Semi-coherent Interface Research Articles

Equilibrium solute segregation to matrix- θ′ precipitate interfaces in Al-Cu alloys from first principles

Particular combinations of solute atoms segregated to the interface of the Al and ${\ensuremath{\theta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Al}}_{2}\mathrm{Cu}$ in Al-Cu alloys can help stabilize ${\ensuremath{\theta}}^{\ensuremath{'}}$ precipitates at high temperatures. Stabilization of such precipitates is determined by a combination of thermodynamics (including driving forces for coarsening and transformation and solute segregation tendencies) and kinetic effects (including solute diffusion and interfacial mobility in the presence of interfacial solute segregation). For some alloys such as recent Al-Cu-based alloys, multiple solutes segregate in significant quantities to interfaces, and solute-solute interactions at the interface are important, with multiple types of solutes competing for similar interfacial sites. To treat this situation, we develop and apply a statistical mechanics approach to calculate the temperature-dependent equilibrium solute atoms distribution near the coherent and semicoherent interfaces between the Al matrix and the ${\ensuremath{\theta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Al}}_{2}\mathrm{Cu}$ precipitates. The developed approach is applied to the investigation of Si, Mn, and Zr segregation at the interface, as particular combinations of these elements affect the thermal stability of the ${\ensuremath{\theta}}^{\ensuremath{'}}$ precipitates. We demonstrate that because Si and Mn atoms segregate on the same semicoherent interface, the presence of Si reduces the concentration of Mn solutes at the interface. Si atoms preferably occupy the first layer of the interface and compete with Mn atoms for one type of particular sites in the layer. Mn atoms preferably occupy the second layer of the semicoherent interface, and the Mn-Mn interaction plays an important role in their distribution. Zr atoms mostly segregate on one of the two nonequivalent sites of the second layer of the coherent interface. Due to symmetry properties of the coherent interface, the calculations show that the segregation Zr of atoms to this interface will likely lead to the formation of ${\mathrm{L}1}_{2}$ ordered ${\mathrm{Al}}_{3}\mathrm{Zr}$ layer.

Continuous and Discontinuous Yielding Behaviors in Ferrite-Cementite Steels

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macro-scale deformation examinations. The continuous yielding behavior was observed in heat-treated specimens with a lamellar or cementite-spheroidal structure even after strain-aging treatment. However, the discontinuous yielding behavior accompanied by the Luders elongation appeared in ferrite-cementite steel that was recrystallized after cold rolling. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.

High-mass-proportion TiCp/Ti6Al4V titanium matrix composites prepared by directed energy deposition

Titanium matrix composites (TMC) have potential applications in the aerospace industry because of their excellent performance. The comprehensive performance of TMC mainly depends on the matrix, reinforcement and interface characteristics. Hence, this study discussed the knowledge of microstructure-property relationships in detail. Crack-free high-mass-proportion TiCp/Ti6Al4Vcomposites were successfully prepared by directed energy deposition (DED). As the TiCp mass fraction increasing from 0 to 50%, the quantity of primary TiC and unmelted TiC (UMT) increased. Meanwhile, the refined α-Ti in the composites had a relatively weak texture. In addition, the interface between primary TiC and α-Ti was a semi-coherent interface, exhibiting a 112-0 α-Ti // [110]TiC, 1-100 α-Ti // 1-11 TiC orientation relationship, which facilitated the heterogeneous nucleation of Ti and improved bonding of primary TiC with the matrix. With the increase in microhardness taking the form of a cubic function, the wear mechanism was found to transform from abrasive wear to slight delamination wear. Due to the fact that both UMT and primary TiC bonded well with Ti64 matrix, they shared partial friction to protect matrix from severe abrasion, resulting in an excellent wear resistance of composites. Moreover, the thermal conductivity of 50% TiCp/Ti6Al4V was 9.063 W∙m-1∙K-1, which was nearly 26.5% higher than that of Ti6Al4V. Owing to the premature cracking of brittle UMT and dendritic TiC, the tensile strength and elongation of the composite with 50% TiCp were 515.5 MPa and 1.83%, which decreased by 45.8% and 78.8%, respectively. Adding a high proportion of TiCp can significantly improve the hardness and wear resistance of TMC, whereas it is detrimental to the tensile performance of TMC. The study have significant implications for the design of novel TMC, particularly for the aerospace industrial applications.

