Low silicon U(Al,Si)3 stabilization by Zr addition

Previous knowledge states that (U,Zr)Al₃ and U(Al,Si)₃ phases with Zr and Si content higher than 6 at.% (7.7 wt%) and 4 at.% (1.4 wt%), respectively, does not partially transform to UAl₄ at 600 °C. In this work, four alloys within the quaternary system U-Al-Si-Zr were made with a fixed nominal 0.18 at.% (0.1 wt%) Si content in order to assess the synergetic effect of both Zr and Si alloying elements to the thermodynamic stability of the (U,Zr)(Al,Si)₃ phase. Heat treatments at 600 °C were undertaken and samples were analyzed by means of XRD, EPMA and EDS techniques. A remarkable conclusion is that addition of 0.3 at.% Si in the (U,Zr)(Al,Si)₃ phase reduces in 2.7 at.% the necessary Zr content to inhibit its transformation to U(Al,Si)₄.     

Development of ultra-fine grained W-(0.25-0.8)wt%TiC and its superior resistance to neutron and 3 MeV He-ion irradiations

W-(0.25-0.8)wt%TiC with equiaxed grain sizes of 50-200 nm and nearly full density of 99% was fabricated utilizing mechanical alloying (MA) in different gas atmospheres of H₂, Ar and N₂ and hot isostatic pressing. Microstructural and mechanical property examinations were conducted before and after irradiations with neutrons at 600 °C to 2 × 10²⁴ n/m² and 3 MeV He-ions at 550 °C to 2 × 10²³ He/m². It is found that TiC additions and MA atmospheres significantly affect grain refinement and baseline mechanical properties. The room-temperature fracture strength takes a maximum of 2 GPa for W-(0.25-0.5)%TiC with MA in H₂ (W-(0.25-0.5)TiC-H₂). At 1400-1700 °C superplastic behavior occurs for W-0.5TiC-H₂, but is suppressed for W-0.5TiC-Ar. No neutron irradiation hardening is recognized in W-0.5TiC-H₂ and W-0.5TiC-Ar. The critical fluence for surface exfoliation by He irradiation for W-0.3TiC-H₂ is more than 10 times as large as that for commercially available W materials. These results suggest that ultra-fine grained W-TiC is capable of improved performance as the spallation neutron source solid target.     

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.     

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.     

Oxidation and creep failure of alloy 617 foils at high temperature

The microstructure of thermally grown oxides (TGO) and the creep properties of alloy 617 were investigated. Oxidation and creep tests were performed on 100 μm thick foils at 800-1000 °C in air environment, while the thickness of TGO was monitored in situ. According to energy dispersive X-ray (EDX) mapping micrographs observation, superficial dense oxides, chromia (Cr₂O₃), which was thermodynamically unstable at 1000 °C, and discrete internal oxides, alumina (α-Al₂O₃), were found. Consequently, the weight of the foil specimen decreased due to the spalling and volatilization of the Cr₂O₃ oxide layer after an initial weight-gaining. Secondary and tertiary creeps were observed at 800 °C, while the primary, secondary and tertiary creeps were observed at 1000 °C. Dynamic recrystallization occurred at 800 °C and 900 °C, while partial dynamic recrystallization at 1000 °C. The apparent activation energy, Q_app, for the creep deformation was 271 kJ/mol, which was independent of the applied stress.     

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.     

TEM investigations of MN nitride phases in a 9% chromium ferritic/martensitic steel with normalization conditions for nuclear reactors

The investigations on the precipitate phases in a 9%Cr ferritic/martensitic steel under different normalization conditions have been made by using a transmission electron microscope and an energy-dispersive X-ray spectroscopy. Hot-rolled steel samples were normalized at 1050-1200 °C for 1-2 h followed by an air cooling to room temperature. MN vanadium nitride precipitates with a plate-like morphology and a chemical formula of about (V_0.4Nb_0.4Cr_0.2)N have been observed at triple junctions, grain boundaries and within matrix in the steel samples normalized at 1050-1150 °C for 1-2 h, but they were dissolved out at 1200 °C within 1 h. Vanadium nitride is a stable phase at 1050 °C according to thermocalc prediction of equilibrium phases in the steel. With increasing normalizing temperature and time, there was no a striking change in the chemical composition of metallic elements in the MN phase, but a considerable increase in the size of the MN precipitate.     

