Phase Dislocations Research Articles

Contribution of microstructure to yield strength of X-750 alloy used for garter spring in Canada deuterium uranium reactor

The role of microstructures on the overall yield strength of garter-spring coil X-750 alloy used for the fuel channel structure in the Canada deuterium uranium reactor is investigated in this study. We perform a comprehensive characterization of the residual microstructures in the garter-spring coil X-750 alloy and heat-treated plate X-750 alloys using electron microscopy. Simultaneously, we perform small-scale mechanical tests and bulk mechanical tests to evaluate the yield strength of the compact garter-spring coil component. We compare the mechanical property data obtained via the micro-bending measurement method and derive a correlation between each measured data point to assess the increase in yield strength. The γ′ ordering phase contribution is the most crucial factor in strengthening, followed by dislocation density and boundary-decorated precipitates in the garter-spring coil X-750 alloy. We quantitatively evaluate the effects of the γ′ ordering phase and initial dislocation density on the yield strength of the garter spring coil X-750 alloy because they significantly affect the change in the mechanical properties in the irradiation environment.

The selection of laser beam diameter in directed energy deposition of austenitic stainless steel: A comprehensive assessment

One focus of laser directed energy deposition (L-DED) is to increase productivity, and this can be achieved by using a larger beam diameter and higher laser power. However, it is difficult to build complex metallic components with fine structures using a laser beam of large diameter. Under the trade-off between formability and productivity, the choice of beam diameter needs to be thoroughly evaluated. Herein, three beam diameters (0.75 mm, 1.5 mm, and 3 mm) in building austenitic stainless steel parts have been investigated and compared in four aspects: microstructure, mechanical properties, powder utilization, and as-built part surface quality. Specifically, cooling rate and powder splashing in the building process, and phase composition, grain size and dislocation density of as-built parts are revealed by means of numerical simulation, high-speed camera, electron backscattered diffraction, and X-ray diffractometry. Results show that for a small beam diameter, the microstructure formed has finer grains and less amount of ferrite due to higher cooling rate of the molten pool, and grain refinement and high density of dislocations lead to superior mechanical properties of the as-built part; conversely, for a larger beam diameter, more stable molten pool and less amount of powder splashing brings higher material utilization rate and better surface quality. A brief evaluation of beam diameter selection is accordingly proposed, and a novel hybrid beam diameters building strategy is presented for building complex metallic components with high quality and efficiency.

Mechanical and magnetic properties of TRIP690 steel strengthened by strain-induced martensite

The sensitivity of the induced magnetic signals to microstructures makes it possible to characterize nondestructively the mechanical properties of ferromagnetic materials. In this study, utilizing the hysteresis loop measurement and the magnetic Barkhausen noise measurement, the correlation between the mechanical and the magnetic properties of low-alloy multiphase transformation-induced plasticity (TRIP) steel after plastic deformation was investigated, then the strength of the TRIP steel after plastic deformation was characterized. The results showed that, as the strain level increased from 0 to 0.2, the martensite volume fraction of TRIP690 steel increased from 0.78% to 3.64% which contributed to the enhancement of mechanical properties and the change of magnetic properties, and in particular, hardness and tensile strength had linearly increased by approximately 42% and 20%, respectively. Meanwhile, the peak value of the magnetic Barkhausen noise signal (MBNS) increased linearly with the increasing strain levels, the coercivity increased rapidly and then slowly, while the susceptibility decreased exponentially. By theoretically deriving the effects of the ferromagnetic phase volume fraction, dislocation density and residual stress of the TRIP690 steel on susceptibility and coercivity, the variation of the MBNSs under the influence of phase transformation, dislocation and residual stress was analyzed. The above non-destructive magnetic technique makes it possible to evaluate the mechanical response of the TRIP steel after plastic deformation, which allows it to be further applied to the quality evaluation of steel parts in stamping, such as body-in-white for the automotive industry.

