Interfacial Dislocations Research Articles

Evolution of phase interface microstructure and its effects on stress rupture property of a 4th generation nickel-based single crystal superalloy after thermal exposure

Lattice constants, phase interface microstructure and stress rupture properties of nickel-based single crystal superalloy after thermally exposed at 1100 °C for different time were investigated using synchrotron radiation diffraction and transmission electron microscopy techniques. The results show that the lattice constants of γ and γ' phase at room temperature decrease and the misfit at room temperature becomes more negative after 200 h exposure, and these parameters remain unchanged after exposure for 500 h. The release of misfit stress during thermal exposure is completed by the transformation from coherent γ/γ' interface to semi-coherent interface as well as the formation of misfit dislocation. The misfit dislocation network generated during thermal exposure is mainly rectangular shape, and its formation process is analyzed in detail. Additionally, it is found that the stress rupture lives decrease obviously after thermal exposure, while the morphology and size of interfacial dislocation network of thermally exposed alloys after rupture are very similar with that of unexposed alloy. The relation of γ' phase morphology and interfacial dislocation network to the stress rupture property deterioration has been discussed.

Substructure and crystallography of lath martensite in as-quenched interstitial-free steel and low-carbon steel

In this study, the substructures and crystallographic features of as-quenched lath martensite in interstitial-free (IF) steel and low-carbon steel (0.2C steel) were investigated using focused ion beam-scanning electron microscopy serial sectioning and advanced transmission electron microscopy. A three-dimensional analysis revealed that the morphology of the so-called lath, the smallest structural unit, exhibited a wedge-shaped plate instead of the ideal lath shape. The habit plane orientations were distributed from (5 7 5)A to (1 1 1)A in the IF steel, whereas they were near to that predicted based on the phenomenological theory of martensite crystallography in the 0.2C steel. In the IF steel, well-developed dislocation cells, tangled dislocations, and curved dislocations could be observed. Conversely, straight dislocations and cementite formation were observed in the 0.2C steel. Moreover, the orientation changed significantly inside a given lath in both steels. The lath boundary comprised regularly spaced steps with (0 1 1)M atomistic terrace planes. Interfacial dislocations corresponding to the discontinuity of the (0 1 1)M lattice planes could be observed at steps, and these dislocations were introduced to compensate for the misorientation between adjacent laths. Additionally, carbon segregations were observed at the lath and block boundaries in the 0.2C steel. Our observation results on the dislocation structures, cementite formation, and segregation of carbon atoms on the lath and block boundaries indicated that the substructure changed significantly during quenching. Based on our experimental results, we discuss the origin of the substructures and crystallographic features of lath martensite.

Atomic insights into interface-mediated plasticity and engineering principles for heterogeneous serrated interfaces

Metallic nanolayered composites typically experience substantially enhanced resistance to irreversible deformation as the portion of interfaces increases. Three-dimensional (3D) serrated interfaces possess considerably higher resistance to interface-facilitated plasticity than two-dimensional (2D) planar interfaces; however, the atomistic mechanisms underlying this phenomenon are little explored, while the engineering principles of tailoring atomic serrations are nearly unknown. In this study, two known representative serrated interfaces, i.e., Cu{991}//{112}Nb and Cu{112}//{112}Nb interfaces, are analyzed using atomic-scale simulations and interfacial dislocation theory and comprehensively compared with their planar counterparts. The Cu{991}//{112}Nb and Cu{112}//{112}Nb serrated interfaces exhibit the novel interface-facilitated deformation behaviors of deformation twinning and near-interface dislocation nucleation, respectively. The stress inhomogeneity arising from the geometrical mismatch between Cu and Nb serrations contributes to deformation twinning rather than dislocation nucleation, while the improved symmetry of highly distorted atomic hexagons on the extended Cu{111}//{110}Nb facets dominates the near-interface dislocation nucleation. Both deformation twinning and dislocation nucleation are closely related to the geometry and characteristics of atomic serrations at the interfaces, which differ from those observed in planar interfaces. Further systematic investigations of fourteen serrated interfaces derived from Cu{991}//{112}Nb and Cu{112}//{112}Nb suggest that the screened facet planes, free volume, and Poisson's ratio mismatch may be used as critical descriptors to tailor the mechanical properties and responses, which presents a convenient solution for interface engineering. These findings provide not only novel atomistic mechanisms that explain the localized interface-facilitated plasticity, but also general principles for engineering atomically serrated interfaces.

