Phase Dislocations Research Articles

Generalized stacking fault energy and dislocation properties in NiTi shape memory alloys

Abstract Dislocations in the austenite phase of Nickel-Titanium (NiTi) shape memory alloys (SMAs) are of critical importance mainly due to their connection to the unrecoverable strains accumulated in SMAs during cyclic loadings, and also the key role of dislocations in the structure and energy of grain boundaries in these alloys. In this paper, we investigate the energy and structure of dislocations, and also the generalized stacking fault energies in NiTi austenite by implementing molecular dynamics (MD) simulations. Through our results the validity and accuracy of three different potentials, two EAM potentials with different cutoff radii and one 2NN MEAM potential, for studying defect related properties of NiTi alloys is investigated. It is shown that while none of these potentials are particularly trained for predicting defect properties in NiTi systems, they can be efficiently utilized to study the stacking fault energies, the energy and structure of edge dislocations, stress distributions at the vicinity of edge dislocations, and even the details of dislocation dissociation in NiTi austenite phase. Particularly our results can be used as a tool to select the appropriate potential for studying any problem related to a specific aspect of defects in austenite NiTi SMAs.

Is it possible to create a perfect fractional vortex beam?

Laguerre–Gaussian beams of integer azimuthal index satisfy the fundamental principle of quantization of orbital angular momentum. Here, we consider light-induced orbiting of a trapped microparticle as a probe of the local orbital angular momentum density in both integer- and fractional-index perfect vortex beams. Simulations suggest that the distribution and the corresponding light-induced motion of the particle may be uniform in beams with integer azimuthal index, but fundamentally this cannot be achieved in beams with fractional index. We experimentally verify these predictions by light-induced trapping and rotation of individual microparticles in fractional index beams where we distribute the phase dislocations around the annular profile.

Tuning of Fano resonances by rotation of continuum: Wave faucet

We consider wave transmission in a non-axisymmetric waveguide which consists of cylindrical resonator and two semi-infinite cylindrical waveguides whose axes are shifted relative to each other by azimuthal angle Δϕ. Although rotation by Δϕ does not change the eigenfrequency spectrum of the resonator, it effectively tunes the Fano resonances and respectively gives rise to an analog of a faucet opening and closing wave flux under rotation. Moreover, under the rotation of the waveguide and variation of the length of resonator, numerous events of the Fano resonance collapse occur to argue for the bound states in the continuum. In the nearest vicinity of the bound states scattering wave function with orbital angular momentum shows phase dislocations with giant vortical acoustic flows around.

High temperature deformation of Laves phase precipitates in alumina-forming austenitic stainless steels

Laves phases with the C14 type structure are present in recently-developed, alumina-forming austenitic (AFA) stainless steels. Here, for the first time, we show the Laves phase undergoes plastic deformation when such AFA steels are deformed at 700°C at strain rate of 5×10−4s−1. The gliding dislocations in the Laves phase leave behind stacking faults that we have analyzed using transmission electron microscopy. The stacking faults are on the basal plane of the Laves phase and are bounded by 1/3 〈101¯0〉 partial dislocations. These slip of dislocations are effective plastic deformation and improve the ductility of AFA materials.

Interaction between phase transformations and dislocations at incipient plasticity of monocrystalline silicon under nanoindentation

Structural phase transformation and dislocation slip are two important deformation modes of monocrystalline silicon. In the present work, we elucidate mechanisms of inhomogeneous elastic-plastic transition in spherical nanoindentation of monocrystalline silicon by means of molecular dynamics simulations. The Stillinger-Weber potential is utilized to present simultaneous phase transformations and dislocation activities in the silicon nanoindentation. And a bond angle analysis-based method is proposed to quantitatively clarify silicon phases. The influence of crystallographic orientation on the silicon nanoindentation is further addressed. Our simulation results indicate that prior to the “Pop-In” event, Si(010) undergoes inelastic deformation accompanied by the phase transformation from the Si-I to the Si-III/Si-XII, which is not occurred in Si(110) and Si(111). While the phase transformation from the Si-I to the bct-5 is the dominant mechanism of incipient plasticity for each crystallographic orientation, dislocation nucleation is also an operating deformation mode in the elastic-plastic transition of Si(010). Furthermore, interactions between phase transformations and dislocations are more pronounced in Si(010) than the other two crystallographic orientations.

Robust processing of phase dislocations based on combined unwrapping and inpainting approaches.

