Formation Of Antiphase Boundaries Research Articles

Some advances on precipitation hardening in Waspaloy: Critical review on the contribution of shear, Orowan and coherency mechanisms

The precipitation hardening mechanisms and their dependence on precipitate size (rp) and precipitate/matrix lattice misfit (δ) in γ'/γ systems were studied in nickel-base superalloy using micro-indentations experiments and theoretical models. Metallurgical states were characterized by TEM, SEM-EBSD, XRD and APT. For the precipitate size range studied (radius rp smaller than 110 nm), γ' precipitates are ordered spherical Ni3(Al,Ti) phase, the volume fraction remains relatively constant and equal to 20 % and lattice misfit δ is lower than 0.45 %. Prevalence of order strengthening by anti-phase boundary formation and Orowan bypassing was observed in various rp range, whereas chemistry and coherency strengthening were found insignificant in the nickel-base superalloy studied (Waspaloy®). Shearing by weakly coupled dislocations was dominant for fine precipitates, i.e. rp lower than 18 nm, shearing by strongly-coupled dislocations for an intermediate rp range, i.e. from 18 nm to 98 nm and Orowan bypassing for coarse precipitates with a rp larger than 98 nm.

Towards controllable formation of antiphase boundaries (APBs) in the B2 matrices of the Al-Fe based complex multicomponent alloys

Anti-Phase Boundaries (APBs) formation in B2 (ordered Body Centered Cubic) structures was investigated, focusing on the Al-Fe-Cr/Ni alloys. The current research goal was to propose strategies for controlling APBs formation in multicomponent alloys, following their positive effect on properties of High Entropy Alloys, stated in previous researches. This study is based on our previous investigation of the APBs formation mechanisms in binary B2 Fe50+xAl50-x alloys. There, stable APBs have formed at and above specific Long-Range order parameter. Here, it was established that stabilization of APBs requires disorder at only Al sublattice. Thorough analysis of dislocations revealed that higher complexity accelerates the decrease of order, promoting the formation of APBs. An assessment of disorder in multicomponent systems is challenging and sometimes even impossible, therefore suggested here usage of evaluation of dislocations is an interesting and useful experimental tool to estimate the disorder in these complex systems.

Strain Heterogeneity and Extended Defects in Halide Perovskite Devices.

Strain is an important property in halide perovskite semiconductors used for optoelectronic applications because of its ability to influence device efficiency and stability. However, descriptions of strain in these materials are generally limited to bulk averages of bare films, which miss important property-determining heterogeneities that occur on the nanoscale and at interfaces in multilayer device stacks. Here, we present three-dimensional nanoscale strain mapping using Bragg coherent diffraction imaging of individual grains in Cs0.1FA0.9Pb(I0.95Br0.05)3 and Cs0.15FA0.85SnI3 (FA = formamidinium) halide perovskite absorbers buried in full solar cell devices. We discover large local strains and striking intragrain and grain-to-grain strain heterogeneity, identifying distinct islands of tensile and compressive strain inside grains. Additionally, we directly image dislocations with surprising regularity in Cs0.15FA0.85SnI3 grains and find evidence for dislocation-induced antiphase boundary formation. Our results shine a rare light on the nanoscale strains in these materials in their technologically relevant device setting.

Offcut Substrate-Induced Defect Trapping at Step Edges.

We report step edge-induced localized defects suppressing subsequent antiphase boundary formation in the bulk structure of a trilayer oxide heterostructure. The heterostructure encompasses a layer of La0.66Sr0.34MnO3 sandwiched between a superconducting La1.84Sr0.16CuO4 bottom layer and an insulating La2CuO4 top layer. The combination of a minor a-axis mismatch (0.11 Å) and a pronounced c-axis mismatch (2.73 Å) at the step edges leads to the emergence of localized defects exclusively forming at the step edge. Employing atomically resolved electron energy-loss spectroscopy maps, we discern the electronic state of those structures in the second La0.66Sr0.34MnO3 unit cell near the step edge. In particular, a reduction in the pre-edge region of the O-K edge indicates the formation of oxygen vacancies induced by the strained step edge. This study underscores our capability to control defects at the nanoscale.

