Electron Channeling Contrast Imaging Research Articles

Strain-induced martensite formation in cryorolled AISI 317 L stainless steel

AISI 317 L stainless steel replaces 316 L grade in some applications due to its superior mechanical strength and corrosion resistance. Aiming at expanding its applicability to structural applications, ongoing studies are dedicated to overcoming the trade-off between strength and ductility. The stacking fault energy decreases with deformation temperature and favors stacking faulting, (nano)twinning and strain-induced martensite (SIM) formation, resulting in severe microstructural fragmentation. The effect of temperature on deformation behavior of AISI 317 L steel was investigated in samples rolled at room temperature to thickness reductions of 50% and 85% and at 77 K to reductions in thickness of 10% and 50%. The microstructural evolution was followed by scanning electron microscopy, Vickers microhardness, X-ray diffraction, magnetization, electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). The nucleation sites in the early stages of the transformation sequence γ → ε → α’ were identified in the 10% cryorolled sample. The highest volume fraction of α’-martensite reached 45.8% in the cryorolled steel to 50% rolling reduction. Much lower fractions were obtained for samples rolled to 10% reduction at 77 K (2%) and at room temperature to 50% (0.3%) and 85% reductions (1.6%). The texture components after cryorolling were Goss and Brass for austenite; rotated cube, α- and γ-fibers for δ-ferrite and α’-martensite. The ε-martensite presents the typical texture of hcp metals with a c/a ratio above the ideal value and 〈0001〉 − oriented tilted about 21° from the normal direction towards the rolling direction. The results show cryorolling as an effective method for enhancing SIM formation and promoting severe microstructural refinement in AISI 317 L stainless steel.

Effects of Mo content on the microstructure and mechanical properties of laser cladded FeCoCrNiMox (x = 0.2, 0.5) high-entropy alloy coatings

The laser cladding additive manufacturing technology and high-entropy alloys can serve as ideal techniques and materials for the surface repair of high-speed train axles. Coatings of FeCoCrNiMox (x = 0.2, 0.5) were prepared on EA4T axle steel using laser cladding technology, and their phase, structure, and mechanical properties were analyzed using X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) characterization techniques. Additionally, the mechanical properties of the coatings were tested, and first-principles calculations were used to verify and calculate the material's mechanical performance. The research findings indicate that an increase in Mo content leads to a greater degree of lattice distortion, causing a leftward shift in diffraction peak positions. Significant differences in the microstructure from the substrate to the coating surface were observed, with columnar grain structure at the bottom of the cladding layer and equiaxed dendrites dominating the coating surface. The increase in Mo content promotes the formation of σ phase while also refining the grain size. ECCI results show that both types of coatings consist of a high-density dislocation cell structure and Mo-rich particles. With an increase in Mo content, the peak hardness of the coating increased from 456.5 HV to 469.4 HV, while the impact energy decreased from 56 J to 16 J, attributed to the increase in dislocation density and the greater quantity of σ phase. First principles calculations verified that the comprehensive mechanical properties of FeCoCrNiMo0.2 are superior, providing theoretical guidance for the optimization and design of the coatings.

Assessing fracture toughness of vintage and modern X65 pipeline steels under in situ electrochemical hydrogen charging using advanced testing methodology

The susceptibility of pipeline steels to hydrogen embrittlement, which can significantly impair mechanical performance, especially fracture toughness, is a critical issue in infrastructure integrity. This study evaluates the fracture toughness of modern and vintage API X65 steels under cathodic polarization (CP) conditions, employing methods such as rising displacement testing, stepwise rising constant load testing, and constant load testing. Tearing crack growth was detected at crack tip opening displacements (CTODs) of 0.032 to 0.055 mm while subsequent crack arrest under constant force loading was observed with CTOD thresholds of 0.148 mm and 0.255 mm for modern and vintage X65, respectively. Electron channelling contrast imaging demonstrated that arrested cracks in both steels exhibited reduced plasticity and a higher propensity for crack branching compared to propagating cracks. These findings highlight the need for tailored strategies to mitigate the effects of hydrogen embrittlement in pipeline materials and underscore the differences in damage tolerance between steel generations.

