Antiphase Boundaries Research Articles

Temperature dependence of deformation mechanisms of a new Ni-based superalloy and high-temperature property optimization

The influence of temperature on the deformation mechanisms of a new Ni-based superalloy named K4800 containing 35 vol% γ′ were investigated. We studied the temperature dependence of deformation mechanisms and used the alloying method to optimize the high-temperature properties of the alloy. By analyzing slightly deformed microstructures, the dominant deformation mechanisms of alloy K4800 at the yielding stage were identified as: the anti-phase boundary (APB) shearing from RT to 600 °C, stacking faults (SFs) shearing and Orowan looping at 600–800 °C, and Orowan looping/cross-slip/climbing above 800 °C. We found that it was the activation of thermally assisted processes declined the strengthening effect of γ′ phase and impaired the high-temperature strength of the alloy above 800 °C. First-principles calculations were employed to investigate the effects of alloying elements on the formation and stability of stacking faults in K4800 alloy by using a 48-atom Ni3(Al, Ti) orthorhombic supercell. Considering the effectiveness of elements in lowering stacking faults energy, Co and W elements were chosen to alleviate the strength degradation of K4800 alloy above 800 °C. The deformation mechanism and properties of these three alloys at elevated temperature were also revealed and discussed.

Anisotropy in flow behaviour of directionally solidified Ni-based superalloy under compression at an elevated temperature

Directionally solidified (DS) superalloys consist of coarse columnar grains with <001> preferred orientation parallel to the maximum stress axis; but, grain orientations in the transverse direction are not controlled. However, during service, substantial loading could occur in the transverse direction as well, leading to early intergranular failure of components such as DS turbine blades. The present study investigates the effect of crystallographic orientation as well as specimen orientation (transverse and longitudinal with respect to grain growth direction) on the mechanical response of DS Ni-based superalloy in compression. Compression tests were performed at 700 ºC on transverse (with predominantly <101> or <001> oriented grains) and longitudinal (with <001> oriented grain) samples. Though there were negligible changes in yield stress, variations in hardening behaviour and ductility were observed in all the tested samples. Unlike transverse samples, longitudinal samples showed significant strain hardening with no failure even up to 0.3 true strain. Deformation mechanisms that led to these differences are identified using advance characterization techniques. γ' shearing with antiphase boundary formation was observed in the sample containing <101> grains, whereas γ' shearing with the formation of superlattice intrinsic stacking faults occurred in the sample having <001> grains. In addition, substantial lattice rotation and deformation twinning also contribute to strain hardening, especially in longitudinal samples. The effect of orientation is explained in terms of effective stacking fault energy. The study suggests that grains with <001> orientation undergo significant strain hardening and thereby accommodate large strain as compared to <101> oriented grains in DS alloys.

Revealing direct atomic structure and electrostatic maps of the Ruddlesden-Popper faults in LaNiO3 perovskite

Ruddlesden-Popper (RP) faults in perovskites as one type of the anti-phase boundaries provide a versatile platform to manipulate the magnetic, conducting, and magnetoresistance properties in the complex oxides. Local fine structure at the boundaries containing light atoms is a key factor to understand the underlying structure-function relationship due to their interfaces mediated effects. In this work, the atomic structure, chemical distribution and electronic structure of typical RP faults in perovskite LaNiO3 and its epitaxial nanocomposite films on a SrTiO3 substrate were systematically investigated combining high-resolution scanning transmission electron microscopy (STEM) and density functional theory (DFT) calculations. Integrated differential phase contrast (iDPC) STEM imaging results demonstrate a NiO2-x-(LaO-LaO)-NiO2-x atomic configuration with content fluctuations of oxygen at the RP faults. Atomic electronic energy loss spectroscopy results and DFT calculations illustrate that Ni cations have mixed valence of Ni2+, Ni3+ at the RP faults accompanying with oxygen environment fluctuations affected by the non-stoichiometric bonding. This study offers a comprehensive route to explore intriguing chemical structures and physical properties in the homointerface structure at the atomic level.

