Analysis Of Dislocation Structure Research Articles

Multi-scale study of subsurface fatigue cracking behavior of laser-powder bed fused Inconel 718 at stress ratios and temperatures

Multi-scale subsurface fatigue cracking is a significant lifetime-limiting failure mode, not yet fully understood, of additively manufactured superalloys in mechanical applications. Here, combined with some technologies of electron-backscattered diffraction, focused Ion beam, and transmission electron microscope, a series of axial fatigue tests at different stress ratios and temperatures are conducted to investigate the fatigue failure behavior of as-built superalloy manufactured by Laser-Powder Bed Fusion. The main results show that especially in the assistance of defect (e.g., pore or inclusion) and elevated temperature, the faceted cracking related to the grain feature is a typical failure mode. It is mainly controlled by shear stress and is more popular under a positive stress ratio. Based on the analysis of dislocation structure with faceted cracking, the localized plastic deformation at 25 °C is controlled by anti-phase boundary (APB) shearing, whereas that at 650 °C is caused by a combination of APB shearing, precipitation bypassing, and stacking fault shearing mechanisms. Combined with the definition of threshold value at the crack tip, a crack nucleation life prediction approach associated with the faceted cracking characteristics is proposed, and the predicted results show a good agreement with the experimental results.

Effective Penetration Depths Investigation on Synchrotron X-Ray Topographic Images of Basal Plane Dislocations in 4H-SiC Crystals through Ray-Tracing Simulation

The demand for power electronic devices fabricated from silicon carbide (SiC) has increased rapidly in recent years for a diverse range of existing and emerging applications. The development of device technology highly depends on the quality of substrate and epitaxial material and the number of deleterious defects within, as those are critical factors that impact the device performance and long-term reliability [1, 2]. Therefore, understanding the nature of defects is of great importance for preventing the degradation or premature breakdown of devices. Synchrotron X-ray topography (XRT) is a dominant analytical method for the imaging and characterization of crystallographic defects. For 4° off-axis (0001) 4H-SiC wafers, the grazing-incidence geometry topographic technique is commonly employed to focus on the defect investigation near surface region. In this geometry, a small incidence angles (~1-2°) is used for allowing X-ray to only penetrate the sample with a shallow depth below the surface, therefore, it is essential to establish a procedure to define the effective penetration depth for detailed analysis of dislocation structures and density calculations. In earlier studies, Dudley in 1989 concluded that the observed penetration depths are limited by photoelectric absorption rather than extinction [3]. Ishiji in 2017 estimated the penetration depth based on the photoelectric absorption experienced by the diffracted X-rays [4]. However, only basal plane dislocations (BPDs) with screw segments parallel to the off-cut [11-20]-direction were evaluated in that study, which is insufficient for evaluating other types of BPDs. Therefore, a universal method to determine the effective penetration depth for all dislocation types would be of more practical importance.In this study, a procedure to define this effective penetration depth is demonstrated by investigating physical vapor transport (PVT) grown 4° off-axis 4H-SiC crystals through preforming systematic analysis of topographic and ray-tracing simulated contrast of BPDs with different Burgers vector and line direction combinations. The topographic images are recorded through synchrotron monochromatic beam in grazing incidence geometry. The simulation model is based on the principle of orientation contrast mechanism [5, 6] and developed by considering both the effects of surface relaxation [7] and X-ray absorption [8, 9] for evaluating the contributed factors of dislocation contrast formation. Analysis of the results indicates the observable dislocation contrast depends on the effective misorientation associated with the dislocation modulated by the photoelectric absorption effect. The dislocations with larger effective misorientation are associated with longer projected lengths and corresponding deeper effective penetration depths. A more simplified factor based on an approximate expression for the misorientation is also introduced in this study as an alternative to the full ray-tracing simulation approach. The effective penetration depth determination method presented in this study enables three-dimensional dislocation configuration analysis and accurate density calculations.Reference: Das, H., et al., Materials Science Forum, 2019. 963: p. 284-287.Kimoto, T., et al. 2017 IEEE International Reliability Physics Symposium (IRPS). 2017.Dudley, M., J. Wu, and G.D. Yao, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1989. 40-41: p. 388-392.Ishiji, K., et al., Japanese Journal of Applied Physics, 2017. 56(10): p. 106601.Dudley, M., X.R. Huang, and W. Huang, Journal of Physics D: Applied Physics, 1999. 32(10A): p. A139-A144.Huang, X.R., et al., Journal of Applied Crystallography, 1999. 32(3): p. 516-524.Peng, H., et al., Journal of Applied Crystallography, 2021. 54(2): p. 439-443.Fujie, F., et al., Acta Materialia, 2021. 208: p. 116746.Ailihumaer, T., et al., Materials Science and Engineering: B, 2021. 271: p. 115281. Figure 1

