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Co-Al based superalloys have been considered as the promising alternatives to Ni-Al based superalloys. Appropriate alloy design can stabilize a dual phase structure of face-centered cubic (FCC) Co matrix and L1 2 Co 3 (Al, X) intermetallics and avoid precipitation of B2 CoAl intermetallics for Co-Al based superalloys. It has been shown that stable Co 3 Al single phase compound does not exist. Instead, at such a chemistry, Co-Al binary phase diagram predicts the coexistence of hexagonal close-packed (HCP) Co and B2 CoAl at room temperature. Here we report the synthesis of stoichiometric Co 3 Al via magnetron sputtering. Transmission electron microscopy investigations identified metastable phases of Co 3 Al, dominated by HCP supersaturation with minor L1 2 intermetallics. Furthermore, the phase transformation from HCP-to-FCC Co 3 Al induced by stacking faults in Co/Co 3 Al multilayers is also discussed. This investigation provides new insights on the crystal structures of stoichiometric Co 3 Al and the design of novel Co-Al based alloy systems. [Display omitted] [ABSTRACT FROM AUTHOR]

Various ceramic materials have been successfully flash sintered via mostly direct current (DC) and some by alternative current (AC). However, a direct comparison on the effects of different field types on the overall microstructure and atomistic defect distribution in flash-sintered ceramics is still very limited. In this work, rutile TiO 2 was chosen as a model system to directly compare the effects of DC and AC fields on microstructure and defect distribution. DC flash-sintered TiO 2 presents asymmetrical distributions of grain sizes and defects, while AC sintered TiO 2 have more homogeneous microstructures. More interestingly, we demonstrated a reverse-polarity flash sintering technique can achieve dense TiO 2 bulk samples with homogeneous microstructure and tunable defect gradient, which are previously not attainable from either DC or AC sintering alone. This study shows the importance of field on the microstructures of flash-sintered ceramics and the great potential in achieving dense and uniform microstructures via effective field control process. [ABSTRACT FROM AUTHOR]

Metallic materials with a gradient microstructure usually exhibit excellent mechanical properties. However, the radiation response of gradient structural materials is less well understood. Here, room-temperature He ion irradiation up to ~4.5 dpa with ~10 at% He injection was performed on a gradient T91 steel processed by surface severe plastic deformation. In comparison to the coarse-grained ferritic T91, the gradient T91 with the nanocrystalline layers shows improved radiation tolerance in terms of less bubble swelling and radiation hardening. Additionally, bubble distribution along grain boundaries depends on misorientation angle suggesting a strong capacity of non-equilibrium grain boundaries in storing He atoms. The present study provides insight into the design of radiation tolerant gradient steels for nuclear industry applications. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Reversible solid oxide cells (rSOCs) present a promising solution to future energy challenges through the efficient conversion between electrical and chemical energy. To date, the benefits of rSOC technology have been off-limits to portable power and electrolysis applications due to the excessive polarisation resistance of the oxygen electrode at low temperatures, characterised by high area specific resistance (ASR) values below 500 °C. In this work we demonstrate growth of symmetric and complete rSOC structures based on state-of-the-art vertically aligned nanocomposite (VAN) films grown by pulsed laser deposition (PLD) on porous Pt-coated anodised aluminium oxide (AAO) substrates. The symmetric rSOC structures give the first demonstration of an rSOC oxygen electrode with ASR below 0.1 Ωcm2 at temperatures less than 450 °C. This is achieved through oxygen vacancy tuning by annealing, as confirmed by Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA) and Rutherford Backscattering Spectrometry (RBS) measurements. Thus, the present work describes a promising route towards future high-performance rSOC devices for portable power applications. [Display omitted] • Dense, defect free thin films can be deposited on porous aluminium oxide substrates by pulsed laser deposition. • Vertically aligned nanocomposite oxygen electrodes reduce activation energy and give superior low-temperature performance. • Area specific resistance values of solid oxide cells reduced by several orders of magnitude by oxygen vacancy engineering. [ABSTRACT FROM AUTHOR]

