M2N nitride phases of 9% chromium steels for nuclear applications

M₂N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600-780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M₂N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the steels. Four types of Cr-rich M₂N phases with different lattice parameters probably existed in the steels. The M₂N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.     

Vanadium nitride precipitate phase in a 9% chromium steel for nuclear power plant applications

Vanadium nitride precipitate phase in a 9% Cr steel was observed and analyzed using transmission electron microscopy and energy dispersive spectroscopy. The steel samples were normalized at 1050 and 1100 °C for 1 h and then tempered at 750 °C for 30 min to 5 h followed by an air cooling. Through the microdiffraction pattern analyses and energy dispersive X-ray data, two kinds of vanadium nitride precipitates were determined to be (V_0.6Nb_0.2Cr_0.2)N and (V_0.45Nb_0.45Cr_0.1)N with the same fcc crystal structure and different lattice parameters ā = 4.070 and 4.232 Å, respectively. Lattice parameters estimated for the precipitates regarding the VN phase agree well with the present data from the microdiffraction patterns, indicating that the precipitates do not belong to the VC phase. Observed (V_0.45Nb_0.45Cr_0.1)N precipitates consisted of undissolved particles remaining after a normalizing and the particles newly precipitated during a tempering, whilst, the observed (V_0.6Nb_0.2Cr_0.2)N precipitates were formed during a tempering. These two vanadium nitrides seem to be a stable phase, and not an intermediary phase.     

Identification of precipitate phases in a 11Cr ferritic/martensitic steel using electron micro-diffraction

The precipitate phases in a 11Cr ferritic/martensitic steel normalized at 1050 °C for 1 h followed by tempered at 750 °C for 2 h have been studied through electron micro-diffraction combined with EDX analysis. Two niobium-rich carbonitride phases, (Nb_0.75V_0.15Cr_0.1)(C,N) and (Nb_0.5V_0.4Cr_0.1)(C,N), with the same f.c.c. structure were identified. Three vanadium-rich carbonitride phases were identified to be (V_0.65Nb_0.15Cr_0.2)(C,N), (V_0.45Nb_0.45Cr_0.1)(C,N) and (V_0.55Nb_0.25Cr_0.2)(C,N) with the same f.c.c. structure. Chromium-rich carbonitride M₂(C,N) and chromium-rich carbide M₂₃C₆ phases with a hexagonal and a f.c.c. crystal structure as well as a typical chemical formula of (Cr_0.8V_0.2)₂(C,N) and (Cr_0.7Fe_0.25W_0.05)₂₃C₆, respectively, were also identified in the steel sample. The size of each kind of precipitate phase was examined. The morphologies and precipitation sites along with the amount of observed precipitate phases were described and discussed.     

Precipitate phases of a ferritic/martensitic 9% Cr steel for nuclear power reactors

One of the reasons that ferritic/martenstic steels have been considered as candidate materials for nuclear power reactors is their superior creep resistance at elevated temperature. The creep rupture strength of 9% chromium steel could be improved by a fine dispersion of secondary precipitate phase. The precipitate phases in extra-low carbon 9% chromium steel with tempered conditions were investigated by transmission electron microscope and energy-dispersive X-ray analysis. The steel specimens were normalized and then tempered at different temperatures. Niobium-rich MN nitrides (Nb_0.6V_0.3Cr_0.1)N, and two kinds of vanadium nitrides, (V_0.6Nb_0.2Cr_0.2)N and (V_0.45Nb_0.45Cr_0.1)N having a f.c.c. crystal structure, were identified in the steel specimens tempered at 600-780 °C, and 750 or 780 °C respectively. Hexagonal chromium-rich M₂N precipitate phases with different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the tempered steel specimens. The M₂N phase showed a decrease/an increase in its chromium/vanadium content as the tempering temperature was increased. The influence of precipitates and heat treatment conditions on the high temperature properties of 9% Cr steel was discussed.     