Dislocation structure analysis in the strain gradient of torsion loading: a comparison between modelling and experiment

Complex stress states due to torsion lead to dislocation structures characteristic for the chosen torsion axis. The formation mechanism of these structures and the link to the overall plastic deformation are unclear. Experiments allow the analysis of cross sections only ex situ or are limited in spacial resolution which prohibits the identification of the substructures which form within the volume. Discrete dislocation dynamics simulations give full access to the dislocation structure and their evolution in time. By combining both approaches and comparing similar measures the dislocation structure formation in torsion loading of micro wires is explained. For the ⟨100⟩ torsion axis, slip traces spanning the entire sample in both simulation and experiment are observed. They are caused by collective motion of dislocations on adjacent slip planes. Thus these slip traces are not atomically sharp. Torsion loading around a ⟨111⟩ axis favors plasticity on the primary slip planes perpendicular to the torsion axis and dislocation storage through cross-slip and subsequent collinear junction formation. Resulting hexagonal dislocation networks patches are small angle grain boundaries. Both, experiments and discrete dislocation simulations show that dislocations cross the neutral fiber. This feature is discussed in light of the limits of continuum descriptions of plasticity.

Chromian spinel neomineralisations and the microstructure of plastically deformed ophiolitic peridotites (Kraka massifs, Southern Urals, Russia)

Results of a microstructural study of spinel peridotite samples obtained from the Kraka massif in the Southern Urals, involving findings of Cr-spinel neomineralisations within intensive ductile deformed silicates (olivine and orthopyroxene), are presented. The new-formed Cr-spinel grains show different stages of syn-deformation growth as evidenced by investigations combining petrography, decorated dislocation structure analysis, scanning electron microscopy and electron backscatter diffraction (EBSD). Initial precipitations appearing as rods or lamellae are observed to form around structural defects of host silicate grains (olivine and orthopyroxene) by means of impurity segregation or heterogeneous nucleation in the most distorted lattice regions (i.e., in the predominant recrystallisation zones). Syn-deformational crystal growth leads to a complication and coarsening of the grain morphology by coalescence due to a reduction in grain boundary (interfacial) energy. While in the process of growing, the Cr-spinel grains capture fragments of silicate matrix in the solid-state process. The final stage of Cr-spinel growth involves a change in morphology resulting in their characteristic crystallographic forms (spheroidisation). The presence of euhedral Cr-spinel grains, typical for ophiolitic dunite bodies, is a result of interfacial energy reduction in areas of grain boundaries of the hardest phase. The general trend of the observed stages relates closely with the localisation of deformation zones in the upwelling upper mantle (diapir), which are composed by the weakest phase of olivine (dunites). The concentration of the weakest olivine phase in the mobile zones, which is energetically beneficial, explains why dunite bodies having euhedral chromite grains comprise the dynamic equilibrium rocks in the plastic flow localisation zones in upper mantle diapirs. Conversely, assemblages having pyroxene phases, which are stronger and larger in size compared to olivine, are not stable in these zones.