Atomic-Scale Structural Properties in NiCo2O4/CuFe2O4 Bilayer Heterostructures on (001)-MgAl2O4 Substrate Regulated by Film Thickness.

Changing film thickness to manipulate microstructural properties has been considered as a potential method in practical application. Here, we report that atomic-scale structural properties are regulated by film thickness in an NiCO2O4(NCO)/CuFe2O4(CFO) bilayer heterostructure prepared on (001)-MgAl2O4 (MAO) substrate by means of aberration-corrected scanning transmission electron microscopy (STEM). The misfit dislocations at the NCO/CFO interface and antiphase boundaries (APBs) bound to dislocations within the films are both found in NCO (40 nm)/CFO (40 nm)/MAO heterostructures, contributing to the relaxation of mismatch lattice strain. In addition, the non-overlapping a/4[101]-APB is found and the structural transformation of this kind of APB is resolved at the atomic scale. In contrast, only the interfacial dislocations form at the interface without the formation of APBs within the films in NCO (10 nm)/CFO (40 nm)/MAO heterostructures. Our results provide evidence that the formation of microstructural defects can be regulated by changing film thickness to tune the magnetic properties of epitaxial bilayer spinel oxide films.

Investigation of conventional exchange bias in (Nd1-xHox)2CoMnO6 (x = 0.0–0.4) double perovskite

In this article, we have investigated the characteristics related to structure, magnetism and exchange bias behavior in single phased polycrystalline samples of (Nd1-xHox)2CoMnO6 (x = 0.0–0.4) (NHCMO) series. The samples were prepared using the conventional solid state reaction technique and they crystalize in monoclinic structure with P21/n space group. The x = 0.0 sample shows the ferromagnetic transitions at TC1 = 139K and TC2 = 170K. These values decrease to 136K and 160K respectively for x = 0.1 sample. However, for other Ho substituted samples we observed only one transition. For x = 0.2, TC is 129K which decreases to 118K for x = 0.4 sample. The ferromagnetic super exchange interaction that arises within the network of Co2+ and Mn4+ ions bridged by O2− ions is the reason of occurring of these transitions. The presence of anti-site defects with antiferromagnetic interactions and subsequent formation of anti-phase boundaries within the system cannot be ignored. To gain a deeper insight into the underlying magnetic interactions, the M − T plots, under both the ZFC and FC conditions, were measured by applying different external magnetic fields. Saturation magnetization (MS) value of 3.68 μB/f.u. is observed in parent sample. The MS value increases to 7.22 μB/f.u. for x = 0.4 sample. Such mixed magnetic ground state of the samples motivates us to perform the exchange bias field measurement revealing an exchange bias field of 725Oe is found for x = 0.0 sample at 5K.

Compositional effects on the antiphase boundary energies in B2-type NiTi and NiTi-based high-entropy intermetallics

High-entropy intermetallics (HEIs) have experimentally exhibited excellent room- and high-temperature strengths. Superior strengths of HEIs are mainly ascribed to the enhanced resistance of dislocation nucleation and slip caused by large antiphase boundary (APB) energies. In this work, first-principles calculations were performed to investigate the compositional effects on the APB energies in B2-type NiTi and NiTi-based HEIs. The site preferences of solutes in NiTi alloy are determined, and the effects of solutes on the APB energies (APBE) are predicted to correlate with their site preference. Solutes with definite site preference enhance the APB energies of NiTi alloy, while solutes possessing composition-dependent site preference reduce the APB energies. Furthermore, the correlations between APB energies, formation enthalpy, and valence electron concentration (VEC) in B2-type intermetallics are established. APB energies decrease with the increasing formation enthalpy in B2-type intermetallics. The VEC-APBE rule shows that B2-type intermetallics with a VEC range of 6–7 possess large APB energies. Our findings provide theoretical basis for composition design of HEIs to achieve superior strength-plasticity synergy.

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.