On the role of carbon in molybdenum manufactured by Laser Powder Bed Fusion

Carbon is known to improve the ductility and grain boundary strength of molybdenum. Scientific consensus to explain this phenomenon is that carbon either replaces oxygen impurities at grain boundaries or suppresses the diffusion of oxygen towards the grain boundaries. The aim of this study is to clarify the mechanism on how the carbides interact with the oxygen impurities and the molybdenum matrix, thus to provide a better understanding for designing molybdenum alloy compositions. Molybdenum alloyed with 4500 μg/g carbon was manufactured in the additive manufacturing process of Laser Powder Bed Fusion (LPBF). The cellular grain structure of molybdenum cells with surrounding molybdenum carbide caused by constitutional undercooling during the solidification process was investigated by Transmission Electron Microscopy (TEM) for morphology, composition and crystal structure. The carbide was identified to be orthorhombic ζ-Mo2C, which has a solubility for oxygen. ζ-Mo2C shows a semi-coherent interface with α-Mo. No cracks could be found at the α-Mo / α-Mo or the α-Mo / ζ-Mo2C interfaces. Oxygen could not be detected at grain boundaries, but only within ζ-Mo2C, which traps the oxygen, thus avoiding oxygen segregation to the grain boundaries and improving the grain boundary strength.

Microstructures and mechanical properties of oxide dispersion strengthened CoCrFeNi high-entropy alloy produced by mechanical alloying and spark plasma sintering

1.2 Hf and 1.5 Y2O3 (wt.%) were added into CoCrFeNi (CCFN) high-entropy alloy (HEA) to fabricate oxide dispersion strengthened CoCrFeNi (ODS-CCFN) composite by mechanical alloying and spark plasma sintering. Microstructures, including XRD, SEM, EBSD, and TEM of CCFN and ODS-CCFN HEAs were investigated to identify the effects of introducing nano-sized oxides into CCFN HEA. These HEAs compose of FCC matrix and a small amount of Cr7C3 and Cr2O3, and a few of MnCr2O4 is only detected in CCFN HEA. High-density Y2Hf2O7 with the average diameter of 6.9 ± 4.2 nm uniformly distribute in the matrix and Cr7C3 of ODS-CCFN. Lattice misfit calculations indicate that Y2Hf2O7 particles exhibit a coherent interface with the matrix, and a semi-coherent interface with Cr7C3. The average sizes of all phases, including grain, Cr7C3 and Cr2O3, are highly refined in ODS-CCFN. The effects of microstructure changes on properties of CCFN HEA were also detected. Compared with CCFN, the tensile yield strength of ODS-CCFN increases from 978 MPa to 1.25 GPa with a reasonable elongation decreases from 2.6% to 1.9%. The increase in yield strength for ODS-CCFN is mainly ascribed to the precipitation strengthening of Y2Hf2O7 and highly refined grains, Cr7C3 and Cr2O3. The electrical resistivity of ODS-CCFN is lower than that of CCFN, the reason may be that the existence of Y2Hf2O7 particles reduce the lattice distortion of the matrix, and make the electrical resistivity decrease. The corrosion resistance of ODS-CCFN HEA is highly improved in both sulfuric acid solution and sodium chloride solution because its passive current density is largely reduced by the addition of Hf and Y2O3.

HRTEM study of interfacial structure in SiCp/A390 composites

SiCp/A390 composite was fabricated by powder metallurgy method and hot extrusion process. The interfacial microstructure of SiCp/A390 composites after heat treatment was characterized by HRTEM. The results show that the interface between SiC and Al matrix has an orientation relationship of [410]SiC//[01]Al, (100) SiC//(11) Al and forming a semi-coherent interface. The crystallographic orientation relationship between SiC and Si in the SiCp/A390 composites is [010]SiC//[001]Si, (004)SiC//(220)Si. The interplanar spacing of Al matrix on lattice plane (111) is close to the twice interplanar spacing of the Si phase on lattice plane (400). They keep a semi-coherent crystal plane, with an orientation relationship of [01]Al//[001]Si, (111)Al//(400)Si.