Взаимосвязь микроструктуры с ударной вязкостью металлов сварного шва трубных высокопрочных низколегированных сталей (обзор исследований)

Introduction. The modern pipeline industry requires the development of materials of high strength and toughness for the production of steels for oil and gas pipelines. Changes in steel production and rolling technologies have become a challenge for welding consumables and joining technologies. This is more critical for strength levels above 830 MPa, where there are no specific regulations for the approval of welding consumables. Research methods. The failure of stainless steel pipeline welds is becoming a serious problem in the pipeline industry. Multiphase microstructures containing acicular ferrite or an acicular ferrite-dominated phase exhibit good complex properties in HSLA steels. This paper focuses on the results obtained using modern methods of scanning electron microscopy for microstructural analysis, backscattered electrons (BSE) for electron channel contrast imaging (ECCI) and orientation microscopy based on electron backscatter diffraction (ORM), as well as characteristic X-rays for compositional analysis using energy-dispersive X-ray spectroscopy (XEDS) and secondary electrons (SE) for observing surface morphology. Results and discussion. This paper analyzes the characteristics of the microstructure of the weld and its relationship with impact toughness. It is shown that predicting impact toughness based on the microstructural characteristics of steel weld metals is complicated due to the large number of parameters involved. This requires an optimal microstructure of the steel. Satisfactory microstructure depends on several factors, such as chemical composition, hot work processing, and accelerated cooling. Alloying elements have a complex effect on the properties of steel, and alloying additives commonly added to the steel composition include Mn, Mo, Ti, Nb and V. From a metallurgical point of view, the choice of alloying elements and the metallurgical process can greatly influence the resulting microstructure. A longer cooling time tend to improve the toughness and reduce the mechanical strength of weld deposits on high-strength steels. Welding thermal cycles cause significant changes in the mechanical properties of the base material. The analysis showed that impact toughness strongly depends on the microstructure of the multi-pass weld of the material under study, which contains several sources of heterogeneity, such as interdendritic segregation, and the effective grain size can also be a significant factor explaining large deviations in local impact toughness values. Acicular ferrite nucleated in intragranular inclusions has been shown to produce a fine-grained interlocking arrangement of ferrite plates providing high tensile strength and excellent toughness, and is therefore a desirable microstructural constituent in C-Mn steel weld metals. At the same time, discussion regarding the relationship between acicular ferrite and toughness is very complex and still open at present. Relating impact toughness to acicular ferrite, taking into account the top bead, is not a reliable procedure, even for single-pass deposit welding. Impact strength depends on several factors, and the strong effect of acicular ferrite is generally recognized due to its fine-grained interlocking structure, which prevents the propagation of brittle cracks by cleavage. The large-angle boundaries and high dislocation density of acicular ferrite provide high strength and toughness. However, for the same amount of acicular ferrite, different viscosity values may be observed depending on the content of microalloying elements in the steel. An analysis of the results of various studies showed that other factors also affect the impact strength. For example, microphases present along the Charpy-V notch are critical for the toughness of weld metals. The combination of OM, SEM and EBSD techniques provides an interesting method for metallographic investigation of the refined metal microstructure of stainless steel pipeline welds. Conclusion. This review reports the most representative study regarding the microstructural factor in the weld of pipe steels. It includes a summary of the most important process variables, material properties, regulatory guidelines, and microstructure characteristics and mechanical properties of the joints. This review is intended to benefit readers from a variety of backgrounds, from non-welding or materials scientists to various industrial application specialists and researchers.