Scattering Analysis of AlGaN/AlN/GaN Heterostructures with Fe-Doped GaN Buffer.

The results of the study of the influence of Fe segregation into the unintentionally doped GaN channel layer in AlGaN/AlN/GaN heterostructures with Fe-doped GaN buffer layer on the electrical properties of two-dimensional electron gas are presented. A set of several samples was grown by metal-organic vapor-phase epitaxy and characterized by the van der Pauw method. The dependence of concentration and mobility of the two-dimensional electron gas on the channel layer thickness was analyzed theoretically by self-consistent solving of 1D Poisson and Schrödinger equations and scattering rate calculations within the momentum relaxation time approximation. It was found that both concentration and mobility decreases were responsible for the increase in the sheet resistance in the structures with a thinner channel layer, with a drop in mobility being not only due to ionized impurity scattering, but also due to a combined effect of weakening of screening, lower carrier energy and change in form-factors on scattering by interface roughness, dislocations and polar optical phonons.

The effect of loading axes on deformation behavior of Cu/Al multilayer

The effect of loading axes on deformation behavior of Cu/Al multilayer is studied via molecular dynamics. The yield strength under x-axis or y-axis loading parallel to interface is lower than that under z-axis loading perpendicular to interface, while tensile strength is opposite. Partial dislocations preferentially nucleate at nodes of interfacial misfit dislocations under parallel loading, but explosively nucleate below interface under perpendicular loading. Deformation mechanisms under loading along x, y, and z axes are partial dislocation and deformation twinning, extended full dislocation, as well as partial dislocation, respectively. Strip-like voids form at interface under parallel loading and sphere-like voids form below interface under perpendicular loading, respectively.

Effect of solution cooling rate on the microstructure and creep deformation mechanism of a rhenium-free second-generation single crystal superalloy

• The cubic degree and size of γ′ phases were inversely proportional to the solution cooling rate. • Air cooling method significantly improved the creep lifetime at 800 °C/750 MPa and 1100 °C/137 MPa. • Air cooling facilitated the formation of dislocation locks and retarded γ′ phases to coarsen at 800 °C/750 MPa. • The influence mechanism that air cooling facilitated the denser interface dislocation networks was explored in combination with the long-term ageing test. • The nucleation and precipitation of secondary γ' phase might well be influenced by the solution cooling rate. Various cooling scenarios (water, oil, air and furnace) were employed to study the impacts of the solution cooling rate (SCR) on the microstructure and creep behavior of a novel single-crystal (SX) superalloy. The results showed that the cubic degree and size of the γ′ phases were inversely proportional to the SCR. The creep life first increased and then dropped dramatically with a reduction in the SCR. The creep life of the sample cooled with air cooling (AC) was the highest, up to 144.90 h at 800 °C/750 MPa and 160.15 h at 1100 °C/137 MPa. During creep at 800 °C/750 MPa, the improved creep life of the AC sample was mainly attributed to the fine cubic γ′ phases, which decreased the rate of γ′-phase coarsening and favoured plastic deformation by promoting the active movement of dislocations. The AC helped the γ′ phases become rich in Al, Ti and Ta while depleted in Co and Cr, which enhanced its stacking fault energy, thus promoting the formation of dislocation locks. Meanwhile, the largest negative lattice misfit caused by AC induced denser γ/γ′ interface dislocation networks at 1100 °C/137 MPa, which efficiently reduced the minimum creep rate. The calculated average dislocation spacing results indicated that the smallest density of excess dislocations corresponded to the AC sample, proving its greatest creep resistance. Interestingly, the size of the secondary γ′ phases first decreased and then increased sharply with decreasing SCR during creep at 1100 °C/137 MPa, when fine secondary γ′ phases had a positive role in the blockage of dislocation movement in the matrix. Eventually, the comprehensive SCR effect was explored to provide more guidance in the design of Re-free SX superalloys.

