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Revealing the magnetic coupling nature of boundary defects is crucial for in-depth understanding of the behavior and properties of magnetic materials and devices. Here, magnetite (i.e., Fe3O4) thin films were grown epitaxially on (100) SrTiO3 single-crystal substrates by pulsed laser deposition. Atomic-scale scanning transmission electron microscopy characterizations reveal that three types of antiphase boundaries (APBs) are formed in the Fe3O4 thin film. They are the (100) APB that is formed on the (100) plane with a crystal translation of (1/4)a[01 1 ¯ ], the type I and type II (110) APBs that are both formed on the (110) plane with the same crystal translation of (1/4)a[101] but different terminated atomic planes. The type I (110) APB is terminated at the atomic plane with mixed tetrahedral- and octahedral-sites Fe atoms, the type II (110) APB is terminated at the octahedral-site Fe plane. First-principles calculations reveal that the (100) APB and the type I (110) APB tend to form the ferromagnetic coupling that will not decrease the spin polarization of Fe3O4 films, while the type II (110) APB prefers to form the antiferromagnetic coupling that will degrade the magnetic properties. The magnetic coupling modes of the APBs are closely related to the Fe-O-Fe bond angles across the boundaries.

Cold-drawn pearlitic steel wire is irreplaceably used in industry owing to its outstanding mechanical property which is dominated by the cementite/ferrite (Fe3C/Fe) interfaces in the material. However, the fine structures of the Fe3C/Fe interfaces in the deformed wires are less known to date. In this work, transmission electron microscopic investigation was performed on the atomic structures of the interfaces with the Isaichev orientation relationship (OR) in the wires with progressive deformation strains. In addition to the effect of the dislocation/interface interactions, this work revealed that the deformation-induced partial decomposition of cementite plays an important role in the interface reconstruction during deformation. The interfacial carbon vacancies generated by cementite decomposition and particularly, the amorphization of cementite layers in the sample with e > 1 could effectively annihilated the interfacial dislocations and consequently relaxed the interfacial stress. The correlations between the interface structure changes and the mechanical properties of the wires were discussed.

Recently, the eutectic high-entropy alloy (EHEA), AlCoCrFeNi2.1, can reach a good balance of strength and ductility. The dual-phase alloy exhibits a eutectic lamellar microstructure with large numbers of interfaces. However, the role of the interfaces in plastic deformation have not been revealed deeply. In the present work, the orientation relationship (OR) of the interfaces has been clarified as the Kurdjumov-Sachs (KS) interfaces presenting 111 B 2 | | 110 F C C and 110 B 2 | | 111 F C C independent of their morphologies. There exist three kinds of interfaces in the EHEA, namely, (321)B2||(112)FCC, (01 1 ¯ )B2||(33 2 ¯ )FCC, and (23 1 ¯ )B2||(552)FCC. The dominating (321)B2||(112)FCC interface and the secondary (01 1 ¯ )B2||(33 2 ¯ )FCC interface are both non-slip planes and atomistic-scale faceted, facilitating the nucleation and slip transmission of the dislocations. The formation mechanism of the preferred interfaces is revealed using the atomistic geometrical analysis according to the criteria of the low interfacial energy based on the coincidence-site lattice (CSL) theory. In particular, the ductility of the dual-phase alloy originates from the KS interface-induced slip continuity across interfaces, which provides a high slip-transfer geometric factor. Moreover, the strengthening effect can be attributed to the interface resistance for the dislocation transmission due to the mismatches of the moduli and lattice parameters at the interfaces.

Understanding the interface between strengthening precipitates and matrix in alloys, especially at the atomic level, is a critical issue for tailoring the precipitate strengthening to achieve desired mechanical properties. Using high-resolution scanning transmission electron microscopy, we here clarify the semi-coherent interfaces between the matrix and long-period stacking ordered (LPSO) phases, including 18R and 14H, in Mg–Zn–Y alloys. The LPSO/Mg interface features the unique configuration of the Shockley partial dislocations, which produces a near zero macroscopic strain because the net Burgers vectors equal zero. The 18R/Mg interface characterizes a dissociated structure that can be described as a narrow slab of 54R. There are two dislocation arrays accompanied to the 18R/54R and 54R/Mg interface, resulting a slight deviation (about 2.3°). The 14R/Mg interface exhibits the dislocation pairs associated with solute atoms. We further evaluate the stability and morphology of the corresponding interfaces based on elastic interaction, via calculating the mutual strong interactions between dislocation arrays, as well as that between the dislocations and solute atoms. The synchronized migration of interfacial dislocations and solute atoms, like move-drag behavior, dominates the lateral growth of LPSO phases in Mg alloys.