Spectromicroscopic study of the transformation with low energy ions of a hematite thin film into a magnetite/hematite epitaxial bilayer

The search of new properties in novel oxide heterostructures requires the exploration of new fabrication methods and the study, at the microscopic level, of the processes involved during the synthesis. We present a synchrotron-based spectromicroscopic investigation of a magnetite/hematite bilayer on Pt(111) grown in a two-step process by thermal evaporation and Low Energy Ion Bombardment (LEIB). The characterization includes the study of structural, electronic, chemical, and magnetic properties using X-ray Absorption Spectroscopy (XAS), Low Energy Electron Microscopy (LEEM), Photoemission Electron Microscopy (PEEM), or X-ray Magnetic Circular Dichroism (XMCD). The aim is to obtain microscopic information of the thin film before, during, and after the ion bombardment. Ion bombardment gradually transforms the topmost layers of the hematite thin film into a defective sub-oxide, where magnetite nuclei grow and coalesce with increasing ion doses. Two rotational domains of magnetite coexist, which are typically a few tens of nanometres large and do not grow significantly with temperature annealings. The incoherent growth of the magnetite nuclei favours the formation of stable twin boundaries (TBs) and antiphase boundaries (APBs). Dichroic spectra show the characteristics of the ferrimagnetic (FiM) order of magnetite, and the spatial distribution of magnetic domains shows no apparent correlation with the structural image, displaying smooth domains separated by diffuse frontiers. These findings illustrate the importance of a spectromicroscopic characterization of novel oxide heterostructures for potential future applications.

Atomistic insights into the synergistic effect of nanotwins and nano-precipitates on the mechanical behavior of superalloys

As one of the typical nanostructures, nanotwins (NTs) have been utilized to evade the trade-off between strength and ductility, which is also applicable to improve the mechanical properties of superalloys. However, the optimization capability of mechanical properties by tailoring NTs has encountered bottleneck for the reasons such as inverse Hall-Petch effect and anisotropic effect. Based on the experimental observation of a special distribution pattern of NTs and nano-precipitates (NPs) in superalloys, that is, NPs are located at the twin boundaries (TBs), our work aims to study the distinctive synergistic deformation mechanism between NTs and NPs, which could become a new microstructure design strategy to optimize the mechanical properties of superalloys. Specifically, molecular dynamic (MD) method was employed to unveil the synergistic deformation mechanism of NTs and NPs in a model Ni-based superalloy. As expected, the introduction of NPs onto TBs can inhibit dislocation motion along or parallel to TBs, despite the softening deformation mode of NTs is activated. Moreover, the existence of NPs onto TBs can greatly improve the strengthening efficiency of NTs by impeding twin thickening/thinning. On the other hand, both phase boundaries and NPs interior have obstructive effect on dislocation motion, which could also be beneficial for material strengthening. More interestingly, anti-phase boundary (APB) structure will form along with the dislocations cutting through NPs. Subsequently, the APB structure can act as channels for dislocation slip and dislocation nucleation site induced by a kind of TB confinement effect, which may effectively accommodate plastic deformation. Our research outcomes could be utilized to facilitate the development of high-performance superalloys via rational collaborative design of NTs and NPs.

High-temperature structural and mechanical stability of refractory high-entropy alloy Nb40Ti25Al15V10Ta5Hf3W2

Refractory high-entropy alloys (RHEAs) with superior mechanical properties have been considered as a potential next-generation candidate for high-temperature structural materials. However, the structural stability of RHEAs at high temperatures is still noncommittal so far. Especially, the Al containing RHEAs with coherent BCC/B2 microstructure usually form intermetallic phases following short/long term aging at elevated temperatures, which deteriorate the mechanical performance. In this work, we systematically determine the microstructural evolution, including intergranular precipitates, BCC/B2 morphology, and corresponding mechanical properties changes of the Nb40Ti25Al15V10Ta5Hf3W2 RHEA from 923 K to 1123 K. The density of antiphase domain boundaries (APBs) in the RHEA after aging at 923 K is significantly higher than at 1023 K and 1123 K. Only a low volume fraction of tetragonal (Nb, Hf)2Al phase precipitates along grain boundaries after aging at 1123 K. However, the microhardness and yield strengths of the RHEA after aging at 923 K - 1123 K are close to those of the as-cast RHEA, which proves that the APBs and low-density (Nb, Hf)2Al precipitates have a negligible effect on the mechanical performance of the RHEA. Therefore, the Nb40Ti25Al15V10Ta5Hf3W2 RHEA exhibits superior structural stability from 923 K to 1123 K, which also imparts the mechanical stability. This work guides towards tailoring the composition and microstructure of RHEAs for even better high-temperature mechanical performance.