Interfacial damage of bilayer graphene under shear deformation: Theory, experiment, and simulation

Damage at micro- and nanoscale plays a key role in the failure of a solid, but such behaviors at the interfaces of two-dimensional (2D) materials are still not fully explored. In this work we systematically study the interfacial damage of bilayer graphene (BLG) beyond its interfacial shear strength, covering aspects of theory, experiment, and simulation. We extend the well-established shear-lag model for monolayer graphene on a polymer surface to a graphene/graphene/polymer system, whose numerical results demonstrate that the deformations of two graphene layers are strongly coupled with strain localizations and affected by the edges, and they both remain deformed even after the interface has been damaged. Using Raman spectroscopy and the carbon-isotope substitution technique, the interfacial damage of BLG is monitored experimentally, confirming the non-zero strains in the graphene layers with a damaged interface. Finally, molecular dynamics simulations reveal that with an increased tension, the shear stress concentration near the BLG edge increases the density of interfacial dislocation, initiating the damage and expanding to the central part. This work helps clarify the interfacial damage process in a 2D material system with a cross-scale picture, and we believe that it will provide valuable guidance for the understanding of other basic issues in solid mechanics.

High-Temperature Creep Behavior of a Nickel-Based Single-Crystal Superalloy with Small-Angle Deviation from [111] Orientation

By means of tensile creep tests and microstructure analysis, the creep behavior and deformation mechanism of a nickel-based single-crystal alloy with small-angle deviations from the [111] orientation at 1040°C/137 MPa were investigated. The results suggested that in the early stage of creep, proliferating dislocations generated by different slip systems get accumulated in the γ phase. Dislocations mainly slipped in the γ phase and climbed over the γ′ phase, and they began to form a dislocation network at the interface. γ′ phases were connected to each other, and the coherence relationship was destroyed. In the steady-state stage of creep, the solid interfacial dislocation network was formed, and the γ′ phase transformed into a lamellar raft structure, which hinders the slip and climb of dislocations in the matrix channel. In the accelerated creep stage, the dislocation network was destroyed, and dislocations sheared into the γ′ phase in the way of dislocation pairs, which could react to form a superdislocation node and decompose to form APBs on the <111> plane or cross slip from the <111> plane to the <100> plane to form K-W locks.

Decomposition and phase transformation mechanisms of α2 lamellae in β-solidified γ-TiAl alloys

The decomposition of (α2/γ) lamellae during service is an important microstructure degradation phenomenon that is worth studying. In this study, the decomposition mechanism of (α2/γ) lamellae was systematically investigated by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques. The results showed facilitated α2 →βo phase transformation by [112¯0]α2/[01¯1]γ semi-coherent α2 /γ interface due to its larger misfit than that of [112¯0]α2/[1¯10]γ interface. Ellipsoidal ωo particles were formed at βo /α2 interface and nucleated at the interface dislocations. The energy-dispersive X-ray spectroscopy (EDS) results demonstrated feasible depletion of Mo and enrichment of Ti at α2 /βo interface to form ωo phase. In addition, lath-shaped ωo precipitates were observed between two precipitated large βo grains formed by direct α2 →ωo phase transformation. The formation mechanism of the lath ωo phase was attributed to both element segregation and strain accommodation caused by βo precipitate in α2 lamellae. The α2 /βo interface served as preferential nucleation sites for ωo phase due to the atomic structure of the interface, which crystallographically was closely associated with the structure of ωo phase. Moreover, βo precipitates at low stress introduced stacking faults in α2 lamellae, while delaying the parallel decomposition of α2 lamellae at high stress. In sum, these findings provided profound insights into the decomposition mechanism of (α2/γ) lamellae, relevant as theoretical basis for controlling the microstructures and improving the performance of β-solidified γ-TiAl alloys.

