Dislocation Character Research Articles

An interplay between a hydrogen atmosphere and dislocation characteristics in BCC Fe from time-averaged molecular dynamics.

The interplay between hydrogen and dislocations (e.g., core and elastic energies, and dislocation-dislocation interactions) has implications on hydrogen embrittlement but is poorly understood. Continuum models of hydrogen enhanced local plasticity have not considered the effect of hydrogen on dislocation core energies. Energy minimization atomistic simulations can only resolve dislocation core energies in hydrogen-free systems because hydrogen motion is omitted so hydrogen atmosphere formation can't occur. Additionally, previous studies focused more on face-centered-cubic than body-centered-cubic metals. Discrete dislocation dynamics studies of hydrogen-dislocation interactions assume isotropic elasticity, but the validity of this assumption isn't understood. We perform time-averaged molecular dynamics simulations to study the effect of hydrogen on dislocation energies in body-centered-cubic iron for several dislocation character angles. We see atmosphere formation and highly converged dislocation energies. We find that hydrogen reduces dislocation core energies but can increase or decrease elastic energies of isolated dislocations and dislocation-dislocation interaction energies depending on character angle. We also find that isotropic elasticity can be well fitted to dislocation energies obtained from simulations if the isotropic elastic constants are not constrained to their anisotropic counterparts. These results are relevant to ongoing efforts in understanding hydrogen embrittlement and provide a foundation for future work in this field.

ГЕОЭЛЕКТРИЧЕСКОЕ СТРОЕНИЕ ГУСИНООЗЕРСКОЙ И СОПРЕДЕЛЬНЫХ ВПАДИН ЗАПАДНОГО ЗАБАЙКАЛЬЯ ПО РЕЗУЛЬТАТАМ ИНТЕРПРЕТАЦИИ АРХИВНЫХ ДАННЫХ ВЭЗ

The paper presents the reinterpretation results of vertical electrical soundings unique archival data performed in depressions of West Transbaikalia (Gusinoozerskaya, Ubukunskaya, Orongoyskaya, Ivolginskaya depressions). The relevance of this paper is due to the not fully understood deep structure of Mesozoic depression; thicknesses and electrical resistivities of individual rock complexes, characteristics of multidirectional fault dislocations, relief of basement are undefined. Sections of measurement profiles are constructed on the basis of 1D interpretation and two-dimensional inversion of VES data. Disjunctive dislocations are identified, many of which are consistent with geological data, and previously unknown faults have also been identified. Clarification of the structure of depressions located almost on the border of the Baikal rift zone is of undoubted interest to a wide range of specialists in both geological, geophysical and related disciplines, it is important for studies of seismic safety, geodynamics and mineral prospecting.

Analysis of dislocation characteristics and inside-outside contrasts in irradiated and annealed tungsten as a fusion reactor material

