Formation Of Stacking Faults Research Articles

Selective Laser Melted High Mg‐Content Al–Si–Mg–Zr Aluminum Alloy with the Formation of Stacking Faults

The currently applied selective laser melted (SLMed) Al–Si–Mg aluminum alloys generally exhibit low strength. Increasing the Mg content is an effective method to enhance the strength of those alloys. However, an excessive Mg content reduces their processability and plasticity. Herein, Zr is chosen as the alloying element to enhance the processability and plasticity of the SLMed high Mg‐content Al–Si–Mg alloy. The results demonstrate that the alloying of the Zr element is beneficial for improving the alloy's processability and mechanical properties through grain refinement. Besides, the high‐concentration solid solution of low‐stacking fault energy (SFE) elements, including Mn, Zr, and Mg, effectively reduces the SFE of the alloy, thereby promoting the formation of stacking faults. The yield strength, ultimate tensile strength, and elongation of the alloy are 388 ± 3, 503 ± 6 MPa, and 5.1 ± 1%, respectively. The strength of the present alloy is higher than that of previously reported SLMed Al–Si–Mg aluminum alloys.

Ultrafast lattice engineering for high energy density and high-rate sodium-ion layered oxide cathodes

Sodium-ion batteries attract significant interest for large-scale energy storage owing to abundant sodium reserves, while challenges remain in the high synthesis energy consumption, long synthesis period, and poor electrochemical performance of sodium-ion layered oxide materials. This study presents a general high-temperature thermal shock (HTS) strategy to synthesize and optimize sodium-ion layered oxides. The rapid ramping, sintering, and cooling processes minimize volatile sodium loss during HTS, facilitating the improvement of phase purity and effectively optimizing the microstructure of materials in a non-equilibrium state. As a proof of concept, Mn-based Na0.67MnO2 treated with HTS (NMOHTS) suppresses Mn ion vacancy within transition material layers, thereby increasing the redox centers and lowering the Mn 3d orbital energy level. Besides, the formation of transition metal layer stacking faults mitigates the structural transformation and Na+-vacancies ordering arrangement during cycling. Consequently, the energy density of the NMOHTS increases by 21.5 % to 559 Wh kg-1, with an outstanding rate capability of 108 mAh g-1 at 10C and an impressive capacity retention of 93.7 % after 300 cycles at 1C. In addition, we demonstrate the universality of HTS in synthesizing various other sodium-ion layered oxides, including nickel-based and iron-based cathodes, as well as in activating degraded materials.

Role of pressure-induced stacking faults on the magnetic properties of gadolinium

Experimental data show that under pressure, Gd goes through a series of structural transitions hcp→Sm-type (close-packed rhombohedral) →dhcp that is accompanied by a gradual decrease of the Curie temperature and magnetization till the collapse of a finite magnetization close to the dhcp structure. We explore theoretically the pressure-induced changes of the magnetic properties, by describing these structural transitions as the formation of fcc stacking faults. Using this approach, we are able to describe correctly the variation of the Curie temperature with pressure, in contrast to a static structural model using the hcp structure. Published by the American Physical Society 2024

Influence of grain boundary segregation on the deformation behavior of nanocrystalline Ni50Co50 solid solution alloys investigated by molecular dynamics simulations

Molecular dynamics simulations were carried out to investigate the tensile deformation behavior of Ni50Co50 solid solution nanocrystalline alloys with varying degrees of grain boundary segregation. A comprehensive analysis disclosed that the observed strengthening in the Ni-rich grain boundary model is primarily due to the formation of Lomer-Cottrell locks and dislocation networks which impede dislocation motion. Plastic deformation, on the other hand, is predominantly governed by enhanced grain boundary migration, grain rotation, and deformation twinning. In the Co-rich grain boundary model, the interactions among twins, dislocations, and grain boundaries, stemming from the formation of extensive stacking faults and twinning, dictate the plastic deformation and strengthening. Fluctuations in grain boundary composition promote stress concentration, thereby facilitating plastic deformation and dislocation accumulation within the grain, which in turn enhances strain hardening. The study reveals that the Ni50Co50 solid solution nanocrystalline alloys exhibit both high tensile strength and favorable plasticity for models with grain boundary segregation around ±10%. These insights provide valuable guidance for the development of Ni50Co50 nanocrystalline alloys with optimized strength and toughness.

