Semi-coherent Interface Research Articles

Effects of TiB2 Particles on the Microstructure Evolution and Mechanical Properties of B4C/TiB2 Ceramic Composite.

B4C/TiB2 ceramic composites reinforced with three size scales (average particle size: 7 μm, 500 nm, and 50 nm) of TiB2 were prepared by using a pressureless sintering furnace at 2100 °C under Ar atmosphere for 60 min. The results demonstrated that during the sintering process, TiB2 located on the boundaries between different B4C grains could inhibit the grain growth which improved the mass transport mechanism and sintering driving force. A semi-coherent interface between B4C and SiC was found, which is supposed to help to reduce the interface energy and obtain good mechanical properties of the B4C/TiB2 ceramic composite. On sample cooling from sintering temperature to room temperature, the residual tensile stress fields formed at the TiB2 interfaces owning to the thermo-elastico properties mismatched, which might have contributed to increase the ability of the sample to resist crack propagation. The results showed that the relative density, Vickers hardness, and fracture toughness of the composite with 20 wt.% submicron and 10 wt.% nano-TiB2 were significantly improved, which were 98.6%, 30.2 GPa, and 5.47 MPa·m1/2, respectively.

Mechanistic insights into interface-facilitated dislocation nucleation and phase transformation at semicoherent bimetal interfaces

The nucleation of lattice dislocations and interface sliding at bimetal interfaces are two fundamental mechanisms of plasticity that are responsible for the mechanical responses of nanostructured materials; however, the interface-facilitated phase transformation is rarely considered owing to its relatively high energy barrier for activation. Taking the bimetal hcp/bcc interfaces with Pitch-Schrader and Burgers orientation relationships (ORs) as an illustration, we show that both non-basal dislocation nucleation and hcp-to-bcc phase transformation can be activated at the interface under external loading when the basal slip systems are effectively suppressed. The non-basal dislocation nucleation is shown to be closely related to the dynamic evolution of misfit dislocation patterns at the semicoherent interface, in which the 1/6[022¯3¯] pyramidal dislocation is not strictly parallel to the (011¯1) stacking fault plane owing to the corrugated feature. In contrast to non-basal dislocation nucleation, phase transformation requires specific crystallographic ORs of the constituent metals under certain loading conditions, which corresponds to the process of alternate shuffle and shear deformation that involves atomistic migration. To further reveal the competition between non-basal dislocation nucleation and phase transformation, a series of twisted interface models were constructed to systematically investigate the optimal condition of the interface geometry for phase transformation. The phase transformation occurred only when the dislocation nucleation was further hindered at some specific twist angles, suggesting a strong dependence of phase transformation on the interface structure. These findings provide a foundation to the atomistic mechanism of various interface-mediated deformation and a solution to tune interface-facilitated plasticity via interface engineering.

Microstructure Evolution and Properties of an In-Situ Nano-Gd2O3/Cu Composite by Powder Metallurgy.

Gadolinia (Gd2O3) is potentially attractive as a dispersive phase for copper matrix composites due to its excellent thermodynamic stability. In this paper, a series of 1.5 vol% nano-Gd2O3/Cu composites were prepared via an internal oxidation method followed by powder metallurgy in the temperature range of 1123–1223 K with a holding time of 5–60 min. The effects of processing parameters on the microstructure and properties of the composites were analyzed. The results showed that the tensile strength and conductivity of the nano-Gd2O3/Cu composite have a strong link with the microporosity and grain size, while the microstructure of the composite was determined by the sintering temperature and holding time. The optimal sintering temperature and holding time for the composite were 1173 K and 30 min, respectively, under which a maximum ultimate tensile strength of 317 MPa was obtained, and the conductivity was 96.8% IACS. Transmission electron microscopy observations indicated that nano-Gd2O3 particles with a mean size of 76 nm formed a semi-coherent interface with the copper matrix. In the nano-Gd2O3/Cu composite, grain-boundary strengthening, Orowan strengthening, thermal mismatch strengthening, and load transfer strengthening mechanisms occurred simultaneously.