Dislocation structures in a Ti–6Al–4V alloy subjected to cyclic tensile deformation

The present work investigates dislocation structures developed within both the α and β phase of a Ti–6Al–4V alloy undergoing cyclic tensile straining. The cyclic loading-unloading was performed using the stress levels of 830 and 580 MPa (R∼0.7) at a strain rate of 2 × 10-3 s-1 to 15125 cycles, as part of a synchrotron in-situ experiment. The macroscopic yield stress was about 950 MPa. The dislocation arrangement, type and density were studied post-mortem employing transmission electron microscopy, together with the automatic determination of crystal lattice orientations via precession nanobeam diffraction. The deformation took place exclusively through dislocation slip and no presence of deformation twins was detected within either of the studied phases. The α phase dislocation structure displayed some similarity to that reported for monotonic deformation and no evidence of the formation of regular planar dislocation networks, proposed in the literature, has been found. The prevalent deformation modes were prismatic and basal glide, based on the experimentally determined Burgers vectors together with the global Schmid factor and reported critical resolved shear stress values. The <a>-type dislocations observed after deformation generally possessed a large screw component, with most of them being of a pure screw type. There were also some <c+a> dislocations present within the α phase and these typically displayed a mixed character. Dislocation Burgers vectors present after straining within the β phase frequently deviated from the Schmid factor rule and these dislocations were often wavy and largely possessed a significant screw component.

Design of metastable β-Ti alloys with enhanced mechanical properties by coupling αS precipitation strengthening and TRIP effect

A strain-transformable microstructure was successfully designed in a metastable β Ti–7Mo–3Nb–3Cr–3Al alloy with enhanced mechanical properties, by introducing ∼28% α precipitates coupled with the TRIP effect, overcoming the traditional trade-off dilemma between strength and ductility in most metastable β-Ti alloys. The as-designed lamellar microstructure was predominantly deformed by stress-induced martensitic (SIM α") phase transformations, martensitic twinning and dislocation slip of the parent β grains and α laths. The β→α" transformation followed the [113]β//[112]α"//[-310]α"//[1-21]α" orientation relationship, with the {133}β habit plane predicted by the Phenomenological Theory of Martensite Crystallography (PTMC). A novel <211>α" type II martensitic twinning mode was also found, in addition to the {111}α" type I mode at SIM α"/α impinging region. The results show that, not only the lamellar α precipitates play a major role in precipitation strengthening, but they can also effectively block SIM α" propagation at the initial stages of deformation. However, SIM α" transmission across the α laths was also observed for large strains. Moreover, <c+a> pyramidal slip and shear of the α laths also contributed to the accommodation of internal stresses. Therefore, the origin of the enhanced tensile mechanical properties can be attributed to the combined effects of α precipitation strengthening coupled with the TRIP softening effect and the extra interaction stresses introduced by the α laths and other deformation products, validating the design concept. The current investigation may provide a novel strategy for designing new high-performance metastable β-Ti alloys.

Morphology of fibrous structures formed in the course of superplastic deformation of the 01420T alloy with the original bimodal grain structure

