Transmission Kikuchi Diffraction Research Articles

Mechanisms of grain boundary α precipitation in the metastable β-titanium Ti-5Al-5Mo-5V-3Cr

In this study, we investigated the early nucleation and growth of grain boundary α during the aging heat treatment of Ti-5553. Through a correlative approach using scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, transmission electron microscopy, and atom probe tomography, high-angle β-grain boundaries were investigated in detail to understand the evolution of the precipitation of grain boundary α through nucleation and subsequent growth. It is found that at grain boundaries there is a pronounced nucleation of α precipitates with similar orientations. Growth of these similarly orientated α precipitates, in the later stages, leads to their agglomeration and the formation of grain boundary α. This investigation provides insights into the formation of grain boundary α in its early stages of nucleation and growth.

Delineating the Ultra-Low Misorientation between the Dislocation Cellular Structures in Additively Manufactured 316L Stainless Steel.

Sub-micro dislocation cellular structures formed during rapid solidification break the strength-ductility trade-off in laser powder bed fusion (LPBF)-processed 316L stainless steel through high-density dislocations and segregated elements or precipitates at the cellular boundaries. The high-density dislocation entangled at the cellular boundary accommodates solidification strains among the cellular structures and cooling stresses through elastoplastic deformation. Columnar grains with cellular structures typically form along the direction of thermal flux. However, the ultra-low misorientations between the adjacent cellular structures and their interactions with the cellular boundary formation remain unclear. In this study, we revealed the ultra-low misorientations between the cellular structures in LPBF-processed 316L stainless steel using conventional electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and transmission electron microscopy (TEM). The conventional EBSD and TKD analysis results could provide misorientation angles smaller than 2°, while the resolution mainly depends on the specimen quality and scanning step size, and so on. A TEM technique with higher spatial resolution provides accurate information between adjacent dislocation cells with misorientation angles smaller than 1°. This study presents evidence that the TEM method is the better and more precise analytical method for the misorientation measurement of the cellular structures and provides insights into measuring the small misorientation angles between adjacent dislocation cells and nanograins in nanostructured metals and alloys with ultrafine-grained microstructures.

Hydrogen promoted Ti3Al precipitation during Ti-6Al-4V martensite tempering

Temporary hydrogen alloying (THA) treatments are extensively used to refine coarse Ti-6Al-4V microstructures. The present study focuses on a THA treatment which promotes martensite decomposition; it highlights the effect of hydrogen on tempered martensite evolution by comparing martensite decomposition in hydrogenated and control samples. Microstructure evolution was characterised using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). Bulk XRD measurements indicate that quenching hydrogenated Ti-6Al-4V from the beta region causes a partial beta to hexagonal (α’) plus orthorhombic (α’’) martensite transformation. The EBSD, TKD and HRTEM analyses show that hydrogen saturated plate martensite decomposes to form a nanobanded structure which consists of nanosized, periodical Al-rich, and V-rich HCP regions that stabilise D019 Ti3Al within 3 h of tempering. In comparison, typical martensite decomposition is observed in the control experiment. Quenching non-hydrogenated Ti-6Al-4V from the beta region causes a full beta to hexagonal (α’) martensite transformation. Elemental partitioning occurs, but not to the extent that D019 Ti3Al is promoted and compositional banding is not observed. Formation of the nanobanded structure suggests that hydrogen altered the transformation pathway in hydrogenated martensite because of the presence of α’’possibly promoting spinodal decomposition.

Study of the epitaxy between ɛ-Fe2O3, hematite and spinel in brown-glazed Chinese ceramics using electron diffraction mapping techniques

Black-to-brown Chinese ceramics have sparked the interest of material scientists since the widespread discovery of complex dendritic structures containing mainly the rare ɛ-Fe2O3 polymorph in their glazes. This study proposes to tackle this issue using two electron diffraction mapping techniques, transmission Kikuchi diffraction and automated crystal orientation mapping–precession electron diffraction, which have not been fully exploited despite their relevance for studying these structures at the scales of interest. It first demonstrates the efficiency of applying these techniques to characterize the structures at the nanoscale and discusses some limitations that may be encountered, in particular due to the specificity of heritage samples. It then focuses on the crystalline orientations obtained with these techniques, which highlight epitaxial relationships among the crystalline planes (001) of the ɛ-Fe2O3 phase, (0001) of the hematite and (111) of the spinel. Finally, it discusses the growth of such complex multi-phase structures by proposing two possible mechanisms, one based on the transformation of ɛ-Fe2O3 into hematite and spinel, and the other based on the growth of hematite and spinel on already crystallized ɛ-Fe2O3 dendrites. It also compares the dendritic structures of the two modern samples with those found in an ancient sample of Jian ware.