Thermomechanical treatment (TMT) can effectively improve the mechanical properties of high-Cr ferritic/martensitic (F/M) steels, which has been mainly attributed to a dense dispersion of nano-sized precipitates. Precipitate phases in high-Cr F/M steels produced by TMT require further investigations. Precipitates in commercial F/M steel P92 produced by a TMT process, warm-rolled at 650 °C plus tempered at 650 °C for 1 h, were investigated by transmission electron microscopy. Nano-sized precipitates with a high number density in the steel after the TMT were found to be Cr-rich M 2 (C,N) carbonitride, rather than MX or M 23 C 6 phase. The M 2 (C,N) carbonitride has a hexagonal lattice with the lattice parameters about a / c = 0.299/0.463 nm. These M 2 (C,N) carbonitrides with a typical composition of (Cr 0.85 V 0.06 Fe 0.06 Mo 0.03 ) 2 (C,N) have an average diameter smaller than 30 nm, and mainly distribute on dislocations and at the boundaries of equiaxed ferrite grains in the TMT steel. The TMT process inhibits the precipitation of M 23 C 6 and M 5 C 2 phases. Enhanced creep properties of the P92 steel after the TMT, as reported previously, were considered to be mainly attributed to plenty of nano-sized Cr-rich M 2 (C,N) carbonitrides produced by the TMT rather than to MX and M 23 C 6 precipitates. [ABSTRACT FROM AUTHOR]

Recent radiation studies on nanotwinned pure metals suggest that twin boundaries are appealing defect sinks, as they can transport and eliminate radiation-generated defects. However, radiation response of twin boundaries in nanotwinned steels is less well understood. Here, we investigate the microstructural evolution of a nanotwinned steel using in situ Kr++ irradiation to 4 dpa at ambient temperature. The in situ results show that nanoscale deformation twins in steels can effectively suppress the formation of ordered dislocation loop rafts. Furthermore, the formation of dislocation loop rafts and Frank loops depends prominently on twin spacing. The underlying mechanisms of the enhanced radiation resistance are discussed. The present study makes a positive step forward for the potential application of nanotwinned structural steels to nuclear reactors. [Display omitted] [ABSTRACT FROM AUTHOR]

Helium is a common nuclear reaction product, and it plays an important role in radiation-induced void swelling. Although extensive studies have been conducted to investigate the helium effects in metals and alloys, most of them were based on the post-irradiation analyses or computer simulations. Conclusive evidence that can demonstrate how voids evolve in the presence of He under high-temperature radiation remains limited. In this work, we performed in situ heavy ion radiation studies in a transmission electron microscope to compare the radiation response of the preexisting nanovoids in single crystal copper subjected to a single beam of 1 MeV Kr ions and a sequential dual beam of 14 keV He ion and 1 MeV Kr ion at 350 °C. Our studies revealed that the nanovoids would contract continuously until being eliminated when irradiated by a single beam of Kr ions. In comparison, the nanovoids in helium-injected Cu could reach a stable state and eventually stopped shrinking. Moreover, the influence of helium on the kinetics of void/bubble evolution under heavy ion irradiation was discussed within the framework of a proposed critical bubble model and bubble coarsening model. Graphical Abstract [Display omitted]. [ABSTRACT FROM AUTHOR]

[Display omitted] FeCrAl alloy is a promising accident tolerant fuel cladding material for next generation nuclear reactors. Grain refinement may lead to FeCrAl alloys with improved mechanical properties. However, the addition of Al retains the ferrite phase and makes the grain refinement difficult through conventional thermomechanical treatments. In this study, a model FeCrAl alloy, C35M, was processed by surface mechanical grinding treatment. Microscopy studies show the formation of gradient microstructures, consisting of surface nanolaminate layer and ultra-fine-grained sublayer. In situ micropillar compression tests reveal the gradient FeCrAl alloy has a high flow stress, exceeding 1.4 GPa, and excellent work hardening capability. A modified Kocks-Mecking model was utilized to explain the grain size-dependent work hardening behavior. This study sheds lights on the design of gradient ferritic steels with high-strength and good work hardening ability for various industrial applications. [ABSTRACT FROM AUTHOR]