Tungsten is an important candidate of plasma-facing material for fusion reactors. Its irradiation response, especially the post-irradiation annealing (PIA) behavior needs further investigating. In addition, the practice of the “inside-outside” contrast method of determining the characteristics of irradiation induced dislocation loops has not been utilized frequently, and the present research serves as an example to present some practical considerations. In the present work, a tungsten thin-foil specimen is irradiated at 400 ℃ with 58 keV D<sup>+</sup> to a final fluence of 1× 10<sup>17</sup> cm<sup>–2</sup>, corresponding to a dose of about 0.1 dpa. The specimen is prepared through the electro-polishing method by using a NaOH based electrolyte. The ion irradiation is carried out directly on the electro-polished specimen. The irradiated specimen is followed by isothermal annealing at 900 ℃ for 1 h. The as-irradiated and post-irradiation annealing modified defects are investigated with a transmission electron microscope (TEM) operated at 200 kV. The irradiation defects are characterized by using TEM bright-field imaging for the same imaging field with different <i> <b>g</b> </i> vectors around the three major zone axes: the [001], [111], and [011] zone axis of the body-center cubic lattice of W. For each <i> <b>g</b> </i> vector, the ±<i> <b>g</b> </i> are characterized, and the corresponding contrast-extinctions and “inside-outside” contrasts of selected dislocation loops are identified. The indices of the <i> <b>g</b> </i> vectors around different zone axes are assigned consistently with the Kikuchi map. As a result, the D<sup>+</sup> irradiation increases a fine distribution of dislocation loops with an average size of (11.10±5.41) nm and a bulk density of 2.40×10<sup>22</sup> /m<sup>3</sup>. Voids are not observed obviously. The post-irradiation annealing causes the loop size to increase and the loop density to decrease, with numbers of (18.25±16.92) nm and 1.19×10<sup>22</sup> /m<sup>3</sup>, respectively. Through the contrast-extinction analysis, the dislocation loops in the irradiated and annealed specimen are identified to be <i> <b>a</b> </i>/2<inline-formula><tex-math id="Z-20230218170937">\begin{document}$\langle {111} \rangle$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20222124_Z-20230218170937.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20222124_Z-20230218170937.png"/></alternatives></inline-formula>-type dislocation loops. The post-irradiation annealing also causes the coalescence of large loops and forms large irregular-shaped dislocation loops. Voids with typical sizes of 1–2 nm are also observed in the annealed specimen. The PIA modified microstructure is consistent with the stage IV or stage V characterization of classical PIA induced microstructures. Through the “one-step” inside-outside contrast method, the dislocation loops are identified as an interstitial type. The influcing factors for the “one-step” method are discussed and the importance of consistent indexing is also noted.

Atomic-level quantification of twinning shears in magnesium alloy

Unlike dislocation slips with lattice translation vectors, the shear vector of twinning dislocation or disconnection (TD) is a non-lattice translation vector. Corresponding to the crystallography of twinning, geometrical analysis such as topological model has been widely adopted to define possible TDs that have both step and dislocation character. More importantly, multiple TDs can be defined to achieve the same crystal reorientation but generate different twinning shears. Thus, an accurate measurement of twinning shear allows us to define the character of elementary TD. Here, we quantitatively measured twinning shear of three twinning modes at an atomic level through tracing the twinning induced shear in nanoscale lamellar precipitates under laser shock peening and correspondingly determined the character of elementary TD. According to the measured angle for characteristic planes of nanoscale lamellar precipitates before and after twinning, we determined the twinning shear of 0.617 for{112¯1} twinning, which corresponds to a 1/2-layer TD with the Burgers vector of 0.024<1¯1¯26> as identified in the topological model. Using the same method, a twinning shear of 0.129 is determined for {101¯2} twinning, which corresponds to a two-layer TD with the Burgers vector of 0.080<1¯011>. More importantly, this result rules out the so-called pure-shuffle mechanism for {101¯2} twinning. A twinning shear of 0.136 is determined for {101¯1} twinning, which corresponds to a four-layer TD (or a reassembly of two-layer TDs) with the Burgers vector of 0.113<101¯2¯> as identified in topological model. These findings may obviate the dissent of twinning mechanisms, and provide an effective method to determine other complex shear mechanisms at an atomic level.

Data-driven investigation of microstructure and surface integrity in additively manufactured multi-principal-element alloys

The great variability of the additively manufactured quality is becoming a major challenge to limit its wide application. Here, we combine the high-throughput molecular dynamics simulations with the artificial neural network to search for the high surface quality and damage minimization of the selective laser melted (SLMed) FeCrNi multi-principal-element alloy (MPEA), based on the complex microstructure obtained from experiment characterization. By adjusting the laser scanning speed and laser power density, a large amount of data related to the surface roughness, dislocation characteristics, and residual stress in the SLMed MPEAs is obtained for optimizing the SLM process. Atomic simulation results show that there are various degrees of Cr segregation under different processing parameters, which leads to the uncertainty of residual stress distribution and dislocation characteristics. Thus, the surface roughness and subsurface damage is highly sensitive to the SLM parameters. According to the training results of the artificial neural network, the laser scanning speed plays a more key role in regulating the surface integrity and subsurface damage in the SLMed MPEAs. The overall prediction results exhibit that the relatively slow laser scanning speed effectively reduces the residual stress. Importantly, a complex relationship between the SLMed parameters and properties is built. The current work provides a predictive recommendation to optimize the quality and process design of the SLMed MPEAs with high surface integrity and low subsurface damage.