Achieving enhanced toughness of a nanocomposite coating by lattice distortion at the variable metallic oxide interface

Ceramic coatings are in general a kind of brittle material because they are predominantly made up of ionic crystals that avoid dislocation motion caused by lattice distortion. In this regard, a remarkable toughened ZrO2/MgO nanocomposite coating is obtained by the plasma electrolytic oxidation (PEO) process and in-situ synthesized ZrO2 with quantitative control approach. It is revealed that the toughening behavior of the ZrO2/MgO coating is related to the coordination and diversion of lattice distortion at the metallic oxide interface, which induces distinct dislocation motion at the interface. The semicoherent interface between m-ZrO2 and MgO is verified to act as a buffer to realize toughening of the nanocomposite coating through dislocation slipping induced by lattice coordinated distortion. Simultaneously, significant interfacial lattice distortion transfer and dislocation pinning are discovered at the semicoherent interface between t-ZrO2 and MgO, which are beneficial to toughness enhancement of the nanocomposite coating. The results indicate that the toughening effect occurs along with dislocation slipping and pinning caused by lattice distortion of the ZrO2/MgO semicoherent interface, which enables the toughness of novel nanocomposite coating to reach 2.7 times of the traditional PEO coating.

Microstructural stability and creep properties of middle Nb γ-TiAl alloy with a modulated microstructure

In this work, a modulated structure composed of nano-scale Ti2AlNb (O phase) and α2 phase was obtained in middle Nb γ-TiAl alloy. The O phase nucleates at the α2/γ interface and is orthogonal to the α2/γ interface. The creep testing results show that the modulated structure exhibited the better creep performance compared with the fully lamellar microstructure, manifesting in a smaller minimum creep rate and creep strain after creep for 200 h. The O phase in α2 lamellae suppressed the α2 →γ phase transformation and constructed the coherent interface with adjacent γ lamellae, which replaced part of the semi coherent α2/γ interface, to reduce the generation of interfacial dislocation loops, thereby improving the creep resistance. Besides, discontinuous precipitations (DPs) caused by (α2/γ) → β0(ω0) + γ phase transformation were observed in the crept samples, contributing to the microstructure degradation. The present work shows significant engineering significance and scientific value for improving the creep performance and microstructure stability for middle Nb–TiAl alloys.

Dislocation modelling of the plastic relaxation and thermal ratchetting induced by zirconium hydride precipitation

The precipitation of hydrides in zirconium alloys is accompanied by a significant and anisotropic volumetric expansion. Previous literature quantified the misfit both theoretically and experimentally, but these values differ greatly; the experimental values are consistently lower. One possibility is that the experimental measurements include the effect of dislocations generated by the hydride, which relax the transformation stresses. To test this hypothesis, it is important to determine the stress field of a hydride and its associated dislocations, combined. A simple planar dislocation model was developed of the hydride—dislocation ensemble in α-Zr. By capturing details of the dislocation structures given in the literature, it is shown in this study that including the interfacial dislocations largely reconciles the predicted and experimental values. Discrete dislocation plasticity is then used to model the diffuse plastic relaxation associated with hydride formation. The effects of plastic relaxation on the equilibrium hydrogen profile, hence the implications for subsequent hydride precipitation, are discussed. In particular, precipitation–dissolution cycles were simulated to calculate the magnitude of the residual hydrostatic tension, which is argued to be the primary cause of the “memory effect” for the re-precipitation of both γ and δ hydrides.

Mid-infrared InAs/InAsSb Type-II superlattices grown on silicon by MOCVD

In this work we report the growth of the InAs/InAsSb type-II superlattice (T2SL) onto Si substrates via the use of a GaSb/GaAs/Si buffer layer structure all grown by MOCVD. Transmission electron microscopy (TEM) was used to show the effectiveness of the buffer layer structure in reducing threading dislocation density and to verify the formation of an interfacial misfit dislocation array between the GaSb and GaAs layers. Electron channelling contrast imaging was used to measure a threading dislocation density of 6.73 × 108/cm2 at the surface of the T2SL. TEM and X-ray diffraction show that the T2SL itself was grown to a high quality considering the large mismatch of the heteroepitaxy. Fourier transform infrared spectroscopy was used to measure the photoluminescence performance of the T2SL which was found to have a FWHM of 50 meV at a peak wavelength of 4.5 µm at 77 K. These results are a step forward towards integration of full InAs/InAsSb T2SL device structures onto Si substrates via MOCVD.

