Density Of Stacking Faults Research Articles

Exploring defect behavior in helium-irradiated single-crystal and nanocrystalline 3C-SiC at 800°C: A synergy of experimental and simulation techniques

In this study, single crystal (sc) and nanocrystalline (nc) 3C-SiC samples were subjected to 30 keV He ion irradiation across various doses while maintaining a temperature of 800 °C. Employing techniques including Raman spectroscopy, transmission electron microscopy (TEM), and nanoindentation, the alterations in microstructure and hardness resulting from He irradiation with various fluences were examined.In sc-SiC, irradiation prompted the formation of He platelets, resulting in a hardness increase of 7 GPa. In contrast, nc-SiC, characterized by a higher stacking fault density, exhibited the formation of bubbles, primarily at grain boundaries (GBs), with fewer occurrences within the grain interior, leading to a hardness increase of 1 GPa. Notably, in both sc- and nc-SiC, hardness reached saturation and subsequently stabilized or declined with increasing He fluence.Through molecular dynamics (MD) cascade simulations, we discerned that various planar defects do not uniformly contribute to enhancing radiation resistance. For example, intrinsic stacking faults (ISF) and twins in SiC played a substantial role in altering defect density and configurations, thereby facilitating point defect annihilation. Conversely, extrinsic stacking faults (ESF) and Σ3 GBs had a limited impact on defect production during a cascade.Furthermore, calculations of cluster diffusivity revealed an accelerated movement of He-vacancy towards GBs compared to bulk material and other planar defects. Moreover, the scarcity of point defects and constrained mobility of He atoms towards stacking faults in nc-SiC elucidated the marked tendency of He to form platelets in sc-SiC.Additionally, our findings established a correlation between the calculated indentation hardness and the geometry of He defects, consistent with experimental results from nanoindentation. These results significantly contribute to ongoing efforts to design SiC materials with heightened radiation tolerance.

(Invited) From Inside Out: How Synthesis-Structure Correlations Conspire to Determine Photoluminescence Traits and Operational Robustness in Non-Blinking Giant Quantum Dots

“Giant” or thick-shell core/shell quantum dots (gQDs) are an important class of solid-state quantum emitter. Without any encapsulation gQDs are characterized by strongly suppressed blinking, or even non-blinking behavior, and resistance to photobleaching at room temperature. In addition, non-radiative Auger processes are significantly reduced in this class of luminescent nanomaterial. Together, these qualities lead to novel functionality as photon sources for a range of ensemble and single-emitter applications, including down-conversion phosphors, direct excitation light-emitting diodes, single-biomolecule tracking, and single-photon generation. For many applications, operation at elevated temperature and under intense photon flux is also desired, and we have shown that gQDs outperform other emitters in this key metric of operational robustness. Interestingly, performance under these conditions is strongly dependent on the synthetic method employed for shell growth. Previously we revealed the mechanistic pathways responsible for thermally-assisted photodegradation, distinguishing effects of hot-carrier trapping and QD charging (ACS Nano 2018, 12, 5, 4206), and subsequently used solid-state spectroscopy to obtain kinetic and thermodynamic parameters of the photothermal degradation process (ACS Appl. Mater. Interfaces 2020, 12, 27, 30695). Here, I will discuss a comprehensive analysis of gQD structural properties from the inside of the nanocrystal to its surface and demonstrate correlations between specific structural features and both causal synthetic parameters and resulting performance metrics. Two synthetic methods—successive-ionic-layer-adsorption-and-reaction and high-temperature continuous-injection—will be compared. Investigated structural features will include interfacial alloying, stacking-fault density and surface-ligand identity, while correlated functionalities will address quantum yield, single-gQD photoluminescence under thermal/photo-stress, charging behavior and quantum-optical properties. I will show several unexpected conclusions, including the surprising finding that while interfacial alloying is the strongest indicator of gQD stability under stress, the parameter is not the determining factor for Auger suppression. Furthermore, quantum yield is strongly influenced by surface chemistry and can approach unity even in the case of a gQD comprising a thick shell with a high defect density with proper choice of excitation wavelength. Interestingly, we find a strong correlation between the existence of zinc-blende stacking faults and whether the gQD exhibits charged-state or purely excitonic emission, and the interplay between charged emission and excitonic emission can be precisely tuned by “mixing” shell-growth methods (Small Sci. 2023,3, 2300092).