Segregation-assisted yield anomaly in a Co-rich chemically complex intermetallic alloy at high temperatures

Chemically complex intermetallic alloys (CCIMAs) have emerged as promising materials for achieving exceptional softening resistance at elevated temperatures, owing to their unique superlattice structures and elemental synergism. However, the lack of experimental endeavors and understanding regarding their temperature-dependent mechanical behaviors hinders their advancement for high-temperature applications. Here, we conducted a systematic investigation on the mechanical properties and associated deformation mechanisms of an L12-type Co-rich CCIMA using transmission electron microscopy. The Co-rich CCIMA exhibits superior strength across a wide temperature range of 700-900°C, with an anomalous peak yield strength (YS) of ∼1.0 GPa at 700°C, which surpasses most previously reported L12-type intermetallic alloys. This superior softening resistance can be primarily attributed to a combination of cross-slip hardening and segregation-assisted hardening. Specifically, massive Kear-Wilsdorf (K-W) locks formed by multiple cube cross-slips of screw dislocations prevail at 700-900°C, providing strong barriers for dislocation movements and enhancing the yield strength (YS) accordingly. More interestingly, we revealed a segregation-assisted formation of superlattice stacking faults and nanotwins at the peak yield temperature (700°C). The interlocking of these substructures and the associated element dragging effect further impeded the propagation of dislocations, contributing to the observed yield anomaly. This work provides in-depth insight into the high-temperature performance and deformation behaviors of CCIMAs, which paves the way for the future development of novel heat-resistant structural alloys.

Generating stacking faults in 4H–SiC junction transistor by indentation and forward biasing

Stacking faults in silicon carbide have been widely studied due to their negative impact on the application of silicon carbide in the power electronics industry. In this work, with the assistance of forward biasing, we observe several triangular shaped structures emerging near the indenter imprint in two separate 4H–SiC bipolar junction transistor samples that were deformed by nanoindentation. Based on the study of electroluminescence spectra on one of the samples, the emission peak at 420 nm indicates the formation of single Shockley stacking faults inside deformed transistors. We conclude that the use of indentation can provide a method to study recombination induced stacking faults in silicon carbide junction devices by intentionally introducing dislocations at selected areas of interest.

Effect of carbon content on the microstructure and stress rupture properties of a 4th-generation nickel-based single crystal superalloy

The balance between the stress rupture properties and castability of a 4th-generation Ni-based single crystal superalloy can be achieved by addition of appropriate carbon content. In this work, the effects of carbon content on the microstructure and stress rupture properties of a 4th-generation Ni-based single crystal superalloy were investigated in detail. The results showed that minor carbon addition could aggravate the dendrite segregation, while the addition of carbon had no obvious effect on the rafting and distortion of γ′ phase during stress rupture deformation. At 760 °C/810 MPa and 1100 °C/210 MPa, the stress rupture lives of the three experimental alloys initially increased and subsequently decreased with the increasing carbon content. However, under condition of 1140 °C/137 MPa, the increased carbon content resulted in the reduction of the stress rupture lives of alloy. It was discovered that minor carbon elements addition (0.045 wt%) can promote formation of interfacial dislocation networks and improve creep resistance at high temperatures. At intermediate temperature, the introduction of carbon could reduce the stacking fault (SF) energy of γ matrix, thus facilitating the formation of SFs in γ matrix. The M6Cgranular carbides precipitated at high temperature, which was considered to be beneficial to stress rupture properties.

Microstructure Evolution of the Interface in SiCf/TiC-Ti3SiC2 Composite under Sequential Xe-He-H Ion Irradiation and Annealing Process.