Interfacial structures and strengthening mechanisms of in situ synthesized TiC reinforced Ti6Al4V composites by selective laser melting

The in-situ nanoscale TiC particles reinforced Ti6Al4V composites was prepared by selective laser melting (SLM) from the mixture of Ti6Al4V alloy powders and graphene powders, the microstructure and interface bonding of the composites were studied. The composites were mainly composed of α, α' martensite and α+β structures. More TiC were formed in the overlapping regions than that in the molten pool, the solidification rate in overlapping regions was slow, which was beneficial to TiC nucleation and growth. The TiC and Ti matrix displayed following orientation relationships: axis [0 0 1]β-Ti∥[-1 -1 1]TiC∥[0 -1 1 0 ]α-Ti, lattice plane (2 2 2)TiC //(0 0 0 2)α-Ti, lattice plane (1 1 1)TiC//(0 0 0 2)α-Ti, lattice plane (1 1 1)TiC//(1 0 -1 0)α-Ti. The TiC and Ti matrix have a small degree (6.2%) of mismatch between (1 1 1)TiC and (0 0 0 2)α-Ti, and the semi-coherent interface was occurred. TiC preferentially nucleated and grew up along its (1 1 1) plane. The situ synthesized TiC was added into the Ti6Al4V alloy, refined the grains, and increased the microhardness of the alloy. The microhardness of the composites was 13.6% higher than that of Ti6Al4V alloy. The TiC effectively pinned the dislocations and grain boundaries were responsible for the enhanced mechanical properties of the composites.

Modification of microstructure and performance via doping Ti in W–1TiC fine-grained alloy

The Ti-doped TiC-dispersion-strengthened W alloys are prepared by mechanical alloying in this work. The influence of Ti content on the performance and microstructure of W–1TiC alloys is systematically investigated. Results show that the ultra-fine and homogenized W–1TiC–0.5Ti alloy powders with average size of 212.3 nm have high sintering activity. The relative density of its pressed sample sintered at 1930 °C is nearly full densification (over 99%). Introducing Ti in W–1TiC alloy forms a Ti-rich region, which inhibits the growth of the TiC and maintains the uniform distribution of the TiC, around the TiC particles in sintering. The dispersed TiC decreases the average grain size of sintered W–1TiC–0.5Ti alloy into 2.71 μm. Moreover, the W(110) and TiC(111) form semi-coherent interface at W/TiC phase boundary to modify the interface compatibility between W matrix and TiC ceramic phase. The stable semi-coherent interface strengthens the phase boundary and limits the growth of the TiC particles and W grains. Accordingly, the ultimate tensile strength and microhardness of W–1TiC–0.5Ti alloy at RT are 426 MPa and 860 ± 11 HV0.2, respectively. The improvements of W–1TiC–0.5Ti alloy in mechanical properties are attributed to the synergistic effect of fine-grained strengthening, dispersion strengthening, and interface strengthening.

Effect of in situ synthesis TiC on the microstructure of graphene/Ti6Al4V composite fabricated by selective laser melting

The graphene/Ti6Al4V composite was prepared by selective laser melting, and the effect of the addition of graphene on the microstructure of the composite was studied. The in situ synthesis of graphene and Ti6Al4V occurred, and the TiC was formed. Compared with other areas of the molten pool in the composite material, the grain of the overlap zone becomes smaller and a large number of basket-weave structures are formed and more TiC in the remelting zone than the melting zone. The addition of graphene plays a role in heterogeneous nucleation, which makes the internal structure of the composite finer than pure Ti6Al4V. The formation of the semi-coherent interface between TiC and α-Ti leads to strong interface bonding.