• Response of solidification cellular structures in an additively manufactured 316LN SS was investigated under annealing and heavy ion irradiation at 450 ˚C. • Effects of cellular structures on the formation of Frank loops were studied. • Cr depletion, Ni and Si enrichment along the cellular walls were first observed in irradiated additively manufactured 316LN SS. • Experimental results were interpreted by a comparison with previous neutron irradiation studies on wrought 316L and 316LN SSs. Additive manufacturing has become an appealing technique to fabricate three-dimensional metallic materials and components for nuclear reactors. However, response of additively manufactured alloys to high-dose heavy ion irradiations at elevated temperatures is still not well understood. Here, an additively manufactured 316LN austenitic stainless steel with high-density solidification cells was irradiated using 3.5 MeV Fe ion to a peak dose of 220 dpa at 450 ˚C. Microscopy studies show a lower Frank loop density and smaller size in the additively manufactured sample compared with its cold worked counterpart, and the cellular structures may largely suppress the formation of perfect loops and dislocation networks and reduce the magnitude of solute segregations comparing with high angle grain boundaries. The present work advances the understanding on the high-temperature irradiation response of additively manufactured steels for nuclear reactor applications. [ABSTRACT FROM AUTHOR]

To raise mechanical strength of metallic materials, methods such as grain refinement and amorphization are usually utilized but sacrifice plasticity. Strength and plasticity are largely dictated by dislocation-defect interactions in crystalline materials and by shear banding in metallic glasses, depending significantly on the length scales of microstructural features. Here we report on the evolution of microstructures and multistage phase transformations from micro/nanocrystalline to an entirely amorphous structure, realized by tailoring the composition of Cr in co-sputtered Al 1 00 -x Cr x (x = 0–25 at%) alloys. The associated Cr segregation caused the formations of different dual-phase nanocomposites. In-situ micromechanical experiments revealed that the flow stress of Al–Cr alloys can reach 2.4 GPa and the deformation behaviors varied drastically with Cr composition. This study established the mechanistic connection between the Cr composition-dependent evolution of microstructure and ultrahigh strength as well as plasticity, and revealed the benefits of building nanocomposites through a multistage phase transformation to improve the plasticity of nanocrystalline and amorphous materials. • Cr segregates at grain boundaries in Al–Cr alloys with a wide compositional range. • The segregation enables multistage phase transformation: FCC-Al 7 Cr-amorphous. • Microstructure-dependent flow stress could reach 2.4 GPa for Al alloys. • Nanocomposites could improve plasticity of nanocrystalline and amorphous materials. • Mechanistic connection between property and microstructure is established. [ABSTRACT FROM AUTHOR]

BiFeO 3 is a promising multiferroic material for versatile device applications due to its co-existence of magnetic (i.e., antiferromagnetic) and ferroelectric ordering at room temperature. While its functional properties have been extensively investigated, the exploration of its mechanical behavior was limited mostly to the thin-film form of BiFeO 3. In this work, we conducted in situ micropillar compression experiments to investigate the deformation behavior of La-doped BiFeO 3 (La-BFO) samples processed by both conventional and flash sintering methods. The conventionally sintered La-BFO exhibited limited deformability at room temperature and 450 °C. In contrast, the deformability of the flash-sintered La-BFO specimens was substantially improved by nearly 100% at both testing temperatures. Detailed post-mortem studies suggest that preexisting dislocations and wide anti-phase boundaries introduced during flash sintering can toughen flash-sintered La-BFO by easing dislocation migration and ferroelastic domain switching. This study provides a fresh perspective to design an advanced multifunctional system with improved mechanical properties. • Flash sintering introduced high population of pre-existing dislocations and anti-phase boundaries into La-BFO. • The flash sintered La-BFO deformed at room temperature exhibited an over 100% deformability enhancement. • At 450 °C, the flash-sintered La-BFO outperformed the conventionally sintered counterpart by showing plastic deformability. [ABSTRACT FROM AUTHOR]