Structure and RF performance evolution of polycrystalline silicon charge capture layer for advanced RF-SOI in in-situ annealing process

In this paper, the structures and radio-frequency (RF) properties of high resistivity silicon (HR-Si) wafers with a polycrystalline silicon (Poly-Si) charge capture layer after in-situ annealing were investigated. The thermal treatment significantly improves the wafer warpage and optimize the resistivity of Poly-Si/HR-Si interfaces. By using spreading resistance profiling (SRP), energy dispersive spectrometer (EDS) and secondary ion mass spectrometry (SIMS) technology, the rise of interfacial resistivity may be as results of the pyrolysis of the native oxide and the diffusion of hydrogen atoms. The transmission Kikuchi diffraction (TKD) analysis exhibited that the grains were recrystallized during the annealing, leading to the transformation from the small angle boundaries (SAGBs) into low-Σ coincidence lattice grain boundaries (CSL GBs). The drop of the bulk resistivity of the Poly-Si layer is attributed to the reduction of the total grain boundary (GB) density and the increase of the low electro-activity GB density. Moreover, the microwave losses and non-linearity of the RF-SOI with embedded Poly-Si layer was evaluated by on-chip testing based on CPW. It was determined that the decrease in the Poly-Si resistivity only worsens the microwave losses but not the non-linearity.

A multi-scale microscopic study of phase transformations and concomitant α/β interface evolution in additively manufactured Ti-6Al-4V alloy

Ti-6Al-4V alloy produced by laser-based powder bed fusion (LPBF) technique consists of an α′-martensite phase which is usually brittle and hence required, microstructure optimization by heat treatment to improve ductility. In this study, detailed microstructure analysis in the as-built condition and its evolution after sub-transus annealing was investigated. On annealing, the α′-martensite decomposes into α- and β-phases with the evolution of the α/β interface. Considering that the composition and microstructure play a crucial role in deciding the mechanical properties and service performance, a multi-scale correlative microscopy approach involving Transmission Electron Microscopy (TEM), Transmission Kikuchi Diffraction (TKD), and Atom Probe Tomography (APT) was employed. In the as-built condition, predominant segregation of β-stabilizers at the defect site especially along grain boundaries was observed; and these act as potential nucleation sites for β-phase during subsequent annealing. Low temperature annealing (750ᵒC) transforms the V-rich regions into the β-phase in the form of nanoscale precipitates. However, as the annealing temperature increases above 800ᵒC, the β-phase adopts a rod-like morphology. Comparing α-stabilizers, it is realized that β-stabilizers have a twofold greater contribution to elemental partition. Further, the composition and width of the α/β interface significantly impacts the mechanical properties of the alloy. Annealing at 750 °C and 850 °C resulted in significant improvement in ductility by 57.5% and 62.7% respectively without much of a trade-off in strength (>1 GPa). The strength and ductility are seen to decline when annealing temperature is raised above 850 °C due to a rise in compositional fluctuation and broadening of the α/β interface.

Crack nucleation and dislocation activities in titanium alloys with the strong transverse texture: Insights for enhancing dwell fatigue resistance

Cold dwell sensitivity of near α titanium alloys has posed a significant challenge to the engineering safety within the aerospace industry. However, there exists inconsistency regarding the critical facet formation mechanism between basal and prismatic nucleation. A key revelation in this paper is that the controversy surrounding the facet nucleation plane primarily arises from variations in texture: when the loading direction is parallel to the c-axis or at a 45 ° angle, it leads to basal plane cracking, whereas loading direction perpendicular to the c-axis results in prismatic plane cracking. What's more, a large anisotropy is observed in the dwell fatigue performance in descending order: rolling direction (RD) > transverse direction (TD) >45° direction. To be specific, the dwell fatigue life of RD specimens was 2 times and 4.07 times longer than that of TD and 45 ° specimens, which exhibits excellent dwell fatigue resistance. Furthermore, the high prismatic dislocation density near the facet of RD specimen was confirmed through the Transmission Electron Microscope (TEM) and Transmission Kikuchi Diffraction (TKD) map, which is evidenced that prismatic slips lead to higher strain hardening during cyclic loading. The basal plane of facet grain is generally parallel to the facet line, indicating that the elliptical facet developed by the transgranular fracture through basal plane. Twinning activation in hard grain to accommodate deformation in Ti60 alloy is firstly observed under dwell fatigue tests, which is a product of high stress level in the hard grain. This paper unveils the significant impact of texture on dwell fatigue resistance, offering novel insights into enhancing dwell fatigue performance through texture control.