Exotic domain-wall phenomena make ferroelectrics the candidates for nanoelectronics. The local symmetry and structure anomalies at domain walls have raised interest in the unusual functionalities, such as domain wall chirality and conductivity. Especially, the spontaneous lattice distortion and symmetry breaking at domain walls activate them as the location of structural transformation. However, the routes to achieve ferroelectric-antiferroelectric phase transition via ferroelectric domain walls remain challenging, which are important to develop materials for energy storage and conversion. Here, we have observed stepwise antiferroelectric phase transition in strained pure BiFeO3 ultrathin films derived from ferroelectric domain walls. Aberration-corrected transmission electron microscopy observation reveals that the resultant phase transition is mediated by dense 180° domain walls via providing the antiparallel cation displacement, cooperating with the enhanced interfacial oxygen octahedral clamping. First-principles calculations further confirm the critical role of interfacial oxygen octahedral coupling during this transition. These findings report an alternative route for antiferroelectric phase transition. Besides, our results provide fresh insights into functionalities of ferroelectric domain walls and open a venue for developing energy-storage materials based on domain engineering.

High-entropy alloys (HEAs) have displayed numerous unique features, however, high strength and high ductility are still the urgently desired mechanical property. The current study reports an outstanding combination of high strength and excellent ductility achieved in a eutectic high-entropy alloy (EHEA) AlCoCrFeNi2.1. The great achievement relays on a dual-phase heterogeneous structure strengthened with nano-precipitates. The heterogeneous structure provides the alloy a good combination of strength and ductility, and nano-precipitates in both of the face-centered cubic (FCC) and B2 phases generate an extra significant contribution to strength.

Retention failure is deleterious on ferroelectric domain stability, resulting in data loss in ferroelectric memories. However, the understanding of the origin of retention failure remains a challenge for the lack of a direct experimental evidence. Here, using a combination of piezoelectric force microscopy, atomic-scale scanning transmission electron microscopy and reciprocal space mapping, we report that the polarization retention failure is caused by a strain gradient induced flexoelectric field in tetragonal ferroelectric films. Atomic imaging reveals that the strain gradient is introduced by tensile strains, which is resultant from the vertically distributed Pb-rich anti-phase domains. This strain gradient couples with polarizations and results in flexoelectric fields in the films, leading to the retention failure in the films. Our study directly underlines the atomic mechanisms behind the strain gradient induced macroscopic retention failure behavior and clarifies the effect of local strain state on domain relaxation processes, thus can shed light on further understanding and improving the retention properties of oxide ferroelectrics.

Phase coexistence in ferroelectric oxide films displays complex phase competitions and transformation which suggest new multiple-coupled properties. Here we have successfully engineered the coexistence of flux-closure and a1/a2 phases in tensile-strained PbTiO3 films sandwiched between GdScO3 substrate and a SrTiO3 layer. Moreover, by using in-situ electron beam illumination, the unusual transformation from a1/a2 phase to the flux-closure phase was directly observed. In detail, there are two types of transformations: One is the nucleation, growth, and expansion of the flux-closure phase from the internal region of the a1/a2 phase. The other feature is a “dislocation gliding” like behavior: a thin lamella of a1/a2 shrinks like “partial dislocation pairs” and finally the gradually disappeared a1/a2 lamella forms a “perfect dislocation” in the flux-closure domain matrix. Phase-field simulations suggest that the a1/a2 to flux-closure transition is induced by the decrease of the depolarization field, which is screened by the injected electrons from electron beam irradiation. These results directly confirm the phase interconversion under an external stimulus at the nanometer scale, which shed new light on the fabrication of new polar textures.

Converse flexoelectricity is the mechanical strain induced by electric polarization gradients, which is largely omitted compared with the relatively well-known direct flexoelectricity. Thoroughly understanding the converse effect may not only explain peculiar structures generated by electric polarization or field gradients, but also stimulate new ideas to the design of novel electromechanical devices. In this work, the converse flexoelectricity around ferroelectric domain walls has been studied elaborately by synergetically integrating the aberration-corrected transmission electron microscope (TEM), first-principles calculations, and the Landau-Ginzburg-Devonshire (LGD) theory, taking prototypical tetragonal ferroelectric PbTiO3 as an example. We not only uncovered the important role of converse flexoelectricity on the asymmetric structure around 90° domain walls, but also quantified the flexoelectric coefficients. This quantification is deterministic in both the magnitude and sign of flexoelectric coefficients, by the mutual verification of the atomic mapping and first-principles calculations. Our results suggest that the converse flexoelectricity cannot be neglected for understanding ferroelectric DWs and other boundaries in ferroelectric materials.

Voids, helium bubbles and stacking fault tetrahedra (SFTs) are common irradiation-induced defects in face-centered cubic (FCC) metals and their alloys that have detrimental effects on their deformation behavior and lifetime. The formation mechanisms of voids and SFTs have been investigated in single crystals but the potential augmentation of these mechanisms by a heterophase interface has not been well studied. Here, using transmission electron microscopy (TEM), we report on the stability of both SFTs and voids at interfaces in an irradiated Cu/Ag nanolayered composite. With atomistic simulations, we show that the heterophase interface can promote the transformation of voids (

B4C particulate-reinforced 6061Al composite was fabricated by powder metallurgy method. The as-rolled composite possesses high tensile strength which is comparable to that of the peak-aged 6061Al alloy. More importantly, the microstructures and mechanical properties are thermally stable during long-term holding at elevated temperature (400 °C). The microstructual contributions to the strength of the composite were discussed. Transmission electron microscopy (TEM) analysis indicates that the in-situ formed reinforcement Mg(Al)B2, as products of the interfacial reactions between B4C and the aluminum matrix, show not only good resistance to thermal coarsening but also strong pinning effect to the grain boundaries in the alloy matrix.