A new type of SiCf/TiC-Ti3SiC2 composite was prepared by the Spark Plasma Sintering (SPS) method in this work. The phase transformation and interface cracking of this composite under ion irradiation (single Xe, Xe + He, and Xe + He + H ions) and subsequent annealing were analyzed using transmission electron microscopy (TEM), mainly focusing on the interface regions. Xe ion irradiation resulted in the formation of high-density stacking faults in the TiC coatings and the complete amorphization of SiC fibers. The implanted H ions exacerbated interface coarsening. After annealing at 900 °C for 2 h, the interface in the Xe + He + H ion-irradiated samples was seriously damaged, resulting in the formation of large bubbles and cracks. This damage occurred because the H atoms reduced the surface free energy, thereby promoting the nucleation and growth of bubbles. Due to the absorption effect of the SiCf/TiC interface on defects, the SiC fiber areas near the interface recovered back to the initial nano-polycrystalline structure after annealing.

Hot oscillatory pressing of SiC ceramics with B4C and C as sintering additives

SiC ceramics with 0.5 wt% B4C and 0.5 wt% C as sintering additives were prepared by hot oscillatory pressure (HOP) and hot pressure (HP). The mechanical properties and microstructures of the resultant ceramics were investigated. The results showed that the oscillatory pressure could effectively suppress the abnormal growth of SiC grains and promote the formation of high-density dislocation and stacking faults. The hardness of SiC ceramics made by HOP at 2000oC reached 28.9 GPa, which was much higher than that (25.9 GPa) of the sample made by HP at the same temperature. The current study suggests that HOP is a promising technique for preparing high-performance SiC ceramics.

Near-α titanium alloy dwell load-induced deformation twinning to coordinate the deformation mechanism associated with crack initiation

In this study, we identified a specific phenomenon of coordinated deformation of twins in near-α titanium alloys during dwell fatigue (DF). The main crack source regions and internal cracks in the micro-texture region (MTR) and no-MTR samples with inconsistent orientation characteristics were characterized. The results demonstrate that the main cracks in all specimens are aligned with the (0001) basal plane, which is associated with basal slip. Notably, twins are found and confirmed to be involved in the DF crack initiation process in high Al content near-α Ti60 alloys where deformation twins are rare, and tend to nucleate at the DF basal cracks. Further in-situ dwell investigation reveals and proposes that the dislocation pile-up and prismatic-basal (PB) interfacial features between soft/hard grains lead to deformation twin nucleation and growth from hard grain boundaries. Concurrently, the pyramidal 〈c+a〉 dislocation slip is observed to be in concurrent operation with the induced twins. These findings suggest that deformation twins not only coordinate basal grain deformation but also hinder the initiation and propagation of basal cracks caused by basal 〈a〉 slip. Moreover, a high density of pyramidal 〈c+a〉 dislocations within the twin and their dissociation to form basal stacking faults (SFTs) with a specific nanometric spacing make the twinning process accompanied by significant lattice distortions inside the twin. These newly formed nanotwin boundaries and SFTs act as barriers to dislocation motion, enhancing the strength and DF lifetime of near-α Ti60 alloys and effectively reducing the dwell sensitivity of no-MTR alloys. Our findings extend the understanding of the coordinated roles of dislocation slip, twin nucleation and formation of basal SFTs in near-α titanium alloys in dwell fatigue.

Non-Destructive Quantification of In-Plane Depth Distribution of Sub-Surface Damage on 4H-SiC Wafers Using Laser Light Scattering

The development of non-destructive quantitative evaluation techniques for the in-plane depth distribution of sub-surface damage (SSD) layer induced by mechanical processing of chemical mechanical polishing (CMP) finished SiC wafers is essential to reduce the occurrence of crystal defects during epitaxial growth. Until now, no wafer inspection method has been able to nondestructively and quantitatively assess the in-plane depth distribution of the SSD. This study investigates the correlation between the scattered light intensity measured nondestructively by the Laser light scattering (LLS) method and the SSD depth estimated by destructive inspection using the Dynamic AGE-ing® method, a sublimation-controlled etching and growth process, to develop a novel non-destructive SSD inspection method. As a result, it was found that there is an exponential relationship between the scattered light intensity by the LLS method on the bare wafer surface and the depth of the SSD layer that contributes to the formation of in-grown stacking faults (IGSF) during subsequent epitaxial growth. The results show that SiC wafer inspection using the novel LLS method, which introduces this relational equation, enables non-destructive and quantitative evaluation of SSD depth and in-plane distribution.