Effects of solutes on dislocation nucleation and interface sliding of bimetal semi-coherent interface

Misfit dislocations at bimetal interfaces play a key role in interface-induced deformation mechanism, which in turn determines the strengthening and softening at different length scales. Although a variety of interfaces have been explored to reveal distinct misfit dislocations and resultant deformation mechanisms, an ideal clean bimetal interface model without any solute atoms is generally assumed in modeling and simulations. Taking bimetal semi-coherent Cu{111}//{111}Ag interface as an illustration, we reveal that solute segregation at the interface could change the underlying mechanism of interface-facilitated dislocation nucleation and interface sliding. In contrary to the clean interface, the energy barrier is much lower for dislocation nucleation at compositionally diffused interfaces due to the appearance of more preferable nucleation sites at the boundaries of solute clusters than those around the nodes of misfit dislocations. We also find that solutes may modify the preferred slip systems via the formation of more complicated distribution of localized shearing regions. Further exploration of the solute effect on the resistance of interface sliding indicates that the solute segregation may also provide a strong barrier for the relative sliding between two constituent metals and change the pathway of sliding via the strong interaction between solute clusters and intersection nodes of misfit dislocations. Our results provide a foundation and emphasize the necessity to include the effect of solutes on the plastic deformation of bimetal interfaces in understanding the interface-dominated plasticity in a more realistic way.

Self-assembling behavior and interface structure in vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x films on (001) (La,Sr)(Al,Ta)O3 substrates

Heteroepitaxial oxide-based nanocomposite films possessing a variety of functional properties have attracted tremendous research interest. Here, self-assembled vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x (x = 0.2 and 0.5) films have been successfully grown on single-crystalline (001) (La,Sr)(Al,Ta)O3 substrates by the pulsed laser deposition technique. Self-assembling behavior of the nanocomposite films and atomic-scale interface structure between Pr0.5Ba0.5MnO3 matrix and CeO2 nanopillars have been investigated by advanced electron microscopy techniques. Two different orientation relationships, (001)[100]Pr0.5Ba0.5MnO3//(001)[1-10]CeO2 and (001)[100]Pr0.5Ba0.5MnO3//(110)[1-10]CeO2, form between Pr0.5Ba0.5MnO3 and CeO2 in the (Pr0.5Ba0.5MnO3)0.8:(CeO2)0.2 film along the film growth direction, which is essentially different from vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)0.5:(CeO2)0.5 films having only (001)[100]Pr0.5Ba0.5MnO3//(001)[1-10]CeO2 orientation relationship. Both coherent and semi-coherent Pr0.5Ba0.5MnO3/CeO2 interface appear in the films. In contrast to semi-coherent interface with regular distribution of interfacial dislocations, interface reconstruction occurs at the coherent Pr0.5Ba0.5MnO3/CeO2 interface. Our findings indicate that epitaxial strain imposed by the concentration of CeO2 in the nanocomposite films affects the self-assembling behavior of the vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x films.

Homogenization method for microscopic characterization of the composite magnetoelectric multiferroics

Tuning of magnetization or electrical polarization using external fields other than their corresponding conjugate fields (i.e., magnetic field for the former or electric field for the latter response) attracts renewed interest due to its potential for applications. The magnetoelectric effect in multiferroic 1–3 composite composed of alternating magnetic and ferroelectric layers operating in linear regime consequent to external biasing fields is simulated and analysed theoretically. Two-scale homogenization procedure to arrive at the equilibrium overall physical properties of magnetoelectric multiferroic composite is formulated using variational analysis. This procedure is extended to quantify the underlying local (microscopic) electric, magnetic and elastic fields and thereby compute local distribution of stresses and strains, electrical and magnetic potentials, the electric and magnetic fields as well as the equivalent von Mises stresses. The computational model is implemented by modifying the software POSTMAT (material postprocessing). Computed local stress/strain profiles and the von Mises stresses consequent to biasing electrical and magnetic fields provide insightful information related to the magnetostriction and the ensuing electrical and magnetic polarization. Average polarization and magnetization against magnetic and electric fields respectively are computed and found to be in reasonable limits of the experimental results on similar composite systems. The homogenization model covers multiferroics and its composites regardless of crystallographic symmetry (with the caveat of assuming an ideal and semi-coherent interface connecting the constituent phases) and offer computational efficiency besides unveiling the nature of the underlying microscopic field characteristics.