Mechanism of mean stress sensitivity in Ti-6Al-4V under different stress ratios in high cycle fatigue

Initially, this article precisely determines that the “facet” on the fracture surface of the Ti-6Al-4V alloy during high cycle fatigue (HCF) loading under positive stress ratios, as determined by white light interferometry and transmission Kikuchi diffraction (TKD), originates from cracking along the slip planes that exhibit a grain orientated α grain in the loading direction, predominantly involving shear deformation along the crystallographic planes. The article emphasizes the crack initiation mechanisms in Ti-6Al-4V alloy related to internal microstructures under positive stress ratio HCF loading, elucidating relationship between texture-induced crack initiation and growth, and anomalous mean stress sensitivity. Under positive stress ratio fatigue loading, preferentially orientated texture regions tend to generate irreversible dislocations parallel to the basal plane, gradually accumulating within multiple locations to form persistent slip bands (PSBs), which, under the influence of internal stresses, sequentially collapse and rupture, leading to nucleation of microcracks. Upon encountering unfavorably oriented texture regions or grains, PSBs or existing microcracks experience dislocation accumulation, which in turn induce grain refinement, thereby causing further crack nucleation. Consequently, microcracks, through aggregation and coalescence during the fatigue loading sequence, form the main crack, ultimately precipitating fatigue failure and engendering the anomalous mean stress sensitivity observed in the Ti-6Al-4V alloy.

Diffraction-Based Multiscale Residual Strain Measurements.

Modern analytical tools, from microfocus X-ray diffraction (XRD) to electron microscopy-based microtexture measurements, offer exciting possibilities of diffraction-based multiscale residual strain measurements. The different techniques differ in scale and resolution, but may also yield significantly different strain values. This study, for example, clearly established that high-resolution electron backscattered diffraction (HR-EBSD) and high-resolution transmission Kikuchi diffraction (HR-TKD) [sensitive to changes in interplanar angle (Δθθ)], provide quantitatively higher residual strains than micro-Laue XRD and transmission electron microscope (TEM) based precession electron diffraction (PED) [sensitive to changes in interplanar spacing (Δdd)]. Even after correcting key known factors affecting the accuracy of HR-EBSD strain measurements, a scaling factor of ∼1.57 (between HR-EBSD and micro-Laue) emerged. We have then conducted "virtual" experiments by systematically deforming an ideal lattice by either changing an interplanar angle (α) or a lattice parameter (a). The patterns were kinematically and dynamically simulated, and corresponding strains were measured by HR-EBSD. These strains showed consistently higher values for lattice(s) distorted by α, than those altered by a. The differences in strain measurements were further emphasized by mapping identical location with HR-TKD and TEM-PED. These measurements exhibited different spatial resolution, but when scaled (with ∼1.57) provided similar lattice distortions numerically.

The characterisation of dental enamel using transmission Kikuchi diffraction in the scanning electron microscope combined with dynamic template matching

The remarkable physical properties of dental enamel can be largely attributed to the structure of the hydroxyapatite (HAp) crystallites on the sub-micrometre scale. Characterising the HAp microstructure is challenging, due to the nanoscale of individual crystallites and practical challenges associated with HAp examination using electron microscopy techniques. Conventional methods for enamel characterisation include imaging using transmission electron microscopy (TEM) or specialised beamline techniques, such as polarisation-dependent imaging contrast (PIC). These provide useful information at the necessary spatial resolution but are not able to measure the full crystallographic orientation of the HAp crystallites. Here we demonstrate the effectiveness of enamel analyses using transmission Kikuchi diffraction (TKD) in the scanning electron microscope, coupled with newly-developed pattern matching methods. The pattern matching approach, using dynamic template matching coupled with subsequent orientation refinement, enables robust indexing of even poor-quality TKD patterns, resulting in significantly improved data quality compared to conventional diffraction pattern indexing methods. The potential of this method for the analysis of nanocrystalline enamel structures is demonstrated by the characterisation of a human enamel TEM sample and the subsequent comparison of the results to high resolution TEM imaging. The TKD – pattern matching approach measures the full HAp crystallographic orientation enabling a quantitative measurement of not just the c-axis orientations, but also the extent of any rotation of the crystal lattice about the c-axis, between and within grains. Results presented here show how this additional information highlights potentially significant aspects of the HAp crystallite structure, including intra-crystallite distortion and the presence of multiple high angle boundaries between adjacent crystallites with rotations about the c-axis. These and other observations enable a more rigorous understanding of the relationship between HAp structures and the physical properties of dental enamel.

