High Density Of Stacking Faults Research Articles

The important role of extremely low content of soluted Ce in the wide stacking faults formation in Al-Ce alloy

The performance of the material can be significantly improved by introducing stacking faults and nanotwins. However, introducing high-density stacking faults and twins into Al remains difficult due to its ultrahigh stacking-fault energy. In this study, it was discovered for the first time that the addition of Ce promotes the formation of stacking faults in Al. The broadening of stacking faults is attributed to the extremely low content of Ce in Al and the local stress concentration during the rolling process. The solubility at eutectic temperature was determined to be 0.0084at.%. The solid solution of Ce has a great influence on the stacking fault energy of Al, which is attributed to the solute Ce disturbing the lattice orientation and electronic environment of the matrix atoms. The local stress concentration in Al matrix near the eutectic Al11Ce3 phase also provides a favorable condition for the formation of stacking faults.

A high Fe-containing AlSi12 alloy fabricated by laser powder bed fusion

The high Fe-containing AlSi12 alloy was processed by additive manufacturing of laser powder bed fusion (LPBF) to understand the features of microstructures and mechanical properties under as-fabricated condition. The Fe impurity was found to be beneficial for mechanical property enhancement in the LPBFed samples. The parameters including the combination of laser power of 200 W, scanning speed of 1110 mm/s, hatch spacing of 0.15 mm, layer thickness of 0.03 mm and laser volumetric energy density of 40 J/mm3 were optimized to achieve a high relative density of 99.7%. The as-LPBFed AlSi12FeMn alloy was featured by a high density of significantly refined spherical α-Al(Fe,Mn)Si phase (10–50 nm), which was coherent with the Al matrix. Meanwhile, the as-LPBFed AlSi12FeMn alloy can deliver superior mechanical properties including the yield strength of 305 MPa, the ultimate tensile strength of 485 MPa and the fracture strain of 6.1%. The improved mechanical properties are attributed to synergistic strengthening mechanisms, including solid solution strengthening, grain boundary strengthening and precipitation strengthening. Moreover, the formation of high-density stacking faults (SFs) and Lomer-Cottrell locks (LCs) in localized regions can also offer strengthening in the as-LPBFed AlSi12FeMn alloy.

Atomic-level study of {[formula omitted]} deformation twinning in pure Ti and Ti-5at.% Al alloy

In this work, pure Ti and Ti-5at.%Al alloy are both cold rolled, and then characterized to observe deformation twins {101¯1}. Many two-layer steps are observed at the {101¯1} twin boundary (TB) in pure Ti by high-resolution transmission electron microscopy (HRTEM). However, in addition to two-layer steps, a large number of four-layer steps are observed at the {101¯1} twin boundary in Ti-5at.%Al alloy. To understand the experimental observations, molecular dynamics (MD) simulations are performed to analyze the {101¯1} deformation twinning at the atomic scale. MD simulations are also showing similar results to the experimental observations in pure Ti and Ti-5at.%Al alloy. The two-layer and four-layer steps are twinning dislocations (TDs) with the Burgers vectors of 12×4γ2−93+4γ2<101¯2¯> ±16<12¯10> (b⇀2) and 4γ2−93+4γ2<101¯2¯> (b⇀4) by topological analysis, respectively. For pure Ti, the twin growth is carried out only by the slip of TDs b⇀2. For Ti-5at.% Al alloy, twinning is carried out by the gliding of TDs b⇀2 and TDs b⇀4. Due to the poor mobility of TDs b⇀4, the nucleation of the twinning dislocations b⇀4 (4-layer steps) requires some special nucleation sites, such as intersections between the stacking fault (SF) and TB (SF-TB intersection) in Ti-5at.% Al alloy. The SF-TB intersection is a sessile imperfect disconnection, and this defect is immobile, which leads to high stress at the SF-TB intersections to facilitate the nucleation of TD b⇀4. The Al element enhances the high density of basal stacking faults (BSFs) and SF-TB intersections at the twin-tip, and indirectly promotes the nucleation of TD b⇀4 in Ti-5at.% Al alloy. In addition, Al element also facilitates the nucleation of matrix dislocations, resulting in twin-dislocation interactions that promote the generation of TDs.