Ab initio study of helium behavior near stacking faults in 3C-SiC

First-principles calculations are used to investigate the effects of stacking faults (SFs) on helium trapping and diffusion in cubic silicon carbon (3C-SiC). Both extrinsic and intrinsic SFs in 3C-SiC create a hexagonal stacking sequence. The hexagonal structure is found to be a strong sink of a helium interstitial. Compared to perfect 3C-SiC, the energy barriers for helium migration near the SFs increase significantly, leading to predominant helium diffusion between the SFs in two dimensions. This facilitates the migration of helium towards interface traps, as confirmed by previous experimental reports on the nanocrystalline 3C-SiC containing a high density of SFs. This study also reveals that the formation of helium interstitial clusters near the SFs is not energetically favored. The findings from this study enhance our comprehension of helium behavior in faulted 3C-SiC, offering valuable insights for the design of helium-tolerant SiC materials intended for reactor applications.

Cathodoluminescence studies of the optical properties of a zincblende InGaN/GaN single quantum well

Zincblende GaN has the potential to improve the efficiency of green- and amber-emitting nitride light emitting diodes due to the absence of internal polarisation fields. However, high densities of stacking faults are found in current zincblende GaN structures. This study presents a cathodoluminescence spectroscopy investigation into the low-temperature optical behaviour of a zincblende GaN/InGaN single quantum well structure. In panchromatic cathodoluminescence maps, stacking faults are observed as dark stripes, and are associated with non-radiative recombination centres. Furthermore, power dependent studies were performed to address whether the zincblende single quantum well exhibited a reduction in emission efficiency at higher carrier densities—the phenomenon known as efficiency droop. The single quantum well structure was observed to exhibit droop, and regions with high densities of stacking faults were seen to exacerbate this phenomenon. Overall, this study suggests that achieving efficient emission from zinc-blende GaN/InGaN quantum wells will require reduction in the stacking fault density.

Superior tensile properties at cryogenic temperature induced by dual-graded structures in medium entropy alloys

The dual-graded structure with both gradients of grain size and coherent L12 precipitation was applied to a (CoCrNi)96Al3Ti3 medium-entropy alloy for achieving superior tensile properties. Simultaneous improvement on yield strength and uniform elongation is observed in the dual-graded structure at cryogenic temperature, compared to those at room temperature. Stronger hetero-deformation-induced hardening and higher density of geometrically necessary dislocations are observed at cryogenic temperature in both single-grain-size-graded and dual-graded structures. The hardening mechanisms at room temperature are characterized by low density of deformation twins and stacking faults (SFs) on one slip system, while are dominated by high density SFs at two slip systems and formation of Lomer-Cottrell (L-C) locks at cryogenic temperature for the single-graded structure. The hardening mechanisms at both room and cryogenic temperatures are revealed by high density of SFs and L-C locks in the dual-grade structure. These SFs and L-C locks can provide strong hardening themselves as dynamic Hall-Petch effect on one hand, and can interact with coherent L12 precipitates for intense precipitation hardening on the other hand. The better tensile properties for the dual-graded structure at cryogenic temperature should be attributed to a special SF-induced plasticity, i.e., the formation of SF networks and their interactions with nanoprecipitates.

Effect of SiC particle content on the mechanical behavior and deformation mechanism of SiCp/Al composite under high-frequency dynamic loading

SiCp/Al composites exhibit excellent structural properties but suffer from inherent brittleness, which has brought rigorous challenges on the materials processing. Ultrasonic-vibration-assisted approach offers a promising solution to improve the processability. However, the underlying mechanisms of mechanical response and defect suppression under ultrasonic action have been rarely studied for SiCp/Al composites, hindering the development of high-efficiency processing. This work aims to investigate the mechanical behavior and microstructural evolution in this class of composites with varying SiC volume fractions up to 45% under high-frequency dynamic load induced by ultrasonic vibration. The results showed that the deformation stress of SiCp/Al composites under ultrasonic compression was significantly reduced to only about 1/10 of that under conventional quasi-static loading. Moreover, as the SiC fractions increasing, this ultrasonic softening effect of SiCp/Al became more pronounced. The plastic limit of SiCp/Al with 45% SiC fraction nearly doubled under ultrasonic compression without any macroscopic cracks. The subsequent nanoindentation tests revealed a transition in work-hardening behavior of SiCp/Al with different SiC fractions under ultrasonic loading. Microstructure characterization was conducted to analyze the grain refinement, recrystallization, and deformation textures within the deformed areas, revealing the deformation mechanism of SiCp/Al under different loading mode. TEM observations confirmed the distribution of dislocations in Al matrix and dense stacking faults in SiC particles after ultrasonic compression, indicating that the plastic behaviors were enhanced in both Al matrix and SiC phase under the high-frequency cyclic stress. This research revealed the distinct deformation mechanisms of SiCp/Al composites under high-frequency dynamic loading, exhibiting extremely low deformation stress accompanied with significantly enhanced plasticity. The results provide theoretical insights for the high-efficiency processing and rapid property regulation of particle-reinforced-composites based on ultrasonic-vibration-assisted technologies.