This Letter proposes a robust processing of phase dislocations to recover continuous phase maps. The approach is based on combined unwrapping and inpainting methods. Phase dislocations are determined using an estimator based on the second order phase gradient. The algorithm is validated using a realistic simulation of phase dislocations, and the phase restoration exhibits only weak errors. A comparison with other inpainting algorithms is also provided, demonstrating the suitability of the approach. The approach is applied to experimental data from off-axis digital holographic interferometry. The phase dislocation from phase data from a wake flow at Mach 0.73 are identified and processed. Excellent phase restoration can be appreciated.

Promotion of powder crystallinity and its influence on the properties of Nd:YAG transparent ceramics

The Nd:YAG raw powders of various crystallinities were obtained from the co-precipitated precursor by calcining at 1100–1250 °C for 2–6 h. XRD, HRTEM and EDX were employed to analyze the powder crystallinity, e.g., its phases, changes of lattice constants, surface defects and morphologies. The subsequent ceramics were fabricated by simply vacuum sintering at 1780 °C for 10 h to well compare the initial intrinsic effect of various powder crystallinities. The induced defect states and corresponding properties of ceramics were explored and discussed. The results show that poorly crystallized powder involves a thick layer on particle surface, which is not well crystallized but can escape XRD detection. This poorly crystallized powder contributes more the inhomogeneous ceramic sintering and introduces more the defects in final ceramics, e.g., impure phase inclusions and dislocations. The results reveal that powder crystallinity should be promoted and considered as a further way to improve ceramic properties.

STRUCTURE-PHASE STATES, MECHANICAL AND TRIBOLOGICAL PROPERTIES OF THERMOMECHANICALLY STRENGTHENED BEAM

Using the methods of modern materials science the researches and the comparative analysis are performed for the structural and phase states, dislocation substructure, mechanical and tribological properties of the surface of the thermo-mechanically strengthened and non-strengthened I-beam DP155 from low carbon steel used for mine monorails. It was found that accelerated cooling of the beam section in line 450 of the mill at “EVRAZ – Consolidated West-Siberian Metallurgical Plant” (rolling speed is 6 m/s, the water pressure on the sections of shelf cooling is 0.22 – 0.28 MPa, the temperature before the refrigerator is approximately 800 °С) forms a high defect surface layer structure characterized by higher (relatively to the unhardened state) values of hardness, wear resistance and scalar dislocation density. In the non-heat-resistant state, the microhardness of the samples is 2.70 ± 0.33 GPa, and Young’s modulus is 269.6 ± 27.1 GPa. Thermo-mechanical hardening of the material leads to a decrease in its microhardness up to 3.30 ± 0.29 GPa and to an increase in the Young’s modulus up to 228.2 ± 25.7 GPa, respectively. In addition, an increase in the range of microhardness values from 2.20 – 3.80 GPa to 2.64 – 4.60 GPa and a decrease in the Young’s modulus range from 208.0 to 403.0 GPa to 184.1 to 278.2 GPa is established during thermomechanical hardening of steel. It is shown that the thermomechanical strengthening of steel leads to an increase in the wear resistance of the surface layer in approximately 1.36 times (the wear rate varies from 5.3·10 –5 mm 3 /N·m to 2.9·10 –5 mm 3 /N·m) and an increase in the friction coefficient by 1.36 times (from 0.36 to 0.49). In the non-strengthened state the dislocation chaos structure is observed (the dislocation scalar density is (0.9 . 1.0)·10 10 cm –2 ). High-temperature rolling and subsequent accelerated cooling of the samples lead to the formation of a strip dislocation substructure in the grains of ferrite and a reticular dislocation substructure in martensite grains (the average scalar dislocation density in the surface layer is 4.5·10 10 cm –2 ). The possible reasons for the observed regularities are discussed.

Phase slip process and charge density wave dynamics in a one dimensional conductor

In this paper, we study the phase slip effect on the charge density wave (CDW) dynamics in a one-dimensional conductor in the weak pinning limit. A considerable enhancement of JCDW is observed in the presence of phase slips. In addition, a spatial dependence of the CDW current density JCDW is also studied showing that a decrease of JCDW with distance from the current contact occurs. The results are discussed in terms the relationship between additional phase slips and the mobility of phase dislocations nucleated at electrical contacts.

The molecular Aharonov–Bohm effect redux

A solvable molecular collision model that predicts Aharonov–Bohm (AB) like scattering in the adiabatic approximation is introduced. For it, we propagate coupled channel wave packets without resorting to a Born–Oppenheimer (BO) approximation. In those, exact, solutions we find evidence of topological phase dislocation lines that are independent of the collision energy and provide definitive signatures of AB-like scattering. The results of these simulations contrast with the conclusions of a recent study that suggests survival of the molecular Aharonov–Bohm effect only in the adiabatic limit in which the nuclear reduced mass . We discuss generalizations of this model and consider possible screening of the Mead–Truhlar vector potential by the presence of multiple conical intersections (CI). We demonstrate that the Wilson loop phase integral has the value −1 if it encloses an odd-number of CI's, and takes the value +1 for an even number. Within the scope of this model, we investigate the ultra-cold limit of scattering solutions in the presence of a conical intersection and comment on the relevance of Wigner threshold behavior for s-wave scattering.