A Multitechnique Study of C2H4 Adsorption on Fe3O4(001).

The adsorption/desorption of ethene (C2H4), also commonly known as ethylene, on Fe3O4(001) was studied under ultrahigh vacuum conditions using temperature-programmed desorption (TPD), scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT)-based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermodynamics. C2H4 adsorbs intact at all coverages and interacts most strongly with surface defects such as antiphase domain boundaries and Fe adatoms. On the regular surface, C2H4 binds atop surface Fe sites up to a coverage of 2 molecules per (√2 × √2)R45° unit cell, with every second Fe occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD spectra at this coverage, which is approximately 0.1-0.2 eV lower than the value calculated by DFT + U with van der Waals corrections. Additional molecules are accommodated in between the Fe rows. These are stabilized by attractive interactions with the molecules adsorbed at Fe sites. The total capacity of the surface for C2H4 adsorption is found to be close to 4 molecules per (√2 × √2)R45° unit cell.

Chemical short-range order dependence of micromechanical behavior in CoCrNi medium-entropy alloy studied by atomic simulations

Chemical short-range order (CSRO) is a typical microstructural characteristic commonly found in multi-principal element alloys (MPEAs). For MPEAs with the face-centered cubic (FCC) structure, the presence of CSRO has been verified and characterized meticulously by using electron microscope. However, it is still challenging to reveal the effect of CSRO structure on the deformation microstructure and micromechanical behavior of those alloys. By applying atomic scale simulations, this study explores the micromechanical behavior of an equiatomic CoCrNi medium entropy alloy with random solid solution and three different degrees of CSRO under plane strain compression, respectively. The results show that CSRO can effectively promote planar slip of dislocations in the FCC matrix and elevate flow stress of the alloy. The planar slip and stacking of partial dislocations then lead to the formation of hexagonal-close-packed (HCP) structure lamellae. CSRO also modulates the morphology of dislocations. When dislocations cut through CSRO, it hinders the movement of dislocations while being destroyed by atomic plastic flow, causing CSRO and atomic clusters at the interface of the matrix to embed each other and generate antiphase boundaries which can significantly enhance the bulk yield strength. These findings provide fundamental insights into the interaction between CSRO and dislocations in CoCrNi MEA, guiding the design of MPEAs with superior combinations of strength and ductility.

Creep Behavior and Deformation Mechanism of a Third-Generation Single Crystal Ni-Based Superalloy at 980 °C

As the primary choice for aero-engine turbine blades, creep resistance is an important mechanical property for the developing third-generation single crystal Ni-based superalloys. The creep behavior of the superalloy in the [001] orientation was studied at 980 °C under a series of stress levels, accompanied with scanning electron microscope (SEM) and transmission electron microscope (TEM) observation to investigate the microstructure and deformation mechanism. The deformation mechanism of the alloy is found to be dislocation gliding, propagating and forming a dislocation network in the γ/γ′ interface. Dislocation networks could hinder the movement of dislocation and decrease the creep rate to a constant during the steady-creep stage. The formation of dislocation networks was analyzed due to the interaction of &lt;110&gt; {111} dislocations. Then dislocations cut into γ′ phases as individual &lt;110&gt; super-dislocations, anti-phase boundary dislocation pairs, and stacking faults. The &lt;110&gt; super-dislocation in the γ′ phase may cross-slip into the {001} plane from the {111} plane to form Kear–Wilsdorf locks, which could inhibit dislocations from gliding or cross-slipping and then enhance the creep resistance.

Defect-Engineering by Solvent Mediated Mild Oxidation as a Tool to Induce Exchange Bias in Metal Doped Ferrites.