Oxidation kinetics of Zircaloy-4 and Zr-1Nb-1Sn-0.1Fe at temperatures of 700-1200 °C

The oxidation characteristics for the Zircaloy-4 and Zr-1.0Nb-1.0Sn-0.1Fe alloys were investigated in the temperature ranges of 700-1200 °C for 3600 s under steam supply conditions, using a modified thermo-gravimetric analyzer. The oxidation at these temperatures generally complied with the parabolic rate law for the examined duration up to 3600 s. However, the parabolic rate was not obeyed in the temperature ranges of 800-1050 °C. The oxidation kinetics were changed depending on the oxidation temperatures due to the phase transformations of the base metal and its oxide. The oxidation rate exponents of the Zr-1.0Nb-1.0Sn-0.1Fe alloy at all the temperatures were higher than those of Zircaloy-4. Considering the data controlled by the parabolic rates at 700, 1100, 1150, and 1200 °C, the oxidation rate constants were the same slopes as the Baker-Just relationship. The rate transition at 800 °C could have resulted from the phase transformation of the base metal and those at 1000 and 1050 °C could have resulted from the lateral cracks in the oxide due to the ZrO₂ phase transformation from a monoclinic structure to a tetragonal structure.     

Channeling study of thermal decomposition of III-N compound semiconductors

GaN thermal stability is the limiting factor of the growth rate for epitaxially grown films and of the thermal annealing of defects. As a consequence, this issue has been extensively studied for more than one decade. There are, however, substantial differences in the reported kinetics and presumed mechanisms of decomposition, which are primarily related to the reactor design thus, reflecting the complexity of chemical reactions involved. We report here on the use of 1.7 MeV He-ion RBS/channeling for the study of thermal decomposition of MOVPE grown GaN and Al_xGa_1−xN (x = 0.05-0.5) layers. These layers with thickness of 320 nm were grown on sapphire substrates with 20 nm AlN nucleation layer. Prior to annealing samples were characterized by RBS/channeling, selected samples were also studied by SEM. Thermal treatment was performed in the MOVPE reactor in the temperature range 900-1200 °C in the N₂ atmosphere. RBS/channeling analysis provided data on layer thickness, composition and evolution of ingrown defects. GaN decomposition starts at 900 °C and results in the reduction of the layer thickness without observable changes of the film composition. The presence of large density of GaN hillocks on the surface was revealed by SEM after annealing at 1000-1050 °C. Remarkable stability of Al_xGa_1−xN was observed, this alloy remains unchanged upon annealing at 1200 °C/6 h even for x as low as 0.05.     

Influence of tempering temperature upon precipitate phases in a 11%Cr ferritic/martensitic steel

The effect of tempering temperature on the precipitate phases in a 11%Cr ferritic/martensitic steel normalized at 1050 °C for 1 h and tempered for 2 h at temperatures ranging from 600 to 780 °C has been investigated using transmission electron microscope and energy-dispersive X-ray spectroscopy. The results show that tempering temperature does not affect the existences of niobium-rich carbonitrides, (Nb_0.7V_0.2Cr_0.1)(C,N) and (Nb_0.55V_0.35Cr_0.1)(C,N), vanadium-niobium-rich carbonitride (V_0.45Nb_0.45Cr_0.1)(C,N), chromium-rich carbonitride (Cr_0.83V_0.12W_0.05)₂(C,N) and chromium-rich carbide (Cr_0.7Fe_0.25W_0.05)₂₃-C₆, whilst the precipitations of vanadium-rich carbonitrides, (V_0.65Nb_0.2Cr_0.15)(C,N) and (V_0.55Nb_0.25Cr_0.2)(C,N) are dependent on tempering temperature, which were detected only at the higher tempering temperatures of 750 and 780 °C. No coarsening was occurred during the temperings for the niobium-rich and spherical vanadium-rich carbonitrides. There was a low coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides with increasing the tempering temperature from 600 to 700 °C and 650 to 780 °C, respectively, and a high coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides at the tempering temperatures 750 through 780 °C and 650 °C, respectively. The compositions show an increase in vanadium and a decrease in niobium and chromium contents for the niobium-rich carbonitrides, and a decrease in niobium and an increase in vanadium and chromium contents for the vanadium-niobium-rich carbonitrides, and an increase in vanadium and a decrease in tungsten contents for the chromium-rich carbonitrides. The chromium-rich carbides show an increase and a decrease in their iron and chromium contents, respectively, with increasing the tempering temperature from 650 to 780 °C.     