Electron Microscope Study of the Pressure-Induced Phase Transformation and Microstructure Change of TiO2 Nanocrystals.

Microstructure transformation of materials under compression is crucial to understanding their high-pressure phase transformation. However, direct observation of the microstructure of compressive materials is a considerable challenge, which impedes the understanding of the relations among phase transformation, microstructure, and material properties. In this study, we used transmission Kikuchi diffraction and transmission electron microscopy to intuitively characterize pressure-induced phase transformation and microstructure of TiO2. We observed the changes of twin boundaries with increasing pressure and intermediate phase TiO2-I of anatase transformed into TiO2-II (α-PbO2 phase) for the first time. The following changes occur during this transformation: anatase (diameter of ∼100 nm) → anatase twins 60° along the [110] zone axis → intermediate TiO2-I twins 60° along the [010] zone axis → TiO2-II twins 90° along the [010] zone axis. These results directly reveal the crystallographic relation among these structures, enhancing our understanding of the phase transformation in TiO2 nanocrystals.

Deformation microstructures and martensitic transformation pathways in cryogenically deformed 316L stainless steel

Liquefied gas storage and transportation, as well as space propulsion, are driving increasing interest in the cryogenic temperature deformation behaviour of 316L stainless steels. This was investigated here during tensile deformation at 15, 50 and 173 K. Complex transformation pathways, including one-step γ-austenite → α′-martensite, two steps γ → ε-martensite → α′ transformation as well as twinning and stacking fault-assisted γ → α′ transformation, are observed. γ grains with a (111) plane normal direction aligned 50–65°from the loading direction appear more likely to form the ε phase. Further, high-resolution transmission Kikuchi diffraction mapping revealed that the nucleation process of α′ can be assisted by ε and stacking faults at all cryogenic temperatures, whereas twins can also serve as sites for α′ nucleation when deformed at 173 K. For two-step transformation, separate lenticular α′ nucleate following Kurdjumov–Sachs orientation relationship (OR) within the shear band, once grown out of the shear band, Pitsch OR is preferable. As for one-step transformation, irregular oval α′ nucleates directly at γ grain boundaries with Nishiyama–Wassermann OR. These findings provide new insights into the correlation between the various transformation pathways and deformation mechanisms, as well as their improved performance at low temperatures.

Mechanical performance and deformation mechanisms of ultrastrong yield strength Fe-Cr-Ni-Mn-N austenitic stainless steel at 4.2 Kelvin

We report the mechanical performance and microstructural characteristics of a Fe-Cr-Ni-Mn-N alloy at cryogenic temperatures. The exceptionally high yield strength of 1.5 GPa combined with a high strain-hardening rate and no deterioration in ductility at 4.2 K was displayed. The evolution of deformation microstructure was examined using electron backscatter diffraction (EBSD), the transmission Kikuchi diffraction (TKD), high-resolution transmission electron microscopy (HRTEM), and aberration-corrected scanning TEM (STEM). The deformation microstructure mainly consisted of dislocation slip with L-C locks, {111} stacking fault formation, {111} deformation nanotwinning, and FCC → HCP shear transformation at 4.2 K. The occurrence of FCC-HCP shear transformation inside/near {111} twins to form γ-γtw-ε dual-phase structure induces a dynamic Hall-Petch effect that promotes the strain-hardening rate and enhances the strength-ductility combination. We believe that this alloy displays outstanding damage tolerance through a progressive synergy of deformation mechanisms leading to exceptional strength which provides a new insight into the commercialized development of high-performance alloys for cryogenic applications.