Phase calibration unwrapping algorithm for phase data corrupted by strong decorrelation speckle noise.

Robust phase unwrapping in the presence of high noise remains an open issue. Especially, when both noise and fringe densities are high, pre-filtering may lead to phase dislocations and smoothing that complicate even more unwrapping. In this paper an approach to deal with high noise and to unwrap successfully phase data is proposed. Taking into account influence of noise in wrapped data, a calibration method of the 1st order spatial phase derivative is proposed and an iterative approach is presented. We demonstrate that the proposed method is able to process holographic phase data corrupted by non-Gaussian speckle decorrelation noise. The algorithm is validated by realistic numerical simulations in which the fringe density and noise standard deviation is progressively increased. Comparison with other established algorithms shows that the proposed algorithm exhibits better accuracy and shorter computation time, whereas others may fail to unwrap. The proposed algorithm is applied to phase data from digital holographic metrology and the unwrapped results demonstrate its practical effectiveness. The realistic simulations and experiments demonstrate that the proposed unwrapping algorithm is robust and fast in the presence of strong speckle decorrelation noise.

Formation of high-order acoustic Bessel beams by spiral diffraction gratings.

The formation of high-order Bessel beams by a passive acoustic device consisting of an Archimedes' spiral diffraction grating is theoretically, numerically, and experimentally reported in this paper. These beams are propagation-invariant solutions of the Helmholtz equation and are characterized by an azimuthal variation of the phase along its annular spectrum producing an acoustic vortex in the near field. In our system, the scattering of plane acoustic waves by the spiral grating leads to the formation of the acoustic vortex with zero pressure on axis and the angular phase dislocations characterized by the spiral geometry. The order of the generated Bessel beam and, as a consequence, the size of the generated vortex can be fixed by the number of arms in the spiral diffraction grating. The obtained results allow for obtaining Bessel beams with controllable vorticity by a passive device, which has potential applications in low-cost acoustic tweezers and acoustic radiation force devices.

Filamentation of an annular laser beam with a vortex phase dislocation in fused silica

The filamentation of a femtosecond laser pulse in fused silica has been numerically investigated for the case of an annular beam with a phase singularity at a wavelength of . The spatiotemporal propagation dynamics of the pulse and the transformation of its frequency-angular spectra are analysed. It is shown that a tubular structure with a radius of , peak intensity of about , and maximum plasma density on the order of is formed in the nonlinear focus; the length of this structure significantly exceeds the waist length in the linear case. The results of the analysis are compared with the data obtained for an annular beam free of phase dislocations and for a Gaussian beam.

The Gaussian distribution of lattice size and atomic level heterogeneity in high entropy alloys

It has been shown that heterogeneity at the micro-structure level can effectively enhance the strength and ductility of materials. Here we demonstrate via density-functional theory calculations that high entropy alloys (HEAs) exhibit atomic level heterogeneity in bond length and stacking fault energy (SFE) which could lead to HEAs’ excellent hardenability and deformability. The bond length of HEA FeCoNiCrCu follows a Gaussian distribution, in contrast to the single-valued SFE and bond length in conventional crystalline counterparts. Its SFEs also fall into a wide span, which could introduce sequential and dispersed nucleation of different plastic sources including phase transformation, deformation twinning, and dislocations, giving rise to their exceptionally high hardenability and ductility. The atomic level heterogeneity in HEAs further gives rise to solid solution strengthening during dislocation motion. The concept of making atomic level heterogeneity can also be utilized in other metallic alloys for better hardenability and ductility.