Development and characterisation of a new ODS ferritic steel for fusion reactor application

This paper describes the microstructure, tensile properties and Charpy impact resistance of a reduced activation oxide dispersion strengthened ferritic steel Fe-14Cr-2W-0.3Ti-0.3Y₂O₃ produced by mechanical alloying of a pre-alloyed, gas atomised steel powder with Y₂O₃ particles, compaction by hot extrusion at 1100 °C, hot rolling at 700 °C and heat treatment at 1050 °C for 1 h. At room temperature the material exhibits a high ultimate tensile strength of about 1420 MPa and high yield strength of about 1340 MPa in the transverse direction. In the longitudinal direction the values are about 10% lower, due to the anisotropy of the microstructure (elongated grains in the rolling direction). At 750 °C the material still exhibits relatively high yield strengths of about 325 MPa and 305 MPa in the longitudinal and transverse directions, respectively. The material exhibits reasonable uniform and total elongation values over the temperature range 23-750 °C, in both transverse and longitudinal directions. The material exhibits weak Charpy impact properties in the transverse direction. Charpy impact properties are slightly better in the longitudinal direction, with upper shelf energy of about 4.2 J and a ductile-to-brittle transition temperature of about 8.8 °C.     

Study on microstructural changes in thermally-aged stainless steel weld-overlay cladding of nuclear reactor pressure vessels by atom probe tomography

The effect of thermal aging on microstructural changes was investigated in stainless steel weld-overlay cladding composed of 90% austenite and 10% δ-ferrite phases using atom probe tomography (APT). In as-received materials subjected to cooling process after post-welding heat treatments (PWHT), a slight fluctuation of the Cr concentration was already observed due to spinodal decomposition in the ferrite phase but not in the austenitic phase. Thermal aging at 400 °C for 10,000 h caused not only an increase in the amplitude of spinodal decomposition but also the precipitation of G phases with composition ratios of Ni:Si:Mn = 16:7:6 in the ferrite phase. The chemical compositions of M₂₃C₆ type carbides seemed to be formed at the austenite/ferrite interface were analyzed. The analyses of the magnitude of the spinodal decomposition and the hardness implied that the spinodal decomposition was the main cause of the hardening.     

Precipitation in AISI 316L(N) during creep tests at 550 and 600 °C up to 10 years

The precipitation behaviour in the gauge lengths and in the heads of initially solution annealed type 316L(N) austenitic stainless steel specimens tested in creep at 550 and 600 °C for periods of up to 85 000 h has been studied using several metallographic techniques. Three phases were detected: M₂₃C₆, Laves, and sigma phase. The volume fraction of the precipitated sigma phase was significantly higher than that of carbides and the Laves phase. M₂₃C₆ carbide precipitation occurred very rapidly and was followed by the sigma and Laves phases formation in the delta ferrite islands. Sigma and Laves phases precipitated at grain boundaries after longer times. Two different mechanisms of sigma phase precipitation have been proposed, one for delta ferrite decomposition and another for grain boundary precipitation. Small quantities of the Laves phase were detected in delta ferrite, at grain boundaries and inside the grains.     

Long-term properties of reduced activation ferritic/martensitic steels for fusion reactor blanket system

Thermal aging properties of reduced activation ferritic/martensitic steel F82H was researched. The aging was performed at temperature ranging from 400 °C to 650 °C up to 100,000 h. Microstructure, precipitates, tensile properties, and Charpy impact properties were carried out on aged materials. Laves phase was found at temperatures between 550 and 650 °C and M₆C type carbides were found at the temperatures between 500 and 600 °C over 10,000 h. These precipitates caused degradation in toughness, especially at temperatures ranging from 550 °C to 650 °C. Tensile properties do not have serious aging effect, except for 650 °C, which caused large softening even after 10,000 h. Increase of precipitates also causes some degradation in ductility, but it is not critical. Large increase in ductile-to-brittle transition temperature was observed in the 650 °C aging. It was caused by the large Laves phase precipitation at grain boundary. Laves precipitates at grain boundary also degrades the upper-shelf energy of the aged materials. These aging test results indicate F82H can be used up to 30,000 h at 550 °C.     

Characterization of the interaction layer grown by interdiffusion between U-7wt%Mo and Al A356 alloy at 550 and 340 °C

U(Mo) alloys are under study to get a low-enriched U fuel for research and test reactors. Qualification experiments of dispersion fuel elements have shown that the interaction layer between the U(Mo) particles and the Al matrix behaves unsatisfactorily. The addition of Si to Al seems to be a good solution. The goal of this work is to identify the phases constituting the interaction layer for out-of-pile interdiffusion couples U(Mo)/Al(Si). Samples γU-7wt%Mo/Al A356 alloy (7.1 wt%Si) made by Friction Stir Welding were annealed at 550 and 340 °C. Results from metallography, microanalysis and X-ray diffraction, indicate that the interaction layer at 550 °C is formed by the phases U(Al,Si)₃, U₃Si₅ and Al₂₀Mo₂ U, while at 340 °C it is formed by U(Al,Si)₃ and U₃Si₅. X-ray diffraction with synchrotron radiation showed that the Si-rich phase, previously reported in the interaction layer at 550 °C near U(Mo) alloy, is U₃Si₅.     