Influence of Interlayer Cation Ordering on Na Transport in P2-Type Na0.67-xLiy Ni0.33-zMn0.67+zO2 for Sodium-Ion Batteries.

P2-type Na2/3Ni1/3Mn2/3O2 (PNNMO) has been extensively studied because of its desirable electrochemical properties as a positive electrode for sodium-ion batteries. PNNMO exhibits intralayer transition-metal ordering of Ni and Mn and intralayer Na+/vacancy ordering. The Na+/vacancy ordering is often considered a major impediment to fast Na+ transport and can be affected by transition-metal ordering. We show by neutron/X-ray diffraction and density functional theory (DFT) calculations that Li doping (Na2/3Li0.05Ni1/3Mn2/3O2, LFN5) promotes ABC-type interplanar Ni/Mn ordering without disrupting the Na+/vacancy ordering and creates low-energy Li-Mn-coordinated diffusion pathways. A structure model is developed to quantitatively identify both the intralayer cation mixing and interlayer cationic stacking fault densities. Quasielastic neutron scattering reveals that the Na+ diffusivity in LFN5 is enhanced by an order of magnitude over PNNMO, increasing its capacity at a high current. Na2/3Ni1/4Mn3/4O2 (NM13) lacks Na+/vacancy ordering but has diffusivity comparable to that of LFN5. However, NM13 has the smallest capacity at a high current. The high site energy of Mn-Mn-coordinated Na compared to that of Ni-Mn and higher density of Mn-Mn-coordinated Na+ sites in NM13 disrupts the connectivity of low-energy Ni-Mn-coordinated diffusion pathways. These results suggest that the interlayer ordering can be tuned through the control of composition, which has an equal or greater impact on Na+ diffusion than the Na+/vacancy ordering.

The Effect of AlN Epilayer Growth Rate on the Growth of Semipolar (11–22) InGaN/GaN Light Emitting Diode

This article presents the effect of aluminium nitride (AlN) epilayer (EPL) growth rate on the growth of semipolar (11–22) InGaN/GaN light emitting diode (LED). Three samples are grown with different AlN EPL growth rates, which are low (L1, 1.0 nm s−1), intermediate (L2, 1.3 nm s−1), and high (L3, 1.6 nm s−1) growth rate. The root‐mean‐square (RMS) and peak‐to‐valley roughness show an increasing trend with the increasing AlN EPL growth rate which is attributed to an increment in indium composition as justified by the photoluminescence results. The sample with AlN EPL growth rate of 1.3 nm s−1 (L2) exhibits an outstanding crystal quality in terms of defect densities reduction, with a minimum basal stacking fault density of 1.60 × 104 cm−1 as calculated. Besides that, the obvious fringes of InGaN/GaN peaks observed in 2θ−ω scan proves that a high abruptness multi‐quantum well is attained. Sample L2 also shows a single peak profile in both photoluminescence and electroluminescence spectra with green wavelength emission of 506 and 537 nm, respectively, indicating a homogeneous indium composition. As the injection current is swept from 5 to 40 mA for sample L2, a weak blueshift of ≈6 nm is observed which indicates a reduction in quantum‐confined Stark effect (QCSE).

High‐Quality Nonpolar a‐Plane AlGaN Film Grown on Si‐Doped AlN Template by Metal Organic Chemical Vapor Deposition

High crystalline quality and flat a‐plane aluminum gallium nitride (AlGaN) films are obtained on Si‐doped AlN templates with a moderate silane (SiH4) flow rate by metal‐organic chemical vapor deposition (MOCVD). The effects of the SiH4 flow rate on the surface morphology, crystalline quality, stress state, and optical property of a‐plane AlN templates and AlGaN films are comprehensively investigated. As the SiH4 flow rate increases from 0 to 7.0 nmol min−1, the full width at half maximum of X‐ray rocking curve values along [0001]/[1–100] directions for the AlGaN films are monotonically reduced to 1124/1143 arcsec. Meanwhile, the surface root mean square roughness value is decreased to 0.88 nm. These achievements are attributed to the suppression of the anisotropy degrees and in‐plane stress of AlN templates. In addition, an excess SiH4 flow rate leads to deteriorated surface morphologies and increased basal plane stacking fault (BPSF) densities for both AlN and AlGaN films. This work suggests that doping AlN layers with an appropriate SiH4 flow rate is a promising route to obtain high‐quality a‐plane AlGaN films for efficient nonpolar deep ultraviolet (DUV) devices.