The Microstructure and Tensile Properties of New High-Manganese Low-Activation Austenitic Steel

Using X-ray diffraction, scanning and transmission electron microscopy, the microstructure of a new low-activation chromium-manganese austenitic steel with a high content of manganese and strong carbide-forming elements is studied. Its structure, dislocation character and particle composition are detailed. The processes taking place in the steel under cold-rolling deformation are described. It is shown that the mechanical properties of the new high-manganese steel revealed by testing at 20 and 650 °C are comparable with those of well-known analogs or exceed them. Relying on the structural studies, this is attributed to the dispersion and substructural strengthening. Better plastic properties of the steel are associated with the twinning-induced plasticity effect. It is shown that the steel fracture after tension at the test temperatures is mainly ductile dimple transcrystalline with the elements of ductile intercrystalline fracture (at 20 °C), while at 650 °C the signs of the latter disappear. The low-activation chromium-manganese austenitic steels characterized by increased austenite stability are thought to be promising structural materials for nuclear power engineering.

Dislocation characteristics and dynamic recrystallization in hot deformed AM30 and AZ31 alloys

Zn addition to Mg alloys activates non-basal slip or twinning with solute softening effects. On the other hand, the effects of the Zn solute on the macroscopic dislocation behavior and dynamic recrystallization are not completely understood. Moreover, it is unclear if <c+a> slip can be affected by changes in the c/a ratio of solute atoms. This study was conducted to understand the solute strengthening of Zn addition and its effects on the dislocation characteristics and dynamic recrystallization. A hot torsion test was performed on both AM30 and AZ31 alloys up to a high strain to investigate the Zn solute effect on the dislocation characteristics and dynamic recrystallization. The dislocation components of the hot torsioned alloys were evaluated by X-ray line profile analysis and electron backscatter diffraction. The results showed that the Zn solutes slightly accelerate strain accumulation at the initial stages of hot deformation, which accelerated recrystallization at high strain. The dislocation characteristics were changed dynamically by Zn addition: fortified <c+a>-type slip, dislocation arrangement and strain anisotropy parameters. The most important point was that the dislocation characteristics were changed dramatically at the critical strain for recrystallization and high strain regions. The addition of Zn also acted greatly in these strain areas. This indicates that the rapid formation of <c+a>-type slip at the serrated grain boundaries occurs at the initiation of dynamic recrystallization and the increase in the grain triple junction because grain refinement has a great influence on the dislocation characteristics at high strain.

Molecular dynamics perspective of the effects of laser thermal configurations on the dislocation and mechanical characteristics of FeNiCrCoCu HEA through powder bed fusion process

The concept of alloying multiple principal elements at high concentrations has led to the development of High Entropy Alloys (HEAs) with exceptional mechanical properties, making them the focus of major recent scientific endeavors. Geometrically complex HEAs with tailored microstructural characteristics can be produced using additive manufacturing technologies such as powder bed fusion (PBF). However, an in-depth study on the effect of process thermal conditions during PBF is required to effectively fabricate HEAs with desirable mechanical characteristics. Thus, in our present molecular dynamic (MD) study we have explored the implication of PBF process thermal conditions on the mechanical characteristics of FeNiCrCoCu HEA by systematically varying laser scan speed from 0.4 {\AA}/ps to 0.1 {\AA}/ps, unidirectional and reversing laser passes from 1 to 4, and laser power from 100 microwatts to 220 microwatts. Our investigation suggests that reducing the laser scanning speed up to a critical velocity of 0.2 {\AA}/ps considerably improves mechanical strengths, with further reduction creating severe surface defects. Decreased ultimate tensile strength (UTS) is associated with the annihilation of the bulk sessile dislocations during tensile straining marking an early yield failure. Alternately, the material's strength could be improved by annealing with several unidirectional laser passes over the same target region, resulting in enhanced UTS due to subtler yield points. Increasing laser power aids in ameliorating material density ultimately leading to higher UTS even in non-dislocation-free structures. These findings will assist researchers to understand the underlying effects and optimize process thermal parameters to fabricate superior HEAs utilizing additive manufacturing.