Strain mapping in symmetrical core-shell gold nanorods from HRTEM images

Strain distribution, interface type and the causes of strain in gold core-shell nanorods were studied by employing high-resolution electron transmission microscope (HRTEM) and Geometric Phase Analysis (GPA). Anisotropic Strain distributions in gold core-shell nanorods were affected by l-cysteine (Cys) concentration and AA/Au3+ ratio which determines shell growth kinetics. Strain circles were generated at the core-shell interfaces of gold nanorods. The {002} strain distributions of gold nanorods were very sensitive to the increase of Cys concentration and AA/Au3+ ratio. Strain distributions with high strain values on {111} planes were more significant than that on {002} planes. This state was related to the preferential adsorption of thiols on {111} planes. The core-shell interface of gold nanorods was determined to be a mix of distorted coherent interface and semi-coherent interface, which was mainly produced via the slip of {111} planes along <110> slip direction. The gold shell covered the whole core at the growth conditions of different Cys concentrations and AA/Au3+ ratios provided in this work via seed-mediated symmetric epitaxial growth. The strain at the core-shell interface was mainly generated by Au–S bonds and lattice distortion.

Shear band-driven precipitate dispersion for ultrastrong ductile medium-entropy alloys

Precipitation strengthening has been the basis of physical metallurgy since more than 100 years owing to its excellent strengthening effects. This approach generally employs coherent and nano-sized precipitates, as incoherent precipitates energetically become coarse due to their incompatibility with matrix and provide a negligible strengthening effect or even cause brittleness. Here we propose a shear band-driven dispersion of nano-sized and semicoherent precipitates, which show significant strengthening effects. We add aluminum to a model CoNiV medium-entropy alloy with a face-centered cubic structure to form the L21 Heusler phase with an ordered body-centered cubic structure, as predicted by ab initio calculations. Micro-shear bands act as heterogeneous nucleation sites and generate finely dispersed intragranular precipitates with a semicoherent interface, which leads to a remarkable strength-ductility balance. This work suggests that the structurally dissimilar precipitates, which are generally avoided in conventional alloys, can be a useful design concept in developing high-strength ductile structural materials.

Effect of heterointerface on the indentation behavior of nano-laminated c-BN/diamond composites

Heterointerface is generally accepted to tune the mechanical properties of metallic nano-laminated composites (MNLCs). Recent experiments have shown that the ceramic NLCs composed of c-BN and diamond can also possess unprecedented comprehensive performance. However, the physical image of the heterointerface formed by such two hard ceramics is still unclear, and how the interface can affect the mechanical behavior of such CNLCs is unknown. In this work, atomistic simulations are adopted to explore the above problems. The heterointerface is found to be semi-coherent interface composed of periodic parallelogram networks. Each network is further composed of coherent region, stacking faults, Shockley partial dislocations and nodes. Then, the effect of semi-coherent interface on the mechanical property of the CNLCs is subsequently studied. It is found that the growth and transition of interfacial defects dominate the mechanical behavior when the indentation depth is small, while at large depth, the nucleation of full dislocations from the semi-coherent interface instead of the location below the indenter plays a leading role and the mechanical behavior of the CNLCs is governed by the interaction between the semi-coherent interface and the indentation-induced dislocations. The present results should be helpful for the deep understanding of the mechanical behavior of CNLCs.

The significance of Fe2Zr interface precipitation in governing δ-ferrite refinement in low-density Fe-4Al ferritic steel

We address the gap in the knowledge in regard to the significance of refinement δ-ferritic grain in low-density steel by understanding grain boundary precipitation of Fe2Zr. In this regard, the effect of Fe2Zr on refinement δ-ferritic grain was studied and compared with Zr-free steel. The results of the experiments suggest that Fe2Zr exhibits a strong and effective pinning of δ-ferritic grain, and leads to refinement of δ-ferritic grains in Zr-containing steels as compared to Zr-free steel. Furthermore, the precipitation analysis suggested that there was a semi-coherent interface between Fe2Zr and δ-ferrite in a specific direction, and the orientation relationship between them was determined.