C15–Al<sub>2</sub>Ca Precipitation in a Mg–Al–Ca Alloy

The coarsening kinetics of a fine C15–Al2Ca Laves phase with a plate-like morphology precipitated within the primary α-Mg grains were investigated for Mg–5Al–1.5Ca alloy aged at 523 K. The Al2Ca precipitate coarsened as its coherence was retained in the aging time below 300 h, and a quantitative relationship was obtained between precipitate length (l) and aging time (t) as l ∝ t0.23. However, the α/C15 coherent interface changed into a semi-coherent interface by introduction of misfit dislocations on the planar surface of the precipitates in the aging time above 300 h, which resulted in the promotion of Al2Ca coarsening. The Al2Ca phase was assumed to precipitate through a nucleation and growth mechanism rather than spinodal decomposition, and its coarsening was explained using the terrace-ledge-kink mechanism. The result shows that the aspect ratio of the Al2Ca precipitates was predominantly determined by the aging temperature, and it decreased at higher aging temperatures.

Interface characteristics in an α+β titanium alloy

The alpha/beta interface in Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) is investigated via centre of symmetry analysis, both as-grown and after 10% cold work. Semi-coherent interface steps are observed at a spacing of 4.5 +/-1.13 atoms in the as-grown condition, in good agreement with theory prediction (4.37 atoms). Lattice accommodation is observed, with elongation along [-1 2 -1 0]alpha and contraction along [1 0 -1 0]alpha . Deformed alpha exhibited larger, less coherent steps with slip bands lying in {110}beta. This indicates dislocation pile-up at the grain boundary, a precursor to globularisation, offering insight into the effect of deformation processing on the interface, which is important for titanium alloy processing route design.

Effects of local geometry distortion at the Al/Al2Cu interfaces on solute segregation.

Interfaces in materials play a critical role in a wide range of functional properties. The heterophase semicoherent interface is one of the most difficult systems in density functional theory (DFT) simulations. We have studied the segregation energies of different solutes (Si, Mg and Zn) at the Al/Al2Cu (semi)coherent interfaces based on the linear combination of atomic orbitals (LCAO). Our DFT-LCAO computed segregation energies of solutes at the Al/Al2Cu coherent interface are in good agreement with the DFT-PAW calculations. However, at the Al/Al2Cu semicoherent interface, results from the LCAO calculations are significantly different to those obtained using the PAW method. The local geometry distortion in the semicoherent interface region significantly influences the solute energetics. Considering the distortion, the segregation energies of solutes well explain the trend of the experimentally measured solute concentrations, such as the higher solute Si concentration at the semicoherent interface than that at the coherent interface, and the similar Si concentration across the semicoherent interface. The lattice mismatch effect on the solute segregation energy is investigated as well. At low lattice mismatch (0.25%), the segregation energies of Si and Mg are almost compatible. Our results shed light on improving the theoretical predictions for interfaces to determine accurate functional properties.