Hydrogen-enhanced fatigue crack growth behaviors in a ferritic Fe-3wt%Si steel studied by fractography and dislocation structure analysis

The effect of hydrogen (H) on the fatigue behavior is of significant importance for metallic structures. In this study, the hydrogen-enhanced fatigue crack growth rate (FCGR) tests on in-situ electrochemically H-charged ferritic Fe-3wt%Si steel with coarse grain size were conducted. Results showed strong difference between the H-charged and the non-charged conditions (reference test in laboratory air) and were in good agreement with the results from literature. With H-charging, the fracture morphology changed from transgranular (TG) type to “quasi-cleavage” (“QC”), with a different fraction depending on the loading frequency. With the help of electron channeling contrast imaging (ECCI) inside a scanning electron microscope (SEM), a relatively large area in the failed bulk specimen could be easily observed with high-resolution down to dislocation level. In this work, the dislocation sub-structure immediately under the fracture surfaces were investigated by ECCI to depict the difference in the plasticity evolution during fatigue crack growth (FCG). Based on the analysis, the H-enhanced FCG mechanisms were discussed.

Rotational-Electron Channeling Contrast Imaging analysis of dislocation structure in fatigued copper single crystal

The dislocation structure of copper single crystal during cyclic fatigue has been characterized by the Rotational-Electron Channeling Contrast Imaging (R-ECCI) method. This technique is based on the acquisition of series of BackScattered Electron (BSE) images during the sample rotation. It facilitates the determination of orientation conditions in the Scanning Electron Microscope (SEM) for fast and accurate dislocation characterization, regardless of the initial orientation of the sample. The technique was applied to copper, observed in its as-received state as well as after several cyclic fatigue loadings. The evolution of the dislocation structure is described as a function of the applied stress.

Towards bend-contour-free dislocation imaging via diffraction contrast STEM

Dislocation imaging using transmission electron microscopy (TEM) has been an invaluable tool for characterizing crystallographic defects in metals. Compared to conventional TEM imaging, diffraction contrast imaging scanning transmission electron microscopy (DCI STEM) with appropriate setting can provide better defect contrast with almost negligible bend contour artifacts, enabling more effective analysis of dislocation structures. Here, we investigated why STEM can suppresses bend contour, and how dislocation contrast behaves along with different STEM imaging parameters. Using a body-centered cubic HT-9 ferritic/martensitic alloy as an example, a simple procedure and operational theory are described at the beginning to help set up DCI STEM experiments. Comparing with conventional TEM and the STEM strictly complying with the principle of reciprocity, we found that a pair of STEM convergence and collection semi-angles, αS and βS, a few milliradians in size is essential for bend-contour-free defect imaging. It works in concert such that the convergence STEM probe opens up the reciprocal space, and then a comparable collection region evens out the rocking-curve oscillation and alleviates bend contours from the reciprocal space. This fundamental advantage is unique in DCI STEM. Practical guidelines regarding STEM parameters and specimen orientation and thickness are then provided for DCI STEM dislocation imaging. Lastly, we show that coupling DCI STEM with spectrum images of electron energy loss spectroscopy and of energy-dispersive X-ray Spectroscopy offers a comprehensive characterization of crystallographic defects and chemical information of complex microstructures.

Analysis of Dislocation Structures in 4H‐SiC by Synchrotron X‐Ray Topography

SUMMARYCurrent 4H‐SiC wafers contain certain amount of dislocations, stacking faults, and other lattice‐defects. These defect structures evolve during various processes for power device fabrication. It is very important to examine evolutions of dislocation structures during power device processes, as well as the effect of dislocations on performances of fabricated power devices. Since lattice defects can be observed at only subsurface regions selectively by Berg–Barrett X‐ray topography, we have applied this useful observation technique to 4H‐SiC technology to solve various technological issues. Appearances of scratch‐like surface morphology after epi‐film growth on very flat substrate surface by chemomechanically polish were examined by this technique, and the mechanism was discussed. Analyses of dislocation structure evolutions during epi‐film growth and also basal‐plane dislocation glide by recombination of electrons and holes were discussed. Effects of threading‐screw dislocations on reliability of MOS structures and on current leakage in pn‐junction under reversed bias condition were investigated and discussed. Various important suggestions for 4H‐Sic industries were obtained.