Analysis of Residual Stresses and Dislocation Density of AA6082 Butt Welds Produced by Friction Sir Welding

The Friction Stir Welding (FSW) method was employed to join AA6082 sheets. The welds were produced with different tool traverse speed (200 and 250 mm/min), rotational speed (1000 and 1250 RPM) and tool tilt angle (0 and 2 deg). Based on the analysis of XRD patterns, the total precipitation volume fractions in the nugget zones and the base material were calculated. The FSW process resulted in a reduction in the fraction of precipitates up to 64 pct compared to the parent material. Based on the Williamson–Hall analysis and indentation tests, the residual stresses were calculated. The highest tensile residual stresses of − 89.09 ± 6.19 MPa were observed for the base material, and the welding process reduced the residual stresses. The calculated dislocation density in the parent material AA6082 was equal to 8.225 × 1013 m−2, while in the welds a decrease was observed up to the value of 1.419 × 1013 m−2. In addition, the FSW process changed the nature of dislocations with edge-type dislocations dominating, while screw dominating character of dislocations were prevalent in the parent material. The mobility of dislocations in the studied welds was higher and reached the value of 16.78 × 10–7frac{m}{s}, while the dislocation mobility in the parent material was equal to 3.19 × 10–7frac{m}{s}. Process parameters during welding have a crucial effect on the amount of heat and strains introduced during the process, and thus influence the residual stresses, dislocation density and mobility, which might have a fundamental impact on the properties of the produced welds.

Molecular Dynamics Simulation of Nanoindentation of Nb-Zr Alloys with Different Zr Content

To understand the nanomechanical behaviors of the Nb-based alloys with Zr addition at room/high temperature, the molecular dynamics simulations of nanoindentation are conducted. In this work, the load-unload displacement curve, hardness, and dislocation characteristics of Nb-Zr alloys with varying Zr content ranging from 0 to 5 wt.% are studied. The simulation results are found to closely agree with the experimental one at 1 wt.%, therefore showing the reliability of the simulation. Moreover, considering distinct responses of alloys to different service temperature, the high-temperature nanoindentation are performed. The effects of Zr addition on the mechanical deformation under both temperatures are compared. The same phenomenon is found such that the optimum concentration range yielding the greatest hardness is 1–3 wt.%. The elastic modulus of NbZr alloy improves with elevated concentration at room temperature, while the hardness at higher temperature exhibits the opposite trend. This is attributed to the higher amplitude of atomic vibrations at high temperatures, which is more likely to deviate atoms from their equilibrium positions and weaken the pinning effect under external loading. Therefore, we believe that our studies on the nanomechanical mechanisms of materials at room/high temperature will provide an effective way for the alloying optimization design.

Domain boundaries in ScAlMgO4 single crystal observed by synchrotron radiation x-ray topography and reticulography

We performed synchrotron radiation x-ray topography (XRT) and reticulography to observe dislocations and crystal misorientation at domain boundaries (DBs) in a scandium magnesium aluminate single-crystal substrate grown via the Czochralski method. By inserting a fine-scale x-ray-absorbing mesh between the sample and the topograph-recording medium, reticulography was clearly observed allowing for direct measurement of twist and tile domain misorientation >5 × 10−5 rad. The results show that the angular resolution can be further improved to better than 2 × 10−5 rad by applying a different mesh-to-film distance M and then comparing the relative displacement in their reticulographs. The character of dislocations aligning at the DBs revealed by XRT analysis is consistent with the reticulography results. Finally, we demonstrate a method that combines reticulography and XRT into a single reticulographic XRT image, from which dislocation observation and misorientation measurement can be performed simultaneously.