Passive metals have superior resistance to general corrosion but are susceptible to pitting attack in certain aggressive media, leading to material failure with pronounced adverse economic and safety consequences. Over the past decades, the mechanism of pitting corrosion has attracted corrosion community striving to study. However, the mechanism at the pitting initiation stage is still controversy, due to the difficulty encountered in obtaining precise experimental information with enough spatial resolution. Tracking the accurate sites where initial dissolution occurs as well as the propagation of the dissolution by means of multi-scale characterization is key to deciphering the link between microstructure and corrosion at the atomic scale and clarifying the pitting initiation mechanism. Here, we review our recent progresses in this issue by summarizing the results in three representative materials of 316F, and Super 304H stainless steel as well as 2024-Al alloy, using in situ ex-environmental TEM technique.

In this work, we investigate the tensile strength and ductility of bulk Cu/Nb nanolaminates after exposure to high temperatures. We show that the interface transforms from flat to wavy at a transition temperature of 700 °C, and tensile strength is linearly proportional to H-1/2 (H = layer thickness). Moreover, the wavy interfaces give rise to a higher slope of the Hall-Petch law. This result can be attributed to greater resistance to slip transmission across wavy interfaces compared to planar interfaces. After 1000 °C for 1 h, the material still exhibits a high strength of 468 MPa and enhanced elongation.

Ferroelectric flux-closure domains have attracted great attention due to their potentials in high density data storage. For their future applications, it is important to understand their evolution with different factors, such as misfit strain. In this work, a flux-closure domain consisting of a vertical 180° domain wall with two symmetric a domains in both ends (“I” type closure) was observed in nearly unstrained SrTiO3/PbTiO3 multilayers by aberration corrected Transmission Electron Microscopy, which was speculated to be mainly induced by the strong depolarization field near the interface. With the tensile strain increasing, the small a domains in “I” type flux-closures grow gradually and eventually change to the well-known “V” type flux-closures. On the basis of a combination of experimental results and phase field simulations, the phase diagram of the stabilized domain arrays versus the strains in PbTiO3 films are established. These results provide significant information on understanding the formation mechanism of flux-closure domains and shed light on their controlled growth. In addition, it paves a way to reduce the economic cost in their commercial applications because the SrTiO3 substrate is easy to synthesize and much cheaper than scandate substrates.

Titanium with hexagonal close-packed (hcp) structure can undergo phase transformation to face-centered cubic (fcc) structure under mechanical straining at room temperature. In this work, we identified two orientation relationships (ORs) between fcc phase and hcp matrix in commercial pure Ti, i.e., P-type OR [ 1 ¯ 2 1 ¯ 0]hcp||[1 1 ¯ 0]fcc and (10 1 ¯ 0)hcp||(110)fcc, and B-type OR [ 1 ¯ 2 1 ¯ 0]hcp||[1 1 ¯ 0]fcc and (0001)hcp||( 1 ¯ 1 ¯ 1)fcc. The P-type interface is flat while the B-type interface is stepped, which are ascribed to the pure-shuffle mechanism and the partial gliding mechanism, respectively. Most intriguingly, the B-type fcc lamella can penetrate the P-type fcc lamella through gliding of the dislocation with Burgers vector ½[110] on (001)fcc plane.

High entropy alloys (HEAs) have presented potential applications in nuclear power plants owing to their novel atomic structure based high irradiation resistance. However, understanding of He-ion irradiation of HEAs is still lacking. In this work, we reveal He-ion irradiation resistance of HEA CrMnFeCoNi by comparison study with a pure Ni and a 304 stainless steel (304SS). It is found that the damage structure in the three materials can be characterized with He bubbles and stacking faults/stacking fault tetrahedrons ((SFs/SFTs), which show a similar depth distribution after He-ion irradiation at both RT and 450 °C. Although the He bubbles have a similar size about 2 nm after irradiation at RT, the He bubble sizes of the HEA, 304SS, and Ni increase to 4.0 ± 0.9, 5.3 ± 1.0 and 6.7 ± 1.0 nm after irradiation at 450 °C, respectively. Moreover, the density of SFs/SFTs displays in an order of Ni