Effects of the coherency of Al3Zr on the microstructures and quench sensitivity of Al–Zn–Mg–Cu alloys

Effects of the coherency of Al3Zr on the microstructures and quench sensitivity of Al-8.3Zn-2.3Mg-2.4Cu (AA7055) alloys were investigated. These alloys were subjected to two cold-rolling ratios (i.e., 5% and 20%) in order to vary the coherency of Al3Zr particles. The main objectives of this study were to reduce the quench sensitivity, refine the grain size, and retain the coherency of Al3Zr for high-strength Al–Zn–Mg–Cu production alloys. Results reveal that with the increase in the cold-rolling ratio, the quench sensitivity of the Zr-containing alloys increases. At a high cold-rolling ratio, the interface between Al3Zr particles and the matrix became semi-coherent after solution treatment. This semi-coherent interface led to the heterogeneous nucleation of solute atoms during quenching, which was the leading cause for the high quench sensitivity. By contrast, at a low cold-rolling ratio, the alloys did not exhibit any noticeable quench sensitivity. The Al3Zr particles retained coherence in the matrix and did not afford heterogeneous nucleation sites. On the other hand, regardless of the cold-rolling ratio, the alloys without Zr did not exhibit a high quench sensitivity. However, from the electron backscatter diffraction (EBSD) analysis, the grain size became coarser, and the recrystallization fraction was higher for alloys without Zr.

Atomic structures of interfacial solute gateways to θ′ precipitates in Al-Cu alloys

Many materials employed in critical structural applications depend upon metastable strengthening precipitates that transform or dissolve at elevated temperatures. Herein, aberration-corrected scanning transmission electron microscopy and first-principles calculations are used to accurately determine the atomic structure of the highly mobile, semi-coherent precipitate interfaces that control this process in the classic θ′ (Al2Cu) precipitate in the Al-Cu system. Semi-coherent {110} interfaces are found to be composed of an array of unexpected misfit dislocations that are arranged in two different structural units. Dislocations accommodate nearly all of the misfit between the Al matrix and strengthening phase. Cu is observed to segregate to the compressed edge of the dislocation cores at specific sites in this interface. First-principles calculations revealed the energetic landscape that facilitates these sites to become entry and exit gateways of Cu atoms in this semi-coherent interface. This investigation reveals critical features within semi-coherent interfaces that determine the thermal stability of precipitation-hardened alloys.

Superb high-temperature strength of aluminum-based nanocomposite with supra-nano stacking faults/twins

Improving high-temperature mechanical properties of Al-based materials is highly desired due to the significant decrease in strength of these materials as temperature is over 473 K, which limits their high-temperature applications. Here, we report a novel nanoparticles reinforced Al-based nanocomposite containing supra-nano stacking faults/twins fabricated via high energy ball milling followed by supersolidus liquid phase hot pressing. The strength of the nanocomposite is as high as 310 MPa at 673 K, which is at least ~2.5 times higher than that of Al-based materials ever reported. The record-high mechanical improvement may arises from the formation of semi-coherent interface between nanoparticles and Al matrix, along with large amounts of supra-nano stacking faults/twins in the matrix.

A carbide-reinforced Re0.5MoNbW(TaC)0.8 eutectic high-entropy composite with outstanding compressive properties

Refractory high-entropy alloys are promising high-temperature materials owing to their excellent softening resistance. However, most RHEAs that can withstand softening at 1200 °C usually suffer from apparent brittleness at room temperature. Here, a high strength-ductile matching Re0.5MoNbW(TaC)0.8 eutectic high-entropy composite (HEC) consisting of body-centered-cubic and multi-component carbide phases was fabricated by arc melting. The HEC exhibits a high yield strength (1340 MPa), ultimate compressive strength (2347 MPa) and plasticity (8.90 %), which exceeds those of previously reported RHEAs and HECs. Quantitative calculations based on experimental observations suggest that semi-coherent phase interface plays an essential role for strength enhancement, and interfacial dependent strengthening dominates among the interface strengthening terms. Our study envisages a strategy to design strong and ductile HECs aiming at high-temperature applications.