In-situ neutron diffraction study on a dislocation density in a correlation with strain hardening in Al–Mg alloys

It is well known that variations of alloy and microstructural parameters i.e., alloy composition and grain size are favorable ways to improve the mechanical properties in non-heat-treatable aluminum alloys including Al–Mg alloys. It leads to a change in stress-strain curve behaviors. The flow stress is mainly correlated with dislocation interactions. In this study, in-situ neutron diffraction experiment was conducted to study evolution of dislocation density and characteristics during tensile deformation of Al–Mg. The dislocation density and related characteristic i.e., dislocation rearrangement were quantitatively analyzed by using neutron diffraction line profile analysis with using Convolution Multiple Whole Profile (CMWP) method. Variations of solute (Mg) contents and grain sizes in Al–Mg were investigated in a correlation with the dislocation densities and dislocation interactions. The higher Mg content and smaller grain size resulted to higher dislocation density and, consequently, higher flow stress and strain hardening rate.

Symmetric or asymmetric glide resistance to twinning disconnection?

Successive gliding of twinning disconnections (TDs) creates three-dimensional twins in parent crystal and accommodates shear deformation. It is generally recognized that TD is subject to the same Peierls stress as it glides forward or backward because of its dislocation character and the twofold rotation symmetry of the twin plane. Based on atomistic simulations, we demonstrate that the glide of TDs may be subject to a symmetric or asymmetric resistance corresponding to step character, symmetric resistance for A/A type steps but asymmetric resistance for A/B type steps, where A and B represent crystallographic planes in twin and matrix. Furthermore, we experimentally demonstrate that the asymmetric resistance results in asymmetric propagation and growth of twins in Mg alloys.

Following Cu Microstructure Evolution in CuZnO/Al2O3(−Cs) Catalysts During Activation in H2 using in situ XRD and XRD‐CT

AbstractUnderstanding how the microstructure of the active Cu0 component in the commercially applicable Cu/ZnO/Al2O3(−Cs2O) low‐temperature water‐gas shift catalyst evolves under various H2 partial pressures in the presence/absence of a Cs promoter during thermal activation has been investigated. Time‐resolved XRD and spatially‐resolved XRD‐CT data were measured as a function of H2 concentration along a packed bed reactor to elucidate the importance of the zincite support and the effect of the promoter on Cu sintering mechanisms, dislocation character and stacking fault probability. The rate of Cu reduction showed a dependency on [Cs], [H2] and bed height; lower [Cs] and higher [H2] led to a greater rate of metallic copper nanoparticle formation. A deeper analysis of the XRD line profiles allowed for determining a greater edge character to the dislocations and subsequent stacking fault probability was also observed to depend on higher [H2], smaller Cu0 (and ZnO) crystallite sizes, increased [ZnO] (30 wt.%, sCZA) and lower temperature. The intrinsic activity of Cu/ZnO/Al2O3 methanol synthesis catalysts has been intimately linked to the anisotropic behaviour of copper, and thus the presence of lattice defects; to the best knowledge of the authors, this study is the first instance in which this type of analysis has been applied to LT‐WGS catalysts.

Evaluation of texture weakening and microstructural evolution in an Fe–10Ni–7Mn martensitic steel severely deformed by six turns of high-pressure torsion