Molecular dynamics simulation of migration behavior of FCC-BCC atomic terrace-step phase boundary in iron-based alloy

The martensitic transformation between the high-temperature face-centered cubic (FCC) phase and the low-temperature body-centered cubic (BCC) phase in iron-based alloys has been studied for years, which plays a critical role in controlling microstructures and hence properties of the alloys. Generally, the BCC structure martensitic phase forms from the FCC parent phase, involving a collective motions of atoms over a distance less than the interatomic distance in the vicinity of the interphase boundary. Thus the structure of interphase boundary separating the FCC and BCC phases is the key characteristics to quantitatively understanding the mechanism and kinetics of martensitic transformation. Due to the difficulty in observing the atomic motions taking place at a velocity as high as the speed of sound, the experimental investigation on the migration of FCC/BCC interphase boundary during the transformation is as yet limited. Noteworthily, molecular dynamics (MD) simulation has been applied to studying the martensitic transformation, in particular for investigating the mobility of the FCC/BCC interphase boundary in iron. However, in most of the MD studies the atomistically planar interfaces of {111}<sub>FCC</sub> // {110}<sub>BCC</sub> are considered as the initial configuration of the interphase boundary between FCC and BCC phases, which is in contradiction to the high-resolution TEM observations. In fact, the FCC/BCC interphase boundary, which is known as the macroscopic habit plane, is a semi-coherent interface consisting of several steps and terrace planes on an atomic scale. In the present work, the atomic configuration of a terrace-step FCC/BCC interphase boundary of iron is built in terms of the topological model. The MD simulation is conducted to clarify the mechanism of interphase boundary migration in the FCC-to-BCC transformation. The results show that the FCC/BCC boundary migrates along its normal at the expense of FCC phase as a result of the lateral motions of the transformation dislocations. Meanwhile, the interphase boundary maintains the stable terrace-step structure during the transformation. Further examinations reveal that the transformation dislocations move steadily at a velocity as high as (2.8 ± 0.2) × 10<sup>3</sup> m/s, affecting the migration of the interphase boundary with a constant velocity of about (4.4 ± 0.3) × 10<sup>2</sup> m/s. The effective migration velocity of FCC/BCC interface exhibits dynamic properties consistent with the characteristic features commonly observed in a displacive martensitic transformation. Additionally, the motion of transformation dislocations gives rise to the macroscopic shape strain composed of a shear component <inline-formula><tex-math id="M3">\begin{document}$ {\varGamma _{{\rm{yz}}}} = 0.349$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M3.png"/></alternatives></inline-formula> parallel to the boundary and a dilatation <inline-formula><tex-math id="M4">\begin{document}$ {\varGamma _{{\rm{zz}}}} = 0.053$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M4.png"/></alternatives></inline-formula> normal to the boundary in the MD simulation, which is close to the crystallographic calculations by the topological model.

Extraction, Distribution, and Precipitation Mechanism of TiN–MnS Complex Inclusions in Al-Killed Titanium Alloyed Interstitial Free Steel

The TiN–MnS complex inclusions in Al-killed titanium alloyed interstitial free steel were extracted from the slabs using an electrolytic aqueous solution method. Scanning electron microscopy with energy spectroscopy analysis and an automatic scanning electron microscope were employed to analyze the size and distribution of TiN–MnS inclusions in the thickness direction of the slab. It was found that TiN–MnS complex inclusions were primarily concentrated in the 1/4 thickness direction from the inner and outer surfaces, and the size of the complex inclusions in the slab center was approximately 8 μm. TiN began to precipitate when the solidification rate reached 0.646–0.680 in the δ phase, and MnS would appear in the γ phase when the solidification rate exceeded 0.450 through thermodynamic analysis. Also, the formation mechanism of the complex TiN–MnS was investigated under a crystal structure during solidification. The TiN–MnS complex inclusions in Al-killed titanium alloyed interstitial free steel were extracted from the slabs using an electrolytic aqueous solution method. Scanning electron microscopy with energy spectroscopy analysis and an automatic scanning electron microscope were employed to analyze the size and distribution of TiN–MnS inclusions in the thickness direction of the slab. It was found that TiN–MnS complex inclusions were primarily concentrated in the 1/4 thickness direction from the inner and outer surfaces, and the size of the complex inclusions in the slab center was approximately 8 μm. Due to the similar plane of atoms ((100)TiN||(100)MnS and [001]TiN||[001]MnS) and a semi coherent interface between TiN and MnS, a series of edge dislocations would provide a comfortable accommodation for misfits parallel to the interface.