Additively manufactured (AM) 316 L stainless steel (SS) after hot isostatic pressing (HIP) was found to exhibit superior resistance to irradiation-assisted stress corrosion cracking (IASCC) in high-temperature water, as compared to wrought 316 L SS. The well-accepted IASCC factors of radiation-induced segregation (RIS) and radiation hardening are not accurate descriptions of IASCC susceptibility in this case. A decreased strain localization along grain boundaries, caused by dislocation channel broadening, was confirmed to suppress crack initiation. A unique distribution of irradiation-induced defects in HIP AM 316 L SS eased dislocation cross-slip compared to those in the wrought counterpart, thus increasing the channel width near the grain boundaries. For the first time, this study highlights the importance of dislocation channel broadening as a potential mechanism to further improve the IASCC resistance of 316 L SS and provides direct experimental evidence based on commercial-grade materials. Dislocation channel broadening decreased strain localization near grain boundaries and enhanced the IASCC resistance of additively manufactured 316 L stainless steel treated by hot isostatic pressing. Different sizes and densities of radiation induced loops and precipitation behavior are believed to be the underlying causes. [Display omitted] [ABSTRACT FROM AUTHOR]

Understanding the void evolution in irradiation environment is of great interest and significance, as irradiation-induced voids typically lead to pronounced volumetric swelling and degradation of mechanical properties. In situ studies on the irradiation response of nanovoids at elevated temperature remain limited. In this work, we performed systematic in situ 1 MeV Kr++ irradiations on Cu with nanovoids in a transmission electron microscope up to 350 °C. The in situ studies revealed intriguing void spheroidization, shrinkage and migration. Furthermore, the morphology evolution and migration of nanovoids showed a strong dependence on irradiation temperature and initial void size. Post-irradiation analyses identified defect clusters in the form of stacking fault tetrahedrons, and the remaining large faceted nanovoids. The underlying mechanisms of irradiation-induced void spheroidization and shrinkage were discussed based on phase-field modeling. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Nanoprecipitates or grain refinement can effectively enhance the mechanical strength of Al alloys, but the room-temperature strengths of precipitation hardened and nanocrystalline Al alloys often fall below 1 GPa. Furthermore, they are largely plagued by precipitous mechanical softening at elevated temperature below 300°C, mostly due to degraded microstructural stability. Here, we report a mechanism of coupled solute effect in nanotwinned Al-Fe-Ti alloys that enables stability of nanograins up to 400°C and an unprecedented high-temperature flow stress of ~ 1.7 GPa at 300°C. The supersaturated Fe solutes in Al act as effective grain refiner, forming superstrong solid solution alloys. More importantly, empirical evidence combined with first principle calculations indicate that the Ti solutes delay the agglomeration of Fe solutes, thereby remarkably extending the temperature window for the stability of nanograins in nanotwinned Al alloys. This study highlights the opportunity to design ultrastrong and stable nanostructured alloys for potential high temperature applications via a coupled solute effect. Image, graphical abstract [ABSTRACT FROM AUTHOR]

FeCrAl alloy is one of the promising cladding materials in the advanced nuclear reactors. Here we compare the irradiation response of the nanolaminate (NL) and coarse-grained (CG) FeCrAl alloys, by performing He ion irradiation followed by in situ Kr ion irradiation in a transmission electron microscope at 450 °C. NL alloy has reduced He bubble induced swelling and decreased the density of dislocation loops in comparison to the CG alloy. The distribution of He bubbles appears to be dependent on misorientation angles of high angle grain boundaries. A new type of nanoprecipitates was identified in the irradiated CG FeCrAl without noticeable chemical segregation. Chemical segregation occurred along the laminate boundaries in the irradiated NL FeCrAl alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