Multi-microscopy characterization of threading dislocations in CVD-grown diamond films

We present site-coincident imaging of a high dislocation density boron-doped chemical vapor deposition-grown homoepitaxial (001) diamond film using electron channeling contrast imaging (ECCI), cathodoluminescence, transmission electron microscopy (TEM), and scanning secondary ion mass spectroscopy (SIMS). With growing interest in large area heteroepitaxial substrates, we show that ECCI is a promising technique to accurately evaluate dislocations in diamond films. We find the electron backscatter yield is sufficient to distinguish individual threading dislocations in ECCI despite the low atomic number of carbon, and we have generally good agreement between dislocations observed in ECCI and TEM of the same region. Importantly, relying on luminescence in the 430 nm dislocation-related A-band alone results in an underestimation of the defect density as we find only 20%–40% of the threading dislocations luminesce. We further show that dislocations do not perturb the spatial uniformity of boron doping measured by SIMS, even when the dislocations are clustered with high density, and we can tentatively rule out strong segregation effects at the dislocations. Our results establish the complementary use of microscopy and microanalysis methods to rapidly characterize and understand the impact of dislocations in diamond thin films.

Influence of high‐strain deformation on major element mobility in garnet: Nanoscale evidence from atom probe tomography

AbstractGarnet is a common rock‐forming mineral that occurs in a variety of rock types and over a wide range of pressure (P)–temperature (T) conditions in the Earth's lithosphere. Because garnet is considered a high‐strength mineral stable across an extensive range of conditions (1–25 GPa, <300–2000°C), it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, garnet is commonly used as a geothermobarometer and geochronometer to provide P–T and timing constraints on tectonic events. Herein, we study garnet from an eclogite facies mylonite (central Australia) to investigate the mechanisms of element mobility during high‐strain deformation under relatively dry, lower crustal conditions. Electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) reveal evidence of crystal plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low‐angle grain boundaries developed over length scales of 20–50 μm in the rims of garnet porphyroclasts. Atom probe tomography (APT) analysis of a low‐angle grain boundary within a highly strained portion of a clast shows Ca enrichment and Mg depletion along dislocations, whereas APT data along the rim of a mostly undeformed clast reveal a homogeneous distribution of garnet major components in the specimen matrix with the exception of Ca, Fe and Mg enrichment within a healed microfracture. The above‐mentioned results suggest that under relatively dry conditions, crystal plasticity enhances bulk element mobility via pipe diffusion, highlighting the importance of deformation‐induced microstructures on element mobility, with important implications for the robust and reliable use of garnet as a petrological tool.

On fatigue crack origin with a fine granular area in matrix without defect

Fatigue crack initiation in metallic materials during very high cycle fatigue has been characterized by a subsurface crack origin with defect such as an inclusion. In this paper, fatigue damage behavior in an austenitic stainless steel has been studied using a novel progressive stepwise load increasing test method with each cycle step higher than 108 cycles. Subsurface crack origin with a fine granular area has formed in the matrix without defect. This is a new phenomenon. The mechanism has been investigated using Focused Ion Beam cross-sectioning and electron channeling contrast imaging techniques. Strain localization, grain fragmentation and local plasticity exhaustion are the main factors that cause fatigue damage and crack initiation in the matrix. This study provides a fundamental understanding how material damage and crack initiation occur in material matrix during very high cycle fatigue.

Observation and quantitative analysis of dislocations in steel using electron channeling contrast imaging method with precise control of electron beam incident direction.