Full-operating-temperature tensile mechanisms of [111] oriented single-crystal superalloy: New intermediate temperature toughening behavior against ductility losing

Single crystal (SX) superalloys are required for full-operating-temperature mechanical properties. However, the quest for intermediate temperature (IT) resistance often encounters a perplexing phenomenon: anomalous yielding behavior coupled with an unexpected loss of ductility. This study delved into the tensile behavior of a [111]-oriented SX superalloy from room temperature (RT) to 1150 °C, uncovering temperature-dependent tensile mechanisms where the interplay among phases and deferent defects governs plastic deformation. Desirable high strength-ductility properties were observed at IT, showcasing comparable strength with increased ductility. Microstructural evidences show that the primary strengthening effects stem from coupled interface boundary strengthening and anti-phase boundary (APB) strengthening, while the plasticity arises from planer defects transitioning from the stacking fault (SF) within γ phase at small strains, to superlattice SFs, ultimately to the erasure of superlattice SFs, leaving cutting dislocation pairs in γʹ phase. Energy analysis of APB and SF, along with adherence to Schmid laws, reinforce the plausibility of such intricate defect interactions. The strength-ductility balance can be ascribed to the collective effect of preferentially generated dislocations and prompt formation of SF. This strategy of sequential defects’ competition provides a new route for solving the strength-ductility trade-off of alloys.

Effect of stacking fault energy (SFE) of single crystal, equiatomic CrCoNi and Cantor alloy on creep resistance

Compared to mechanisms like solid solution strengthening, the stacking fault energy (SFE) should be considered as a further factor that influences the material properties. The effect of SFE of alloys or individual elements on strength and resistance can vary considerably. In the high-temperature regime above 700 °C, there are still significant gaps in the knowledge about the effect of the SFE on the mechanical properties of single-phase alloys. The effect of SFE on creep resistance of two face-entered cubic equiatomic medium and high entropy alloys, CrCoNi and CrMnFeCoNi, respectively, is evaluated to fill parts of these gaps. Using the Bridgman solidification process, the alloys were produced as single crystals and crept under vacuum at 700 °C up to 1100 °C. This work shows a significant impact of the lower SFE of CrCoNi on the creep behavior compared to the results of previous investigations of CrMnFeCoNi. The creep resistance of the former is higher over the complete temperature range. At very high temperatures, the strengthening effect of the stacking faults is significantly present. The formation of tetragonal stacking faults and extended dislocation nodes can be identified as the reason for this effect.

Improving Epitaxial Growth of γ‐Al2O3 Films via Sc2O3/Y2O3 Oxide Buffers

Heteroepitaxial growth of γ‐Al2O3 on Sc2O3/Y2O3/Si (111) is achieved with oxygen plasma‐assisted molecular beam epitaxy in order to prevent polycrystalline grain boundary formation caused by lattice mismatch. Substrate temperature as well as oxygen flow are adjusted to optimize epitaxial growth conditions around 715–760 °C and 1.9 sccm, respectively. Epitaxial growth is monitored in situ by reflection high‐energy diffraction, while surface morphology is studied by scanning electron microscopy ex‐situ. X‐ray diffraction indicates epitaxial out‐of‐plane 111 orientation with oxygen flow above 0.6 sccm. However, transmission electron microscopy shows stacking fault formation for high oxygen flows. Finally, nanobeam electron diffraction confirms Smrčok model of a spinel‐like γ‐Al2O3 crystal structure.