Alloy 718 has been intensively investigated for additive manufacturing. While previous studies on additively manufactured alloy 718 typically have metallic phases only, in this work, we show that oxide nanoparticles can be introduced into alloy 718 via a reactive sintering mechanism. The introduction of micron scale Cr 2 O 3 powder in alloy 718 powder led to the formation of a homogeneous distribution of Al and Ti rich oxide nanoparticles. This surprising finding arises from the chemical reaction between oxide powders and metal matrix containing Al and Ti. In comparison to the control alloy 718 without oxides, the presence of oxide nanoparticles dramatically alters the microstructure, leading to a relatively more equiaxed grain structure, while also reducing the residual stress. While the mechanical testing between the two material groups demonstrates comparable mechanical strength and tensile ductility at room temperature, the high-temperature (900 °C) mechanical properties of the oxide containing 718 samples were significantly improved over the control 718 samples. The apparent possibilities of using this reactive process to introduce oxide nanoparticles into metallic materials may have a general implication for the fabrication of oxide dispersion in various metal matrices via reactive additive manufacturing. • Production of nano oxide dispersion strengthened alloy 718 • Reactive formation of oxide nano particles creates a (Al,Ti) 2 O 3 chemistry • Reduction of residual stress via the oxide particles accommodating residual stress • Increased high-temperature mechanical properties up to 900 °C [ABSTRACT FROM AUTHOR]

FeCrAl alloys have attracted extensive attention as one of the potential cladding materials for the commercial light water reactors. Although the creep behaviors of coarse-grain FeCrAl alloys have been investigated at elevated temperatures, little is known on the deformation mechanism of fine-grain FeCrAl alloys. Here, we performed surface mechanical grinding treatment to fabricate nanolaminated ferritic FeCrAl alloys. In situ micropillar compression tests were applied to determine the underlying deformation mechanism of nanolaminated FeCrAl alloys at elevated temperatures. The strain rate sensitivity and activation energy for the dominant deformation mechanism were estimated considering the presence of a threshold stress. [Display omitted] [ABSTRACT FROM AUTHOR]

Recent studies show that flash sintering can introduce different types of defects in various ceramics. We investigate the effects of pre-flash incubation on the microstructure of flash-sintered ceramics by comparing flash-sintered TiO 2 samples under constant field (CF) and instant field (IF). The CF sample underwent an incubation period before flash sintering, while IF sample "flashed" instantaneously after the application of electric field. The CF induced a higher relative density and greater grain size than those in the IF flash-sintered specimens. Raman spectroscopy and transmission electron microscopy analyses showed that the CF sample has more types of defects, such as dislocations and stacking faults. The correlation between microstructural defects and the incubation period is discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

ZnO has been widely used for various industrial applications. Compared to the extensive investigations on the functional properties of ZnO, the mechanical behavior of ZnO is less well understood. Here we investigated temperature-dependent deformation mechanisms of flash-sintered and conventionally sintered ZnO by in-situ microcompression test up to 600 °C. When tested at room temperature, prominent transgranular cracking occurred in the flash-sintered ZnO at a flow stress of 1.2 GPa, higher than that for the conventionally sintered ZnO (0.9 GPa). A high density of dislocations formed near grain boundaries and fracture surfaces led to the high strength and work-hardening capability in the flash-sintered ZnO. At 200 °C, the flow stress of the flash-sintered ZnO decreased to 0.7 GPa, and the fracture mode evolved to intergranular cracking. When compressed at 400 °C, severe deformation localized at the pillar top was dominated by grain boundary-mediated processes. At 600 °C, plastically deformed grains and intergranular cracks increased prominently, accompanied by a decrease in the density of geometrically necessary dislocation. This study shed lights on understanding the influence of the flash sintering process on the mechanical behaviors of ZnO. Image, graphical abstract [ABSTRACT FROM AUTHOR]