La2Ti2O7 has recently received extensive interest due to its attractive ferroelectric and photocatalytic properties. Here, high-quality La2Ti2O7 thin films on (110) SrTiO3 substrates are prepared via oxidation of LaTiO3 thin films that are fabricated by pulsed-laser deposition. X-ray diffraction analyses reveal that the La2Ti2O7 thin film grows epitaxially on the (110) SrTiO3 substrate and has a good crystallinity. Atomic force microscopy investigations suggest that the surface of La2Ti2O7 thin film turns into a stepped and terraced structure after the annealing. X-ray photoelectron spectroscopy analyses indicate that the La2Ti2O7 thin film has a high purity and contain only Ti4+ ions (i.e., without Ti3+ ions). The band gap of the La2Ti2O7 film is measured to be ∼ 3.2 eV by UV-visible spectra. Atomic and electronic structures of the La2Ti2O7/SrTiO3 heterointerface have been studied by a combination of transmission electron microscopy and first-principles calculations. The La2Ti2O7/SrTiO3 heterointerface is atomically abrupt and coherent. Electronically, this heterointerface contributes to the decrease of band gap in the La2Ti2O7 film. The reduction of spontaneous polarization in La2Ti2O7 is localized in two layers of TiO6 octahedra near the interface due to the discontinuous Ti-O bonds between the perovskite layers of La2Ti2O7.

Tuning multiple strain and polar states of ferroelectrics by using strain engineering is an essential approach for designing multifunctional electric devices such as multiple state memories. However, integrating multiple strain states is still a challenge, and in addition, revealing such strains and the resultant polar behaviors on the atomic level remains difficult. In this work we prepare PbZr0.52Ti0.48O3/PbTiO3 (PZT/PTO) superlattices on SrRuO3-buffered SrTiO3(001) substrates. Aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) reveals that the superlattice is coherent in both c (out-of-plane polar direction) and a (in-plane polar direction) domains. We find that the strain states of both PZT and PTO in c and a domains are variant, leading to four special strain states. For example, the tetragonality for PTO in c and a domains is 1.061 and 1.045, respectively. In contrast, PZT in c domains displays a tetragonality as giant as 1.107, which corresponds to 110 µC cm−2 spontaneous polarization, much larger than the bulk PZT; while PZT in a domains exhibits 1.010 tetragonality with about 70 µC cm−2 polarization. This study reveals a practical way to integrate multiple strain states and enhanced polarizations in ferroelectric films, which could be used as multifunctional electric elements.

Nanoindentation-induced phase transformation between silicon carbide (SiC) polymorphs has been investigated by a combined study of transmission electron microscopy and first principles calculations. In situ nanoindentation of 4H-SiC single crystals revealed the occurrence of a phase transformation from 4H to 3C in the vicinity of a crack. First principles calculations pointed out an extremely low energy barrier for this phase transformation, which could be attributed to the very smooth transition in the atomic and electronic structures across the 3C/4H interface. The phase transformation from 4H to 3C may retard the propagation of cracks and reduce the band gap of SiC.

Secondary phases precipitated in diffusion affected zones (DAZs) of a transient liquid phase (TLP) bonded Ni-based superalloy have a large impact on materials properties. Here we report an atomic-scale analysis of the crystal structures and elemental distributions within DAZ precipitates in a TLP-bonded single-crystal superalloy using a range of electron microscopy techniques. The predominant precipitate phases are found to be M3B2- and M5B3-type borides, where M is a mixture of transition metal elements. Atomically-resolved energy-dispersive X-ray spectroscopy in an aberration-corrected scanning transmission electron microscope (STEM) enabled the distributions of metal atoms within the precipitates to be determined. The observation that different metals occupy preferred sites within the crystals indicates that these phases are not uniform solid solutions, but instead are chemically ordered. Both borides contain two distinct metal lattice sites such that the phases can be described as L2SB2 and L4SB3, where L and S are elements with larger (e.g., W, Mo) and smaller (e.g., Cr, Co, Ni) atomic numbers, respectively. Ordering of W and Cr is found to be particularly strong. A systematic search for stable phases within the chemical space of the W-Cr-B ternary system using the crystal structure prediction (CSP) program USPEX provided further insights into the nature of the observed chemical ordering. This combination of STEM and CSP techniques provides a direct and robust way of determining chemical features and local structures of multicomponent phases with atomic resolution even when particle sizes are too small for analysis using conventional methods.

The dissolution behavior of γ/Laves eutectic within a typical Ni-based superalloy (Inconel 718) was investigated by in-situ and conventional transmission electron microscopy (TEM) techniques. The results show that the Laves phase dissolved firstly at 700°C, but a new Ni7Nb2 phase possessing similar composition with δ-Ni3Nb phase precipitated near Laves phase did not dissolve at temperatures up to 950°C. The new Ni7Nb2 phase with lattice parameters: a=0.473 nm, b=0.794 nm, c=1.21 nm and β=91.3°, shows a C2/m space group symmetry. The crystal structure of the new Ni7Nb2 phase can be understood by “triple layers” as the Ni7Zr2-type structure does, stacking perpendicular to the (001) planes. Instead of stacking with one single shift vector s corresponding to the Ni7Zr2-type structure, the stacking sequences of the new Ni7Nb2 can be characterized by alternate occurrence of +s and -s. Moreover, the structural change of γ/Laves eutectic constituent before and after heat treatment was investigated. The Ni7Nb2 could remain after heat treatment. Since the experimental Ni7Nb2 structure was shown to be unstable in the Ni-Nb binary system by calculation, it is suggested that Zr, Si and Co promote the formation of Ni7Nb2 phase in superalloys according to the segregation behaviors of elements. Moreover, as Ni7Zr2 is considered to be detrimental to the mechanical properties, Ni7Nb2 phase should also have negative influences and should be eliminated through possible approaches. The finding of the new phase should shed light on the microstructural controlling and heat treatment process formulating of superalloys.