Tensile Deformation Behaviors and Microstructure Evolution Under Various Temperatures for MAR‐M247 Nickel‐Based Superalloy

The study aims to analyze the tensile behavior and microstructural changes in MAR‐M247 nickel‐based superalloy across different temperatures. Tensile behavior is examined under a constant strain rate of 2.5 × 10−4 s−1 at varying temperatures. Results indicate a temperature‐dependent nature of the alloy's tensile properties. At 550 °C, a Portevin–Le Chatelier effect is observed, attributed to twinning nucleation and carbon atom diffusion. Dynamic recrystallization occurs at 950 °C, manifesting in a sinusoidal stress–strain curve. At room temperature, the primary fracture mechanism involves dislocation shearing in the γ matrix, with minor dislocation shearing in the γ′ phase. At 550 °C, carbides along grain boundaries and the γ′ phase impede dislocation motion. Concurrently, dislocations shear the γ′ phase, leading to the formation of superlattice intrinsic stacking faults and Kear–Wilsdorf locks, thereby enhancing tensile strength. However, at 950 °C, dislocation motion primarily involves climb, carbides decompose, the γ′ phase softens and enlarges, resulting in a notable decrease in tensile strength.

Precipitation strengthening and thermal stability in a conventionally non-heat treatable AA3xxx aluminium alloy

We report an unexpected yet significant precipitation-strengthening effect with remarkable thermal stability in an otherwise conventionally non-heat treatable AA3104 aluminium alloy. We reveal that a minor addition of Cu plays a critical role in the alloy's strength and thermal stability, and explain the underpinning mechanisms. The primary strengthening β′ phase can be engineered to exhibit distinctive structural characteristics, including regularly-spaced Cu atom columns at the coherent β′/Al interfaces, stacking faults and resultant domains. Our atomistic simulations rationalise that these microscopic characteristics are directly promoted by the Cu addition. The Cu-decorated interfaces promote β′ phase nucleation and result in strong, directional covalent-like Cu-Si bonding at the β′/Al interfaces. Specifically, the interfacial Cu columns increase the work of separation of the habit-plane interfaces and reduce the interface energy, resulting in significant strengthening and stabilising effects. The Cu columns also facilitate the stacking fault formation in the β′ phase. Collectively, these features enhance the strengthening effect and thermal stability of the precipitates. This study demonstrates the potential for exploring "new" features in "old" alloys to advance alloy science and technology.

A machine learning interatomic potential for high entropy alloys

High entropy alloys (HEAs) possess a vast compositional space, providing exciting prospects for tailoring material properties yet also presenting challenges in their rational design. Efficiently achieving a well-designed HEA often necessitates the aid of atomistic simulations, which rely on the availability of high-quality interatomic potentials. However, such potentials for most HEA systems are missing due to the complex interatomic interaction. To fundamentally resolve the challenge of the rational design of HEAs, we propose a strategy to build a machine learning (ML) interatomic potential for HEAs and demonstrate this strategy using CrFeCoNiPd as a model material. The fully trained ML model can achieve remarkable prediction precision (>0.92 R2) for atomic forces, comparable to the ab initio molecular dynamics (AIMD) simulations. To further validate the accuracy of the ML model, we implement the ML potential for CrFeCoNiPd in parallel molecular dynamics (MD) code. The MD simulations can predict the lattice constant (1 % error) and stacking fault energy (10 % error) of CrFeCoNiPd HEAs with high accuracy compared to experimental results. Through systematic MD simulations, for the first time, we reveal the atomic-scale deformation mechanisms associated with the stacking fault formation and dislocation cross-slips in CrFeCoNiPd HEAs under uniaxial compression, which are consistent with experimental observations. This study can help elucidate the underlying deformation mechanisms that govern the exceptional performance of CrFeCoNiPd HEAs. The strategy to establish ML interatomic potentials could accelerate the rational design of new HEAs with desirable properties.

Dual heterogeneous structure enabled ultrahigh strength and ductility across a broad temperature range in CrCoNi-based medium-entropy alloy