Quantification of the temperature-dependent evolution of defect structures in a CoNi-base superalloy

Quantifying the individual contributions of specific deformation mechanisms to the total plastic strain is of primary importance to understand the underlying deformation behavior and assess the significance of the different mechanisms. However, this quantification is nearly impossible in Ni-base superalloys due to the inherent difficulties in determining the propagation distances of the respective shearing dislocations. In this study, the occurrence of stacking faults (SFs) at temperatures between 750 °C and 1000 °C in the single crystalline CoNi-base superalloy ERBOCo-4 provides an opportunity to systematically quantify the evolution of the defect structures as a function of temperature and strain after constant strain-rate compressive tests. The quantification approach revealed that deformation is still localized in the matrix at 750 °C. With increasing temperature, the main deformation mechanism changes to SF shearing, which contributes 74% of the total deformation during the initial yielding stage (∼ 0.3% plastic strain) at 900 °C. Deformation spreads significantly more homogeneously across the γ/γ’ microstructure with increasing plastic strain. Extremely high stacking fault densities and frequent fault interactions cause a pronounced work-hardening anomaly at 850 °C. Structural characterization on the atomic scale confirms the strengthening effect of SFs as barriers to partial dislocation propagation. A strong correlation between the possible diffusion length of segregating solutes and the operant shearing mechanisms reveals the significance of segregation on the occurring deformation mechanisms.

Formation of high-density stacking faults in ceramic films induced by Ti transition layer

Stacking faults and twins are extremely difficult to form in ceramic materials. At the same time, previous studies often used the metal transition layer to improve the film-substrate bonding force, but ignored its influence on the film structure and mechanical properties. Our research shows that a large number of stacking faults are formed in the Ti transition layer grown on (110) MgO substrate, which induces high density tacking faults in nitride ceramic films with high stacking fault energy, thereby increasing the nanoindentation hardness of the ceramic films. Whether the stacking faults in the Ti transition layer can enter the ceramic films depends on the stacking fault density. This work is of great significance for studying how to induce the formation of stacking faults and twins in ceramics with high stacking fault energy to improve the mechanical properties.

Processing and mechanical properties of B4C-SiCw ceramics densified by spark plasma sintering

Homogenous distribution of whiskers in the ceramic matrix is difficult to be achieved. To solve this problem, B4C-SiCw powder mixtures were freeze dried from a slurry dispersed by cellulose nanofibrils (CellNF) in this work. Dense B4C ceramics reinforced with various amounts of SiCw up to 12 wt% were consolidated by spark plasma sintering (SPS) at 1800 °C for 10 min under 50 MPa. During this process, CellNF was converted into carbon nanostructures. As iron impurities exist in the starting B4C and SiCw powders, both thermodynamic calculations and microstructure observations suggest the dissolution and precipitation of SiCw in the liquids composed of Fe-Si-B-C occurred during sintering. Although not all the SiCw grains were kept in the final ceramics, B4C-9 wt% SiCw ceramics sintered at 1800 °C still exhibit excellent Vickers hardness (35.5 ± 0.8 GPa), flexural strength (560 ± 9 MPa) and fracture toughness (5.1 ± 0.2 MPa·m1/2), possibly contributed by the high-density stacking faults and twins in their SiC grains, no matter in whisker or particulate forms.

Synergistic strengthening of heterogeneous structures and dual-morphology nano-precipitates in Co1.5CrNi1.5Al0.2Ti0.1V0.1 medium-entropy alloy

In this paper, we designed a novel non-equiatomic medium-entropy alloy (MEA) with heterogeneous grain structures (HGS) and dual-morphology L12 nano-precipitates through sever plastic deformation followed by directly annealing at 600 °C for 4 h, which exhibited the ultra-high yield strength of 1750 MPa, the ultimate tensile strength of 1890 MPa and the ductility as high as 23.9%. The research on the reveal that the ultra-high strength mainly results from precipitation strengthening and hetero-deformation induced (HDI) strengthening. The HGS effectively induced the HDI strengthening of over 795 MPa during deformation. And the highly coherent lamellar and spherical L12 nano-precipitates provided the yield strength of 770 MPa. Meanwhile, the low stacking fault energy of FCC matrix could make it easy to form a high-density stacking faults and Lomer-Cottrell locks. The Lomer-Cottrell locks not only play a role as strong obstacles to pin the dislocation motion effectively, but also can be sources for dislocation multiplication, enhancing work hardening capacity continuously and delaying the onset of necking. Our work offers an extremely effective strategy to enhance the strength-ductility of non-equiatomic MEAs.