The morphology of the fibrous structures formed in the working parts of the 01420T alloy samples with the initial bimodal grain structure, deformed to fracture under optimal conditions of superplastic deformation at a temperature Т = 520°С and flow stress σ = 4,5 MPa is investigated. The maximum elongation of specimens deformed to failure δ is 670%. It has been suggested that the specific type of fibrous structures found in the specimens of the investigated alloy 01420T probably depends on the volume of the metastable liquid-solid phase, which was concentrated in the form of inclusions at some grain boundaries and made a viscous flow during superplastic deformation, its shear viscosity , the characteristics of its surface tension, the degree of dynamic oxidation of the melt, and the kinetics of the development of this process. The final view of the fibers and their shape, likely, depends not only on the nature of the viscous flow of the liquid-solid material, but also on the process of its crystallization during the cooling of the specimen in air to room temperature after mechanical tests. It was found that in view, all fibrous structures found in the working parts of the specimens can be conditionally divided into the following: cylindrical fibers; tapered fibers; cylindrical fibers on which there is a thickening or one or more drop-like formations; ribbon-like fibers; fibers that look like stalactites or stalagmites. The reasons for the formation of cracks on ribbon-like fibers are considered. It is assumed that they were formed as a result of relaxation of internal stresses, which were not fully minimized in the course of recrystallization, which was carried out when the sample was cooled. The reasons for the formation of droplets on the fibers are considered. It has been suggested that fibrous structures similar to stalactites and stalagmites were formed from a viscous material, which, in the course of superplastic deformation, as a result of crystallization, occured in local microvolumes of fibers, gradually turned from liquid-solid to solid-liquid. This led to the fact that in the crystallized microvolume of this fiber, the viscous homogeneous flow of the material probably turned into a localized flow, which is characteristic of the plastic flow carried out as a result of displacement of dislocations in the solid phase, and leads to the formation of stalagmitic fibers.

Thermally driven structural phase transformation and dislocation density of CdS nanoparticles precipitated without surfactant in KOH alkaline medium

The present research demonstrates a detailed discussion for the effect of annealing temperature on the structural transformation and surface morphology of the CdS nanoparticles synthesized using the precipitation method without surfactant in KOH alkaline medium. The annealing temperature used was in the range of 160 – 480 oC. The samples structural, functional group and morphological properties were investigated by using XRD, FTIR and SEM techniques. XRD analysis reveals that the CdS has gradually been transformed from the pure cubic to hexagonal polycrystalline structure as well as improved crystallinity upon increasing the temperature. Besides, the parameters of average crystallite size and dislocation density were calculated using the established Debye- Scherrer equation. The average crystallite size was in nano-dimension and increases gradually with temperature. The FTIR spectra indicate that the characteristic vibration band of CdS emerged in the lower wavenumber region of 650 and 500 cm-1, and the band becomes stronger as the temperature rises. Also, the SEM images demonstrate that the CdS exhibits uniform spherical morphology and the particle size grows larger at elevated temperatures. The improved crystallinity and structural properties tuning ability against temperature allows beneficial optical applications as solar cells, photocatalysts, non-linear optics, light emitting diodes and optoelectronic devices.

Phase discontinuities induced scintillation enhancement: coherent vortex beams propagating through weak oceanic turbulence

Under the impact of an infinitely extended edge phase dislocation, optical vortices (screw phase dislocations) with tight spacing induce scintillation enhancement. As the simplest case, the scintillation index of an interfering beam consisting of two Gaussian vortex beams with topological charges through weak oceanic turbulence is researched via derivation and phase screen simulation to present this phenomenon. Different combinations of two types of phase discontinuities can be obtained by changing the overlapping degree and the phase difference of two coherent Gaussian vortex beams. The scintillation indexes for them verify that the formation condition of the phenomenon is the coexistence of two types of phase discontinuities. And the enhanced scintillation index can be several orders of magnitude larger than that of a plane wave under weak perturbation (Rytov variance). This phenomenon could be useful for both optical vortex detection and perturbation measurement.

Ni/Ni3Al interface-dominated nanoindentation deformation and pop-in events

Nickel-based single crystal alloys have excellent mechanical properties due to its unique two-phase structure and interface. Therefore, molecular dynamics methods were used to simulate nanoindentation and microstructural evolution. We found the indenter reaction force and hardness of the Ni3Al phase is the largest. The pop-in event in Ni3Al phase is more obvious than that in the Ni phase and Ni/Ni3Al phase. Because lots of dislocations in the Ni3Al phase break through the barrier of the interface and cut into the Ni phase, while dislocations in the Ni phase only slip inside the Ni phase. Moreover, we found that the position of the starting point of the adhesion force recovery is mainly related to the elastic recovery of the material. The stronger the elastic recovery of the phase, the smaller the depth value corresponding to the starting point of the recovery. We further studied the variation of potential energy with indentation depth and found that the change of wave trough of the load–displacement (P–h) curve is related to stacking fault energy. This study has important theoretical guiding significance for the in-depth understanding and engineering application of the mechanical properties of nickel-based single crystal alloys.

