Electron Channeling Contrast Imaging Research Articles

An in-situ investigation of microstructure neighborhood effects on hydrogen-induced dislocation activity

The interactions between hydrogen and crystallographic defects are the key to understanding hydrogen induced localized plasticity or damage mechanisms. However, unraveling such interactions based on the resulting mechanical effects alone can be challenging, due to the spectrum of various effects hydrogen can create in a given microstructure. Pre-deformation or post-mortem characterization of hydrogen segregation has similar limitations, as they can only provide snapshots of these complex processes with challenging spatial and temporal scales. To tackle this, we design in situ experiments that combine electron channeling contrast imaging (ECCI), electron backscattered diffraction (EBSD), and desorption spectroscopy, to investigate hydrogen defect interactions, under no external loading. We show that the inhomogeneous hydrogen distribution within microstructures can cause long-range (>200 nm) dislocation rearrangements. Interpretation of these results as manifestations of boundary-segregation induced stress suggests that microstructural characteristics on either side of the boundary should also play an important role. We provide quantitative assessments of these microstructure neighborhood effects and discuss additional contributions from hydrogen-induced changes in stacking fault energy and elastic modulus.

Direct observation of the evolution behavior of micro to nanoscale precipitates in austenitic heat-resistant steel via electron channeling contrast imaging

Precipitation directly determines the high-temperature properties of the austenitic heat-resistant steels. The precipitations of MX, Z-phase, M23C6 and σ-phase, ranging from micrometers to nanometers, are commonly characterized using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). However, details of their evolution behavior are still unclear due to the limited SEM resolution using the traditional sampling method and the limited spatial resolution of TEM. In this work, field emission SEM-based electron channel contrast imaging (ECCI) techniques with flexible sampling routes were introduced to observe the precipitation evolution behavior of HR3C steel after aging at 700 °C for 8095 h. We showed that the coarse primary-MX and Z-phase in the as-received steel are relatively stable during aging. The coarse M23C6 and tiny secondary Z-phase dispersions were rapidly formed along the grain/twin boundaries and within dislocation arrays inside the grain interior, respectively. We further found that the M23C6 at grain boundaries would change from continuous to semi-continuous due to the formation of σ-phases, while in twin boundaries, it would become continuous over aging. Moreover, we showed that the σ-phases were in-situ transformed from M23C6 at the grain boundaries via its dissolution, facilitating the nucleation of tiny Z-phase inside the σ-phase grains, and this phenomenon has not been reported so far. We have demonstrated that ECCI with an electrolytic-polishing-based sampling route is effective in revealing multi-scale precipitation with high-throughput efficiency, and it allows the direct observation of the complex behavior of precipitates down to the nanoscale using a bulk sample. This method can be used as an efficient way for the quantitative microstructure study.

Damage-tolerant oxides by imprint of an ultra-high dislocation density

Dislocations in ductile ceramics offer the potential for robust mechanical performance while unlocking versatile functional properties. Previous studies have been limited by small volumes with dislocations and/or low dislocation densities in ceramics. Here, we use Brinell ball scratching to create crack-free, large plastic zones, offering a simple and effective method for dislocation engineering at room temperature. Using MgO, we tailor high dislocation densities up to ∼1015 m−2. We characterise the plastic zones by chemical etching, electron channelling contrast imaging, and scanning transmission electron microscopy, and further demonstrate that crack initiation and propagation in the plastic zones with high-density dislocations can be completely suppressed. The residual stresses in the plastic zones were analysed using high-resolution electron backscatter diffraction. With the residual stress being subsequently relieved via thermal annealing while retaining the high-density dislocations, we observe the cracks are no longer completely suppressed, but the pure toughening effect of the dislocations remains evident.

Exploring microcrack formation in cyclic and dwell fatigue in MTR-lean Ti 6242 alloy

This study examines cyclic and dwell fatigue crack initiation and growth in well forged, low microtexture intensity Ti 6242 alloy. The statistics of crack occurrence have been explored in cross-sections of samples that failed in cyclic and dwell fatigue. Cracks are observed dominantly on the basal planes of the primary α grains with a high Schmid factor for basal slip in both cyclic and dwell fatigue and also in hard α grains in dwell fatigue. The crystallography of early crack initiation and extension is described. Electron channelling contrast imaging shows that basal slip dominates low strain behaviour in this alloy, leading to the observed crack formation.