Electron channeling contrast imaging (ECCI) was applied by precisely controlling the primary electron beam incident direction of the crystal plane in scanning electron microscope (SEM), and the dislocation contrast in steel materials was investigated in detail via SEM/ECCI. The dislocation contrast was observed near a channeling condition, where the incident electron beam direction of the crystal plane varied, and the backscattered electron intensity reached a local minimum. Comparing the dislocation contrasts in the visualized electron channeling contrast (ECC) images and transmission electron microscope (TEM) images, the positions of all dislocation lines were coincident. During the SEM/ECCI observation, the dislocation contrast varied depending on the incident electron beam direction of the crystal plane and accelerating voltages, and optimal conditions existed. When the diffraction condition g and the Burgers vector b of dislocation satisfied the condition g⸱b = 0, the screw dislocation contrast in the ECC image disappeared. An edge dislocation line was wider than a screw dislocation line. Thus, the SEM/ECCI method can be used for dislocation characterization and the strain field evaluation around dislocation, like the TEM method. The depth information of SEM/ECCI, where the channeling condition is strictly satisfied, can be obtained from dislocation contrast deeper than 5ξg, typically used for depth of SEM/ECCI.

Influence of pre-existing configurations of dislocations on the initial pop-in load during nanoindentation in a CrCoNi medium-entropy alloy

ABSTRACT The origin and mechanisms responsible for incipient plasticity in metals are still poorly understood. Moreover, the reasons for the recently reported large scattering of the initial pop-in load remain unclear. Hence, this study addresses these issues through a combination of nanoindentation tests and electron channelling contrast imaging characterisation considering a CrCoNi medium-entropy alloy. Experimental findings were also supported by elastic calculations that consider both the indentation and dislocation stress fields. A wide scatter in the maximum shear stress underneath the indenter, as expected, was observed for the analysis based on dislocation density. As a consequence, the spatial arrangement of dislocations within the indented region or local dislocation configuration is introduced as a new parameter to overcome overly simple analysis based on the dislocation density. The maximum shear stress underneath the indenter increased from 6 GPa for dislocation closer to the indentation axis to 11 GPa at 600 nm for dislocation far away from it. Additionally, elastic calculations revealed that the response to the incoming nanoindenter was different for dislocations with different configurations. Thus, the complex interactions of stress fields due to configurations of dislocations and indentation account for the large scatter of the maximum shear stress beneath the indenter.

Characterization of irradiation-induced dislocation loops and helium bubbles via electron channeling contrast imaging

Characterizing irradiation-induced defects is essential to understand damage mechanisms and develop irradiation-resistant materials. Prevailing strategies rely on transmission electron microscopy (TEM) observations which require rigorous prerequisite to prepare high-quality foils, making it difficult for rapid screening of irradiation-resistant materials. Here, we propose that the electron channeling contrast imaging (ECCI) is expected to provide a novel strategy for characterizing surface irradiation defects of ion-irradiated materials. The comparison between ECCI and conventional cross-sectional TEM for the observation of dislocation loops and helium (He) bubbles is analyzed in a common FeCoNiCr high-entropy alloy. The results indicate that the ECCI images of irradiation-induced defects are optimal at the extra-high tension of 20 kV, with a limit resolution of ∼1.5 nm for visible defects. The irradiation-induced dislocation loops show sharper contrast than bubbles in ECCI images, and the acceptable misorientation was within 4° of sample tilt. These findings offer a promising solution to the challenges for rapid assessment of irradiation damage in ion-irradiated materials.

Microstructure and residual stress analysis as heat treatment of additive manufactured A356.2 alloy by powder bed fusion

Aluminum alloys are commonly used in additive manufacturing (AM) because of their high specific strength and low weight. However, the high energy used in the AM process to overcome heat reflectivity induces thermal gradients and residual stress in the products. This study, A356.2 alloy was used for AM, and the residual stress and microstructure generated during this process were observed. X-ray diffraction (XRD) and nanoindentation were used to measure the surface residual stress, while the residual stress inside the material was measured using neutron diffraction. During the nanoindentation tests, we varied the indentation load conditions and compared the results with the residual stress values measured using XRD. To relieve the residual stress, we conducted a heat treatment and analyzed the changes. For surface residual stress, tensile residual stress (27∼91 MPa) in the as-built samples changed to compressive residual stress (‐28∼49 MPa) after heat treatment, with higher heat treatment temperatures producing stronger compressive residual stress. As for the internal residual stress, it was generally relaxed after heat treatment. As indentation depth increased, the residual stress results became similar to the residual stress values measured by XRD. In addition, the electron channeling contrast imaging (ECCI) technique was used for observation of lattice defects within precipitates generated and coarsened by heat treatment. We assessed the efficiency of each measurement method by comparing the results obtained through different techniques.