Investigation of strengthening mechanisms in an additively manufactured Haynes 230 alloy

There are abundant studies on additive manufacturing of Ni alloys, such as Inconel 718. However, the studies on the microstructure and the deformation behavior of solid solution strengthened Ni alloys, such as Haynes 230, are limited. Here we compare the microstructure and the mechanical behavior of as-printed and stress-relieved Haynes 230 Ni alloys prepared by laser powder bed fusion. Transmission electron microscopy analysis revealed M23C6 carbides decorated at the dislocation cell walls in the as-printed material transformed to well-aligned arrays of M6C nanoprecipitates separated by an array spacing of 500 nm after stress-relief (heat treatment). The nanoprecipitates in stress-relieved alloy have formed numerous aligned orientation relationships with the matrix despite their large lattice mismatch. Post-tension microscopy analyses demonstrated high-density dislocation networks in the as-printed samples, whereas high-density stacking faults and nanoscale deformation twins were observed in the stress-relieved counterparts. This study provides insights for understanding the influence of nanoprecipitates on the deformation mechanisms of additively manufactured Ni alloys.

Gradient cell-structured high-entropy alloy with exceptional strength and ductility.

Similar to conventional materials, most multicomponent high-entropy alloys (HEAs) lose ductility as they gain strength. In this study, we controllably introduced gradient nanoscaled dislocation cell structures in a stable single-phase HEA with face-centered cubic structure, thus resulting in enhanced strength without apparent loss of ductility. Upon application of strain, the sample-level structural gradient induces progressive formation of a high density of tiny stacking faults (SFs) and twins, nucleating from abundant low-angle dislocation cells. Furthermore, the SF-induced plasticity and the resultant refined structures, coupled with intensively accumulated dislocations, contribute to plasticity, increased strength, and work hardening. These findings offer a promising paradigm for tailoring properties with gradient dislocation cells at the nanoscale and advance our fundamental understanding of the intrinsic deformation behavior of HEAs.

Ultra-strong nanostructured Co-Ni-V medium entropy alloy thin film designed by interface strengthening

In this work, amorphous and nanocrystalline Co-Ni-V medium entropy alloy (MEA) thin films were prepared by magnetron sputtering via tuning substrate temperature. The nanocrystalline Co-Ni-V thin film deposited at 673 K containing ultrahigh-density stacking faults was found to be more superior to amorphous Co-Ni-V thin film deposited at 473 K and some other face-centered cubic (fcc)-structured materials by exhibiting higher hardness of ∼9.5 GPa and yield strength of ∼4.0 GPa. The strengthening mainly stems from various defects including nanograin boundaries and high-density stacking faults, which were found to possibly serve as strong impediments to dislocation motion. Moreover, the weak boundaries between adjacent nanocolumns with relatively weak bonding is eliminated with increasing substrate temperature, which also contributes to the enhanced strength/hardness for the nanocrystalline Co-Ni-V film as compared with amorphous Co-Ni-V film. The prepared ultra-strong nanostructured Co-Ni-V MEA thin film exhibits high yield strength and micro-compressive plasticity (over 50%), suggestive of the relatively strong material among some reported fcc-structured metals and alloys. These results on nanostructured Co-Ni-V MEA thin film demonstrate that besides refining grain, interface strengthening, i.e., improving nanocolumn interface bonding via substrate temperature and introducing high-density stacking faults via rapid cooling of vapor deposition, could be considered when designing high strength alloy thin films for potential micro-electron-mechanical system applications.

Ultrafast Laser-Induced Atomic Structure Transformation of Au Nanoparticles with Improved Surface Activity.