In-situ electron channeling contrast imaging of local deformation behavior of lath martensite in low-carbon-steel

In-situ tensile Electron Channeling Contrast Imaging (ECCI) observations were performed on low-carbon steel lath martensite to elucidate its plastic deformation mechanisms under tensile loading. Two key deformation processes were identified through the direct observation of dislocation activity: intra-lath crystallographic slip and boundary sliding. The initial dislocation movement signifies the onset of microscopic yielding in lath martensite. As the applied stress increases, macroscopic yielding occurs when dislocations overcome internal barriers like high-dislocation-density walls within the laths. Out-of-lath-plane slip systems exhibit a strong interaction with the lath boundary, hindering their activity after initial small plastic deformation. In contrast, in-lath-plane slip systems predominantly contribute to plastic deformation. After the work hardening triggered by these slip systems, boundary sliding takes over the plastic deformation. The boundary sliding was found to accompany a rapid movement of the dislocation network adjacent to the boundary.

Hydrogen-enhanced microbanding in an austenitic FeMnAlC low-density steel: Effect on hydrogen embrittlement resistance

We have investigated the influence of 101 mass ppm hydrogen content on the room temperature deformation structure and mechanical behavior of an austenitic Fe30Mn6.5Al0.3C (wt.%) low-density steel by several electron microscopy techniques, such as electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The steel exhibits a high hydrogen embrittlement resistance associated with a moderated increase in strength (yield stress increase of 10%) and ductility (increase in the elongation to fracture of ∼ 8%). Analysis of the deformation structure reveals that hydrogen influences the deformation behavior by promoting deformation mechanisms associated with inhomogeneous plasticity (hydrogen-enhanced deformation banding (HEDB)) and strain localization (hydrogen-enhanced microbanding (HEMB)). These deformation mechanisms are ascribed to hydrogen-induced effects on dislocation plasticity, resulting in macroscopic kink bands, sub-micron localized strain gradients, and localized shear at cell blocks. We find that HEMB plays a relevant role in the deformation behavior of sub-micron localized strain gradients by promoting plastic relaxation and the enhanced storage of geometrically necessary dislocations within them. These effects mitigate the activation of damage mechanisms and enhance the strain-hardening capacity, contributing to the high HE resistance of the steel, comparable to that of high HE-resistant fcc alloys and steels.

Effective InAsP dislocation filtering layers for InP heteroepitaxy on CMOS-standard (001) silicon

In this work, we report InAsP-based dislocation filter layers (DFLs) for InP heteroepitaxy on CMOS-standard (001) Si substrates, demonstrating a threading dislocation density of 3.7 × 107 cm−2. The strain introduced by InAsP induces dislocation bending at the InAsP/InP interface, thereby facilitating the reaction and annihilation of dislocations during their lateral glide. Concurrently, the InP spacer exhibits tensile strain, leading to the formation of stacking faults (SFs). With a comprehensive analysis utilizing x-ray diffraction, electron channeling contrast imaging, and transmission electron microscopy, the effects of DFL-induced strain on dislocations and SFs are investigated. Fine-tuning the strain conditions allowed low-dislocation-density while SF-suppressed, anti-phase boundary free InP on Si. This work, therefore, provides a useful buffer engineering scheme for monolithic integration of InP-based electronic and photonic devices onto the industry-standard silicon platform.

Microstructural analysis and tribological characteristics of thermally sprayed Co-Ni-O ternary oxide coatings

Due to their high thermal and chemical stability, ternary oxide coatings have been extensively investigated as potential solutions for high-temperature solid lubrication. In this study, the tribological behavior of the thermally sprayed Co0.75Ni0.25O and Co0.5Ni0.5O oxide coatings was investigated. The dry sliding reciprocating friction tests were performed against an Al2O3 counterface at room and elevated temperatures using a ball-on-flat tribometer. The results indicated that the friction and wear of Co0.75Ni0.25O oxide coatings did not change significantly at atmospheric and elevated temperature conditions, whereas the friction and wear of the Co0.5Ni0.5O increased at elevated temperatures. The correlations between the worn surface morphologies, the subsurface microstructure induced by wear, and the wear behavior of the coatings were discussed based on the characterizations, (i.e., microhardness, scanning electron microscopy (SEM), and electron channeling contrast imaging (ECCI)).