Effect of sintering temperature on the mechanical properties and microstructures of pressureless-sintered B4C/SiC ceramic composite with carbon additive

B4C/SiC (15 wt%) ceramic composite with 2 wt% carbon additive was fabricated via solid-phase pressureless sintering under an argon atmosphere for 60 min between 2100 °C and 2200 °C. The effects of the sintering temperature on the microstructure and mechanical properties of the B4C/SiC ceramic composites were studied in detail. The optimum values of the relative density, Vickers hardness, flexural strength, and fracture toughness of the B4C/SiC ceramic composite sintered at 2150 °C were 95.3%, 25.5 GPa, 296 MPa, and 2.81 MPa m1/2, respectively. The microstructural investigations revealed that when the sintering temperature increases from 2100 °C to 2150 °C, sintered necks begin to grow and the densification zone increases. With sintering temperatures above 2150 °C, the grains grow rapidly such that the pores are not eliminated, causing the formation of large closed pores and resulting in a decrease in the relative density and mechanical properties of the B4C/SiC ceramic composite. The plate-like and irregular SiC grains are uniformly dispersed in the B4C matrix to inhibit B4C grain growth. The interface between SiC and B4C is clean and narrow, and the clean boundary suggests strong binding, which contributes to the mechanical properties of the B4C/SiC ceramic composite. At 2150 °C, the C additive undergoes significant crystallisation, forming lamellar structures with high aspect ratios, thereby promoting the pores in the ceramic composite to be discharged to the outside through volume diffusion to obtain a denser sintered body. The interface between B4C and graphite is a semi-coherent interface that increases the flexural strength with a low defect concentration. The main toughening mechanisms of the B4C/SiC ceramic composite are crack deflection and crack branching around the SiC grains and crack bridging by SiC grains in the wake of the crack tip.

Fabrication of WO3/TiO2 core-shell nanowire arrays: Structure design and high electrochromic performance

Unique WO3/TiO2 core-shell nanowire heterostructures are rationally constructed based on a solvothermal method combining magnetron sputtering. High resolution transmission electron microscopy (HRTEM) analysis evidences the existence of a semi-coherent interface between a hexagonal phase WO3 core and anatase TiO2 shell. X-ray photoelectron spectroscopy and optical absorption spectra suggest an interfacial interaction between the WO3 core and TiO2 shell. The WO3/TiO2 core-shell nanowires exhibit superior electrochromic performance in both visible and near infrared regions with large optical modulation (87.0% at 1500 nm and 85.3% at 633 nm), short bleaching–coloration times (1.1 s and 3.0 s), decent coloring efficiency (102.1 cm2 C−1) and excellent cycling stability (95.6% after 3000 cycles). The obtained WO3/TiO2 core-shell nanowire heterostructures may find great potential in producing electrochromic nanodevices and other energy-efficient applications.

The effect of grain boundary structure on the intergranular degradation behavior of solution annealed alloy 690 in high temperature, hydrogenated water

The environmental degradation of four different types of grain boundaries were investigated on alloy 690 following slow strain rate tensile tests in 360°C hydrogenated water. Random high angle boundaries (RHABs) support fast Cr diffusion that promotes the formation of a continuous surface oxide film and grain boundary migration. Surprisingly, coherent twin boundaries (CTBs) are susceptible to intergranular oxidation and do not exhibit Cr diffusion or grain boundary migration. When CTBs are changed to transformed twin boundaries (TTBs) by cold work, they behave like RHABs. Finally, incoherent twin boundaries (ITBs) undergo intergranular oxidation with limited Cr depletion but no boundary migration beyond the oxide. The Cr diffusivity along grain boundary in this alloy is directly related to the density of coincident site in the grain boundary plane and determines the morphology of oxide formed near the grain boundary. CTBs are still highly resistant to stress corrosion cracking (SCC) due to the semi-coherent interface between the intergranular chromia and grain matrix. In contrast, the intergranular oxides formed along RHABs inherit highly-disordered boundary structure from the original grain boundaries and show much higher SCC susceptibility. The grain boundary structure dependence of SCC resistance should be understood from its effects on solute diffusivity, structure of intergranular oxide and the local stress-strain state.