There are increasing studies that show nanotwinned (NT) metals have enhanced radiation tolerance. However, the mechanical deformability of irradiated nanotwinned metals is a largely under explored subject. Here we investigate the mechanical properties of He ion irradiated nanotwinned Cu with preexisting nanovoids. In comparison with coarse-grained Cu, nanovoid nanotwinned (NV-NT) Cu exhibits prominently improved radiation tolerance. Furthermore, in situ micropillar compression tests show that the irradiated NV-NT Cu has an ultrahigh yield strength of ∼1.6 GPa with significant plasticity. Post radiation analyses show that twin boundaries are decorated with He bubbles and thick stacking faults. These stacking fault modified twin boundaries introduce significant strengthening in NT Cu. This study provides further insight into the design of high-strength, advanced radiation tolerant nanostructured materials. Image 1 [ABSTRACT FROM AUTHOR]

Ferritic/martensitic grade 91 steel (T91 or P91) is one of the candidate materials for primary and secondary loops in sodium cooled fast reactor. Recently, studies show that equal channel angular extrusion (ECAE) and heat treatment can enhance the strength of T91 from 480 MPa to 1600 MPa. However, a detailed refinement mechanism is yet to be clarified. Furthermore, there is a need to correlate the hardness measurement to tensile test results in T91 steels for nuclear application. Here, we report the grain refinement mechanisms in T91 after extensive microstructural analysis. Packet (a group of martensite with the same habit plane) is identified as an elementary unit for grain refinement. The Vickers hardness correlates well with the ultimate tensile strength, with a fitting ratio of ∼3.24. However, the ratio of Vickers hardness to yield strength is a function of ductility in T91 steels. • Microstructural characterization and hardness were conducted on three different planes in ECAE deformed Grade 91 steels. • Packet (a group of martensite with the same habit plane) is identified as an elementary unit for grain refinement. • The H v correlates well with the σ UTS but the ratio of H v to σ y is a function of ductility in TMT Grade 91 steels. [ABSTRACT FROM AUTHOR]

Abstract Nanotwinned metals exhibit outstanding radiation tolerance as twin boundaries effectively engage, transport and eliminate radiation-induced defects. However, radiation-induced detwinning may reduce the radiation tolerance associated with twin boundaries, especially at elevated temperatures. Here we show, via in-situ Kr ion irradiation inside a transmission electron microscope, that 3 at. % Fe in epitaxial nanotwinned Cu (Cu 97 Fe 3) significantly improves the thermal and radiation stability of nanotwins during radiation up to 5 displacements-per-atom at 200 °C. Such enhanced stability of nanotwins is attributed to a diffuse 9R phase resulted from the dissociation of incoherent twin boundaries in nanotwinned Cu 97 Fe 3. The mechanisms for the enhanced stability of twin boundaries in irradiated nanotwinned alloys are discussed. The stabilization of nano-twins opens up opportunity for the application of nanotwinned alloys for aggressive radiation environments. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Al alloys have widespread applications, but often suffer from low yield strength. Recently, nanotwinned Al-Fe solid solution alloys have shown high flow stress (>1.5 GPa), ascribed to nanograins with abundant incoherent twin boundaries and solute-stabilized 9R phase. However, the high temperature mechanical behaviors of high-strength twinned Al-Fe alloys remain unknown. In this study, we show that nanotwinned microstructures are stable up to 280 °C, followed by recrystallization at 300 °C. In-situ uniaxial compression tests in a scanning electron microscope show that the nanotwinned Al alloys retain their high flow stress of ∼1.3 GPa when tested up to 200 °C, and substantial softening occurs at a test temperature of 300 °C. This work reveals the superb thermal stability and high temperature mechanical behaviors of the nanotwinned Al-Fe alloys, and offers a new perspective to design future strong and thermally stable nanostructured Al alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Microstructure of P92 ferritic/martensitic steel irradiated under 3.5 MeV Fe 13+ ion beams with a damage rate of ∼1 × 10 −4 at 700 °C to 1.62 displacements per atom (dpa) was investigated using transmission electron microscopy (TEM). M 23 C 6 and MX precipitates were the dominant phases in the unirradiated samples. Irradiation induced high density Cr-rich Cr 2 (C,N) precipitates were identified in irradiated samples using TEM. The nucleation of the Cr-rich Cr 2 (C,N) precipitates in the irradiated P92 steel at elevated temperature were discussed in detailed. [ABSTRACT FROM AUTHOR]