XRD-Analysis of the Relation of Stacking Fault Formation and the TRIP-Effect in ADI

The mechanical properties of ausferritic ductile iron are strongly influenced by the ability of the austenite to undergo martensite formation. Strain-induced martensite formation occurring under the right circumstances results in transformation induced plasticity (TRIP) that improves ductility and strength. TRIP has been shown to depend on the austenite’s stacking fault energy (SFE), which describes the crystals micromechanical behaviour. In austenitic FeMnAlSi-TRIP steels, TRIP as a reaction to mechanical load only occurs for SFE <20 mJ/m2. For carbon-stabilised austenite as in ADI, the relationship between stacking faults, mechanical properties and martensitic transformation has not yet been established. To investigate the TRIP-effect in ADI, unalloyed ADI with 3.43 wt% C, 2.52 wt% Si and 0.21 wt% Mn was ausferritised and subjected to tensile tests at temperatures between −180 °C and 200 °C. The amount of martensite produced by thermal and mechanical activation, crystalline microstrain and stacking fault density were measured on deformed and undeformed regions of the specimen by XRD and the resulting SFE calculated. Between −70 and 20 °C, the elongation at fracture exceeded 10 % compared to below 2 % in the temperature range of −130 to −180 °C. At temperatures above 80 °C, elongation at fracture gradually decreased to 4.5 % at 200 °C. High sample ductility was associated with lower SFEs as low as 35 mJ/m2, indicating a correlation of stacking fault formation and strain-induced martensite formation in ADI.

Phase-dependent microstructure modification leads to high thermoelectric performance in n-type layered SnSe2

Microstructure control is crucial for thermoelectrics since it is intimately related to the scattering mechanism of both electrons and phonons. Herein, we propose a new strategy to modify microstructure by phase regulation that simultaneously induces high carrier mobility and low lattice thermal conductivity. As demonstrated in layered SnSe2, the addition of Cu can induce a phase transition from space group P3¯m1 to P63mc. Due to the enlarged formation energy of stacking faults in the later phase, the stacking fault density is greatly reduced after heat treatment that leads to an increased grain size. Accordingly, the carrier mobility of SnSe1.97Br0.03–3 % Cu sample is enhanced by 100 % at room temperature. Furthermore, the reduction of stacking fault density is accompanied by the formation of pores in the matrix, which results in low lattice thermal conductivity. As a result, a record peak zT of 1.13 for SnSe2-based materials is achieved at 773 K, and the attained ZTave of 0.62 is a record-high value among n-type polycrystalline layered materials working in intermediate-to-high temperature region. This work provides a feasible strategy to decouple the electron and phonon transport in layered thermoelectric compounds by phase-dependent microstructure modification.

Extra-conventional strengthening mechanisms in non-recrystallized grains of an extruded Mg-Gd-Zr alloy

Mg-RE (RE = rare earth) based alloys generally exhibit outstanding mechanical properties. However, their high-strength seems to be unexplained using classic strengthening mechanisms in some cases. Herein, a Mg-13Gd-0.4Zr (wt%) alloy that was fabricated by a conventional differential thermal extrusion plus artificial aging treatment exhibits ultra-high yield strength over 510 MPa in both tension and compression. Characterizations using Cs-corrected scanning transmission electron microscopy (STEM) show two unusual microstructures in non-recrystallized grains as: a large density of basal stacking faults (SFs) and profuse distortion areas (DAs). Atomic-resolution STEM imaging indicates that basal SFs are consisted of two types of intrinsic SFs, namely I1 and I2, and DAs are self-assembled by 〈c〉 and 〈c + a〉 screw partials. Their strengthening mechanisms are analogous to grain boundary strengthening and dispersion strengthening, respectively, contributing satisfactory yield-strength increments of ∼46 MPa and ∼76 MPa, respectively. Moreover, DAs improved aging hardening by inducing novel clusters at DA-related boundaries, or increasing the number density of βH’ precipitate and promoting their distribution along a certain direction. This work supplements the strengthening mechanisms in traditional high-strength Mg-RE(-Zr) based alloys and provides novel insights in the development of ultra-high-strength Mg alloys.