Cooperative roles of stacking fault energies on dislocation nucleation at bimetal interface through tunable potentials

By tuning consistent potentials for bimetal system, the cooperative roles of stacking fault energies (SFEs) such as intrinsic SFE (ISFE) and unstable SFE (USFE) on the dislocation nucleation are comprehensively explored. It reveals that the dislocation nucleation at semi-coherent interface is determined by the misfit dislocations, but the critical yield stress depends strongly on the values of ISFE and USFE that are function of external loadings. To account for such relationship, a modified analytical model is proposed based on the nucleation and propagation of double kinks at the interface, showing good agreement with the simulation results. These results not only resolve the previous inconsistency in the role of stacking faults on plastic deformation but shed light on evaluating the atomistic mechanism according to the synergistic effect between stacking faults and loading schemes.

Atomic-resolution study on the interface structure and strain state reversion of the Bi2Sr2CuO6+δ/MgO heterostructure

Due to the crucial influence of interface structure and strain on the performance of heterojunctions, they have received extensive attention in recent years. In this article, the interface structure and strain of the Bi2Sr2CuO6+δd(Bi-2201)/MgO superconducting heterojunction prepared by molecular beam epitaxy were investigated by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDX), and geometric phase analysis (GPA). The interfacial atomic sequence is determined as MgO-(CuO-CuO-Cu/BiO)-(Bi-2201)n, where a 0.53 nm thick CuO interlayer accompanied by Bi/Cu atomic co-occupation is observed between the Bi-2201 film and the MgO substrate. In addition, there is a typical coherent lattice-matching epitaxial interface between CuO/MgO with no defects and a semi-coherent domain-matching epitaxial interface between Bi-2201/CuO accompanied by an ideal misfit dislocation network. Because misfit dislocations almost compensate for the strain caused by lattice mismatch, the final Bi-2201 film undergoes in-plane compressive strain (εxx ~ −0.573%) rather than expected tensile strain relative to bulk Bi-2201, which is attributed to the thermal expansion mismatch. The compressive strain gradually releases as the distance from the heterointerface increases.

Excellent plasma electrolytic oxidation coating on AZ61 magnesium alloy under ordinal discharge mode

In the preparation of plasma electrolytic oxidation (PEO) coating, the rapid heating of freely-happened electron avalanche under traditional discharge (TD) mode inevitably results in a strong eruption of electric breakdown melt. The PEO coating is loose and invariably composed of a very thin inner dense layer and an outer loose layer, as a result of which its properties and application have been limited greatly. In this work, for purpose of weakening the eruption of breakdown melt, thickening the inner dense layer, densifying the outer loose layer and improving the performance of PEO coating, ordinal discharge (OD) mode of PEO coating is developed by regulating the mass ratio of MgF 2 to MgO ( α ) and voltage in the PEO investigation on AZ61 magnesium alloy in KF-KOH electrolyte. The formation mechanism under different discharge mode, electrochemical corrosion and wear of PEO coatings are investigated. The results show that the suitable α and voltage for effective OD are 1.3 and 130 V under which the freely-happened electron avalanche in MgF 2 under TD mode can be restricted by the adequate adjacent MgO. Compared with TD mode, the inner dense layer, in which the ( 1 ¯ 0 1 ¯ ) plane of MgF 2 is parallel to the (111) plane of MgO at their well-knit semi-coherent interface, is thickened to 2.4∼7.2 times, the corrosion potential ( E corr ) improvement is enlarged to 3.6∼13.2 times and the corrosion current intensity ( I corr ) is reduced from 10.8∼9.499 to 0.433 (10 −6 A/cm 2 ). The outer loose layer is densified and the wear rate is lessened 65.5%∼89.8% by the evident melioration in surface porosity, impedance and hardness. This work deepens the understanding about the discharge of PEO coating and provides an available OD mode for preparing excellent PEO coating.