Focal beam structuring by triple mixing of optical vortex lattices

On-demand generation and reshaping of arrays of focused laser beams is highly desired in many areas of science and technology. In this work, we present a versatile approach for laser beam structuring in the focal plane of a lens by triple mixing of square and/or hexagonal optical vortex lattices (OVLs). In the artificial far field the input Gaussian beam is reshaped into ordered arrays of bright beams with flat phase profiles. This is remarkable, since the bright focal peaks are surrounded by hundreds of OVs with their dark cores and two-dimensional phase dislocations. Numerical simulations and experimental evidences for this are shown, including a broad discussion of some of the possible scenarios for such mixing: triple mixing of square-shaped OVLs, triple mixing of hexagonal OVLs, as well as the two combined cases of mixing square-hexagonal-hexagonal and square-square-hexagonal OVLs. The particular ordering of the input phase distributions of the OV lattices on the used spatial light modulators is found to affect the orientation of the structures ruled by the hexagonal OVL. Reliable control parameters for the creation of the desired focal beam structures are the respective lattice node spacings. The presented approach is flexible, easily realizable by using a single spatial light modulator, and thus accessible in many laboratories.

Deformation mechanism and tensile properties of nanocrystalline CoCrCuFeNi high-entropy alloy: a molecular dynamics simulation study

For purpose of investigating the damage mechanism and tensile properties of the nanocrystalline CoCrCuFeNi high-entropy alloy, the tension experiment simulations were performed using the molecular dynamics method. The effects of the grain size, strain rate, experiment temperature, and percentage of components were considered in detail. By changing the simulated conditions of the tension experiment, the deformation and the grain growth of the nanocrystalline CoCrCuFeNi high-entropy alloy were mentioned and analyzed. The important mechanical factors such as phase transformation, stress-strain relation, shear strain, tensile strength, dislocation density, and von Mises stress were strongly influenced by changing the simulated conditions and deeply discussed.

Molecular dynamics simulation of the interaction between cracks in single-crystal aluminum

The interaction between cracks, as well as their propagation, in single-crystal aluminum was simulated at the atomic scale by molecular dynamics method and modified embedded atom method. Further, the effects of the distance between two cracks and the difference of crack size on crack propagation and mechanical properties were comprehensively investigated. The results demonstrated that multi-crack propagation in aluminum is a quite complex process, which is accompanied by micro-crack growth, merging, stress shielding, and the crossing of slip bands. During crack propagation, there are interactions between cracks. Such interactions inhibit the phase transition, dislocation and slip bands at the crack tip, and also affect the direction of crack propagation. The intensity of the interaction between two cracks decreases with the increase of the distance between them and increases with increasing crack size. Moreover, the strength limit of aluminum decreases with the increase of the distance between cracks and the difference in their size.

Strain hardening behavior of additively manufactured and annealed AlSi3.5Mg2.5 alloy

The ductility of the Al alloys produced by additive manufacturing (AM) has become a critical property, as the AM Al alloys are increasingly used in the automotive industry. However, the ductility of as-built AM Al alloys is relatively low, even with optimized AM conditions. The post-annealing treatment provides an efficient way to improve ductility. Previous investigation has shown that the annealed AM AlSi3.5Mg2.5 alloy possesses superior ductility. However, the plastic deformation micro-mechanisms of the annealed AM AlSi3.5Mg2.5 alloy remain unclear. In this study, in-situ neutron diffraction was employed to explore the annealed AM AlSi3.5Mg2.5 alloy. The evolutions of phase stresses, dislocation density, and crystallite size in the annealed AM AlSi3.5Mg2.5 alloy during tensile deformation were analyzed. The experimental investigation reveals that the dislocation density in the Al matrix of the annealed AM AlSi3.5Mg2.5 alloy increases slowly in the early plastic deformation stage, and it reaches a saturated level upon the following uniform deformation. The crystallite size decreases quickly in the early deformation stage, and then it decreases slowly. The Kocks-Mecking model and the Voce model can capture the strain hardening behavior well. The determined physical constitutive equations can be applied in continuum mechanical computer simulations.