The crystal site occupancy of different divalent ions and the induction of lattice defects represent an additional tool for modifying the intrinsic magnetic properties of spinel ferrites nanoparticles. Here, the relevance of the lattice defects is demonstrated in the appearance of exchange-bias and in the improvement of the magnetic properties of doped ferrites of 20nm, obtained from the mild oxidation of core@shell (wüstite@ferrite) nanoparticles. Three types of nanoparticles (Fe0.95 O@Fe3 O4 , Co0.3 Fe0.7 O@Co0.8 Fe2.2 O4 and Ni0.17 Co0.21 Fe0.62 O@Ni0.4 Co0.3 Fe2.3 O4 ) are oxidized. As a result, the core@shell morphology is removed and transformed in a spinel-like nanoparticle, through a topotactic transformation. This study shows that most of the induced defects in these nanoparticles and their magnetic properties are driven by the inability of the Co(II) ions at the octahedral sites to migrate to tetrahedral sites, at the chosen mild oxidation temperature. In addition, the appearance of crystal defects and antiphase boundaries improves the magnetic properties of the starting compounds and leads to the appearance of exchange bias at room temperature. These results highlight the validity of the proposed method to impose novel magnetic characteristics in the technologically relevant class of nanomaterials such as spinel ferrites, expanding their potential exploitation in several application fields.

Atomistic analysis of temperature-dependent dislocation dynamics in Ni3Al-based intermetallic alloys

In this study, we investigate the dynamics of superlattice edge dislocations composed of partial dislocations, complex stacking faults, and antiphase boundaries in binary and ternary Ni-based alloys (γ’ phase) with the L12 structure. In the case of Ni3Al, the dislocation velocity decreases with increasing temperature at low applied shear stress, mainly owing to the phonon drag effect. Meanwhile, in the case of ternary alloys constructed by exchanging Ni atoms with Co atoms in Ni3Al, the effects of temperature on dislocation motion are more complex. At low applied shear stress, the dislocation motion is disturbed by solute Co (i.e., solid-solution strengthening). In this case, the temperature promotes dislocation motion because the potential energy barrier dominates the thermally activated dislocation dynamics. In contrast, when the applied shear stress increases, the temperature suppresses dislocation motion owing to the phonon drag effect. The drag effect also increases with increasing Co concentration. The origin of the energy barrier due to solute atoms at low applied shear stress can be explained by the interaction between the partial dislocation and solute Co. Meanwhile, the origin of the drag effect on the superlattice dislocation dynamics is discussed on the basis of atomic vibrational motions and wavy dislocation behaviors. In this study, we propose and construct a map showing the dominant effects on complex superlattice dislocation dynamics. The map has energy-barrier-dominated, phonon-drag-dominated, and sound velocity-dominated regions, suggesting that the dominant factor changes depending on the applied shear stress, temperature, and strength of the solute–dislocation interaction.

Formation and stability of anti-phase boundaries (APBs) in Al–Fe B2

Formation and stability of anti-phase boundaries (APBs) in Al–Fe B2

Influence of in-situ annealing temperature upon structural, micro-structural, magnetic damping properties in sputtered Fe3O4 thin films

The influence of in-situ annealing temperature upon structural, micro-structural, magnetic properties and Gilbert damping parameters of sputtered Fe3O4 thin films (29–49 nm) upon SiO2/Si substrates have been systematically investigated. The oriented Fe3O4 structural phase was evaluated using grazing incidence X-ray diffraction, confocal Raman spectroscopy and X-ray photoelectron spectroscopy data. The atomic force and field-emission scanning electron microscope surface microstructural images reveal an enhanced grain growth with dominant anti-phase boundaries (APB) with in-situ annealing. The macroscopic spin dynamical response is evaluated using co-planar waveguide ferromagnetic resonance (CPW-FMR) spectroscopy from 15 to 35 GHz frequencies. The FMR data reveals a low Gilbert damping (α) ranging from 0.003 ± 0.001 to 0.009 ± 0.006 for all the in-situ annealed Fe3O4 thin films. Further, high inhomogeneity (ΔH0) parameter is correlated to the surface defects and APBs.