Low cycle fatigue behavior and microstructure evolution of a fourth-generation single crystal superalloy at 800 °C

The advanced fourth-generation single crystal superalloys have great service performance at high temperature, including outstanding fatigue, creep and oxidation-resistance. Low cycle fatigue (LCF) behaviors and microstructure evolution of a fourth-generation single crystal superalloy under different strain amplitudes at 800 °C were investigated in this study. As the strain amplitude increased, the cyclic hardening level of the experimental superalloy increased. The crack initiation could be attributed to the exfoliation of the oxidation layers. Furthermore, within strain amplitude range of 0.65% to 1.0%, slipping bands sheared both γ and γ' phases. The process of super-dislocations shearing into the γ' precipitates contributed to cyclic softening and resulted in the severe plastic deformation, thereby reducing the LCF life. Dense stacking faults (SFs) with “straight feature” and “zigzag feature” in the γ channels were observed under strain amplitude of 0.9% and 1.0%. The continuous Lomer-Cottrell lock (L-C lock) is the strong obstacle of dislocation movement, which resulted in cyclic hardening. This investigation and the resulting experimental data serve as a valuable reference for the engineering application of the fourth-generation single crystal superalloy. The findings can provide insights into the behavior and properties of the alloy, aiding in its practical use and development in various engineering applications.

An atomistic study on the strain rate and temperature dependences of the plastic deformation Cu–Au core–shell nanowires: On the role of dislocations

Abstract Recently, Cu–Au core–shell nanowires have been extensively used as conductors, nanocatalysts, and aerospace instruments due to their excellent thermal and electrical conductivity. In experimental studies, various methods have been presented for producing, characterizing, and strengthening these structures. However, the mechanical behavior and plastic deformation mechanisms of these materials have not been investigated at the atomic scale. Consequently, in the present study, we carried out uniaxial tensile tests on Cu–Au nanowires at various tension rates and temperatures by means of the molecular dynamics approach. The Cu–Au interface was found to be the main site for nucleation of perfect dislocations, Shockley partials, and stacking faults due to the stress concentration and high potential energy arising from the atomic mismatch between shell and core layers. It was observed that an increase in the strain rate from 108 to 1,011 s−1 shortened the time required for the nucleation of dislocations, decreasing the dislocation density. This emphasizes that dislocation nucleation and slip mechanisms are time-dependent. Moreover, it was found that the interaction of Shockley partials can lead to the creation of lock dislocations, such as Hirth, Frank, and Stair-rod dislocations, imposing obstacles for the slip of other dislocations. However, as the tension temperature rose from 300 to 600 K, opposite-sign dislocations removed each other due to thermally activated mechanisms such as dislocation climb and dislocation recovery. Furthermore, the combination of Shockley partial dislocations decreased the stacking fault density, facilitating the plastic deformation of these structures. The yield strength and elastic modulus of the samples increased with the strain rate and substantially decreased as the temperature rose.

Effect of cooling rate on microstructure and properties of SiCP/A359 composites

The microstructure and properties of stirred cast SiCP/A359 composites under different cooling rates were studied. The strengthening mechanism of the composite material under three cooling rates was analyzed, and the failure mechanism of the composite material was elucidated by means of in-situ tension and EBSD testing. The results showed that increasing the cooling rate simultaneously refined the grains, secondary dendrites, and eutectic silicon, resulting in improved distribution uniformity of SiC and the stacking fault density of eutectic silicon. Additionally, the relationship between the secondary dendrite arm spacing (SDAS) and cooling rate (v) of the SiCP/A359 composite was described using the equation SDAS = 56.02v^(-0.3). There was a positive correlation between cooling rate and yield strength, with particle strengthening being the main contributor to the strength of composites with high cooling rates, resulting in significantly higher strength than samples with low cooling rates. The geometrically necessary dislocations (GND) density at the eutectic Si-Al boundary was found to be higher than other positions after the material was loaded, and the main crack propagated mainly along the eutectic region. Secondary cracks, including SiC cracking, eutectic silicon cracking, interface separation, and shrinkage cracking, may become part of the main crack by bridging.

Preparation and Electromagnetic Wave Absorption Properties of N-Doped SiC Nanowires.