A Comparative Study of Methods for Calculating the Dislocation Density in GaN-on-Si Epitaxial Wafers.

A series of characterization methods involving high-resolution X-ray diffraction (HR-XRD), electron channel contrast imaging (ECCI), cathodoluminescence microscopy (CL), and atomic force microscopy (AFM) were applied to calculate the dislocation density of GaN-on-Si epitaxial wafers, and their performance was analyzed and evaluated. The ECCI technique, owing to its high lateral resolution, reveals dislocation distributions on material surfaces, which can visually characterize the dislocation density. While the CL technique is effective for low-density dislocations, it is difficult to accurately identify the number of dislocation clusters in CL images as the density increases. The AFM technique analyzes surface dislocation characteristics by detecting surface pits caused by dislocations, which are easily affected by sample and probe conditions. A prevalent method for assessing the crystal quality of GaN is the rocking curve of HR-XRD (ω-scan), which calculates the dislocation density based on the FWHM value of the curves. By comparing the above four dislocation characterization methods, the advantages and limitations of each method are clarified, which also verifies the applicability of DB=β29b2 for GaN-on-Si epitaxial wafers. This provides an important reference value for dislocation characterization in GaN-on-Si materials. The accuracy evaluation of dislocation density can truly and reliably reflect crystal quality, which is conducive to further optimization. Furthermore, this study can also be applied to other heterogeneous or homogeneous epitaxial materials.

Evolution of Electron Channelling Contrast Imaging of Plastic Deformation Induced by Berkovich Nanoindentation in Ferrite Steel

Evolution of Electron Channelling Contrast Imaging of Plastic Deformation Induced by Berkovich Nanoindentation in Ferrite Steel

Duplex surface engineering of cold spray Ti coatings and physical vapor-deposited TiN and AlTiN thin films

The feasibility of a duplex coating based on cold spray technology and magnetron sputtering was evaluated for repair applications requiring a ‘thin-on-thick’ layered structure. Commercially pure angular-shaped Ti grade 4 particles are fed to a cold spray gun and accelerated toward a Ti alloy substrate to deposit thick coatings (∼4.5 mm). TiN and AlTiN thin films are deposited on polished cold spray coatings using a four-source closed-field unbalanced magnetron sputtering (CFUBMS) system. Microstructure was characterized using focused ion beam (FIB) lift-out, scanning electron microscopy (SEM), and electron channeling contrast imaging (ECCI). The nanoindentation technique was used to evaluate the mechanical properties of coatings. The H/E ratios and H3/E2 ratios for TiN films were found to be 0.098 and 0.26 GPa, respectively, while those for AlTiN films were measured at 0.066 and 0.052 GPa, respectively, suggesting higher capacity of TiN films to withstand both elastic and plastic deformation. Using scratch testing, the adhesion of TiN and AlTiN thin films to cold spray Ti was investigated, with TiN-Ti duplex coatings exhibiting better performance compared to AlTiN-Ti coatings. Tribological testing was performed on duplex coatings using a reciprocating tribometer equipped with an alumina ball counterface. The wear rate for AlTiN-Ti coatings after 2000 sliding cycles was found to be (1.0 × 10−3 ± 0.1 × 10−3 mm3/Nm), three orders of magnitudes higher than that for TiN-Ti (8 × 10−6 ± 2 × 10−6 mm3/Nm. SEM was used to reveal worn surface morphologies and cross-sectional analysis of the wear track. Subsurface microstructural changes due to wear were examined using focused ion beam cross-sectioning, revealing bending cracks and tribofilm formation.