Atomistic scale interface structures could tailor the dislocation behaviors at interface, as well as interface strengthening. In this work, we design a disordered interface by introducing an ultrathin amorphous layer in the Cu/Nb multilayers. Compared to the sharp interface, the disordered interface can smear the impinging dislocation with relaxing the in-plane and out-of-plane Burgers vectors, thus inducing a remarkable hardening in the Cu/Nb multilayers. The findings not only prove the hardening effect induced by dislocation interface smearing, but also shed light on developing advanced materials with ultrahigh hardness via disordered interface.

Electron beam irradiation is well known to induce damage in materials. The structural transformation involved in the damage is usually believed to be an irreversible solid state chemical reaction. Here we use in situ transmission electron microscopy (TEM) combined electron-energy loss spectroscopy (EELS) technique in an aberration-corrected TEM to track the structural transformation in spinel Mn3O4 induced by electron beam irradiation. It is clarified that spinel Mn3O4 is transformed to rocksalt structured MnO by irradiation and the reversed recovering transition from rocksalt MnO to spinel Mn3O4 can occur by aging in the gentle electron beam circumstance. The mechanisms including the role of O desorption/adsorption and the displacement of Mn and O involved in the reversible transformation processes are discussed. The work presents an implication that electron beam can modify the structure at atomic dimension yielding diverse assemblies of surfaces, interfaces and colorful properties.

Fully lamellar Ti-6Al-4V alloys comprise body-centered cubic (BCC) β lamellae in large-sized, hexagonal close-packed (HCP) α colonies and exhibit outstanding toughness. Although α/β interfaces are considered to play a key role in plastic deformation connected to the toughness, the interface effects have not been revealed so far. In this work, we studied underlying deformation mechanisms of interface-related deformation modes at an atomic scale. After the cyclic loading, {1‾102} deformation twins were observed in the vicinity of fatigue crack surfaces. Moreover, the α/β interface structures before and after cyclic loading deformation were characterized via transmission electron microscopy (TEM). The initial α/β interfaces can be described by the terrace ledge kink model, consisting of (01‾10)α||(‾121)β terrace plane and (‾1100)α||(‾101)β ledge plane. TEM investigations reveal that deformation twins nucleate at the α/β interface and the corresponding nucleation is ascribed to the dissociation of basal type dislocations. More importantly, these twins can continuously propagate through multiple β phase lamella. The continuous propagation of twinning is accomplished through double kinking mechanism. In this manner, twinning in α phases and sequential kinking in β phases can effectively release the stress intensification at the crack tip and dissipate plastic work/energy, correspondingly enhancing fracture toughness of fully lamellar Ti-6Al-4V.

Hybrid multiferroic systems composed of complex oxides have drawn sustained attention for decades due to their intriguing physical effects and potential technological exploitations. Designing innovative nanostructures of multiferroic nanocomposites to overcome inherent shortcomings for conventional layered (e.g. clamping effect) and vertically aligned (e.g. leaky) films are instrumental to realize genuine multiferroic behaviors. Here the self-assembled architecture with three-dimensional (3D) framework of heterostructures is fabricated, wherein well-ordered multiferroic e-Fe2O3 (e-FO) nanoboats are embedded in a ferroelectric perovskite PbTiO3 (PTO) matrix. The biphasic system combines strong interfacial magnetoelectric couplings of vertically aligned heterostructure and addressed leakage issue via preferential epitaxy of a high-resistance transition layer of PTO. Additionally, the two phases maintain full lattice coherence along vertical interfaces allowing for efficient interfacial strain coupling. Ferroelectricity and piezoelectric switching of the 3D nanostructured PTO:e-FO film have been corroborated macroscopically by polarization hysteresis (Ps ~ 45 μC cm−2) and locally by piezoresponse force microscopy, and strong magnetoelectric coupling has been manifested as a sizable modification of piezoelectric switching characteristics via applying a DC magnetic field, all conducting at room temperature. The novel 3D multiferroic-ferroelectric heterostructure offers great potential for nanoengineering of multiferroic composites, thus opens an avenue towards superior microelectronics and spintronics.