The effect of high-pressure torsion (HPT) on textural components and microstructural evolution of an Fe–10Ni–7Mn steel was investigated. Six turns of HPT, with a rotation rate of 1 rpm and with a pressure of 6.0 GPa, were applied to a solution annealed steel. Microstructural and textural evolution, including dislocation characteristics and grain size/misorientation measurements, were accomplished using electron backscatter diffraction (EBSD). The results showed that a fully martensitic structure, containing a high density of dislocations, was developed during the solution annealing stage and subsequently the application of six turns of HPT process converted the low-angle grain boundaries (LAGBs) of the solution annealed condition into high-angle grain boundaries (HAGBs). Thus, the sub-grain boundaries established in the solution annealed condition were altered into very sharp HAGBs. The grain size was also decreased from ∼ 25 μm in the solution annealed situation to ∼210 nm in the severely deformed condition of six-turn high-pressure torsion state. Yield strength, and tensile strength were increased from 765 MPa, and 840 MPa in the solution annealed case to 1890 MPa, and 2230 MPa in the six-turn HPT-processed sample, respectively. Under such a condition, ductility was decreased from 15.2% to 6.1% in the severely deformed state. Applying six turns of HPT developed a predominantly {111}//ND texture. However, the overall texture intensity of the HPT-processed sample was weakened to a value of about one-quarter of the as-quenched solution treated sample. This was related to dynamic recrystallization and grain subdivision as well as a dislocation density reduction in the HPT-processed sample. The texture components of strong Copper ({112}<111>), S ({123}<634>) and some average intensity Cube {100}<0001> were observed in the {100} pole figure in the solution annealed condition. As well, γ-fibre (<111>//ND) and β-fibre were seen in this condition. However, {110}<uvw> fibres were mostly developed in the six-turn sample with the texture components of E1{011‾}<111>, E2{01‾1}<111>, J1{01‾1}<2‾11>, J2+{11‾0}<1‾1‾2>, D2{1‾1‾2}<111> , D1{112‾}<111> and F {110}<001>. In addition, the Cu{112}⟨111⟩ component was significantly diminished. This weakened Cu{112}⟨111⟩ component would have caused enhancement of the mechanical properties, particularly the formability and toughness, as previously reported by the current authors. Finally, it was found that applying severe plastic deformation (SPD) in the form of HPT reduces the effect of martensitic texture in the solution treated steel.

Exploration of the dislocation-electrochemistry relation in LiFePO4 cathode materials

Defects, such as dislocations, in electrode materials play a significant role in the performance of lithium-ion batteries. The dislocation-electrochemistry relation has only been observed experimentally and not been fully clarified. Computational studies on this mechanism were also very limited, especially the altered cyclic voltammetry behaviors and associated effective diffusivity. This work focuses on the influences of few characteristics of dislocations on the electrochemical performance of an anisotropic cathode material, lithium iron phosphate (LiFePO4). Utilizing linear elastic mechanics and the superposition principle, we study stress and displacement fields of a LiFePO4 particle containing different densities and orientations of dislocations. With the mechanical-electrochemical coupling effects expressed by the modified Butler-Volmer equation and using the finite different method, the cyclic voltammetry curves for different dislocation configurations in the particle are investigated. Our results show that introducing dislocations can shift and distort the cyclic voltammetry curves, especially at one specific dislocation orientation. It is also found that the Li-ion molar fraction-dependent partial molar volume is an important prerequisite of the distortion in cyclic voltammetry curves. Moreover, the altered cyclic voltammetry curves at different scanning rates indicate the improvements of electrical power, stored electrical energy, and the effective diffusivity of lithium. Our discrete dislocation model indicates that the capacity loss of LiFePO4 nanoparticles can be alleviated by introducing tailored dislocations. This study assists the understanding of electrode materials with pre-existing dislocations and provides strategies of using defect engineering to improve the kinetic performance in lithium-ion batteries.

Evaluation of Defects in a SiC Substrate Using the Photoluminescence Measurement Method

Photoluminescence (PL) imaging provide defect and dislocation characteristics not only in a 4H silicon carbide (SiC) epilayer but also in a substrate. In this work, to detect a large-pit or a bar-shaped stacking fault (BSF) before epilayer growth, we employed PL under various detection conditions. A large-pit was detected as a dark spot on a 4H-SiC substrate due to dislocations orthogonal to a micropipe. The BSF was clearly observed as a bright rectangle by tuning the observation conditions. The finding indicates that device killing defects or dislocations should be detected as soon as possible and thus improve the yield.

Clinical Characteristics of recurrent intraocular lens dislocation after scleral-fixated sutured intraocular lens and long-term outcomes of intraocular lens re-fixation.