Accurate determination of active slip systems using a novel lattice rotation analysis: Quasi-in-situ EBSD/ECCI study on the homogeneous/heterogeneous deformation of polycrystalline zirconium

The accurate determination of slip systems is crucial for understanding deformation mechanisms of metals in particular, how slip/twining transfer or blocking occurs. This paper proposes a novel crystallographic lattice rotation analysis, which combines quasi-in-situ Electron Back Scatter Diffraction (EBSD) and Electron channeling contrast imaging (ECCI) techniques to precisely predict the active slip system with specific slip directions in polycrystalline Zr. Large data sets for hundreds of grains were quantitatively analyzed by this method to statistically gain deep insight on the deformation mechanism of Zr. Our results show that prismatic 〈a〉 slip is the primary slip system in most grains, and pyramidal 〈a〉 slip plays a secondary role in coordinating the local stress state of plastic deformation. The predicted slip direction is the same as the direction in which dislocation density increases. This method can serve as a complementary tool to the intragranular misorientation axis (IGMA) method, bridging the gap between macro-mechanical response and microstructural deformation mechanisms.

Fracture of the C15 CaAl2 Laves phase at small length scales

The cubic C15 CaAl2 Laves phase is an important brittle intermetallic precipitate in ternary Mg–Al–Ca structural alloys. Although knowledge of the mechanical properties of the co-existing phases is essential for the design of improved alloys, the fracture toughness of the C15 CaAl2 intermetallic has not yet been studied experimentally due to limitations posed by macroscale testing of defect-free specimens. Here, miniaturised testing techniques like micropillar splitting and microcantilever bending methods are used to experimentally determine the fracture toughness of the CaAl2 Laves phase. It is found that the toughness value of ~ 1 MPa·√m obtained from pillar splitting with a sharp cube corner geometry is largely insensitive to sample heat treatment, the ion beam used during fabrication, micropillar diameter, and surface orientation. From correlative nanoindentation and electron channelling contrast imaging supported by electron backscatter diffraction, fracture is observed to take place mostly on {011} planes. Atomistic fracture simulations on a model C15 NbCr2 Laves phase showed that the preference of {011} cleavage planes over the more energetically favourable {111} planes is due to lattice trapping and kinetics controlling fracture planes in complex crystal structures, which may provide insights into the experimental results for CaAl2. Using rectangular microcantilever bending tests where the notch plane was misoriented to the closest possible {112} cleavage plane by ~ 8° and the closest {001}, {011}, and {111} planes by > 20°, a toughness of ~ 2 MPa·√m was determined along with the electron microscopy observation of significant deviations of the crack path, demonstrating that preferential crystallographic cleavage planes determine the toughness in this material. Further investigation using pentagonal microcantilevers with precise alignment of the notch with the cleavage planes revealed similar fracture toughness values for different low-index planes. The results presented here are the first detailed experimental study of fracture toughness of the C15 CaAl2 Laves phase and can be understood in terms of crack plane and crack front-dependent fracture toughness.Graphical

Microstructure and mechanical behavior in a spherical L12 nanoprecipitates strengthened Ni35Co27.5Cr27.5Al5Ti5 high-entropy alloy fabricated by direct casting and heat treatment

Intricate thermomechanical processing (TMP) including casting, homogenization, cold rolling, annealing and aging, has been widely used to achieve L12-strengthened CoCrNi-based medium- and high-entropy alloys (MEAs/HEAs). However, a large number of parts are usually manufactured by direct casting. We fabricated a non-equiatomic Ni35Co27.5Cr27.5Al5Ti5 HEA consisting of spherical L12 nanoprecipitates and FCC matrix using casting, homogenization and aging. Notably, both the homogenized and aged Ni35Co27.5Cr27.5Al5Ti5 HEA samples exhibit equiaxed grains, with the latter showcasing uniformly distributed spherical L12 nanoprecipitates in the FCC matrix. The average size and volume fraction of these nanoprecipitates in the aged HEA are ∼10.9 nm and ∼41.9 vol%, respectively. Mechanical testing reveals remarkable improvements in the aged HEA, with a yield strength of ∼916 MPa, an ultimate tensile strength of ∼1220 MPa, and a total elongation of ∼21.5 %. The observed enhancement is primarily attributed to precipitation strengthening mechanism. Digital image correlation (DIC) analysis of tensile deformation demonstrates uniform strain distribution in both the homogenized and aged HEA samples. In addition, electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) examinations unveil a complex interplay of deformation mechanisms, including multiple planar slip, dislocation shearing coherent L12 precipitates, superpartials, stacking fault (SF) networks, and Lomer-Cottrell (L-C) locks. These mechanisms collectively impede dislocation motion, thereby contributing to the excellent strain-hardening ability and outstanding strength-ductility synergy of the HEA.