Low cycle fatigue in aluminum single and bi-crystals: On the influence of crystal orientation

Aluminum single crystals with three different double-slip orientations ([2̅32], [3̅3̅2], [130]) and two aluminum bi-crystals – one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured by in-situ digital image correlation (DIC). Dislocation structure analysis was performed by electron channeling contrast imaging (ECCI) and the evolution of local misorientations was recorded by high resolution electron backscatter diffraction (EBSD). The DIC results show a homogeneous strain amplitude distribution in the single crystals while the measured strain amplitude in the low-angle grain boundary bi-crystal sample differs significantly. ECCI observations reveal the presence of dislocation cells elongated along the trace of the primary {111} slip plane in all investigated crystals and the formation of deformation bands parallel to the trace of {110} planes. Deformation bands (DB) were observed in all samples but their frequency and misorientation with respect to the matrix was found to sensitively depend on the crystal orientation and the local strain amplitude. Our results on the bi-crystals show that the grain orientation mainly determines the local stresses and therefore also the formation of the associated dislocation structures rather than the grain boundary character.

4H-SiC中の転位組織の放射光トポグラフ法による解析

4H-SiC中の転位組織の放射光トポグラフ法による解析

Effect of Strain Rate on Ductile-to-Brittle Transition Temperature of a Free-Standing Pt-Aluminide Bond Coat

The ductile-to-brittle transition temperature (DBTT) of a free-standing Pt-aluminide (PtAl) bondcoat was determined using the microtensile testing method and the effect of strain rate variation, in the range 10−5 to 10−1 s−1, on the DBTT studied. The DBTT increased appreciably with the increase in strain rate. The activation energy determined for brittle-to-ductile transition, suggested that such transition is most likely associated with vacancy diffusion. Climb of 〈100〉 dislocations observed in analysis of dislocation structure using a transmission electron microscope (TEM) supported the preceding mechanism.

TEM analysis of dislocation structures formed in the Cr-doped grain boundary of alumina

An alumina bicrystal with a Cr-doped {1−100}/[0001] 2° low-angle tilt grain boundary was fabricated by diffusion bonding at elevated temperatures, and the microstructures around the grain boundary were observed by transmission electron microscopy. It was confirmed that an approximately 200 nm width Cr-rich secondary phase was formed along the grain boundary. Not only an array of triply dissociated dislocations was introduced along the grain boundary but also misfit dislocations were introduced along the heterointerface between the Cr-rich secondary phase and the alumina matrix. By observing the heterointerface from the direction which is perpendicular to the plane, it was found that all the misfit dislocations dissociate into partial dislocations. The dissociation width was several times larger than that in alumina {1−100} planes reported so far.

Dislocation Structure Analysis of Low Angle Tilt Grain Boundaries in Alumina by Elastic Theory

Structure and configuration of boundary dislocations on the low angle tilt grain boundaries in alumina were considered based on the ideas that the boundary is composed of regularly arrayed edge dislocations and that the dislocations could dissociate into partial dislocations as well as glide dislocations in bulk. Moreover, the structure of the dissociated boundary dislocations were evaluated by the calculations based on an elastic theory. The calculations indicated that the largeness of the stacking fault region between partial dislocations formed by the dissociation will decrease with increasing tilt angles. It can be said that the idea and calculations used here will be powerful in considering the dislocation structure of low angle tilt grain boundaries that are not or are difficult to be identified.

Dislocation Structure, Phase Stability and Yield Stress Behavior of Ultra-High Temperature L12 Intermetallics: Combined First Principles-Peierls-Nabarro Approach

ABSTRACTWe present results of comparative studies of the dislocation properties and the mechanical behavior for a class of intermetallic alloys based on platinum group metals (PGM) which are being developed for ultra-high temperature applications: Ir3X and Rh3X (where X = Ti, Zr, Hf, V, Nb, Ta). For the analysis of dislocation structure and mobility, we employ a combined approach based on accurate first-principles calculations of the shear energetics and the modified semi-discrete 2D Peierls-Nabarro model with an ab-initio parametrization of the restoring forces. Based on our analysis of dislocation structure and mobility, we provide predictions of temperature yield stress behavior of PGM-based intermetallics, show that their dislocation properties are closely connected with features of the electronic structure and the L12 → D019 structural stability, and demonstrate the dramatic difference in dislocation structure and the mechanical behavior between PGM alloys with IVA and VA group elements.