Diffusion controlled early-stage L12-D023 transitions within Al3Zr

Antiphase boundary (APB) in Al3Zr was regarded to play an important role in the L12-D023 phase transition, the formation of which has long been explained through the shearing/slipping mechanism. Herein, the detailed atomic structures of conservative {100} APBs within L12-Al3Zr particles forming at the early-stage of L12-D023 transition in an Al-Zr alloy were systematically studied by high angle annual dark-field scanning transmission electron microscope (HAADF-STEM). As a strong evidence of diffusion-limited phase transformation process, significant de-ordering of atoms in APBs and the neighboring atomic planes have been found. By analyzing the two possible pathways, it is suggested that L12-D023 phase transition has been achieved through a diffusion mechanism, instead of shearing mechanism while the formation of APB is an intermediate stage of the phase transition process. First principles density functional theory (DFT) reveals that the formation of APBs and subsequent phase transition are energetically favorable, which provide the driving force for diffusion.

Direct investigation of the atomic structure and decreased magnetism of antiphase boundaries in garnet

The ferrimagnetic insulator iron garnets, tailored artificially with specific compositions, have been widely utilized in magneto-optical (MO) devices. The adjustment on synthesis always induces structural variation, which is underestimated due to the limited knowledge of the local structures. Here, by analyzing the structure and magnetic properties, two different antiphase boundaries (APBs) with individual interfacial structure are investigated in substituted iron garnet film. We reveal that magnetic signals decrease in the regions close to APBs, which implies degraded MO performance. In particular, the segregation of oxygen deficiencies across the APBs directly leads to reduced magnetic elements, further decreases the magnetic moment of Fe and results in a higher absorption coefficient close to the APBs. Furthermore, the formation of APBs can be eliminated by optimizing the growth rate, thus contributing to the enhanced MO performance. These analyses at the atomic scale provide important guidance for optimizing MO functional materials.

Insight into the stacking fault energy, dislocation, and thermodynamic properties of L12-Al3X(X[dbnd]Sc, Ti, V) intermetallics from first-principles calculations

The dislocation and thermodynamic properties of L12-Al3X(XSc, Ti, V) intermetallics were studied based on first-principles calculations. The computed stacking fault energies indicate the energy barrier for dislocation formation in Al3Sc is higher than that of Al3Ti and Al3V. Also, the negative anti-phase boundary(APB) energy for Al3V suggests the formation of APB is energetically favored, and this finding is further evidenced by electron localization function(ELF) and densities of states(DOS). Moreover, the computed Peierls energies and Peierls stress suggest the movement of edge dislocation is easier than screw dislocation in Al3Sc, Al3Ti, and Al3V. Further, the < 110 > (111) screw dislocation of Al3Sc and the < 110 > (111) edge dislocation of Al3V is the most difficult and easiest to move in all the studied systems, respectively. It is consistent with the computed ideal shear stress(τiss) for Al3Sc, Al3Ti, and Al3V. Moreover, we speculated the Al3Sc and Al3Ti in aluminum alloy may possess a better strengthening effect in comparison with Al3V. Finally, the thermodynamic properties of Al3Ti was found less dependent on temperatureare than that of Al3Sc and Al3V, the Al3Ti can serve as a better candidate for improving the heat resistance of aluminum alloys.

On the formation of antiphase boundaries in Fe4Al13 intermetallics during a high temperature treatment

In this paper, we report atomic scale observations and formation mechanisms of a high-density of antiphase boundaries (APBs) within an ultra-fine-grained Fe4Al13 intermetallic layer at an Al/steel interface after a heat treatment at 596 °C. The results reveal that the APBs are formed by nucleation and the glide of partial dislocations with Burgers vector of b/3[010] (b = 12.47 Å). The intensive activation of APBs locally transforms the Fe4Al13 structure from the quasicrystal approximant structure to a quasicrystal. Very few stacking faults and nanotwins are observed indicating that the formation of planar defects is mainly driven by this transformation. This new insight on the formation of high density of APBs could possibly lead to an improvement in toughness by increasing the strength/ductility balance of this intermetallic.