Separation of Damage Mechanisms in Full Forward Rod Extruded Case-Hardening Steel 16MnCrS5 Using 3D Image Segmentation.

In general, formed components are lightweight as well as highly economic and resource efficient. However, forming-induced ductile damage, which particularly affects the formation and growth of pores, has not been considered in the design of components so far. Therefore, an evaluation of forming-induced ductile damage would enable an improved design and take better advantage of the lightweight nature as it affects the static and dynamic mechanical material properties. To quantify the amount, morphology and distribution of the pores, advanced scanning electron microscopy (SEM) methods such as scanning transmission electron microscopy (STEM) and electron channeling contrast imaging (ECCI) were used. Image segmentation using a deep learning algorithm was applied to reproducibly separate the pores from inclusions such as manganese sulfide inclusions. This was achieved via layer-by-layer ablation of the case-hardened steel 16MnCrS5 (DIN 1.7139, AISI/SAE 5115) with a focused ion beam (FIB). The resulting images were reconstructed in a 3D model to gain a mechanism-based understanding beyond the previous 2D investigations.

Comparative investigation of fatigue properties between additively and conventionally manufactured Invar 36 alloy

The mechanical properties of ASTM F1684 (Invar 36) alloy manufactured via laser beam powder-bed-fusion (PBF-LB) technique are strongly influenced by the non-equilibrium solidification microstructure. Due to the complexity of printed microstructure, e.g., the presence of melted pools, nano-precipitates, and high-density dislocations, it is very challenging to disentangle the specific mechanisms responsible for the fatigue properties. Here, a comparative investigation of the fatigue properties between PBF-LB and conventionally manufactured (CM) samples was conducted. Dedicated characterizations, including electron channeling contrast imaging (ECCI), and high-angular resolution EBSD (HR-EBSD), were performed. The mid-cycle (104–105 cycles) fatigue behaviors of both samples are comparable, while the fatigue resistance of PBF-LB sample becomes inferior at high-cycle range (≥106 cycles). This disparity can be ascribed to the competing effect between crack nucleation and propagation. PBF-LB samples are more susceptible to fatigue crack initiation due to the printed macro-defects. Whereas, the homogeneous plastic deformation and the grain fragmentation, both triggered by the ultrafine cellular structure, efficiently mitigate crack propagation rates. Based on these findings, the current understanding of the underlying mechanisms governing the fatigue performance of PBF-LB Invar 36 alloy is deepened, emphasizing the significant advantages of non-equilibrium solidification microstructure in retarding fatigue crack propagation.

Revealing the inhomogeneous nature of microstructure and texture evolution in the cold-rolled CoCrFeMnNi alloy during static recrystallization

In the present investigation, equiatomic CoCrFeMnNi high entropy alloy (HEA) was subjected to 80% cold rolling (as-rolled) followed by isothermal annealing treatment at 700 °C for different time periods. Microstructural characterization was performed using electron back-scattered diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques on the as-rolled and annealed samples. The as-rolled microstructure consisted of elongated grains mainly composed of strong α-fiber and weak γ-fiber textures. The as-rolled sample showed the formation of ingrain SBs in the γ-fiber grains, which served as preferential sites for grain nucleation. The annealing treatment of as-rolled samples at 700 °C cause static recovery (SRV) before 5 minutes of heat-treatment, whereas static recrystallization (SRX) was observed after 5 minutes of heat-treatment in the CoCrFeMnNi alloy samples. A faster rate of recrystallization kinetics was observed after 15 minutes of heat-treatment. The annealing treatment reduced the intensity of α-fiber and enhanced the Copper, Cube and P ({011}<122>) texture components. Copper components originated from coarse recrystallized grains, which were the results of localized grain growth phenomena. Unusual behavior of banded featured α-fiber grains was observed during the annealing process. These banded grains in the partially annealed samples consisted of subgrain-boundaries and showed a delayed response to recrystallization as compared to grains with other orientations. Stored energy (SE) and Taylor factor (M) calculations were also used to understand the delayed recrystallization response of the banded featured/retained deformed grains. The microhardness value of the as-rolled sample was approximately 450 Hv, which decreased to around 230 Hv for the fully recrystallized grains.