In-Situ Study of Temperature- and Magnetic-Field-Induced Incomplete Martensitic Transformation in Fe-Mn-Ga

Significant interest in the stoichiometric and off-stoichiometric Fe2MnGa alloys is based on their complex phase transition behavior and potential application. In this study, temperature- and magnetic-field-induced phase transformations in the Fe41.5Mn28Ga30.5 magnetic shape memory alloy were investigated by in situ synchrotron high-energy X-ray diffraction and in situ neutron diffraction techniques. It was found that incomplete phase transformation and phase coexistence behavior are always observed while applying and removing fields in Fe41.5Mn28Ga30.5. Typically, even at 4 K and under 0 T, or increasing the magnetic field to 11 T at 250 K, it can be directly detected that the martensite and austenite are in competition, making the phase transition incomplete. TEM observations at 300 K and 150 K indicate that the anti-phase boundaries and B2 precipitates may lead to field-induced incomplete phase transformation behavior collectively. The present study may enrich the understanding of field-induced martensitic transformation in the Fe-Mn-Ga magnetic shape memory alloys.

Planar slip triggered by successive dislocation-precipitate interaction in titanium alloys

The planar slip associated with the interaction between dislocations and α2 precipitates in titanium alloys was investigated using atomistic simulation and transmission electron microscopy. The simulation reveals that <a>-dislocations experience different critical resolved shear stress (CRSS) during their passing through the α2-Ti3Al particle. The CRSS of the first pass increases linearly with the particle diameter both in the prismatic and basal planes, resulting in the formation of the anti-phase boundary within particle, which decreases the CRSS of the second pass. This triggers a slip planarity that ends up of dislocation pairs. The present results provide a deeper understanding on the plasticity and fatigue of titanium alloys containing α2 precipitates.

Periodic anti-phase boundaries and crystal superstructures in PtCu3 nanoparticles as fuel cell electrocatalysts

To reduce the cost of the electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells, supported metal alloy nanoparticles are commonly used to decrease the amount of the scarce and expensive Pt. Since the structure of the nanoparticles has a crucial influence on the catalytic properties, advanced characterization methods help elucidate these relationships and ultimately enable more rational synthesis. This work focuses on investigating the presence of one particular planar defect, namely periodic anti-phase boundaries in carbon-supported PtCu3 nanoparticles. The studying of this structural phenomenon was approached with several characterization tools. Ex-situ X-ray diffractograms were used in conjunction with computer simulations and Rietveld analyses to reliably determine the modulated unit cell containing anti-phase boundaries, while their temperature-dependent formation and disappearance were observed with high-temperature X-ray powder diffraction. In addition, their presence was also confirmed by electron diffraction and atomically resolved scanning transmission electron microscopy. Furthermore, the average neighborhood of Cu and Pt atoms was confirmed using extended X-ray absorption fine structure. Finally, the electrocatalytic activity for the oxygen reduction reaction for composites with and without anti-phase boundaries was determined using a thin-film rotating disk electrode, and the performance was found to correlate with the degree of alloy ordering but not with the presence of anti-phase boundaries. Overall, this study represents a significant step towards better control over the atomically precise synthesis of advanced functional metallic materials by providing valuable insight into the formation of planar defects in metallic alloy nanoparticles and their impact on the structure-property relationships of electrocatalysts.

Influence of scandium addition on microstructure and mechanical properties in nickel-based superalloys: An integrated multiscale modeling and experimental approach

Microalloying has long been recognized as an effective method for improving the mechanical properties of metallic materials and developing new ones. In this study, we comprehensively investigate the effects of Sc addition on the microstructural evolution and mechanical performance of nickel-based superalloys using a combination approach of the experiment, first-principles calculation, and multiscale modeling. The results show that Sc addition simultaneously improves the yield strength increasing from 1068 MPa to 1172 MPa (9.7%) and elongation increasing from 13.4% to 19.7% (47%). The microstructural characterizations reveal that Sc addition refines the grain size, increases the volume fraction of primary precipitate, induces the coarsening of secondary precipitates, and produces a new type of Sc-rich particle. First-principles calculations from the atom probe tomography experiment show that the anti-phase boundary energy of secondary precipitate is significantly increased in Sc-containing superalloys. A modified alloying-dependent yield strength model is developed by considering the multimodal precipitate size distribution and microstructure evolution obtained from experiments and atomic simulations. The increased yield strength originates from not only the Sc addition induced increased volume fraction of primary precipitate at grain boundary to enhance grain boundary strengthening, but also Sc addition induced increased anti-phase boundary energy of secondary precipitate to improve the precipitate strengthening. The increased elongation depends on the fact that the Sc element has a good affinity to deleterious elements and forms Sc-rich particle and oxygen-rich precipitate to enhance grain boundary strength. This work integrating multiscale modeling and experiment can be denoted as a universal framework to study the influence of alloy element on the microstructure and properties, which gives guiding significance to screen and design high-performance superalloys.