Developing alloys with exceptional strength-ductility combinations across a broad temperature range is crucial for advanced structural applications. The emerging face-centered cubic medium-entropy alloys (MEAs) demonstrate outstanding mechanical properties at both ambient and cryogenic temperatures. They are anticipated to extend their applicability to elevated temperatures, owing to their inherent advantages in leveraging multiple strengthening and deformation mechanisms. Here, a dual heterostructure, comprising of heterogeneous grain structure with heterogeneous distribution of the micro-scale Nb-rich Laves phases, is introduced in a CrCoNi-based MEA through thermo-mechanical processing. Additionally, a high-density nano-coherent γ' phase is introduced within the grains through isothermal aging treatments. The superior thermal stability of the heterogeneously distributed precipitates enables the dual heterostructure to persist at temperatures up to 1073 K, allowing the MEA to maintain excellent mechanical properties across a wide temperature range. The yield strength of the dual-heterogeneous-structured MEA reaches up to 1.2 GPa, 1.1 GPa, 0.8 GPa, and 0.6 GPa, coupled with total elongation values of 28.6 %, 28.4 %, 12.6 %, and 6.1 % at 93 K, 298 K, 873 K, and 1073 K, respectively. The high yield strength primarily stems from precipitation strengthening and hetero-deformation-induced strengthening. The high flow stress and low stacking fault energy of the dual-heterogeneous-structured MEA promote the formation of high-density stacking faults and nanotwins during deformation from 93 K to 1073 K, and their density increase with decreasing deformation temperature. This greatly contributes to the enhanced strain-hardening capability and ductility across a wide temperature range. This study offers a practical solution for designing dual-heterogeneous-structured MEAs with both high yield strength and large ductility across a wide temperature range.

Manipulating Stacking Fault Energy to Achieve Crack Inhibition and Superior Strength-Ductility Synergy in an Additively Manufactured High-Entropy Alloy.

Additive manufacturing (AM) is a revolutionary technology that heralds a new era in metal processing, yet the quality of AM-produced parts is inevitably compromised by cracking induced by severe residual stress. In this study, a novel approach is presented to inhibit cracks and enhance the mechanical performances of AM-produced alloys by manipulating stacking fault energy (SFE). A high-entropy alloy (HEA) based on an equimolar FeCoCrNi composition is selected as the prototype material due to the presence of microcracks during laser powder bed fusion (LPBF) AM process. Introducing a small amount (≈2.4 at%) of Al doping can effectively lower SFE and yield the formation of multiscale microstructures that efficiently dissipate thermal stress during LPBF processing. Distinct from the Al-free HEA containing visible microcracks, the Al-doped HEA (Al0.1CoCrFeNi) is crack free and demonstrates ≈55% improvement in elongation without compromising tensile strength. Additionally, the lowered SFE enhances the resistance to crack propagation, thereby improving the durability of AM-printed products. By manipulating SFE, the thermal cycle-induced stress during the printing process can be effectively consumed via stacking faults formation, and the proposed strategy offers novel insights into the development of crack-free alloys with superior strength-ductility synergy for intricate structural applications.

Synergistic effects of temperature and strain rate on tensile properties of simulated Ni-6Cu alloy with Σ3 non-Arrhenius grain boundary

ABSTRACT Comprehending the mechanical response of materials on an atomic level is pivotal in the optimisation of advanced materials with superior mechanical properties. This research article utilises the atomistic-scale based molecular dynamics simulations to report the uniaxial tensile behaviour of bicrystalline Ni-6Cu (Nickel – 94% and Copper – 6%) alloy incorporated with pre-existing faceted Σ3 [111] 60° {11 8 5} grain boundaries. The primary aim of this investigation is to comprehend the performance of bicrystalline Ni-6Cu alloy under varying thermodynamic conditions and to assess the influence of pre-existing faceted grain boundaries on its tensile behaviour. This work encompasses a range of strain rates (108 to 1010 1/s) and temperatures (spanning from 100 to 900 K) for the uniaxial tensile deformation simulations. The outcomes unveil that the Young’s modulus of Ni-6Cu alloy (with pre-existing faceted grain boundaries embedded in its domain) was inversely proportional to temperature and constant with respect to strain rate. For the same configuration, yield stress was inversely and directly proportional to temperature and strain rate, respectively. Interestingly, incipient plasticity in the tensile stress–strain response was observed at lower temperature and lower strain rate. From the microstructural point of view, at lower temperatures, the incoherent twin boundary served as a source for the nucleation of stacking faults; however, as the temperature increased, both the incoherent twin boundary and the tips of coherent twin boundary function as the source for stacking faults formation. Our simulations also verified the GB’s anti-thermal (or non-Arrhenius) migration behaviour even under tensile load.