Effect of deformation temperature on strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C low-density steel

We have investigated the influence of the deformation temperature from 25 °C (RT) to -196 °C on the dislocation structures associated with strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel by electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and bright-field transmitted forescattered electron imaging ((BF) t-FSEI) techniques. The characteristics of the dislocation structures were evaluated on the main texture components, i.e. <111>//tensile axis, <112>//tensile axis, and <001>//tensile axis directions. The inhomogeneous character of the plastic behavior is promoted upon cryogenic deformation due to the formation of dislocation structures associated with strain localization, namely microbands (MBs) and deformation bands (DBs). ECCI analysis of the dislocation structure reveals that the deformation temperature has a strong influence on the thermal-assisted dislocation processes controlling the dislocation configurations and MB formation mechanisms. The MB nucleation mechanism evolves from a cross-slip-assisted mechanism at RT deformation conditions to a slip band-assisted mechanism at cryogenic deformation temperatures. This effect has a profound effect on the grain orientation dependence of the MB structure but not on its crystallographic alignment. On the other hand, cryogenic deformation temperatures (-196 °C) enhance the material's mechanical strength and ductility due to the activation of deformation twinning, which is associated with the reduction of the stacking fault energy. We find that MBs have a small contribution to strain-hardening and ductility due to the small mechanical resistance of these dislocation structures against the advance of deformation twins and dense dislocation layers, and the comparatively small plastic strain accommodated by them, respectively. These findings provide new insights into the microband-induced plasticity (MBIP) effect.

Abnormal strain-hardening in Co-rich CoNiV medium-entropy alloys

Two new nonequiatomic Co70Ni20V10 and Co65Ni20V15 medium entropy alloys were designed to investigate the synergistic strengthening effects of the dislocation slip, hexagonal close-packed (HCP) martensitic transformation, chemical short-range order (CSRO), and chemical medium-range order (CMRO), using X-ray diffraction, backscattered electron, electron channeling contrast imaging, electron backscattered diffraction, and transmission electron microscope. The results showed the Co65Ni20V15 alloy presented an abnormally higher strain-hardening rate than the Co70Ni20V10 alloy in the range of 5∼30 % strain, although the Co70Ni20V10 alloy possessed higher dislocation density with extra HCP martensitic transformation strengthening. This abnormality could be attributed to the profound CSRO and CMRO strengthening in the Co65Ni20V15 alloy with more large-sized V atoms, which was evidently observed by a high-resolution transmission electron microscope. It is expected that the strength of the CoNiV medium entropy alloys could be further enhanced via synthetically tuning the CSRO and CMRO effects, together with other strengthening mechanisms.

Local strength of bainitic and ferritic HSLA steel constituents understood using correlative electron microscopy and microcompression testing

HSLA steels with different polygonal ferrite and granular bainite contents resulting from two different cooling rates were investigated. Micropillar compression tests, electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) experiments were performed to reveal microscopic strength differences and their origin. The obtained results indicate that a higher cooling rate caused a smaller granular bainite substructure size and a higher dislocation density for both ferrite and bainite. In addition, the critical resolved shear stress (CRSS) values for both phases were found to be higher for the faster cooling process. This is ascribed to the increased dislocation density for faster cooling rather than the grain size as will be discussed in the manuscript. Interestingly, the macroscopic yield strength can be closely estimated by the CRSS obtained from micropillar compression considering the corresponding phase fractions. The achieved results can be used in future as input variables for crystal plasticity models.