Direct generation of ultrafast vortex beam from a Tm:CaYAlO4 oscillator featuring pattern matching of a folded-cavity resonator.

Optical vortices, beams with spiral wavefronts and screw phase dislocations have been explored in applications in optical manipulation, quantum optics, and the next generation of optical communications. In traditional methods, optical vortices are generated using space light modulators or spiral phase plates, which would sharply decrease the integration of optical systems. Different from previous transverse mode conversion outside the cavity, here we experimentally demonstrate a direct generation of ultrafast vortex beam from a Tm:CaYAlO4 oscillator by pattern matching of a six-mirror-folded-cavity resonator. By accurately adjusted the angle of the end mirror and the distance L between the M4 and the SESAMs to control the beam diameter of laser incidence on the gain medium in the sagittal and tangential planes, a stable 2 µm ultrafast vortex laser emission of annular Laguerre-Gaussian (LG) mode was obtained with a maximum output power of 327 mW and pulse duration of 2.1 ps. A simple YAG crystal plate was used as handedness selector and a homemade Mach-Zehnder (MZ) interferometer has verified the vortical property of the LG01 mode. By furtherly controlling the cavity mode pattern matching, other stable transverse-mode operations for TEM00, high-order Hermite-Gaussian (HG) transverse mode and doughnut-shaped beams were successfully realized. This work provides a flexible and reliable way to generate mid-infrared ultrafast vortex beams and is of special significance for applications in the areas of molecular spectroscopy and organic material processing amongst others.

Influence of cooling rate on ω phase precipitation and deformation mechanism of a novel metastable β titanium alloy

This work investigated the effect of cooling rate (water quenching and air cooling) on the precipitation of ω phase after solution treatment in β-phase region, and its effect on the mechanical properties in a novel metastable β titanium alloy (Ti–5Mo–3Cr–Fe–3Zr). The initial microstructures, phase composition and deformation-induced microstructures have been investigated using SEM, EBSD, XRD, and TEM. The phase composition of water-quenched alloy and air-cooled alloy are β, α", and ω phase. The size and volume fraction of ω phase of air-cooled alloy are larger than that of water-quenched alloy, resulting in an increase in tensile strength and a decrease in ductility. Deformation mechanisms of Ti–5Mo–3Cr–Fe–3Zr alloy with different cooling rate change from stress-induced ω phase transformation and dislocation slip to only dislocation slip. The stress-induced ω lamellas parallel to [1-11]β direction along the [0001]ω1 direction, which is formed by {112}β<111>β slip. Dislocations can cut through the encountered ω phase to form ω-free deformation bands, which accounts for the ductility.

Three-dimensional spiral motion of microparticles by a binary-phase logarithmic-spiral zone plate.

Acoustic vortex beams, which have both linear and angular momentum, can be used to make precise acoustic tweezers. Limited by the symmetry of a normal vortex beam, these tweezers are usually used for trapping or rotating particles in two dimensions. Here, the three-dimensional spiral motion of two soft particles of different sizes was realized using a vortex beam with a twisted focus, which was synthesized by a silicone binary-phase logarithmic-spiral zone plate. Numerical simulations and experimental measurements demonstrated that the beam had anisotropic focuses of crescent transverse intensity profiles and a screw phase dislocation with a singularity at the center. Experiments showed that a small particle (k0r ≈ 1.3) can follow the twisted intensity of the beam, but a large particle (k0r ≈ 4.7) spirals up away from the twisted field pattern. This is attributed to the dominant gradient force for the small particle, whereas the scattering effect induced a scattering force combined with a gradient force for the large particle. This focused twisted beam, which was generated with a structured silicone plate, and the three-dimensional spiral motion of microparticles, advance the development of simple, compact, and disposable acoustic devices for the precise and diverse manipulation of microparticles.