Revisit the Role of Steps/Disconnections on Misfit Cancellation at Semi-Coherent Interface—Bridging the O-Line Model and the Topological Model

It has been a long-standing topic how the lattice misfit is cancelled at a semi-coherent interphase boundary consisting of terraces and steps. Apart from a set of misfit dislocations separating the coherent patches, the role of steps (which is frequently called ‘disconnections’) on misfit cancellation remains ambiguous because these steps do not destroy the continuity of lattice planes across the interface. This paper aims to clarify such ambiguity through identification of a set of secondary dislocations through a rigorous constrained coincidence site lattice (CCSL)/constrained displacive shift complete lattice (CDSCL) analysis. A semi-coherent interface between body-centred cubic (BCC) Cr-rich precipitate and face-centred cubic (FCC) Cu-rich matrix that holds a near N-W orientation relationship (OR) is used as an example to demonstrate the procedure to determine the secondary dislocations that are coincident with steps along the interface. The current approach does not only redefine the disconnections in the topological model, but also extends the description of interface structure from the O-line model. As a result, the ‘discrepancy’ between these two popular crystallographic models can be completely eliminated when the interface is required to contain a pair of parallel close-packed directions.

Colloidal Gelation in Liquid Metals Enables Functional Nanocomposites of 2D Metal Carbides (MXenes) and Lightweight Metals.

Nanomaterials dispersed in different media, such as liquids or polymers, generate a variety of functional composites with synergistic properties. In this work, we discuss liquid metals as the nanomaterials' dispersion media. For example, 2D transition-metal carbides and nitrides (MXenes) can be efficiently dispersed in liquid Ga and lightweight alloys of Al, Mg, and Li. We show that the Lifshitz theory predicts strong van der Waals attraction between nanoscale objects interacting through liquid metals. However, a uniform distribution of MXenes in liquid metals can be achieved through colloidal gelation, where particles form self-supporting networks stable against macroscopic phase segregation. This network acts as a reinforcement boosting mechanical properties of the resulting metal-matrix composite. By choosing Mg-Li alloy as an example of ultralightweight metal matrix and Ti3C2Tx MXene as a nanoscale reinforcement, we apply a liquid metal gelation technique to fabricate functional nanocomposites with an up to 57% increase in the specific yield strength without compromising the matrix alloy's plasticity. MXenes largely retain their phase and 2D morphology after processing in liquid Mg-Li alloy at 700 °C. The 2D morphology enables formation of a strong semicoherent interface between MXene and metal matrix, manifested by biaxial strain of the MXene lattice inside the metal matrix. This work expands applications for MXenes and shows the potential for developing MXene-reinforced metal matrix composites for structural alloys and other emerging applications with metal-MXene interfaces, such as batteries and supercapacitors.

Study on the interface structure between in-situ reaction products and the aluminum matrix of Al-Ti-C-CuO-La2O3 system

The in situ particle reinforced Aluminum Matrix Composites were manufactured by using the contact reaction method in Al-Ti-C-CuO-La2O3 system and 6063 aluminum alloy. The morphology and size of the in situ reaction products and the interface structures between in situ reaction products and the Al matrix were observed by TEM and HRTEM. The results show that the in situ reaction products are TiC, Al2O3 and Ti2Al20La. The shape of TiC is quadrangular and the size is nanosacle. TiC (200) // Al (111) and TiC forms a semi-coherent interface with the Al matrix. The shape of Al2O3 is elliptical and the size is submicron. It is found that Al2O3 (112̄0) // Al (111) and the interface between Al2O3 (112̄0) and Al (111) is in a coherent relationship. Al2O3 (011̄2) and (101̄2̄) form a non-coherent interface with the Al matrix. The shape of Ti2Al20La is square and the size is micron. It is also found that the Ti2Al20La (440) // Al (111) and its interface is in a semi-coherent relationship. The Ti2Al20La (311) // Al (111) and its interface is in a non-coherent interface relationship. Comparing with 6063 aluminum alloy, the hardness value of the Al-Ti-C-CuO-La2O3 system aluminum matrix composites increase from 16.8HB to 36.8HB.