Interface characteristics between TiN and matrix and their effect on solidification structure

Heterogeneous nucleation is an effective way to promote the dispersion and precipitation of second-phase particles in steel and refine the grain size of the solidification structure. Not only refining as-cast structure grain size, but TiN in ferritic stainless steel can also pin grain boundaries and restrain the overgrowth of grains during rolling. The interface characteristics between TiN and heterogeneous phases (high-melting inclusions and ferrite phase) were studied based on the wetting angles between molten steel with different compositions and TiN substrate, and on the matching degree between TiN and ferrite lattice. It was found that, for the molten steel with the same composition, the wetting angle with the TiN substrate was significantly smaller than the contact angles with the other three substrates, while the wetting angle between ferrite phase and TiN was the smallest. The lattice matching was compared among MgAl2O4, TiN and δ matrix by means of a high-resolution transmission electron microscope, which revealed that a coherent or semi-coherent interface was formed between the crystal plane (400) of MgAl2O4 and the crystal plane (200) of TiN, as well as between the crystal plane (200) of TiN and the crystal plane (110) of δ matrix, with a lattice misfit of 5.1% and 3.4%, respectively. Finally, these two characteristics between TiN and ferrite phase were both explained from the perspective of interfacial energy. The microstructure refinement mechanism from high temperature to room temperature can be better reflected by the proposed wetting–lattice misfit theory.

Continuum model for dislocation structures of semicoherent interfaces

In order to relieve the misfitting elastic energy, the hetero-interfaces become semicoherent by forming networks of dislocations. These microscopic structures strongly influence the materials properties associated with the development of advanced materials. We develop a continuum model for the dislocation structures of semicoherent interfaces. The classical Frank-Bilby equation that governs the dislocation structures on semicoherent interfaces is not able to determine a unique solution. The available methods in the literature either use further information from atomistic simulations or consider only special cases (dislocations with no more than two Burgers vectors) where the Frank-Bilby equation has a unique solution. In our continuum model, the dislocation structure of a semicoherent interface is obtained by minimizing the energy of the equilibrium dislocation network with respect to all the possible Burgers vectors, subject to the constraint of the Frank-Bilby equation. The continuum model is validated by comparisons with atomistic simulation results.

Microstructures and mechanical properties of as-extruded Mg–8Gd–2Y–1Zn–6Li alloy

In this study, a high-performance Mg–8Gd–2Y–1Zn–6Li (wt%) alloy having a TYS of 202 ± 2.6 MPa, UTS of 243 ± 2.1 MPa and EL of 10.7 ± 2.3% was successfully fabricated by casting, heat treatment and hot extrusion. The effects of intermetallic compounds on microstructure and mechanical properties were deeply investigated. The results show that the studied alloy consists of coarse deformed α-Mg grains with (0001) basal texture, fine recrystallized β-Li grains with (002) texture and three Mg3RE phase variants, which are bulk Mg3RE phase with a size range of 10–30 µm, spot Mg3RE phase with a size range of 1–2 µm and spherical Mg3RE phase with a size range of 0.3–1 µm, respectively. The spherical Mg3RE phase has a strong dispersion strengthening effect, while the bulk Mg3RE phase acts as crack resources and deteriorates the ductility. A novel island-like β1R phase (FCC, a = 0.78 nm) is found for the first time in the Mg-Gd-Li based alloys. It has a coherent (1̅12)β1R//(101̅0)α interface at the broad facet and a semi-coherent (1̅11)β1R//(112̅0)α interface at the end facet. The β1R phase transforms from βH phase at the semi-coherent interface by atomic shear of RE and Mg atoms. The β1R phase provides a significant precipitation strengthening effect to the studied alloy, while the dissolution of β1R phase results in the age-softening phenomenon. The high-performance of the studied alloy is mainly attributed to the α/β duplex structure, three Mg3RE phase variants, and β1R phase.