Hydrogenation treatment under several gigapascals assists diffusionless transformation in a face-centered cubic steel

The use of hydrogen in iron and steel has the potential to improve mechanical properties via altering the phase stability and dislocation behavior. When hydrogen is introduced under several gigapascals, a stoichiometric composition of hydrogen can be introduced for steel compositions. In this study, a face-centered cubic (fcc) stainless steel was hydrogenated under several gigapascals. When the steel was not hydrogenated, the microstructure after depressurization was an fcc with a hexagonal close-packed (hcp) structure. In contrast, the hydrogenation treatment resulted in a fine lath body-centered cubic (bcc) structure arising from diffusionless transformation. In particular, the bcc phase formed through the following transformation sequence: fcc → hcp → dhcp (double hexagonal close-packed phase) → bcc. That is, the use of hydrogenation treatment realized fine microstructure evolution through a new type of diffusionless transformation sequence, which is expected to be used in future alloy design strategies for developing high-strength steels.

Phase Dislocations in Hollow Core Waveguides

This paper discusses the basic concepts of phase dislocations and vortex formation in the electric fields of fundamental air core mode of hollow core waveguides with specific types of rotational symmetry of the core-cladding boundary. Analysis of the behavior of the electric field phase in the transmission bands shows that the mechanism of light localization in the hollow core waveguides with discrete rotational symmetry of the core-cladding boundary cannot be completely described by the ARROW model. For an accurate description of the phase behavior, it is necessary to account for phase jumps of the magnitude of π when passing through the phase dislocations.

Effect of 1.5 wt% Copper Addition and Various Contents of Silicon on Mechanical Properties of 1.7102 Medium Carbon Steel.

Requirements for mechanical properties of steels are constantly increasing, and the combination of quenching and tempering is the method generally chosen for achieving high strength in medium carbon steels. This study examines the influence of various silicon contents from 1.06 to 2.49 wt% and the addition of copper (1.47 wt%) on the behavior of 1.7102 steel starting with the as-quenched state and ending with the tempered condition at the temperature of 500 °C. The microstructure was characterized by SEM and TEM, the phase composition and dislocation density were studied by XRD analysis, and mechanical properties were assessed by tensile and hardness testing, whereas tempered martensite embrittlement was assessed using Charpy impact test and the activation energy of carbide precipitation was determined by dilatometry. The benefit of copper consists in the improvement of reduction of area by tempering between 150 and 300 °C. The increase in strength due to copper precipitation occurs upon tempering at 500 °C, where strength is generally low due to a drop in dislocation density and changes in microstructure. The increasing content of silicon raises strength and dislocation density in steels, but the plastic properties of steel are limited. It was found that the silicon content of 1.5 wt% is optimum for the materials under study.

Dynamic evolution of edge dislocation and its effect on bcc-hcp martensitic transformation in dual-phase high-entropy alloy

The interactions between phase transformation and dislocation determine the evolution process of microstructures in dual-phase high-entropy (DP-HEA) alloy. However, due to rapid evolution process in martensitic transformation (MT) and the lack of corresponding atomic-scale simulation method, the dynamic evolution processes of dislocation in MT are not yet fully understood. In this work, molecular dynamics (MD) simulation is employed to study the atomic-scale dynamic evolution of edge dislocations and its effect on MT in Ta0.5HfZrTi. The simulation results reveal that the gliding of dislocations to the surface decreases the MT nucleation stress due to increased stress concentration. For the higher global stress (8.0 GPa), the growth of the martensitic variant is found impeded by the new martensitic variant produced near the dislocation core. For the lower global stress (7.1 GPa), the preset edge dislocation will migrate due to elastic strain's incompatibility between dislocation core and phase boundary.