The microstructure, temperature-dependent mechanical properties and deformation behaviors of equiatomic CoCrFeMnNi high-entropy alloy (HEA) additively manufactured by selective laser melting (SLM) ...were investigated. SLM-built HEA had a face-centered cubic (FCC) single-phase random solid solution. In addition, SLM-built HEA was composed of epitaxial growth grains, dislocation network and nano-sized oxides. Room- and high-temperature compression tests confirmed that SLM-built HEA has outstanding mechanical properties in all temperature ranges compared to equiatomic CoCrFeMnNi HEAs reported up to the present. The excellent mechanical properties of SLM-built HEA were achieved with fine grains, high dislocation density and fine precipitates at low temperatures (25 °C to 600 °C), and by high dislocation density and fine precipitates at high temperatures (700 °C or higher). On the other hand, the deformation microstructure showed that slip and deformation twins are the main deformation mechanisms from 25 °C to 600 °C, and slip and partial recrystallization are the main deformation mechanisms above 700 °C. Based on the above findings, this study also discusses correlations among the microstructure, superior mechanical properties and deformation mechanisms of SLM-built equiatomic CoCrFeMnNi HEA.
One of the major challenges of equiatomic CoCrFeMnNi HEA is to manufacture parts with complex geometry that have higher yield strength. Equiatomic CoCrFeMnNi HEA was successfully fabricated in the ...present study with selective laser melting. The unique microstructure and mechanical anisotropy that generally appear in additive manufactured materials were investigated. SLM-built HEA has strongly oriented grains, dislocation networks, and nano-sized oxides. In addition, the average grain sizes were measured as 15.66 μm, 12.93 μm, and 5.98 μm on the plane perpendicular to the scanning direction (SD), transverse direction (TD), and building direction (BD), respectively. A compressive test measured outstanding yield strengths (YS) of 778.4 MPa, 766.4 MPa, and 703.5 MPa in the loading axis of SD, TD, and BD, respectively. These outstanding YSs are the result of a combination of fine grain sizes, high dislocation density and nano-sized oxides. In addition, anisotropy in mechanical properties are result from different values of Taylor factor and grain size according to the loading axis. After a compression test, the geometrically necessary dislocation density was found to differ about 2.5 times on each plane parallel to the loading axis in the same macro strain. Based on such findings, the relationship among microstructure, mechanical anisotropy and deformation mechanism are discussed in the present paper. Furthermore, the mechanical properties of SLM-built equiatomic CoCrFeMnNi HEA are predicted by using multiple strengthening mechanisms considering the microstructural characteristics.
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•SLM-built HEA shows hierarchical microstructure, dislocation network, and nano-sized oxides.•SLM-built HEA shows outstanding yield strength about ∼778.4 MPa.•Anisotropic mechanical response was observed in SLM-built HEA.•Mechanical properties of SLM-built HEA was predicted by using multiple strengthening mechanism.
Selective laser melting (SLM) is an emerging additive manufacturing process for fabricating multifunctional parts by locally melting and consolidating powders in a layer-by-layer manner. Its ...flexibility enables the development of new alloys and metal matrix composites with unique, fine microstructures and complex-shaped components that are all difficult to realize with traditional manufacturing processes. This study investigates how TiB2 nanoparticle reinforcements affect the microstructure and high-temperature deformation behavior of a 316L stainless steel matrix. In SLM-processed pure stainless steel, fully directional columnar grains were observed, whereas TiB2 nanoparticles added to the steel matrix significantly reduced the sizes of the molten pools and grains and disrupted the directional structures. A chemical analysis revealed no compositional difference between the boundary and interior of the molten pool areas of the SLM-processed TiB2/316L nanocomposites, suggesting that elements did not segregate macroscopically. However, for higher TiB2 contents, alloying elements microsegregated at the boundaries of cellular structures due to the particle accumulation structure mechanism. Transmission electron microscopy confirmed the existence of mostly cube-like TiB2 nanoparticles both at the boundaries and in the interiors of the cellular structures. The nanocomposites exhibited high compressive yield strength and ductility at room and high temperatures, except at 600 °C, where embrittlement was observed. Observing the microstructure and flow stress of nanocomposites deformed at 700 and 800 °C shed light on the dynamic recovery and dynamic recrystallization phenomena. However, several conventional strengthening mechanisms could not predict the entire yield strength increase. Thus, a novel microsegregation strengthening mechanism was presented to explain the effect of TiB2 on the refined microstructure, and the novel microsegregation phenomenon and the enhanced mechanical properties were correlated. Hence, the SLM process shows high potential for fabricating new materials with outstanding properties.