Defect structure of gold introduced by high-speed deformation

The effect of deformation speed on defect structures introduced into bulk gold specimens at 298 K has been investigated systematically over a wide range of strain rate from ε′=10 −2 to 10 6 s −1. As strain rate increased, dislocation structure changed from heterogeneous distribution, so-called cell structure, to random distribution. Also, stacking fault tetrahedra (SFTs) were produced at anomalously high density by deformation at high strain rate. The anomalous production of SFTs observed at high strain rate is consistent with the characteristic microstructure induced by dislocation-free plastic deformation, which has been recently reported in deformation of gold thin foils. Thus, the results of the present study indicate that high-speed deformation induces an abnormal mechanism of plastic deformation, which falls beyond the scope of dislocation theory. Numerical analysis of dislocation structure and SFTs revealed that the transition point of variation of deformation mode is around the strain rate of 10 3 s −1.

Application of white x-ray microbeams for the analysis of dislocation structures

The measurement of dislocation structures on mesoscopic length scales is a particularly important application of white-beam Laue microdiffraction. Near a Bragg reflection the intensity distribution in reciprocal space is sensitive to the organization of the dislocations, which occurs at several structural levels. Unpaired geometrically necessary dislocations (GND) and geometrically necessary boundaries (GNB) result in elongated streaks in the Laue image. The direction of the streaks depends on the average orientation of the dislocation arrays and the diffraction vectors. Laue images collected using synchrotron x-ray microbeams are sensitive to the detailed hierarchical distribution of dislocations and can be used to study the orientation and density of individual GNDs and GNBs.

電子顕微鏡法によるL1<SUB>2</SUB>型金属間化合物Al<SUB>3</SUB>Scの転位組織解析

電子顕微鏡法によるL1<SUB>2</SUB>型金属間化合物Al<SUB>3</SUB>Scの転位組織解析

Analysis of dislocation structures after double shear creep deformation of CMSX6-superalloy single crystals at temperatures above 1000 °C

Dislocation structures which form during pure shear creep deformation of the superalloy single crystal alloy CMSX6 at 1025 °C under a shear stress of 85 MPa were analysed using transmission electron microscopy. Two macroscopic crystallographic shear systems were studied: 111〈110〉 and 100〈010〉. At the minimum creep rate (observed at shear values of 0.02) the system 111<110> deforms by a factor of ten faster than the system 100〈010〉. This creep rate ratio could not be rationalised merely on the basis of an external resolved shear stress argument. Shear creep deformation was always associated with multiple slip and nucleation of dislocations was not difficult (absence of incubation periods for creep). A detailed study of dislocation networks around γ′-particles underlined the importance of 〈110〉111 glide and climb processes in the γ-channels in the formation of these networks. Microstructural parameters and processes which must be considered to fully account for the mechanism of creep in superalloy SX are outlined. Channel work hardening always preceded the cutting of γ′-particles which started at the minimum creep rate.

Deformation and Ductility in Alloys Based on Al3Ti

ABSTRACTTitanium trialuminide alloys suffer problems of intense segregation during solidification and extreme brittleness when subjected to tensile stresses. In this study, spray forming techniques have been used to obtain homogeneous materials of fine microstructure, containing various second phase particles, and the deformation behaviour of these materials examined over a range of temperatures.The microstructure and deformation behaviour of an iron-modified titanium trialuminide with the L12structure is reported here.Deformation mechanisms are interpreted by analysis of dislocation structures and further confirmation is obtained by activation volume measurements: the importance of Peierls effects at low temperatures and cross slip at high temperatures is shown. The material remains brittle in tension both at room temperature and at high temperatures: the second phase particles in the material are yet not present in sufficient volume fraction to greatly affect plastic behaviour.

Dislocation structures and in situ observations of dislocation motion in InSb

Dislocation mobility in InSb is investigated both by a microscopic analysis of dislocation structures at 100 kV and by in situ straining in a high voltage microscope. The observed differences in deformation processes between bulk specimens and the foils is discussed. Dislocation pileups observed in thin foils indicate the existence of localized and recurrent dislocation sources.