Giant piezoelectricity has been thought to be originated from polarization rotation, electric-field-induced phase transformation or nano-domain reorientation in diverse complicated morphotropic phase boundaries (MPB) based piezoelectric ceramics. However, the deterministic contribution to the peak piezoelectricity is still elusive. Here, a full stage of piezoelectric response is presented in pure PbTiO3 films epitaxially constrained as the monoclinic phase on SrTiO3(111), where the as-prepared films feature complicated monoclinic nano-domains with {201} domain boundaries. By applying external voltage, the piezoelectric response undergoes three distinct stages which correspond, respectively, to the three existing mechanisms in terms of polarization rotation, nano-domain reorientation, and electric-field-induced phase transformation. A direct correlation between the electric-poling process and the piezoelectric response is established, confirming that the phase transformation from the monoclinic to tetragonal plays a dominant role for the superior piezoelectric properties of the oxide. This study clarifies the role of monoclinic phase by integrating the three individual mechanisms of piezoresponse into a single-phase perovskite oxide, which may facilitate the development of high-performance piezoelectric materials for electronic device applications.

Solute atoms segregation to the interfaces, such as grain boundary or twin boundary, often plays a critical role in modulating the properties of a metallic alloy. Deformation induced segregation to the interfaces has been a subject of significant research, since this is one of the key issues to fully understand the deformation mechanism and microstructure evolution in service of engineering materials. By means of the high-resolution aberration-corrected scanning transmission electron microscopy (STEM), we report the investigations of segregations to symmetrical boundaries, kink boundary (KB) and twin boundary (TB), in the Mg-Zn-Y alloys containing long period stacking ordered (LPSO) phases subjected to a compression at room temperature. We found that Zn atoms preferentially segregate to the deformation-induced symmetrical KBs in the LPSO structures and sandwiched Mg layers, while only a small amount of Y atoms concentrate at KB in LPSO structure. These enriched atoms may be in a random distribution, form nanoscale clusters or in a periodic pattern. Furthermore, solute atoms would rather decorate the segment of coherent TBs than enrich the overlapped TBs. Based on the direct atomic observations, the segregation mechanisms to the featured microstructures are proposed.

Revealing strains on the unit-cell level is essential for understanding the particular performance of materials. Large-scale strain variations with a unit-cell resolution are important for studying ferroelectric materials since the spontaneous polarizations of such materials are strongly coupled with strains. Aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy (AC-HAADF-STEM) is not so sensitive to the sample thickness and therefore thickness gradients. Consequently it is extremely useful for large-scale strain determination, which can be readily extracted by geometrical phase analysis (GPA). Such a combination has various advantages: it is straightforward, accurate on the unit-cell scale, relatively insensitive to crystal orientation and therefore helpful for large-scale. We take a tetragonal ferroelectric PbTiO3 film as an example in which large-scale strains are determined. Furthermore, based on the specific relationship between lattice rotation and spontaneous polarization (Ps) at 180° domain-walls, the Ps directions are identified, which makes the investigation of ferroelectric domain structures accurate and straightforward. This method is proposed to be suitable for investigating strain-related phenomena in other ferroelectric materials.

Boron segregation and boride formation at grain boundaries are generally accepted in Ni-based superalloys. However, the addition of boron was found to result in the formation of a high density of nanosized M5B3 (where M is a mixture of Cr, Mo) particles in γ/γ′ eutectics of a Ni-based superalloy during high-temperature creep in this work. M5B3 phase precipitated at γ/γ′ eutectics keeps a well-defined orientation relationship with the eutectics, which is [001]γ/γ′//[001]M5B3, (020)γ/γ′//(130)M5B3. And these nanosized borides could block dislocation slip, result in stress concentration surrounding γ/γ′ eutectics and facilitate crack initiation at γ/γ′ eutectic/matrix interface during the creep at high temperatures.

The present study reports an atomic level quasi in-situ TEM observation of the redistribution of alloying element Cu in the corrosion process of a B4C-reinforced Al-Mg-Si-Cu composite. Cu atoms were found to combine with Mg and form a thin layer at the interfaces between the aluminum matrix and nano-reinforcements (i.e. Mg(Al)B2 and Al3BC). Dealloying of the Cu/Mg-rich layer was revealed. The following decomposition of Mg(Al)B2 resulted in narrow corrosion trenches forming around Mg(Al)B2 and then the localized pitting of the matrix. In addition, the liberated Cu ions were then redeposited and formed noble Cu nano-particles upon cathodic Mg(Al)B2 and B4C particles.

We investigated the structure, symmetry, and distribution of stacking faults and growth twins in 18R and 14H long-period stacking ordered (LPSO) structures in a near-equilibrium Mg97Zn1Y2 (at.%) alloy by transmission electron microscopy. The stacking faults and growth twins here result from the insertion of the n-Mg components in the LPSO structure, and some of them possess inversion centers or mirror symmetry. The stacking faults and growth twins in 18R LPSO depend on whether the sum of inserted Mg layers of all n-Mg components in defects is even number or odd number, respectively, while only stacking faults appear in 14H LPSO. The gliding of Shockley partial dislocation pairs induces the transformation of stacking faults and motion of twin boundary. Statistically, the periodical arrangement of stacking faults in 18R or 14H LPSO structures results in the formation of superstructures, such as 62L, 86L, 152L, and 235L.