Understanding the microstructural evolution and tensile characteristics of low nickel austenitic stainless-steel welds fabricated by diode LASER

In the present investigation, the utilization of diode laser welding in both continuous wave (CW) and pulse wave (PW) modes was applied to weld sections of 4 mm thick low nickel austenitic stainless steel (LNiASS) material. Subsequently, a comprehensive examination of the microstructural attributes of these welds was conducted by X-ray diffraction, electron backscatter diffraction (EBSD), and electron channeling contrast imaging (ECCI). The microstructural assessment was performed at both as-welded state and after the tensile deformation at three discrete strain rates ranging from 3.3 × 10−4 to 1.0 × 10−2. Furthermore, an investigation into the influence of strain rate variations on the tensile behavior of the two aforementioned types of welded joints, along with an exploration of the occurrence of strain-induced martensitic transformation, was carried out. The outcomes revealed that the PW welds yielded finer grain sizes in comparison to CW welds. It was observed that the martensite content within both CW and PW welds exhibited an augmentation with increasing strain rates. Furthermore, the PW welds exhibited a higher martensite content when compared to the CW welds under all examined strain rates. The PW welds consistently demonstrated higher UTS and total elongation across all three strain rates, while the CW welds exhibited a higher YS. Furthermore, as the strain rates increased, a decline in the strength ratio was observed for both weld types, accompanied by an increasing disparity between UTS and YS, indicative of work hardening phenomena.

In-situ investigation of the interaction between hydrogen and stacking faults in a bulk austenitic steel

The interaction between hydrogen (H) atoms and various microstructural defects remains a key to understand the H-induced damage and the subsequent premature failure of high-strength metallic materials. Previous studies on this subject are mainly focused on the in-situ probing of dislocations in a thin foil placed in an environmental transmission electron microscopy (TEM) cell. Here, a three-point bending test coupled with electron channeling contrast imaging (ECCI) has been applied to investigate the interaction of H with stacking faults (SFs) in a bulk high Mn austenitic steel. The expansion of some SFs, in terms of one partial dislocation movement within a partial dislocation pair, was observed on the H pre-charged sample when kept at a constant loading (i.e., a continuous H migration and likely build-up close to the pre-prepared notch tip). A temporal-resolved cross-correlation EBSD (CC-EBSD) measurement shows that the migration of H towards the notch tip region has a minor effect on the internal stress evolution. However, the local shear modulus (μ) and stacking fault energy (γSF) can be reduced by a local H segregation. Further theoretical calculation of SF ribbon width indicates that the reduction of μ by H results in the shrinkage of SF, while the H-induced reduction of γSF results in SF expansion at a lower resolved shear stress. The observed expansion of SF ribbons can be interrupted by a combined reduction of both μ and γSF due to H.

Exploring structural origins responsible for the exceptional mechanical property of additively manufactured 316L stainless steel via in-situ and comparative investigations

The laser beam powder-bed-fusion (PBF-LB) technique offers the possibility to fabricate metallic materials with unparalleled mechanical properties. However, due to the complexity of nonequilibrium solidification structures, untangling the role of specific structural features in influencing mechanical properties through post-mortem analysis remains a significant challenge. Here, we tracked the complete evolution of deformation microstructures in PBF-LB 316L stainless steel (SS) and conventionally manufactured (CM) counterparts using in-situ electron channeling contrast imaging (ECCI) in conjunction with high-angular resolution electron backscatter diffraction (HR-EBSD). Our findings underscore the importance of the intrinsic dislocation configuration, i.e., faulted dipole, in modulating the plastic deformation behaviors of the PBF-LB sample. The periodic arrangement of faulted dipoles triggers a widespread activation of stacking faults (SFs) with extremely fine spacing at the yielding stage, resulting in a high passing stress (∼433 MPa) that is responsible for the high yield strength. Meanwhile, such configuration facilitates uniform plastic deformation and mediates twinning propensity, thereby alleviating stress concentrations and enhancing ductility. Moreover, by correlating the mechanical response with microstructural evolution, we establish a modified model for multiple strengthening mechanisms in the PBF-LB sample. Additionally, we examine the effect of nonequilibrium solidification structures on the strain rate sensitivity of PBF-LB samples. Taking 316L SS as an exemplar, our results extend the current understanding of the structural origins responsible for the exceptional mechanical properties of metals manufactured by PBF-LB.