Structural study of the oxidation process and stability of NbOxNy coatings

In the present work, we study the oxidation behaviour of NbON multilayer films. The films were deposited by DC magnetron sputtering with a reactive gas pulsing process. The nitrogen flow was kept constant and the oxygen flow was pulsed. Pulse durations of 10 s produced multilayered coatings with a period of λ = 10 nm. Three different films with increasing duty cycles have been deposited.Rutherford backscattering spectroscopy (RBS) was used to study the chemical composition variations at different annealing temperatures (as-deposited, 400 °C, 500 °C and 600 °C) combined with X-ray diffraction (XRD) to identify the crystalline phases formed. At 400 °C, for all films a very thin layer starts to form at the surface with enhanced O concentration. The composition of the deeper part of the samples remains unchanged. At 500 °C, the oxide scale grows, encompassing about half the film thickness. At 600 °C, the process is finished and a single layer is formed with reduced Nb and increased O concentration. Fourier-transformation infrared spectroscopy (FTIR) results confirmed the increase of this surface oxidation, while XRD revealed that crystallization of Nb₂O₅ occurs at 600 °C.     

Structural characterization of buried nitride layers formed by nitrogen ion implantation in silicon

The synthesis of buried silicon nitride insulating layers was carried out by SIMNI (separation by implanted nitrogen) process using implantation of 140 keV nitrogen (¹⁴N⁺) ions at fluence of 1.0 × 10¹⁷, 2.5 × 10¹⁷ and 5.0 × 10¹⁷ cm⁻² into 〈1 1 1〉 single crystal silicon substrates held at elevated temperature (410 °C). The structures of ion-beam synthesized buried silicon nitride layers were studied by X-ray diffraction (XRD) technique. The XRD studies reveal the formation of hexagonal silicon nitride (Si₃N₄) structure at all fluences. The concentration of the silicon nitride phase was found to be dependent on the ion fluence. The intensity and full width at half maximum (FWHM) of XRD peak were found to increase with increase in ion fluence. The Raman spectra for samples implanted with different ion fluences show crystalline silicon (c-Si) substrate peak at wavenumber 520 cm⁻¹. The intensity of the silicon peak was found to decrease with increase in ion fluence.     

Effects of the fabrication process parameters on the precipitates and mechanical properties of a 9Cr-2W-V-Nb steel

The effects of the fabrication process parameters such as a tempering temperature, cold rolling and annealing condition on the precipitates and mechanical properties of a normalized 9Cr-2W-V-Nb steel were evaluated. Nb-rich MX precipitates were found in the specimen tempered at 550 °C while M₂₃C₆, Nb- and V-rich MX ones were observed in the specimen tempered at 750 °C. A cold rolling and an annealing at 750 °C of the specimen tempered at 550 °C induced the formation of large inhomogeneous M₂₃C₆ carbides, causing a reduced tensile strength. However, the cold rolling of the specimen tempered at 750 °C provided fine precipitates due to a fragmentation of some of the M₂₃C₆ carbides, and an annealing at 700 °C for 30 min was found to be suitable to recover the degraded mechanical properties from a cold working.     

Effect of heat-treatment at different temperatures on the phase and morphology of the surface of W-coated C/C composites

W-coated carbon/carbon composites have been considered as an attractive plasma facing material for ITER. W coatings were prepared on carbon/carbon substrates by double glow plasma. W-coated carbon/carbon substrates were heat-treated in a vacuum furnace at 1000, 1100, 1200, 1300 and 1400 °C for 1 h. The coatings were examined by X-ray diffraction for phase identification. The microstructure was observed by scanning electron microscopy. The chemical composition was analyzed by energy dispersive X-ray spectroscopy. The Vickers hardness was measured with a microhardness tester. The results indicated that the critical temperature for generating WC was 1200 °C. When the coatings were heat-treated below 1200 °C, the diffusion of W and C played a dominant role. W grains in 1100 and 1200 °C heat-treated samples presented a diffusion-induced rearrangement around the carbon fibers or the pores on the surface of C/C substrate. The amount of W and the hardness of the heat-treated coatings were reduced as the temperature increased. However, the reaction of W and C played a dominant role above 1200 °C. The amount of WC and the hardness of the heat-treated coatings were greatly increased as the temperature increased.     