The heavily faulted M 2 B-type borides precipitated in the long-term aging Ni-based superalloys with B addition are comprehensively studied at an atomic scale. By means of high angle annular dark field (HAADF) imaging technique in the aberration corrected scanning transmission electron microscope (STEM), atomic configurations of various kinds of structural intergrowth in the profoundly faulted M 2 B phase are directly imaged. The polytypic M 2 B phase is mainly composed of C16, C b and C a structural variants, in which planar defects and the resultant long period stacking order (LPSO) structure are identified. In combination of the advanced spectrometry imaging technique with density functional theory (DFT) calculations, it is proposed that the polytypic feature is attributed to the conservation of the basic polyhedron of anti-square prisms.

The properties of BaO/MgO-type interface in the BaZrO3/MgO(001) heterostructure are studied by aberration-corrected high-resolution transmission electron microscopy combined with first-principles calculations. Experimental evidence demonstrates that cation displacement and vacancies occur at the interface. Our first-principle calculations show that cation displacement results from the electrostatic potential effect at the interface, and cation vacancies could lower the interfacial work of separation and enhance the interfacial stability of BaO/MgO-type interface. The results highlight that the effect of interfacial defects should be taken into account in understanding the film growth kinetics and properties in oxide heteroepitaxy.

The pitting of Al–Cu–Mg alloy is believed to originate from the local dissolution of S-Al2CuMg particles, and the dissolution activity differs from one particle to another. Nevertheless, the initial site where the dissolution of the S phase preferentially occurs and the cause of the heterogeneity in the electrochemical dissolution activity remain unknown, hindering our understanding of pitting initiation of Al–Cu–Mg alloys. In this work, we have applied in situ ex-environmental transmission electron microscopy and identified a large number of nanosized Al20Cu2Mn3 approximants of the decagonal quasicrystal embedded in the S phase. We find that the S phase with Al20Cu2Mn3 inclusions is more active than those free of the approximants. Such a preference is clarified to result from the decomposition of Al20Cu2Mn3 approximant prior to the dissolution of the S phase. In addition, we also find that the electrochemical behavior of Al20Cu2Mn3 approximants is different. The approximants with multiple twins are more active than those with few planar defects.

We studied post-annealed Fe-Ge alloy by X-ray diffraction and transmission electron microscopy. A new Nowotny chimney ladder phase has been discovered in such Fe-Ge system. The crystal structure and the composition have been calculated based on selected area electron diffraction. The formation of the incommensurate structure is attributed to the shrinkage of the Ge sublattice to meet the 14 electrons rule.

As the important structure elements, vacancies are believed to be a crucial factor in the composite multiferroics. In the work, first-principles calculations have been performed to study the effect of vacancies (O, Ti) on the structural, magnetoelectric and electronic properties of the Fe/BaTiO3 interface. The vacancy-type defects are introduced in the first interfacial TiO2 layer. It is found that the interfacial vacancies (O, Ti) will increase the bonding strength of the ferromagnetic/ferroelectric system while the magnetoelectric effect can still keep the same order of magnitude as perfect Fe/BaTiO3, which may be helpful for designing multiferroic thin film materials.

M2B-type borides in nickel-based superalloys containing boron subjected to long-term aging treatments were investigated by means of analytical transmission electron microscopy. Contrast analysis and electron diffraction show that the boride precipitations have dual-phase structures (I4/mcm, C16; Fddd, Cb) with a high density of stacking faults. The faulted planes and displacement vectors are determined as (1 1 0)C16 and [ 1 1 ¯ 1 ] C 16 / 4 in C16 structure and (1 0 0)Cb and [0 1 1]Cb/4 in Cb structure. Structural models based on a polyhedron of square anti-prisms are proposed not only to rationalize the stacking faults, 60° rotation twin and the intergrowth of structural variants, but also to interpret the newly observed structure in M2B-type borides.

Composition and structure of the oxide and its effect on corrosion property in the high entropy alloy CrMnFeCoNi have not been revealed so far. In the present work, oxide in CrMnFeCoNi is identified as MnCr2O4 using transmission electron microscopy. Moreover, the oxide MnCr2O4 is detrimental to pitting resistance indicated by comparison study of CrMnFeCoNi with and without the oxide. This is because that the pitting prefers originating from MnCr2O4/matrix interface, in other words, the matrix around oxides dissolves more easily. From the viewpoint of corrosion, MnCr2O4/matrix forms a local galvanic cell, and the matrix as an anode dissolves quickly.

An accurate understanding of the atomistic structure of interfaces is crucial to explain the mechanism of abnormal grain growth (AGG) in polycrystalline materials. In this work we investigated the atomistic structure of interfaces in a Ti excess BaTiO3 by transmission electron microscopy (TEM) and Cs-corrected scanning transmission electron microscopy (STEM). After sintering at 1300 °C in air, the Ti excess BaTiO3 shows typical AGG morphology with {1 1 1} twin lamellae and {1 1 1} faceted grain boundaries. A sub-nanometer Ti-rich intergranular phase and a second phase, Ba4Ti10Al2O27, at the {1 1 1} faceted grain boundaries were indentified. The intergranular phase and the Ba4Ti10Al2O27 exhibit epitaxial relationships with BaTiO3 in stacking sequences of {1 1 1} BaTiO3/intergranular phase/(1 0 0) Ba4Ti10Al2O27 and {1 1 1} BaTiO3/(1 0 0) Ba4Ti10Al2O27. Additionally, two orientation relationships were identified, [1 1 0] BaTiO3 // [0 1 ¯ 0] Ba4Ti10Al2O27 and [1 1 0] BaTiO3 // [0 1 0] Ba4Ti10Al2O27. The intergranular phase and Ba4Ti10Al2O27 are also shown to exhibit strong structural similarities. Effects of the interfaces on AGG are proposed.