Influence of alloying elements and the state of order on the formation of antiphase boundaries in B2 phases

Alloys with single phase B2 structure exhibit interesting physical properties, such as stability at high temperatures and oxidation resistance, but have low fracture toughness and limited ductility. The presence of antiphase boundaries (APBs) has been proposed as one of the factors improving the ductility of the B2 phases. An example of novel B2/A2 dual-phase alloy, which comprises such defects, is the AlCoCrFeNi multicomponent alloy (also known as high-entropy alloy (HEA)). Presence of APBs positively affected the performance of this alloy; therefore, control over the appearance of APBs is important. Current paper presents a first step in this direction, by studying the energy of APBs formation in binary B2 structures forming in the Al–Co–Cr–Fe–Ni system as a function of doping by ternary element from this system. Calculations, based on Density Functional Theory, took into account atom occupancy of the additives (i.e. in Al or in transition metal's sub-lattice) and position (at the APBs or at grains' interior). Following the presented here calculations, casting and characterization of the alloy, where APBs should be promoted, was made, proving the outcomes of this work. AlFe binary system was found to be the most probable (out of all possible binary prototypes) to form APBs. Furthermore, alloying by Cr (which substitutes Al) and Ni (which substitutes Fe) promote even more APBs formation. Taking into account anti-site defects, frequently forming in the AlFe B2 structure, the degree of disorder was related to the temperature – proposing an interesting aspect of promoting the formation of APBs.

Understanding the formation of antiphase boundaries in layered oxide cathode materials and their evolution upon electrochemical cycling

Understanding the formation of antiphase boundaries in layered oxide cathode materials and their evolution upon electrochemical cycling

Segregation-assisted phase transformation and anti-phase boundary formation during creep of a γ′-strengthened Co-based superalloy at high temperatures

In this study, the creep defects and segregation-assisted local phase transformation processes in a γ′-strengthened Co-based superalloy crept at 982 °C/248 MPa-1% and 1000 °C/137 MPa-1% were analyzed. A non-coplanar deformation configuration, i.e. a repetition of superlattice intrinsic stacking faults and antiphase boundaries (…SISF-APB…), was discovered in a Ni-free Co-based alloy, and found to be assisted by the local elemental segregation and favored more at higher stress conditions. A SISF with a/6<11¯2> type displacement in the (1¯11) plane and APB with a/2<110> type displacement nucleating in the (11¯1) plane were observed to be connected. The γ former segregation-assisted local γ′→γ phase transformation process near LPD was estimated to evolve in the order: γ′→metastable γ→γ+γ′1. The formation of metastable γ generates a new γ/γ′ interface with misfit strain/stress, which is responsible for the nucleation of dislocations, accompanied by the creep stress, thus generating APBs in the γ′ phases. The separation of γ and γ′1 seems to occur by a directional redistribution of γ and γ′ formers along the near-(1¯11) plane and toward the SISF side. The W segregation-assisted γ′→χ phase transformation along the planar fault is estimated to decrease the deformation resistance by forming SISFs with single-layer a/6<112> type displacement, while the Co segregation-assisted γ′→γ phase transformation at the LPD is believed to improve the deformation resistance by trapping the moving dislocation at the local γ/γ′ interface.

From surface data to bulk properties: a case study for antiphase boundaries in GaP on Si(001)

The orientation, atomic structure, and charge state of defects in crystals have a large influence on the functionality of resultant semiconductor devices. Two-dimensional defects are especially challenging to analyze, as they are embedded within the bulk. We developed a data analysis procedure using information from atomically resolved cross-sectional surface data from two non-parallel surfaces, enabling us to draw conclusions about two-dimensional defects within the bulk. Using antiphase boundaries (APBs) in GaP on Si as a model system, we show that this approach can be applied to any material and microscopy method. For the GaP/Si system and for two-side cross-sectional scanning tunneling microscopy as the experimental tool, the procedure shows that the commonly discussed and APBs do not arise in the investigated sample, whereas the data strongly indicates that previously not recognized APBs are present. Furthermore, a statistical analysis allows a calculation of the net excess charge carrier density introduced into the crystal by the APBs. Its value is negligible compared to the density of wrong bonds that characterize the APBs, since negative and positive excess charges compensate each other almost completely. Moreover, we suggest that controlled enhancement of the formation of APBs may also lead to applications in thermoelectric devices.