Metallic nanoparticles (NPs) play a significant role in nanocatalytic systems, which are important for clean energy conversion, storage, and utilization. Laser fabrication of metallic NPs relying on light-matter interactions provides many opportunities. It is essential to study the atomic structure transformation of nonactive monocrystalline metallic NPs for practical applications. The high-density stacking faults were fabricated in monocrystalline Au NPs through tuning the ultrafast laser-induced relaxation dynamics, and the thermal and dynamic stress effects on the atomic structure transformation were revealed. The atomic structure transformation mainly arises from the thermal effect, and the dynamic stress distribution induced by local energy deposition gives rise to the generation of stacking faults. Au NPs with abundant stacking faults show enhanced surface activity owing to their low coordination number. We suggest that this work expands the knowledge of laser-metallic nanomaterial interactions and provides a method for designing metallic NPs for a wide range of applications.

Additive manufacturing of TRIP-assisted dual-phases Fe50Mn30Co10Cr10 high-entropy alloy: Microstructure evolution, mechanical properties and deformation mechanisms

Abstract The metastable dual-phase non-equiatomic Fe50Mn30Co10Cr10 high entropy alloy (HEA) was successfully prepared via laser melting deposition (LMD) technique. The phase composition, microstructure and mechanical performance were systematically characterized at different sections of LMDed HEA samples. The as-printed HEA specimens were dominated by FCC γ phase, plate and needle-like HCP e phase, and high density stacking faults and dislocations were observed at high magnification. The as-printed samples exhibited anisotropic mechanical properties, together with the maximum ultimate tensile strength of 760 MPa and maximum elongation of 28%. The fracture surface along X and Z directions exhibit smooth curve shape, while the fracture surface along Y direction shows saw tooth shape, indicating the boundary of melt pool is weakly bounded. After deformation, the deformation bands were observed in the FCC matrix, showing that the plastic strain was controlled by FCC γ phase. In addition, stacking faults, dislocation increase significantly owing to the formation of stacking fault is caused by sliding of Shockeley partial dislocation, and the stacking fault is the core of HCP martensite nucleation.

Yielding behavior of a single-crystalline γ'-strengthened Co-Ti-Cr superalloy

The anomalous yielding behavior known from Ni3Al or Ni-base superalloys is explained by the formation of immobile dislocation configurations via cross-slip of the leading superpartial of anti-phase boundary-coupled dislocation pairs. A systematic investigation of the anomalous yielding behavior of a single-crystalline Co-11Ti-15Cr superalloy by transmission electron microscopy revealed the absence of similar sessile dislocation configurations during the initial yielding stage. Shearing of γ' precipitates via superlattice stacking fault formation is the dominant deformation mechanism at the peak temperature. Furthermore, the high density of superlattice stacking faults and their interactions also lead to a pronounced work-hardening anomaly.

Effects of skeleton pore size on the microstructure and electromagnetic absorbing property of the SiC nanowires/SiC composites

SiC nanowires exhibited great electromagnetic absorbing property while decorated in porous ceramic skeletons. However, the ceramic skeletons used in former researches always had multi-mode pore structure, so the effects of skeleton pore size on the properties of SiC nanowires could not be researched independently. In this work, porous SiC ceramics with single-mode pore distribution and controllable pore size were applied to prepare SiC nanowires/SiC composites. While the skeleton pore size was decreased from 13.94 μm to 2.18 μm, the SiC nanowires turned from straight to curved. Compared with the straight nanowires, the curved ones presented high-density stacking faults and large surface area, endowing the composites with superior electromagnetic absorbing property.

Exploiting the synergic strengthening effects of stacking faults in carbon nanotubes reinforced aluminum matrix composites for enhanced mechanical properties

Carbon nanotubes (CNTs) reinforced Al matrix composites containing high-density stacking faults were successfully fabricated through spark plasma sintering (SPS) and subsequently hot rolling with high strain rate. It is found that the inclusion of CNTs facilitate the formation of stacking fault in Al matrix. With increasing the CNTs from 0.5 vol% to 1.0 vol%, the density of stacking fault increases, which is mainly because of the reduced stacking fault energy of Al matrix in Al/CNTs composites, as revealed by the density functional theory simulation. Combined with the effective exploitation of the strengthening effectiveness of CNTs due to uniform dispersion of CNTs and well bonded Al-CNTs interface, the highest tensile strength (378 ± 8 MPa) together with good ductility (17.1 ± 1.5%) can be achieved for 1 vol% CNTs reinforced Al matrix composite. Moreover, the enhanced strength deriving from each strengthening mechanism relies on the CNTs content. When the CNTs content is 1.0 vol%, Orowan looping and stacking fault strengthening are the dominant strengthening mechanisms. The reported microstructure design and control can be informative for improving the mechanical properties of CNTs reinforced metal matrix composites.