Nd0.45Sr0.55MnO3 is an A-type antiferromagnetic manganite showing obvious angular-dependent magnetoresistance, which can be tuned by misfit strain. The misfit strain relaxation of Nd0.45Sr0.55MnO3 thin films is of both fundamental and technical importance. In this paper, microstructures of epitaxial Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (110) substrates by pulsed laser deposition were investigated by means of (scanning) transmission electron microscopy. The Nd0.45Sr0.55MnO3 thin films exhibit a two-layered structure: a continuous perovskite layer epitaxial grown on the substrate followed by epitaxially grown columnar nanostructures. An approximately periodic array of misfit dislocations is found along the interface with line directions of both and [001]. High-resolution (scanning) transmission electron microscopy reveals that all the misfit dislocations possess a -type Burgers vectors. A formation mechanism based on gliding or climbing of the dislocations is proposed to elucidate this novel misfit dislocation configuration. These misfit dislocations have complex effects on the strain relaxation and microstructure of the films, and thus their influence needs further consideration for heteroepitaxial perovskite thin film systems, especially for films grown on substrates with low-symmetry surfaces such as SrTiO3 (110) and (111), which are attracting attention for their potentially new functions. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

First-principles calculations, high-resolution transmission electron microscopy (HRTEM) investigations and geometrical phase analysis of lattice strain based on HRTEM images have been carried out on C15 Cr2Nb Laves phase. Asymmetrical nanoscale regions with severe lattice distortion were observed in the vicinity of Shockley partial dislocation cores. These disturbed regions are the result of synchroshear inside the Nb-Cr-Nb triple layers. Reactions of dislocations with twin boundaries (TBs) also result in severe lattice distortion. The resulting local lattice distortion and perturbed strain field deleteriously impact dislocation-governed plasticity. First-principles calculations show that cleavage occurs preferentially between the single Cr layer and the Nb-Cr-Nb triple layer in these materials. TBs, which are on single Cr layers, nucleate and propagate cracks readily in Cr2Nb, as evidenced by the lattice dilatation along a TB interacting with one 60 degrees dislocation. The present study shows atomic level evidence on how crystal defects and their interactions influence the mechanical properties, especially the poor toughness at low temperatures, of C15 Cr2Nb. This provides new insights into the origin of low temperature brittleness of Laves compounds with complex structures. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The microstructures and the corresponding microhardness evolution of a Mg(83)Ni(6)Zn(5)Y(6) (at.%) alloy subjected to a series of heat treatments were investigated. In addition to the 14H long period stacking ordered (LPSO) phase, a diamond-cubic phase with the possible space group of Fd (3) over barm and lattice parameter of a = 0.73nm were identified in the as-cast alloy. The combination of LPSO thin lamellae and the nanometer-sized cubic phase leads to a relatively high Vickers microhardness of about 116.2 +/- 13.7. When annealed at 473K and 573 K, the above diamond-cubic phase partially transforms to its structural variant, Fm (3) over barm, together with a slight change of lattice parameter (a = 0.72 nm). Precipitation of hexagonal Mg(2)Ni lath also happens at these moderate annealing temperatures. The remarkable increase of microhardness at this stage is believed to result from the strengthening of precipitates and the resultant coherent interface between the precipitates and their surrounding medium. When annealed at temperatures ranging from 673K to 773 K, the Mg(2)Ni precipitates were dissolved, grain coarsening in the alloy occurred, Fd (3) over barm phase completely transformed to its Fm (3) over barm counterpart, and consequently microhardness of the alloy was decreased significantly. (C) 2011 Elsevier B. V. All rights reserved.

Suitable TM doping at the Mn site is an important access to manipulate magnetic properties of hexagonal YMnO3, however, it has not yet been systematically explored how the strength of antiferromagnetic interactions and the magnetic transition temperatures (TN) are modified in the doping YMn0.917TM0.083O3 systems. In the work, we have performed first-principles calculations to study the effect of TM doping on the structural and magnetic properties of hexagonal YMn0.917TM0.083O3 bulks; the results are combined with the available experimental results. The calculated results reveal that the planar TM-O bonds and O-TM-O angles of TMO5 bipyramid are both prominent structural features for the transformations of magnetic properties. We have also predicted the Ti, V, Cr and Fe doping effects on magnetic properties and further analyzed the TM electronic structures of TMO5 bipyramid in the YMn0.917TM0.083O3(001)/MgO(001) film configurations, which could provide more understanding towards the designing of new generation multifunctional devices.