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This study examined the surface oxidation properties of Cr-Mn-Fe-Co-Ni high entropy alloy (HEA) at high temperatures (900 °C, 1000 °C, 1100 °C/24 h) in the 21%O2 and 79%N2 atmosphere. An initial ...observation of microstructures confirmed that they consisted of random FCC solid solution, and the average grain size was measured at 245.5 μm. High-temperature oxidation tests of the Cr-Mn-Fe-Co-Ni HEA showed that its weight gained by 1.76, 4.45, and 9.08 mg/cm2 at 900 °C, 1000 °C, and 1100 °C, respectively, and all three temperature conditions represented typical parabolic curves. Observation of its surface found that Cr2O3 and Mn2O3 were main oxides on a specimen tested at 900 °C, while (Mn, Cr)3O4 and Mn3O4 were mostly shown to be main oxides on a specimen tested at 1000 °C and 1100 °C, despite the presence of Cr2O3. The high-temperature oxidation behavior of Cr-Mn-Fe-Co-Ni HEAs was found to have been largely affected by Cr and Mn elements. The resultant kirkendall pore generation, change in the concentration of elements, internal oxidation and Mn2O3 → Mn3O4 transformation were found to affect the oxidation resistance of Cr-Mn-Fe-Co-Ni high entropy alloy.
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•The applicability of high entropy alloy (HEA) in a severe environment is currently focused.•However, studies of high temperature oxidation of HEAs have so far been insufficient.•The surface oxidation behavior of Cr-Mn-Fe-Co-Ni HEA was investigated at high temperatures (900 °C, 1000 °C, 1100 °C/24 h).•The high temperature oxidation mechanism of Cr-Mn-Fe-Co-Ni HEA was suggested.
III–VI metal chalcogenides have garnered considerable research attention as a novel group of layered van der Waals materials because of their exceptional physical properties and potential ...technological applications. Here, the epitaxial growth and stacking sequences of InTe is reported, an essential and intriguing material from III–VI metal chalcogenides. Aberration‐corrected scanning transmission electron microscopy (STEM) is utilized to directly reveal the interlayer stacking modes and atomic structure, leading to a discussion of a new polytype. Furthermore, correlations between the stacking sequences and interlayer distances are substantiated by atomic‐resolution STEM analysis, which offers evidence for strong interlayer coupling of the new polytype. It is proposed that layer‐by‐layer deposition is responsible for the formation of the unconventional stacking order, which is supported by ab initio density functional theory calculations. The results thus establish molecular beam epitaxy as a viable approach for synthesizing novel polytypes. The experimental validation of the InTe polytype here expands the family of materials in the III–VI metal chalcogenides while suggesting the possibility of new stacking sequences for known materials in this system.
This study presents the epitaxial growth and stacking sequences of InTe, an intriguing material within III–VI metal chalcogenides. Utilizing aberration‐corrected STEM, the interlayer stacking modes are directly revealed, leading to the identification of a new polytype. STEM analysis provided evidence for strong interlayer coupling in the new polytype, with layer‐by‐layer deposition identified as the mechanism behind the unconventional stacking order.