This study aimed to investigate the clinical characteristics of patients with recurrent intraocular lens (IOL) dislocation after scleral-fixated sutured IOL implantation and evaluate the long-term outcomes of scleral re-fixation of IOL. The medical records of patients who underwent surgery for IOL dislocation between January 2011 and January 2021 were reviewed. The study included 164 patients (male: 131, female: 33) (176 eyes). Patient demographics, preoperative, intraoperative and postoperative data, and the ocular and systemic conditions associated with IOL re-dislocation were analyzed. The study included 176 consecutive cases of scleral-fixated sutured IOL. Twenty-six eyes (14.8%) showed re-dislocation of IOL after the initial IOL scleral fixation and underwent reoperation (mean 75.5 ± 62.5months after the first surgery); three (11.5%) of them required a third surgery. Younger adults (aged less than 40years), and patients who underwent IOL scleral fixation in complicated cataract surgery or aphakic state had a higher risk of re-dislocation. Diabetes mellitus (DM) was the only statistically significantly higher risk factor in the re-dislocated group (p = 0.041). The complication rate with scleral re-fixation was higher than that in the non-re-dislocated group. No statistically significant differences were observed, except for vitreous hemorrhage (p = 0.024). Caution should be exercised when performing sutured scleral fixation of IOL in younger patients, cases of complicated cataract surgery and aphakia, and patients with DM to prevent IOL re-dislocation. Scleral-fixated sutured IOL in eyes with recurrent IOL dislocation seems to be a safe and effective procedure with a relatively low complication rate.

Revealing the decomposition mechanisms of dislocations and metastable α' phase and their effects on mechanical properties in a Ti-6Al-4V alloy

The metastable α' phase and dislocation characteristics (e.g., density and constituents) are of vital importance for the mechanical responses of (α+β) titanium alloys. In this work, to reveal the in-depth decomposition mechanisms of dislocations and metastable α' and their influences on mechanical properties in a Ti-6Al-4V (α+β) alloy, the thermal stability of different microstructures tailored by various cooling approaches were investigated utilizing scanning electron microscope, electron backscattered diffraction, transmission electron microscope and X-ray diffraction line profile analysis. The results showed that the initial characteristics of α' and α laths and dislocation density were influenced by the cooling methods remarkably. The thermal stability of Ti-6Al-4V alloy increased with decreasing cooling rate. The improvement in thermal stability can be ascribed by the decrement in dislocations, partitioning of the alloying elements and grain orientations variation of the α' lath and α phase. It is also found that the plastic strain accommodation of β→α' transformation was dominated by <a> dislocations. During stabilization annealing treatments, the <a>, <c> and <c+a> dislocations simultaneously decomposed. The decomposition of dislocations and metastable α' phase during various stabilization annealing and the particular twins and stacking faults microstructures formed during quenching have a great influence on the properties of the studied Ti-6Al-4V alloy.

Awake Cranial Traction and Isolated Anterior Cervical Discectomy and Fusion in the Treatment of Traumatic Subaxial Cervical Facet Joint Dislocations: Analysis of a Cohort of 70 Patients and Predictors of Surgical Failure.

Cervical facet dislocations are among the most common traumatic spinal injuries. The management of this type of lesions is still controversial. The objective of the present study was to analyze the results of subaxial cervical facet dislocations submitted to an isolated anterior cervical discectomy and fusion (ACDF) after attempted closed reduction with cranial traction and to identify risk factors for treatment failure. All patients who were operated on in a tertiary trauma center during an 11-year period (2008-2018) for traumatic single-level cervical facet joint dislocation (AO C F4 injuries) were retrospectively reviewed. Age, use of cranial traction, dislocation characteristics, neurologic injury, surgical data, and follow-up records were reviewed. A minimum of 18 months follow-up was required. A total of 70 patients with a mean age of 56 years (18-90) (72% men) were identified. The C6-C7 level was the most frequently affected (36/70 cases). Spinal cord injury (SCI) was present in 34% of the cases. Bilateral dislocations and rigid spines were risk factors for SCI. Cranial traction was performed in 59 cases with success in 52 cases (88%). There were 3 failures after anterior fusion, which required revision surgery with a 360° fusion, all occurring at the C7-T1 level. Cranial traction of the cervical spine is an effective and fast way to achieve closed reduction of cervical facet dislocations. After successful reduction, ACDF, as a single procedure, offers an excellent surgical option. All cases of failure occurred at the C7-T1 level, suggesting that a 360° fusion may be needed at this level.