Interface dislocations and grain boundary disconnections using Smith normal bicrystallography

The CSL/DSCL model for interfaces in crystalline materials offers a unified framework to study interface dislocations in phase boundaries and disconnections in grain boundaries. The model relies on the existence of a coincidence relation between the two lattices that meet at an interface. The model’s ability to quantitatively predict the thermodynamics and kinetics of interfaces has been demonstrated for a limited set of symmetric tilt grain boundaries (STGBs) in cubic materials and twin boundaries. However, the lack of a general framework of interface defects prevents its applicability to arbitrary rational boundaries. In this paper, we present a mathematical framework based on the Smith normal form (SNF) for integer matrices to study the bicrystallography of rational phase and grain boundaries. One of the main results of the paper is a constructive proof of the invariance of the coincident site lattice (CSL) under discrete relative displacements of the parent lattices (of possibly different kind) by a displacement shift complete lattice (DSCL) vector. In addition, we obtain necessary and sufficient conditions on two lattices, related by not only rotations but also lattice distortions, for the existence of a coincidence relation. We first apply these results to explore coincidence relations in arbitrary phase boundaries, and study interface dislocations. In particular, we demonstrate the framework for a phase boundary formed by a strained hexagonal lattice and a square lattice. As a second application, we show how to enumerate all possible (geometric) disconnection modes in arbitrary rational grain boundaries, including glide and non-glide modes in both STGBs and asymmetric-tilt grain boundaries (ATGBs). The constructive nature of the framework lends itself to an algorithmic implementation based exclusively on integer matrix algebra to construct all interfaces that admit CSLs up to a prescribed size, and determine disconnection modes in grain boundaries. We demonstrate the use of SNF bicrystallography on selected bicrystal misorientation axes/angles in face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal (hex) lattices.

Phase transformation generating coherent twin boundaries in titanium alloys

• (10 1 ¯ 1) α and (10 1 ¯ 3) α CTBs can be generated during α precipitation. • (10 1 ¯ 1) α CTB forms when the angle between (0001) α planes in two α variants is 120°. • (10 1 ¯ 3) α CTB is generated when the angle between (0001) α planes in two α variants is 60°. • Generation of the CTBs results in lattic distortion or interfacial dislocations. In this work, α-α precipitates boundaries were characterized using aberration-corrected scanning transmission electron microscope. We reported an interesting view that (10 1 ¯ 1) α and (10 1 ¯ 3) α coherent twin boundaries (CTBs) could be generated by the impingement of (0 0 0 1) α planes in two α variants during α precipitation in a metastable Ti-5Al-5Mo-1Fe-1Cr (wt.%) alloy. When the (0001) α plane in one of the variants is at 120° to that in another variant, the (10 1 ¯ 1) α CTB could form, and when the angle of (0001) α planes in two variants is 60°, the (10 1 ¯ 3) α CTB could be generated. The generation of the CTBs causes (0001) α planes of two variants to rotate 1.4° around their common [2 1 ¯ 1 ¯ 0] α axis, resulting in slight lattic distortion or introducing interfacial dislocations to accommodate the strain. These findings provide a new insight to the formation of CTBs and are applicable in other metastable β titanium alloys.

MOLECULAR DYNAMICS SIMULATION OF TENSILE DEFORMATION OF NANOMETER MULTILAYER Cu/Ta MATERIALS

In this research, the tensile mechanical properties and microstructure evolution of Cu/Ta nanolayered composites were studied using the molecular dynamics simulation method. By analyzing the tensile stress/strain relationship of Cu/Ta with different interface structures and the movement of dislocations during the stretching process, the deformation mechanism of materials with different interface structures and the effect of interface structures on the tensile strength of Cu/Ta nanolayered composites are revealed. The effect of shear localization during extension is also analyzed. The results show that the dislocation structures at the interfaces of Kurdjumov-Sachs-type and Nishiyama-Wasserman-type samples are parallelogram and triangular interface defect arrays, respectively, which can easily induce two Shockley partial dislocations to slide along different (111) planes, forming an intersection and merging into ladder-rod dislocations. However, dislocations between the Kurdjumov-Sachs š112ć-type sample interfaces exhibit parallel array characteristics, while the interfacial dislocations have non-planar interface components, which can induce deformation twinning. The process is dissociated through a set of intrinsic interfacial dislocations. Shockley partial dislocations are then formed by dislocation motion, creating stacking faults (SF1), and then the second set of partial dislocations may nucleate from the interface and slide on the adjacent SF1 plane, eventually forming deformation twinning.