The fabrication of high-entropy alloy (HEA) matrix nanocomposites by additive manufacturing (AM) is challenging due to that the control of defect-low sample having even distribution of reinforcement ...via AM is extremely hard. In this study, we investigated the effect of carbon content on the microstructure evolution, tensile properties, and deformation mechanisms of Cx(Co20Cr20Fe20Mn20Ni20)100–x (x = 0.5, 1.0, and 1.5 at.%) HEA matrix nanocomposites additively manufactured by selective laser melting (hereafter referred to as SLM-built C-HEAs). SLM-built C-HEAs showed epitaxial growth grains, dislocation networks, and nano-sized carbides. In addition, with an increase in carbon content, the number density of nano-sized carbides, and the average grain sizes and columnar widths increased. In addition, the strength, work hardening rate, and elongation of SLM-built C-HEAs were enhanced as the carbon content increased. Dislocation networks in the as-built samples hindered the dislocation motion in the early to later stages of deformation, thus leading to high back stresses in SLM-built C-HEAs. Deformation twins were also formed in the three samples, because the critical stress for twinning was similar to the flow stresses at an early stage of deformation of SLM-built C-HEAs. Further, the yield strengths of SLM-built C-HEAs were predicted using six strengthening mechanisms that considered the microstructural factors. Based on the above findings, we discussed the correlations between the microstructure, mechanical properties, and deformation mechanisms of SLM-built C-HEAs with different carbon contents.
The Association Research Circulation Osseous (ARCO) presents the 2019 revised staging system of osteonecrosis of the femoral head (ONFH) based on the 1994 ARCO classification.
In October 2018, ARCO ...established a task force to revise the staging system of ONFH. The task force involved 29 experts who used a web-based survey for international collaboration. Content validity ratios for each answer were calculated to identify the levels of agreement. For the rating queries, a consensus was defined when more than 70% of the panel members scored a 4 or 5 rating on a 5-point scale.
Response rates were 93.1%-100%, and through the 4-round Delphi study, the 1994 ARCO classification for ONFH was successfully revised. The final consensus resulted in the following 4-staged system: stage I—X-ray is normal, but either magnetic resonance imaging or bone scan is positive; stage II—X-ray is abnormal (subtle signs of osteosclerosis, focal osteoporosis, or cystic change in the femoral head) but without any evidence of subchondral fracture, fracture in the necrotic portion, or flattening of the femoral head; stage III—fracture in the subchondral or necrotic zone as seen on X-ray or computed tomography scans. This stage is further divided into stage IIIA (early, femoral head depression ≤2 mm) and stage IIIB (late, femoral head depression >2 mm); and stage IV—X-ray evidence of osteoarthritis with accompanying joint space narrowing, acetabular changes, and/or joint destruction. This revised staging system does not incorporate the previous subclassification or quantitation parameters, but the panels agreed on the future development of a separate grading system for predicting disease progression.
A staging system has been developed to revise the 1994 ARCO classification for ONFH by an expert panel-based Delphi survey. ARCO approved and recommends this revised system as a universal staging of ONFH.
High entropy alloy (HEA), a new class of materials, has received attention as a substance that can potentially replace conventional alloys. Equiatomic CoCrFeMnNi HEA is an attractive material with ...excellent strength-ductility combination and corrosion resistance, and it achieves greater performance in low temperatures. This study investigated the HCF and tensile deformation behavior of equiatomic CoCrFeMnNi HEA. In order to suggest the possibility of the material's application in an as-homogenized state, coarse-grained (CG) equiatomic CoCrFeMnNi HEA was prepared. Microstructural observation measured an average grain size of 245.5 μm, and it was confirmed to have a face-centered cubic (FCC) random solid solution. A tensile test confirmed that the yield strength and tensile strength are 293.1 MPa and 625.6 MPa, respectively, and change in the work hardening rate according to deformation twin (DT) evolution during tensile deformation was observed. A high-cycle fatigue results shows fatigue strength of 280 MPa, which is close to its yield strength, and this confirmed that the material has outstanding high-cycle fatigue properties considering its yield strength. DT is uniquely formed at cycle loading with a stress level lower than the critical twinning stress (σT), and this can improve yield strength by approximately 95% and tensile strength by approximately 17%. Based on the above findings, this study discussed the role of DTs which affect the high-cycle fatigue and deformation behavior of coarse-grained HEA.