Multi-exposure diffraction pattern fusion applied to enable wider-angle transmission Kikuchi diffraction with direct electron detectors

Diffraction pattern analysis can be used to reveal the crystalline structure of materials, and this information is used to nano- and micro-structure of advanced engineering materials that enable modern life. For nano-structured materials typically diffraction pattern analysis is performed in the transmission electron microscope (TEM) and TEM diffraction patterns typically have a limited angular range (less than a few degrees) due to the long camera length, and this requires analysis of multiple patterns to probe a unit cell. As a different approach, wide angle Kikuchi patterns can be captured using an on-axis detector in the scanning electron microscope (SEM) with a shorter camera length. These ‘transmission Kikuchi diffraction’ (TKD) patterns present a direct projection of the unit cell and can be routinely analysed using EBSD-based methods and dynamical diffraction theory. In the present work, we enhance this analysis significantly and present a multi-exposure diffraction pattern fusion method that increases the dynamic range of the detected patterns captured with a Timepix3-based direct electron detector (DED). This method uses an easy-to-apply exposure fusion routine to collect data and extend the dynamic range, as well as normalise the intensity distribution within these very wide (>95°) angle patterns. The potential of this method is demonstrated with full diffraction sphere reprojection and highlight potential of the approach to rapidly probe the structure of nano-structured materials in the scanning electron microscope.

Re-dissolution behavior of secondary phases and mechanical response of CNT/2024Al composites

Heat treatment is one of the most effective strategies for regulating microstructure and enhancing the mechanical properties of aluminum matrix composites. In this work, the effects of solution treatment on the static recrystallization behavior, characteristics of secondary phases dissolving, and mechanical response of carbon nanotubes reinforced aluminum matrix composites (CNT/2024Al) were systematically investigated using transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). Since plenty of deformation stored energy in as-extruded composites is consumed, the residual energy is insufficient to induce static recrystallization during solution treatment. Besides, various interfacial phases aggregated around the Al2Cu phases, which reduced the effective contact area between the Al2Cu phases and matrix, and severely hindered the dissolution process of the Al2Cu phases. The yield strength, tensile strength, and elongation of as-solution treated composites were 23.2%, 44.9%, and 23.0% higher than those of as-extruded composites, respectively. The enhanced yield strength is attributed to solid solution strengthening induced by the dissolution of secondary phases, thermal mismatch strengthening, and the strengthening originating from the formation of Al4C3 phases.

Refining the mechanistic understanding of microstructural decay during rolling contact fatigue in 52100 bearing steel tempered at high temperature

Subsurface rolling contact fatigue (RCF) failure occurs beneath heavily loaded hard contacts like gears, bearings, and cams. This study investigates microstructural decay beneath a RCF-tested surface in AISI/SAE 52100 bearing steel tempered at 240 ℃. RCF tests were conducted at 100 ℃ with a maximum Hertzian contact pressure of 4 GPa for four stress cycles. Microstructural characterization utilized scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, and transmission electron microscopy. Due to high tempering temperature, white etching bands (WEBs) were observed without preceding dark etching regions. The microstructural decay sequence involved: (1) formation of elongated ferrite and ferrite microbands, (2) complete dissolution of tempered carbides and partial dissolution of residual cementite, (3) formation of WEBs composed of nano-sized ferrite grains (100–300 nm) transformed from ferrite microbands, and (4) appearance of lenticular carbides. Within the WEBs, most nano-sized grains had high-angle grain boundaries, while the fraction of low-angle grain boundaries increased in later stages of RCF. Lenticular carbides formed alongside elongated ferrite and coalesced nano-sized ferritic grains.

Amorphous and anisotropic surface relief formation in tungsten under repeated high-flux hydrogen plasma loads

Facing extreme plasma loads, the structural integrity of the tungsten divertor is crucial for ITER, an engineering marvel in nuclear fusion reactors, to achieve its fusion performance targets. Induced by repeated transient heating from plasmas, the thermal fatigue damage of tungsten–typically accompanied by the formation of surface relief–has been identified as a critical issue but an in-depth understanding is still lacking. Here, we report the formation of amorphous and anisotropic surface relief on ITER-grade tungsten surfaces under ITER-relevant hydrogen plasma loads. Measured by both electron backscatter diffraction over large fields of view and transmission Kikuchi diffraction of site-specific lamellae, such surface relief preferentially forms on grains with {110} planes parallel to the surface. This cannot be explained by the orientation-dependent resolved shear stress according to the Schmid law, threshold displacement energy anisotropy, or oxidation anisotropy. Furthermore, surface relief amorphization is revealed by high-resolution transmission electron microscopy imaging and selected area electron diffraction analysis, and is explained by a novel vacancy-induced amorphization mechanism. The results provide new insights into the thermal fatigue behavior of tungsten for fusion applications.