Understanding the Formation of Antiphase Boundaries in Layered Oxide Cathode Materials and Their Evolution Upon Electrochemical Cycling

Layered Li(Ni1-x-yCoxMny)O2 (NCM, with Ni ≥ 0.8) cathode materials are essential to achieve high energy densities in the next generation of lithium-ion batteries. This increased performance comes at the expense of stability. To extend the materials’ lifetime, it is necessary to understand the role that crystal defects play in the degradation during electrochemical cycling. In this study, NCM851005 (85% Ni) is investigated in the pristine state and after 100 and 200 cycles using scanning transmission electron microscopy (STEM), with the focus being on the defects in the material. The formation of antiphase boundaries (APB) from a dislocation in a pristine sample is proven. After 100 cycles, the APBs’ length and width are enlarged compared to the pristine state. After 200 cycles, APBs further evolve into an intragranular rock salt-like phase, distorting the nearby layered structure. It is suggested that the behavior of APBs plays a critical role in determining the performance of this cathode material with prolonged electrochemical cycling. These findings will help to understand better the role of dislocations and antiphase boundaries with electrochemical cycling, and the role of dopants may then be explored to avoid them.

On the Structural Perfection of Large-Diameter Silicon Carbide Ingots

4H-silicon carbide ingots with high structural perfection have been grown by the modified Lely method (LETI method) on 100-mm-diameter seeds. Using our experimental data and theoretical analysis, we have systematized the key factors responsible for the degradation of the crystal structure of the ingots during the growth process. The formation of parasitic polytype inclusions in the early stages of growth has been shown to lead to the formation of antiphase boundaries and a mosaic substructure in the ingot.

Effects of microstructure evolution on the deformation mechanisms and tensile properties of a new Ni-base superalloy during aging at 800 °C

Tensile properties at room temperature of a new casting Ni-base superalloy during aging at 800 °C for 0-1000 h were investigated. During aging, granular M23C6 carbides presented at grain boundaries and kept growing from dispersed particles to continuous networks. The γ′ phase significantly coarsened, with the morphology of some γ′ phase changed from spherical to rounded cubic shape after 1000 h. Three deformation mechanisms in relation to the γ′ diameter (dγ′) were identified: (i) weakly coupled dislocations (WCD) connected by anti-phase boundary (APB) traveled in pair across the γ/γ′ structure when dγ′ was small in the under-aged alloys; (ii) strongly coupled dislocations (SCD) with reduced spacing compared to (i) sheared γ′ phase when dγ′ increased in the over-aged alloys; (iii) dislocations occasionally by-passed γ′ phase when dγ′ was larger than 97 nm after aging for more than 300 h. The alloy obtained the peak strength when 20 h-aged with dγ′ = 44 nm which was in the transition between (i) and (ii). The aging-induced variation in yield strength was correlated to the coarsening of γʹ phase using a theoretical model of precipitation strengthening in terms of the formation of APB. The calculated results suggested that the γ′ phase with a volume fraction of 23% contributed more than 61% of the peak-aged yield strength. Observation after fracture revealed that the alloys usually fractured at grain boundaries. High stress concentration around carbides resulted in cracks by carbides self-cracking and the initiation of cavities. The undesirable agglomeration of M23C6 at grain boundaries was harmful to the properties of the over-aged alloys.

The role of antiphase domain boundary density on the surface roughness of GaSb epilayers grown on Si (001) substrates

Abstract GaSb epilayers were grown on nominal Si (001) substrates by molecular beam epitaxy, using AlSb nucleation layer (NL) as an interfacial misfit array. The effects of AlSb NL thickness, growth temperature and post annealing on the GaSb epilayers in terms of formation of antiphase boundaries (APBs) were monitored by scanning electron microscopy (SEM). APBs were identified via SEM following a defect sensitive etch solution treatment. SEM images of etched samples were used to calculate the APB densities and the results were correlated with the surface roughness of the samples. We found that the density of APBs together with the area peak-to-valley height has a role on the surface roughness of the GaSb epilayers grown on nominal Si (001) substrates.