•The outstanding high cycle fatigue property of coarse-grained equiatomic CoCrFeMnNi high entropy alloy was suggested.•Deformation twins are uniquely formed by cyclic loading with a stress level lower than the critical twining stress (σT).•Unexpected hardening in the tensile result due to prior cyclic loading was found and explained.
Although the incidence of inflammatory bowel disease IBD is increasing in Asia, data on long-term epidemiological trends are limited. We performed a 30-year longitudinal study to investigate temporal ...trends in the epidemiology of Crohn's disease CD and ulcerative colitis UC in Seoul, Korea.
This population-based study included 1431 IBD patients 418 CD, 1013 UC diagnosed between 1986 and 2015 in the Songpa-Kangdong district of Seoul, Korea. Temporal trends in incidence, prevalence, and disease phenotype at diagnosis were analysed.
The adjusted mean annual incidence rates of CD and UC per 100 000 inhabitants increased from 0.06 (95% confidence interval CI, 0.05-0.07) and 0.29 95% CI, 0.27-0.31, respectively, in 1986-1990 to 2.44 95% CI, 2.38-2.50 and 5.82 95% CI, 5.73-5.92, respectively, in 2011-2015. Average annual percentage change in IBD incidence was 12.3% in 1986-1995, 12.3% in 1996-2005, and 3.3% in 2006-2015. The male-to-female ratio of the adjusted incidence rate was 3.3:1 for CD and 1.2:1 for UC. Perianal fistula/abscess was present in 43.3% of patients before or at CD diagnosis. At diagnosis, 54.3% of UC patients presented only with proctitis. The adjusted prevalence rate in 2015 was 31.59/100 000 95% CI, 31.10-32.07 for CD and 76.66/100 000 95% CI, 75.91-77.42 for UC.
The incidence and prevalence of IBD in Korea have continued to increase over the past three decades. Korean patients have distinct demographic and phenotypic characteristics, including a male predominance and high frequency of perianal fistula/abscess in CD and high proportion of proctitis in UC.
This study investigated the effect of Ti addition on microstructure and high-temperature oxidation property of AlCoCrFeNi high-entropy alloy. Ti content was controlled at 0 at% and 1 at%. The two ...alloys were found to have BCC single phase, and the average grain sizes of Ti0.0 and Ti1.0 were 47.3 µm and 49.7 µm, respectively, showing similarity. The EDS mapping of the inside of grains found that both alloys were characterized to be divided into Al-Ni element rich region and Cr–Fe element rich region. As a result of high-temperature oxidation test at 1100 °C, oxidation weight gains were measured at Ti0.0: 0.75 mg/cm
2
and Ti1.0: 0.17 mg/cm
2
, respectively. The results indicate that Ti addition largely improved high temperature oxidation resistance of AlCoCrFeNi HEA. Ti1.0 alloy, in particular, showed remarkably more excellent 1100 °C high-temperature oxidation resistance than other previously reported major ones such as NiCrAl and FeCrAl. In the surface and cross-section observations after oxidation tests, both alloys were found to have Al
2
O
3
oxides mostly. While the Ti0.0 material was observed to have Al
2
O
3
spallation macroscopically, the Ti1.0 alloy showed Al
2
O
3
spallation only in some local areas. In addition, a unique result was found in AlCoCrFeNiTi
x(x=0,1)
alloy that BCC → FCC phase transformation was accelerated, and FCC phase layer was formed in the surficial area where Al element had been depleted due to high-temperature oxidation. Moreover, as Ti was added, the thickness of FCC layer induced by high-temperature oxidation decreased. Based on the results, it was also discussed on how to improve the high-temperature oxidation resistance of AlCoCrFeNiTi
x
HEA.
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