Comparative study on hydrogen embrittlement resistance in additive manufactured and forged austenitic stainless steel

In this work, the hydrogen embrittlement resistance of additively manufactured (AM) and conventionally manufactured (CM) Cr21Ni6Mn9N (21−6−9) stainless steels were comparatively investigated. The mechanical properties and microstructure evolution of both AM and CM conditions were studied using slow strain rate tensile tests and microanalytical techniques such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and electron channeling contrast imaging (ECCI). The results indicated that compared to the CM condition, the AM stainless steel demonstrated enhanced resistance to hydrogen embrittlement, coupled with a reduction in plasticity loss. The primary factor contributing to this resistance was identified as an increased occurrence of deformation twinning in both steels after hydrogen charging. The superior resistance observed in the AM stainless steel was attributed to the occurrence of higher-density deformation twinning and microbands.

Cyclic deformation response of annealed low-carbon steel: Insights from ratcheting and LCF experiments

Low cycle fatigue (LCF) and ratcheting experiments were carried out on annealed low-carbon steel at room temperature within a laboratory environment, utilising stress and strain control modes. The annealed low-carbon steel consistently demonstrates a cyclic softening response over its LCF lifespan, across all tested strain amplitudes. Notably, it was observed that ratcheting strain rises while ratcheting life declines with both rising mean stress and stress amplitude. Annealed low-carbon steel, being entirely ferritic and lacking precipitation or substitutional solid solution strengthening or hard phase strengthening, exhibits a restricted ability to withstand or alleviate the accumulation of ratcheting strain, particularly under very low mean stress conditions. In both LCF and ratcheting, significant substructure formation was detected. Nevertheless, there was no discernible difference in substructure formation between LCF and ratcheting when employing electron channelling contrast imaging techniques. The existing mean stress-based fatigue life prediction model has successfully forecasted ratcheting and LCF life within the 102–105 cycles range. A novel approach utilising modulus is introduced to characterise the cyclic hardening/softening behaviour of alloys in stress and strain-controlled experiments. The cyclic hardening model based on modulus effectively captures the responses observed in cyclic hardening/softening during LCF and ratcheting experiments.

Novel 3D reciprocal space visualization of strain relaxation in InSb on GaAs substrates

This study introduces the reciprocal space polar visualization (RSPV) method, a novel approach for visualizing x-ray diffraction-based reciprocal space data. RSPV allows for the precise separation of tilt and strain, facilitating their individual analysis. InSb was grown by molecular beam epitaxy on two (001) GaAs substrates—one with no misorientation (sample A) and one with 2° surface misorientation from the (001) planes (sample B). There is a substantial lattice mismatch with the substrate, and this results in the generation of defects within the InSb layer during growth. To demonstrate RSPV’s effectiveness, a comprehensive comparison of surface morphology, dislocation density, strain, and tilt was conducted. RSPV revealed previously unobserved features of the 004 InSb Bragg peak, partially explained by the presence of threading dislocations and oriented abrupt steps. Surface morphologies examined by an atomic force microscope revealed that sample B had significantly lower root mean square roughness. Independent estimates of threading dislocation density (TDD) using x-ray diffraction (XRD) and electron channelling contrast imaging confirmed that sample B exhibited a significantly lower TDD than sample A. XRD methods further revealed unequal amounts of α- and β-type threading dislocations in both samples, contributing to an anisotropic Bragg peak. RSPV is shown to be a robust method for exploring 3D reciprocal space in any crystal, demonstrating that growing InSb on misoriented GaAs produced a higher-quality crystal compared to an on-orientation substrate.