Enhancing the conductivity loss of SiC nanowires through doping is beneficial for improving their electromagnetic wave absorption performance. In this work, N-doped SiC nanowires were synthesized using three different methods. The results indicate that a large amount of Si2ON will be generated during the microwave synthesis of SiC nanowires in a nitrogen atmosphere. In addition, the secondary heat-treatment of the as-synthesized SiC nanowires under nitrogen atmosphere will significantly reduce their stacking fault density. When ammonium chloride is introduced as a doped nitrogen source in the reaction raw material, the N-doped SiC nanowires with high-density stacking faults can be synthesized by microwave heating. Therefore, the polarization loss induced by faults and the conductivity loss caused by doping will synergistically enhance the dielectric and EMW absorption properties of SiC nanowires in the range of 2-18 GHz. When the filling ratio of N-doped SiC nanowires is 20 wt.%, the composite shows a minimum reflection loss of -22.2 dB@17.92 GHz, and an effective absorption (RL ≤ -10 dB) bandwidth of 4.24 GHz at the absorber layer thickness of 2.2 mm. Further, the N-doped SiC nanowires also exhibit enhanced high-temperature EMW absorption properties with increasing temperature.

Structure, microstructure and disorder in low temperature chemical vapor deposited SiC coatings

Chemical vapor deposited SiC coatings were investigated at different scales by X-Ray diffraction, Raman microspectroscopy and transmission electron microscopy. They were prepared under specific conditions explaining the various (micro)structures obtained. The deposits all have a columnar morphology with a preferential orientation and a faulted cubic structure. They differ in how disorder is incorporated in the structure. Fine XRD analyses and stacking fault density assessment by TEM revealed the one-dimensionally-disordered (ODD) polytype in the <111> textured coatings. The frequency and spatial distribution of stacking faults vary and sometimes locally generate periodic alpha sequences. A specific type of disorder was also identified where {111} planes are arranged parallel to the growth direction within the columns. These disorders, more energetic than stacking faults, induce multiple and particularly large Raman modes. Crystal distortions, such as dislocations, are localized at the ODD domain boundaries, which are frequently interrupted as they extend during the growth.

Engineering Dense Stacking Faults in Silver Nanoparticles for Boosting the Oxygen Reduction Reaction

Engineering Dense Stacking Faults in Silver Nanoparticles for Boosting the Oxygen Reduction Reaction

Effects of Buffer Layer on Structural Properties of Nonpolar (112¯0)-Plane GaN Film

Nonpolar (112¯0) a-plane GaN films were grown on semipolar (11¯02) r-plane sapphire substrates using various buffer layers within a low-pressure metal organic chemical vapor deposition system. The structural properties of nonpolar a-plane GaN films were intensively investigated by X-ray diffraction and Raman spectra measurements. A set of buffer layers were adopted from a GaN layer to a composite layer containing a multiple AlN layers and a gradually varied-Al-content AlGaN layer, the full width at half maximum of the X-ray rocking curves measured along the [0001] and [101¯0] directions of a-plane GaN were reduced by 35% and 37%, respectively. It was also found that the basal-plane stacking faults (BSFs) density can be effectively reduced by the heterogeneous interface introduced together with the composite buffer layer. An order of magnitude reduction in BSFs density, as low as 2.95 × 104 cm−1, and a pit-free surface morphology were achieved for the a-plane GaN film grown with the composite buffer layer, which is promising for the development of nonpolar GaN-based devices in the future.

Structures and formation mechanism of borides with varied morphologies in cast γ-TiAl alloys

Borides with varied morphologies and structures obtained by changing B content in as-cast Ti-45Al-2Mn-2Nb alloys are investigated. With the increase of B content, morphologies of borides change from curved to straight, and flake shape, ribbon shape, needle/rod shape, and blocky appear in turn. Curved boride particles have a lamellar structure composed of TiB2 and B2 layers, accompanied by the random appearance and distribution of Ti3B4 and Bf-TiB layers. Except for the absence of the B2 layer, straight needle-like and blocky boride particles have roughly the same structure as curved boride particles, revealing that the formation of curved boride particles is accompanied by divorced eutectic with the matrix and internal irregular eutectic, while no eutectics occurs in straight boride particles. The lack of B atoms during solidification in alloys with lower B content results in a less regular structure and higher stacking fault density of curved boride particles compared to straight boride particles in alloys with higher B content. In addition, some needle-like boride particles are observed to have pure B27-TiB structure without intergrowth. The difference in morphologies and structures can be attributed to the difference in formation mechanism and crystallographic structures of the borides.