Study on the metallurgical reaction law and mechanism of the Ni-xCu alloys/pure Sn solid-liquid interface

In this study, Ni-xCu alloy substrates/pure Sn soldering joints were prepared, and the metallurgical reaction law and mechanism of the Ni-xCu/Sn solid-liquid interface were studied based on electron probe microanalyzer (EPMA) and electron backscatter diffraction (EBSD). Results showed that with the increase of Cu content in the Ni-xCu alloys, the reactants at the Ni-xCu/Sn interface changed from (Ni,Cu)3Sn4 single phase (x ≤ 40 wt%) to (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 two phases (50 wt% ≤ x ≤ 60 wt%), and then transformed into (Cu,Ni)6Sn5 single phase (70 wt% ≤ x). Correspondingly, the thickness of the intermetallic compound (IMC) layer presented four stages: decreased slowly, increased slightly, increased significantly and decreased sharply. The evolution mechanism of the phase structure was clarified according to the critical Cu concentration theory for (Cu,Ni)6Sn5 phase growth. In addition, the evolution mechanism of the IMC layer thickness was explained based on the studies of (Ni,Cu)3Sn4 growth thermodynamics and (Cu,Ni)6Sn5 lattice antiphase boundary array.

Impact of Anionic Ordering on the Iron Site Distribution and Valence States in Oxyfluoride Sr2FeO3+xF1-x (x = 0.08, 0.2) with a Layered Perovskite Network.

Sr2FeO3F, an oxyfluoride compound with an n = 1 Ruddlesden-Popper structure, was identified as a potential interesting mixed ionic and electronic conductor (MIEC). The phase can be synthesized under a range of different pO2 atmospheres, leading to various degrees of fluorine for oxygen substitution and Fe4+ content. A structural investigation and thorough comparison of both argon- and air-synthesized compounds were performed by combining high-resolution X-ray and electron diffraction, high-resolution scanning transmission electron microscopy, Mössbauer spectroscopy, and DFT calculations. While the argon-synthesized phase shows a well-behaved O/F ordered structure, this study revealed that oxidation leads to averaged large-scale anionic disorder on the apical site. In the more oxidized Sr2FeO3.2F0.8 oxyfluoride, containing 20% of Fe4+, two different Fe positions can be identified with a 32%/68% occupancy (P4/nmm space group). This originates due to the presence of antiphase boundaries between ordered domains within the grains. Relations between site distortion and valence states as well as stability of apical anionic sites (O vs F) are discussed. This study paves the way for further studies on both ionic and electronic transport properties of Sr2FeO3.2F0.8 and its use in MIEC-based devices, such as solid oxide fuel cells.

Investigation on tensile deformation and fracture behavior of a Ni-based superalloy specially designed for additive manufacturing

A nickel-based superalloy special developed for additive manufacturing (AM) were prepared by laser directed energy deposition (LDED) technology. Through tensile test at different temperatures, the ultimate tensile strength (UTS), yield strength (YS), and elongation were obtained; The corresponding microstructure and fracture morphologies were characterized as well as the properties. The results show that the abnormal peak values of UTS and YS were found at 700 °C, accompanying with poor ductility. Furthermore, the value of flow stress is little affected by strain rate in the temperature range of 23–700 °C, while it is very sensitive for that of 800–1100 °C. The dominant deformation mechanism of the former is anti-phase boundary (APB) shearing, and the fracture is featured by shear fracture without obvious necking; while for the latter, the dislocation climbing, cross-slip and Orowan looping is dominant, and the fracture mechanism is mainly characterized by the aggregation and growth of micropores and crack propagation, and postmortem specimen exhibit distinct necking phenomena. The abnormal peak values of UTS and YS at 700 °C could be attributed to the formation of Kear–Wilsdorf locks (K-W locks). The experimental results will support the application of the investigated alloy as well as design new alloy for AM.