Delineating dislocation structures and residual stresses in additively manufactured alloys

Dislocation structures and residual stresses have tremendous impact over the properties of alloys produced by fusion-based additive manufacturing (AM) processes. Yet, their origin is still unclear. We quantify and map, for the first time, the spatial distribution of dislocations, including those classified as geometrically necessary dislocations (GND), lattice strains, and type-III residual stresses in body-centered cubic (BCC) Ti-42Nb alloy and face-centered cubic (FCC) stainless steel (SS) 316L using a combination of electron channeling contrast imaging and cross correlation-based electron backscatter diffraction. Our findings reveal distinct residual stresses, lattice strains, and dislocation arrangement along cell walls in both alloys. In particular, Ti-42Nb demonstrates no GND network in cell walls, while SS316L exhibits a high GND density. We show that the Ti-42Nb alloy primarily accommodates solidification strains and cooling stresses elastically while SS316L accommodates them by elasto-plastic deformation and builds GNDs. Our work establishes relationships between dislocations and residual stresses in BCC and FCC LPBF alloys and highlights that not all LPBF-manufactured materials contain a dense dislocation network in their microstructures. These findings can also be extended to other AM alloys, therefore enabling a better control over their properties and performance.

Composition-dependent transformation-induced plasticity in Co-based complex concentrated alloys

While the mechanically-induced martensitic transformation and transformation-induced plasticity (TRIP) effects have various known benefits, transformation of blocky martensite can also accelerate ductile damage nucleation and growth. Some complex-concentrated alloys (CCAs) with negative stacking-fault energy (SFE) demonstrate a rare faulting plasticity behavior with high strain hardening capability, that sets them apart from conventional low, positive SFE alloys that exhibit the TRIP effect. This study investigates the influence of dilute nitrogen on the TRIP mechanisms in a Co-rich CoCrNi CCA, and reveals, among other findings, that nitrogen interstitials can effectively reduce the extension of stacking faults and deformation-induced phase transformation, resulting in higher strain hardenability at early deformation levels and ductility. Thermodynamic calculations are coupled to various experimental analyses based on atom-probe tomography, scanning transmission electron microscopy, electron backscattered diffraction, electron-channeling contrast imaging, in situ high-energy synchrotron X-ray diffraction, and stress relaxation testing, to explore the origins of the observation. This work provides insights into the future design of CCAs with desirable TRIP, faulting plasticity, and mechanical properties.

Automated classification of granular bainite and polygonal ferrite by electron backscatter diffraction verified through local structural and mechanical analyses

Differentiation of granular bainite and polygonal ferrite in high-strength low-alloy (HSLA) steels possesses a significant challenge, where both nanoindentation and chemical analyses do not achieve an adequate phase classification due to the similar mechanical and chemical properties of both constituents. Here, the kernel average misorientation from electron backscatter diffraction (EBSD) was implemented into a Matlab code to differentiate and quantify the microstructural constituents. Correlative electron channeling contrast imaging (ECCI) validated the automated phase classification results and was further employed to investigate the effect of the grain tolerance angle on classification. Moreover, ECCI investigations highlighted that the grain structure of HSLA steels can be subdivided into four grain categories. Each category contained a different nanohardness or substructure size that precluded a nanoindentation-based phase classification. Consequently, the automated EBSD classification approach based on local misorientation achieved a reliable result using a grain tolerance angle of 5°.Graphical abstract