Effect of CuO on CaTiO3 perovskite ceramics prepared using a direct sintering process

Effect of CuO on CaTiO₃ (CT) ceramics prepared using a direct sintering process (reaction-sintering process) was investigated. The mixture of raw materials was pressed and sintered into ceramics without any calcination stage involved. Pure CT could be obtained. The degree of densification in CT via reaction-sintering process is lower than traditional oxide route but the grains grew easier in CT via reaction-sintering process. A density 3.63 g/cm³ (90.3% of ρ_th) is obtained in CT pellets after 1500 °C/16 h sintering. With 3 wt.% CuO addition, density 3.92 g/cm³ (97.5% of ρ_th) is obtained after 8 h sintering at 1500 °C due to the liquid phase sintering. The liquid phase at grain boundaries appeared significantly at a lower sintering temperature for longer soak time.     

Post-annealing effects on the shallow-junction characteristics caused by high-fluence 77 keV BSi molecular ion implantations at room and liquid nitrogen temperatures

In this study, n-type <1 0 0> silicon specimens were liquid nitrogen temperature (LT) and room temperature (RT) implanted with 2 × 10¹⁵ cm⁻² 77 keV BSi molecular ions to produce shallow junctions. Post-annealing methods under investigation included furnace annealing (FA) at 550 °C for 0.5, 1, 2, 3 and 5 h and rapid thermal annealing (RTA) at 1050 °C for 25 s. Post-annealing effects on the shallow-junction characteristics were examined using one-step (FA) and two-step (FA + RTA) post-annealing treatments. Secondary ion mass spectrometry (SIMS), cross-sectional transmission electron microscopy (XTEM), a four-point probe and Raman scattering spectroscopy (RSS) were employed to analyze junction depths (x_j), damage microstructures, sheet resistance (R_s) and damage characteristics, respectively. The results revealed that the shallow-junction characteristics of the LT implant are better than those of the RT one when post-annealing time in FA exceeds 1 h. A post-annealing time of 3 h in FA is needed in order to obtain the optimal one- or two-step post-annealing effects on the shallow-junction characteristics in both the LT and RT implants.     

High-temperature mechanical properties improvement on modified 9Cr-1Mo martensitic steel through thermomechanical treatments

In the framework of the development of generation IV nuclear reactors and fusion nuclear reactors, materials with an improved high temperature (≅650 °C) mechanical strength are required for specific components. The 9-12%Cr martensitic steels are candidate for these applications. Thermomechanical treatments including normalisation at elevated temperature (1150 °C), followed by warm-rolling in metastable austenitic phase and tempering, have been applied on the commercial Grade 91 martensitic steel in order to refine its microstructure and to improve its precipitation state. The temperature of the warm-rolling was set at 600 °C, and those of the tempering heat-treatment at 650 °C and 700 °C thanks to MatCalc software calculations. Microstructural observations proved that the warm-rolling and the following tempering heat-treatment lead to a finer martensitic microstructure pinned with numerous small carbide and nitride particles. The hardness values of thermomechanically treated Grade 91 steel are higher than those of the as-received Grade 91. It is also shown that the yield stress and the ductility of the thermomechanically treated Grade 91 steel are significantly improved compared to the as-received material. Preliminary creep results showed that these thermomechanical treatments improve the creep lifetime by at least a factor 14.     

Effect of sintering conditions on the microstructure and mechanical properties of ZrN as a surrogate for actinide nitride fuels

Pellets of sintered ZrN were studied to optimize the mechanical properties and microstructures needed in nitride fuel pellets, using ZrN as a surrogate for actinide nitrides and as potential component in low fertile and inert matrix fuels. Samples were prepared via sintering in either Ar or N₂ (with and without 6% H₂) and at 1300 °C or 1600 °C. A significant difference in the hardness was measured ranging from 1000 (Kg/mm²) in samples sintered at 1600 °C in argon to 100 (Kg/mm²) in samples sintered at 1300 °C in nitrogen. Samples with 6% hydrogen added to the sintering environment experienced a decrease in hardness, as well as an increase in intergranular cracking as compared to samples sintered without hydrogen, suggesting hydrogen embrittlement. Grain size was more uniform in samples sintered in pure Ar as compared to Ar-H₂, while the latter had a larger fraction of high angle grain boundaries than the former. Cracking around indents had a clear tendency to follow high angle boundaries, which were found to be intrinsically weak in ZrN.