Orientation-tunable InxGa1−xN nanowires with a high density of basal stacking faults for photoelectrochemical/photocatalytic applications

InxGa1−xN nanowires grew along the m-direction (A-NWs) or semipolar-direction (B-NWs) with the presence of a high density of BSFs.

Formation of High-Density Stacking Faults in Ceramic Films Induced by Ti Transition Layer

Formation of High-Density Stacking Faults in Ceramic Films Induced by Ti Transition Layer

Effects of a post-weld heat treatment on the microstructure and mechanical properties of dual-spot laser welded-brazed Ti/Al butt joints

Abstract The influence of a post-weld heat treatment (PWHT), which included a solution treatment and an artificial aging treatment, on the microstructure and mechanical properties of laser welded-brazed Ti/Al joints with Al-12Si filler was investigated. The thickness of the intermetallic compound (IMC) layer at the brazing interface increased with increasing solution treatment time at 530 °C. Si atoms diffused from the weld to the IMC layer during solution treatment, forming a Si-poor region adjacent to the brazing interface. The phases in the IMC changed from Ti(Al,Si)3, nanograined Ti7Al5Si12, and Ti5Si3 in the welded-brazed sample to Ti7Al5Si12 with a high stacking fault density in the solution-treated sample. The net-shaped Al-Si eutectic microstructure in the fusion zone was affected by the PWHT. The size of the Si particles in the fusion zone increased with increasing solution treatment time, and the artificial aging treatment further coarsened the Si particles. The softening of the heat-affected zone (HAZ) in the Al base metal was eliminated by the PWHT. The tensile strength of the Ti/Al joint increased from 177 to 260 MPa. This result of this study indicate that the mechanical properties of Ti/Al welded-brazed joints can be improved by a PWHT.

Formation mechanism of stacking faults and its effect on hardness in M7C3 carbides

Although stacking faults are commonly found in M7C3 carbides, their causes and effects on the properties of carbides are rarely reported. The purpose of this work is to explore the effect of solidification rate on the stacking fault density and study the effect of stacking faults on the carbide microhardness. Hypereutectic high‑chromium cast iron samples were solidified at three different cooling rates, and optical microscopy, scanning electron microscopy, X-ray diffraction, electron probe microanalysis, transmission electron microscopy, and hardness testing were employed to study the changes in carbides. The results show that the density of stacking faults increases with the cooling rate. Partial dislocations due to stacking faults were found in carbides with high fault densities. The Cr/Fe ratio in the carbides continued to decrease with the cooling rate, while the microhardness of the carbides first increased slightly and then decreased. The analysis shows that the significant growth driving force owing to larger degrees of undercooling generated more stacking faults. Notably, hardness is related to stacking faults in carbides. Moreover, low-density stacking faults could increase the microhardness of carbides, while high-density stacking faults produce a large number of vacancies that could soften carbides.

Spontaneous CVD growth of InxGa1-xN/GaN core/shell nanowires for photocatalytic hydrogen generation

InxGa1-xN nanowires (NWs) have drawn great attentions as a promising photocatalyst for H2 generation via solar water splitting. In this article, m-axial NWs with InxGa1-xN/GaN core/shell structure can be formed spontaneously on the n-type Si (111) substrate via Ni-assisted chemical vapor deposition (CVD) method. CVD growth conditions were adjusted by tuning the duration and pressure. The microstructures, surface chemistry, and optical properties of the obtained NWs were characterized using various techniques. The growth mechanism is identified as a two-step process. The InxGa1-xN cores with a high density of basal stacking faults were grown by a vapour-liquid-solid (VLS) mode, while the GaN shells were grown on the sidewalls of InxGa1-xN core through a vapor-solid (VS) mode. The photocatalytic activities of the NWs were evaluated by the amount of H2 generated from water splitting. A thicker GaN shell was revealed to decrease the photoactivity by hindering the charge separation and transfer of electron-hole pairs generated inside the InxGa1-xN cores. A strategy can be proposed to improve the photocatalytic performance of NWs by terminating the VS growth of the GaN shells during the CVD process.