transformation zones, and crystal plasticity. Three-dimensional amorphous/crystalline interfaces serve as effective barriers to the propagation of shear transformation zones and shear bands if formed, diffuse strain localizations, and give rise to improved ductility. Ductility increases with increasing specific interface area. In addition, introducing grain boundaries into the second phase facilitates crystal plasticity, which helps reduce or eliminate mature shear bands in the glass matrix. (Nanotechnology 27, 17 (2016) 175701)
We investigate deformation and damage of a Zr-based bulk metallic glass (BMG) and its Ta particle-reinforced composite (MGMC) under impact loading, as well as quasi-static tension for comparison. Yield strength, spall strength, and damage accumulation rate are obtained from free-surface velocity histories, and MGMC appears to be more damage-resistant. Scanning electron microscopy, electron back scattering diffraction and x-ray computed tomography, are utilized for characterizing microstructures, which show features consistent with macroscopic measurements. Different damage and fracture modes are observed for BMG and MGMC. Multiple well-defined spall planes are observed in BMG, while isolated and scattered cracking around reinforced particles dominates fracture of MGMC. Particle–matrix interface serves as the source and barrier to crack nucleation and propagation under both quasi-static and impact loading. Deformation twinning and grain refinement play a key role in plastic deformation during shock loading but not in quasi-static loading. In addition, 3D cup-cone structures are resolved in BMG, but not in MGMC due to its heterogeneous stress field. (Materials Science and Engineering: A 711 (2018) 284-292)

Graphene and its composites

Graphene, as an important two-dimension (2D) material, has many excellent mechanical properties, such as flexibility, high Young's modulus and high tensile strength, which makes it a good reinforcement in composite materials and graphene-based electronic devices.

We perform molecular dynamic simulations to investigate the effects of orientation and grain boundary (GB) on shock response of graphene, including shock-induced slip and spall. For compression under different loading orientations, the critical resolved shear stress for slip is constant along a given growth direction (armchair or zigzag) in single-crystal graphene, and spall initiates in elastically deformed graphene via tension-induced debonding. For bicrystal graphene, the locations of compression-induced slip and initial spall are influenced by preexisting stress field at GB and the interactions between GB and shock waves, respectively. Both of these two factors are related to GB structure, including the density of pentagon-heptagon pairs at GB. However, bicrystals show negligible anisotropy in shock-compression response. For polycrystalline graphene at high shock strength, slips initiate at GBs and triple junctions under shock compression, develop along GBs or toward grain interior, and serve as nucleation sites for spall damage. (Carbon 132 (2018) 520-528)
Experiments have proved that one solution to improve the ductility of metallic glasses lies in introducing graphene and synthesizing metallic glass (MG) nanolaminates. In this work, molecular dynamic simulations are conducted to investigate layer thickness effects on the tensile behaviors of metallic glass-graphene nanolaminates (MGGNLs). The increase in MG layer thickness leads to the decrease in the ultimate strength of MGGNL and helps to the development of shear bands. Meanwhile, plastic deformation mode transits from homogeneous flow to shear localization. The critical layer thickness related to such a transition can be predicted by the strain energy theory. Once the dissipated energy in MG matrix during shear bands formation exceeds the stored strain energy in graphene, shear localization dominates plastic deformation mode. Besides, fracture strain decreases with increasing MG layer thickness. There also exists a kink for the linear decline in fracture strain, corresponding to the plastic deformation transition. Finally, the equivalent model for layer thickness is proposed to better describe the Hall-Petch effect on the yield strength of MGGNLs. Our study provides the guidance to the synthesis of novel metallic glass nanocomposites. (Computational Materials Science 177 (2020) 109536)
The lack of ductility, mainly due to shear localization, limits the full exploitation of metallic glasses (MGs). Such a weakness can be mitigated via introducing graphene nanoplatelets (GNs), which helps strengthen and toughen MG matrix. Using molecular dynamics simulations, we investigate the tensile deformation of Cu50Zr50 metallic glass-graphene nanoplatelet composites (MGGNCs) regarding the effects of graphene volume fraction and graphene layer number. Increasing fraction of GN in MGGNC enhances both tensile strength and fracture strain. The dominant component changes from MG matrix to GN, corresponding to a failure mode transition from shear banding in MG to brittle fracture in graphene. For a given volume fraction, single-layered GN behaves better than its double-layered counterpart, manifested as a higher fracture strain in MGGNCs. Our study illustrates the strengthening and toughening mechanisms in MGGNCs, and why single-layered GN performs better. All of these results can contribute to the synthesis of novel MG/graphene composites. (Journal of Non-Crystalline Solids 546 (2020) 120284)

Porous Metals

Porous materials, useful for applied purposes owing to their light weight, and balanced stiffness and ductility, are also of fundamental interest as energy-absorption materials.

We investigate the effects of porosity or relative mass density and specific surface area on shock response of open-cell nanoporous Cu foams with molecular dynamics simulations, including compression, shock velocity–particle velocity, and shock temperature curves, as well as shock-induced melting. While porosity still plays the key role in shock response, specific surface area at nanoscales can have remarkable effects on shock temperature and pressure, but its effects on shock velocity and specific volume are negligible. Shock-induced melting of nanofoams still follows the equilibrium melting curve for full-density Cu, and the incipient and complete melting temperatures are established as a function of both relative mass density and specific surface area. (Journal of Applied Physics 118 16 (2015) 165902)

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