Phase decomposition of AlCrFeCoNiCu0.5 HEA thin films by vacuum annealing

The most attractive advantages of high entropy alloy (HEA) thin films are their excellent properties, such as high strength and high corrosion and oxidation resistance, with a lower material cost. However, the thermal stability of the HEA thin films is always controversial. This is critical since the thermal stability of metals and alloys is closely related to the material's performance at high temperatures and also determines the application field of the material. The question of how will the nanocrystalline high entropy alloy thin film perform in terms of thermal stability needs to be clarified urgently. In this research, the thermal stability of AlCrFeCoNiCu0.5 HEA thin film fabricated by cathodic arc deposition was investigated at 500 °C as a function of time during vacuum annealing. X-Ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM) and Transmission Kikuchi Diffraction (TKD) were applied to analyze the chemistry and microstructure of HEA thin films in detail. The results showed that there was no obvious change in the elemental compositions of the films after annealing, but the elemental distribution of the annealed films was different from the as-deposited film, especially at the film surface and film-substrate interface for the 24 h-annealed sample. The existence of the FeCo phase with B2 structure was a significant sign to demonstrate that the spinodal decomposition occurred in the single face-centered-cubic (FCC) phase matrix of the film during annealing. Simultaneously, the segregation of Cu and Cr near the film-substrate interface and the penetration of Ni and Cu towards the Si substrate were clearly observed. These unusual phenomena indicated that the thermal stability of HEA was not always excellent. Also, it was noticed that a few Kirkendall voids forming at the film-substrate interface after 24 h of annealing would affect the adhesion of the film. Additionally, heat recovery and recrystallization were achieved by 500 °C annealing, which were confirmed by the decrease of lattice parameter and the increase of grain size in the 24 h-annealed thin film. Furthermore, the hardness and elastic modulus of the thin films before and after annealing were measured by nanoindentation. The results showed all the thin films exhibited excellent mechanical properties, and the optimal hardness and elastic modulus of 7.7 ± 0.3 GPa and 183.9 ± 4.4 GPa were obtained after 3 h-annealing.

Mechanical Behaviors and Failure Analysis of S38C Steel with Gradient Structure Fabricated by Induction Heating and Quenching

This article proposes a simple and fast method of induction heating and quenching to produce surface gradient structure for S38C steel, and its mechanical behavior and strengthening mechanism are revealed. The variation of the gradient structure from surface to interior is characterized by electron backscatter diffraction, and the tensile behavior of the gradient structure at different depths is acknowledged by the small‐scale tensile tests. The gradient structure is tempered martensite microstructure, which significantly improves the hardness and tensile strength of surface and subsurface regions. Accordingly, with the strengthening of the gradient structure, the general tensile strength and fatigue behavior of the S38C steel are increased close to those of high‐strength steel. Moreover, the fatigue crack initiation mechanism of the gradient structure is studied by energy dispersive spectroscopy, transmission Kikuchi diffraction, and transmission electron microscope characterization on the crack initiation regions. It reveals that the fatigue failure of the gradient structure can be due to stress concentration on the surface and around subsurface inclusions, and the crack initiation modes present surface crack initiation and internal crack initiation, respectively.

Improvement of mechanical properties at cryogenic temperature of CoCrNi medium entropy alloy fabricated by hybrid additive manufacturing technology

The emergence of CoCrNi medium-entropy alloys (MEAs) holds great promise for applications in extreme service environments. However, fabricating CoCrNi MEA with an extraordinary combination of strength and ductility is urgently needed. Herein, we adopt a combination of laser direct energy deposition (L-DED) additive manufacturing and ultrasonic impact treatment (UIT) to fabricate CoCrNi additive parts. The optical microscopy results showed that the proportion of equiaxed crystals increased from 7.2% to 42.1% after UIT, while the proportion of columnar crystals with grain sizes greater than 31 μm decreased from 62.8% to 27.4%. The tensile test results at 298 and 77 K showed that the tensile strength increased by 7.0% and 27.6%, respectively, after UIT. Electron backscatter diffraction (EBSD) results indicated that the texture intensity of the XOY and XOZ planes in the additive parts decreased after UIT. The transmission Kikuchi diffraction (TKD) results showed that 29.9% of the FCC phase transformed into the HCP phase after the tensile test. Meanwhile, the volume fractions of the substructured, recrystallization, and deformation zones in the phase transformation zone and in the FCC structure were 21.2%, 58.7%, and 20.1% and 67.7%, 9.0%, and 23.3%, respectively. The enhanced transformation-induced plasticity effect contributed to the higher strength and ductility of the CoCrNi MEA treated by UIT.