Study of mechanical behavior of AFM silicon tips under mechanical load

In this paper we address critical issues concerning calibration of AFM based methods used for nanoscale mechanical characterization of materials. It has been shown that calibration approaches based on macroscopic models for contact mechanics may yield excellent results in terms of the indentation modulus of the sample, but fail to provide a comprehensive and actual information concerning the tip–sample contact radius or the mechanical properties of the tip. Explanations for the severely reduced indentation modulus of the tip included the inadequacies of the models used for calculations of the tip–sample contact stiffness, discrepancies in the actual and ideal shape of the tip, presence of the amorphous silicon phase within the silicon tip, as well as negligence of the actual size of the stress field created in the tip during elastic interactions. To clarify these issues, we investigated the influence of the mechanical load applied to four AFM silicon tips on their crystalline state by exposing them to systematically increasing loads, evaluating the character of the tip–sample interactions via the load-unload stiffness curves, and assessing the state of the tips from HR-TEM images. The results presented in this paper were obtained in a series of relatively simple and basic atomic force acoustic microscopy (AFAM) experiments. The novel combination of TEM imaging of the AFM tips with the analysis of the load-unload stiffness curves gave us a detailed insight into their mechanical behavior under load conditions. We were able to identify the limits for the elastic interactions, as well as the hallmarks for phase transformation and dislocation formation and movement. The comparison of the physical dimensions of the AFM tips, geometry parameters determined from the values of the contact stiffness, and the information on the crystalline state of the tips allowed us a better understanding of the nanoscale contact.

Analysis of the mechanical behavior of a 0.3C-1.6Si-3.5Mn (wt%) quenching and partitioning steel

A 0.3C-1.6Si-3.5Mn(wt%) steel was subjected to different Q&P treatments, leading to different combinations of initial martensite, bainite, secondary martensite, and retained austenite. In this study, initial martensite refers to the martensite formed during the initial quenching step and then subjected to an isothermal treatment at 400°C; secondary martensite refers to martensite formed during quenching from 400°C to room temperature. The yield strength of each constituent phase was determined by applying physical models to the data obtained from detailed microstructural characterization. The yield strength (uncertainty of 5%) of the Q&P microstructures was calculated by using a composite law to account for the contribution of each constituent phase. The dependence of the yield strength on the microstructural features of the Q&P microstructures was revealed by using the approach developed in this work. For example, initial martensite (which has a high yield strength and is the dominant phase in the microstructures) had the greatest effect on the yield strength of the Q&P microstructures. Furthermore, the phase fraction and dislocation density of this phase increased with decreasing quenching temperature, leading to an increase in the yield strength of the material.

Estimate of the change in the effective beam width by the streamline method for axisymmetric laser beams in a turbulent atmosphere

Results of studying how the initial distribution of the laser beam field affects the change in the effective width of the beam in the process of its propagation in a turbulent atmosphere are presented. The investigations are carried out using the method of streamlines of the average Poynting vector for axisymmetric light beams. The effective beam width in the receiving plane is studied depending on the shape of the initial intensity distribution and presence of the phase dislocation in the initial field. It is shown that parameters of ring and vortex beams can be chosen such that their effective width in the receiving plane will be less than for a Gaussian beam with the same initial effective width in the process of laser radiation propagation in a turbulent atmosphere.

Structure and Physicomechanical Properties of Welded Joints of AISI 321 Steel

We present the results of experimental investigations of the structural-phase state and mechanical properties of welded joints of AISI 321 austenitic steel from thin-walled transformable-volume structures (TVS) of spacecraft application. We estimate the contributions of the phase composition, grain, subgrain, and dislocation structures to the changes in the strength characteristics and the level of local internal stresses in the metal of joints for different conditions of pulse microplasma welding. The optimal technological conditions providing the required mechanical properties of welded structures of the TVS are established.

Duality of Orbital-Symmetry-Allowed Transition States for Thermal Sigmatropic Hydrogen Shifts in Transition Metal Compounds.

Herein, the Zimmerman Möbius/Hückel concept is extended to pericyclic reactions involving transition metals. While sigmatropic hydrogen shifts in parent hydrocarbons are either uniquely antarafacial or suprafacial, we have shown by theoretical orbital topology considerations and quantum chemical computations at DFT level that both modes of stereoselectivity must become allowed in the same system as a consequence of Craig-Möbius-type orbital arrays, in which a transition metal d orbital induces a phase dislocation in metallacycles. This may have fundamental implications for the understanding of reactivity and bonding in organometallic chemistry.

Three-dimensional imaging of shear bands in bulk metallic glass composites.

The mechanism of the increase in ductility in bulk metallic glass matrix composites over monolithic bulk metallic glasses is to date little understood, primarily because the interplay between dislocations in the crystalline phase and shear bands in the glass could neither be imaged nor modelled in a validated way. To overcome this roadblock, we show that shear bands can be imaged in three dimensions by atom probe tomography from density variations in the reconstructed atomic density, which density-functional theory suggests being a local-work function effect. Imaging of near-interface shear bands in Ti48 Zr20 V12 Cu5 Be15 bulk metallic glass matrix composite permits measurement of their composition, thickness, branching and interactions with the dendrite interface. These results confirm that shear bands here nucleate from stress concentrations in the glass due to intense, localized plastic deformation in the dendrites rather than intrinsic structural inhomogeneities.