Effect of amorphous phase on the migration mechanism of basal/prismatic interface in Mg alloys

A dual-phase nanostructured amorphous/crystalline model is an effective method to improve the mechanical properties of Mg alloys. However, the fundamental strengthening mechanism related to the interaction between basal/prismatic (BP) and amorphous phase in the dual-phase Mg alloys is still unclear. Here, the effects of the size and spacing of amorphous nanopillars on the mechanical properties and the BP interface migration behavior of the bicrystalline Mg alloys are investigated by the molecular dynamics simulation method. The results show that due to the attraction of amorphous nanopillar to interfacial dislocations, the introduction of amorphous nanopillar reduces the yield stress of the bicrystalline Mg alloys, and the yield stress decreases with the increase of the amorphous nanopillar radius. The results indicate that the amorphous nanopillar has an obvious blocking effect on the migration of the BP interface, and the larger the radius of amorphous nanopillars (or the smaller the spacing of amorphous nanopillars), the more obvious the strengthening effect. In addition, the migration mechanism of the BP interface in the bicrystalline Mg alloys is analyzed in detail.

The effect of deformation temperature on recrystallization in a Ni-based single crystal superalloys

A series of experiments have been carried out to investigate the effect of deformation temperature on the recrystallization of nickel-based single crystal superalloys using isothermal (compression) and non-isothermal (directional solidification) deformation. The recrystallization features induced by accumulated plastic strain in two temperature ranges (1300–1150 °C and 600–400 °C) are distinctly different from that induced by deformation in one temperature range (1300–1150 °C). The different recrystallized features (area and number of grains) induced by isothermal deformation at temperatures ranging from room temperature to solidus can be attributed to different dislocation features, such as slip band, KW-lock, γ/γ' interfacial dislocations and Low Angle Boundary. Microscopic observations disclose that interactions of multiple slip systems have higher tendency of recrystallization. A close view between deformed and recrystallized samples of different deformation temperatures reveals that the critical kernel average misorientation (KAM) to induce recrystallization is about 0.4°. Furthermore, the number of recrystallized grains is proportional with the average KAM value based on the experimental measurements in a wide temperature range. Isolated recrystallized grains are observed at the stress concentration locations in the low plastic strain zone due to local high misorientation.

Transient dislocation emission from the nanosized interface of Cu–Ag composite under the coupled thermal-mechanical shock: Molecular dynamics simulations study

In this study, we use molecular dynamics to simulate the deformation of Cu–Ag nanocomposite under the coupled thermal-mechanical shock. We compare the dislocation evolution under different loading conditions by changing the strain rate and the heating rate applied to the nanocomposite. The result indicates that for a given strain rate with increasing heating rates, the dislocation density decreases, and so does the interaction between dislocations. The resistance encountered by the dislocation movement decreases as the dislocation movement mode shifts from dislocation slip to climb. The plastic deformation mechanism transforms from dislocation movement to the amorphization of the whole composite, resulting in the disruption of the interfacial dislocation network. These factors conspire to cause a reduction in yield stress. For a given heating rate with increasing strain rates, owing to the strain rate effect and the temperature difference during the deformation process, the yield stress increase. We can conclude that due to the difference in dislocation density, dislocation movement mode, and the plastic deformation mechanism, the dislocation nucleation stress and the yield stress of composite decrease with the strain rate decreases and the heating rate increases (which involves the effect of temperature difference).

The Effect of Dislocation and Interface-Roughness Scattering on Electron Mobility in the MgZnO/ZnO Heterostructure

In this work, the electron mobility in the MgZnO/ZnO heterostructure at room temperature is theoretically studied by considering interface roughness (IFR), dislocation (DIS), and polar optical phonon (POP) scattering. Analytical formulae are introduced to calculate the critical thickness and dislocation density in the barrier layer of MgZnO/ZnO heterostructures. The calculated critical thickness for the MgZnO/ZnO heterostructure is much smaller than that for the AlGaN/GaN heterostructure system. At room temperatures, POP scattering is found to be the most important scattering mechanism. On the other hand, the change of electron mobility limited by IFR as a function of the barrier thickness in the MgZnO layer is found to be quite different to that limited by DIS. High-density (>1013cm−2) 2DEG can be obtained in the MgZnO/ZnO interface by increasing the thickness and Mg composition in the MgZnO layer.