At the present time 9–12% Cr ferritic/martensitic (F/M) steels with target operating temperatures up to 650 °C and higher are being developed in order to further increase thermal efficiency so as to reduce coal consumption and air pollution. An 11% Cr F/M steel was prepared by reference to the nominal chemical composition of SAVE12 steel with an expected maximum use temperature of 650 °C. The precipitate phases of the 11% Cr F/M steel normalized at 1050 °C for 0.5 h and tempered at 780 °C for 1.5 h were investigated by transmission electron microscopy. Except for Cr-/Cr-Fe-Co-rich M 23 C 6 , Nb-/V-/Ta-Nb-/Nd-rich MX, Fe-rich M 5 C 2 , Co-rich M 3 C and Fe-Co-rich M 6 C phases previously identified in the steel, two types of sigma phases consisting of σ-FeCr and σ-FeCrW were found to be also present in the normalized and tempered steel. Identified σ-FeCr and σ-FeCrW phases have a simple tetragonal crystal structure with estimated lattice parameters a /c = 0.8713/0.4986 and 0.9119/0.5053 nm, respectively. The compositions in atomic pct of the observed sigma phases were determined to be approximately 50Fe-50Cr for the σ-FeCr, and 30Fe-55Cr-10W in addition to a small amount of Ta, Co and Mn for the σ-FeCrW. The sigma phases in the steel exhibit various blocky morphologies, and appear to have a smaller amount compared with the dominant phases Cr-rich M 23 C 6 and Nb-/V-/Ta-Nb-rich MX of the steel. The σ-FeCr phase in the steel was found to precipitate at δ-ferrite/martensite boundaries, suggesting that δ-ferrite may rapidly induce the formation of sigma phase at δ-ferrite/martensite boundaries in high Cr F/M steels containing δ-ferrite. The formation mechanism of sigma phases in the steel is also discussed in terms of the presence of δ-ferrite, M 23 C 6 precipitation, precipitation/dissolution of M 2 X, and steel composition. [ABSTRACT FROM AUTHOR]

• M 3 X 2 (Ta 3 C 2 type) phases were found in steel. • Ta-rich precipitate phases in CLAM steel are presented in detail. • Ta-rich M(C,N), MC, M 3 (C,N) 2 and M 3 C 2 phases co-existed in tempered CLAM steel. Tantalum is an essential alloying element in reduced-activation ferritic/martensitic (RAFM) steels being a promising candidate structural material for fusion reactor. Tantalum has an important influence on the microstructure and mechanical properties of RAFM steels by forming tantalum precipitates. The tantalum precipitates in CLAM steel, a type of RAFM steels, normalized for 30 min at 980 °C and tempered for 90 min at 760 °C were investigated using transmission electron microscope. In addition to tantalum-rich M(C,N) carbonitrides and MC carbides with a face centered cubic crystal structure and a typical metallic element composition of about 80Ta, 5-10Cr, 2-5Fe, and 4-5V in atomic pct, tantalum-rich M 3 (C,N) 2 carbonitrides and M 3 C 2 carbides, which are not known to have been reported previously in steels including RAFM steels, were also identified in the steel. Tantalum-rich M 3 (C,N) 2 and M 3 C 2 precipitates have a typical metallic element composition of about 55-60Ta, 25-30Cr, 5-15Fe, and 1-2V in atomic pct, and a spherical morphology, as well as small sizes ranging from 15 to 40 nm in diameter. The experimentally observed tantalum-rich M 3 (C,N) 2 and M 3 C 2 precipitates exhibit a relatively small size, and appear to have a smaller amount compared with the tantalum-rich M(C,N) and MC precipitates observed. The effect of identified precipitate phases on the creep properties of the steel was also discussed. [ABSTRACT FROM AUTHOR]