The interdiffusion and solid-state reaction of low-energy copper ions implanted in silicon

At room temperature, single-crystal silicon was implanted with Cu⁺ ions at an energy of 80 keV using two doses of 5 × 10¹⁵ and 1 × 10¹⁷ Cu⁺ cm⁻². The samples were heat treated by conventional thermal annealing at different temperatures: 200 °C, 230 °C, 350 °C, 450 °C and 500 °C. The interdiffusion and solid-state reactions between the as-implanted samples and the as-annealed samples were investigated by means of Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD). After annealing at 230 °C, the XRD results of the samples (subject to two different doses) showed formation of Cu₃Si. According to RBS, the interdiffusion between Cu and Si atoms after annealing was very insignificant. The reason may be that the formation of Cu₃Si after annealing at 230 °C suppressed further interdiffusion between Si and Cu atoms.     

Post-annealing temperature dependence of blistering in high-fluence ion-implanted H in Si 〈1 0 0〉

This study examined the influence of post-annealing temperature on blister formation and growth in ion-implanted H in Si 〈1 0 0〉. Ion energy levels of 40 and 100 keV and fluences of 2 × 10¹⁶ and 5 × 10¹⁶ cm⁻² were investigated. Post-annealing treatments were performed using the furnace annealing (FA) method with temperatures ranging from 200 to 600 °C for a duration of 1 h. Raman scattering spectroscopy (RSS), optical microscopy (OM), secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (XTEM) were employed to explore the mechanisms behind the smart cut technique. The results revealed that variations among the transformation of the VH₃ (or V₂H₆) defect complex phase into the Si(1 0 0):H bonding configuration phase (RSS results), the appearance of optically detectable blisters and craters (OM results), the average depth of craters (AFM results), the trapping of hydrogen atoms and gettering of oxygen atoms (SIMS results), and the damaged microstructures (XTEM results) against post-annealing temperature were in close correspondence. It was also found that the optimal post-annealing temperature for blister formation and growth was 550 °C.     

Study on chemical structures of poly (tetrafluoroethylene-co-perfluoroalkylvinylether) by soft-EB irradiation in solid and molten state

Poly (tetrafluoroethylene-co-perfluoroalkylvinylether) (PFA) was irradiated by soft electron beam (soft-EB) under nitrogen gas atmosphere in solid-state and its molten state, respectively. The changes of thermal property and chemical structures of irradiated PFA in solid-state and molten state were studied by differential scanning calorimetric analysis (DSC) and solid-state ¹⁹F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. By DSC analysis, the melting temperature shifted to lower temperatures, and crystallinity decreased with increasing soft-EB dose. By solid-state ¹⁹F MAS NMR spectroscopy, the new signals was observed and the detected new signals in irradiated PFA at 315 °C and at 30 °C were due to the tertiary carbon group with branching site (Y-type crosslinking site), perfluoro-propylene site and chain end methylene groups, respectively.Moreover, the molar ratio of perfluoroalkylvinylether (FAVE) structure to -CF₂- units decreased with increasing dose.     

Investigation of high temperature phase stability, thermal properties and evaluation of metallurgical compatibility with 304L stainless steel, of indigenously developed ferroboron alternate shielding material for fast reactor applications

Towards the cause of serving economic power production through fast reactors, it is necessary to bring in functionally more efficient and innovative design options, which also includes exploration of cheaper material alternatives, wherever possible. In this regard, the feasibility of using a commercial grade ferroboron alloy as potential alternate shielding material in the outer subassemblies of future Indian fast reactors has been recently investigated from shielding physics point of view. The present study explores in detail the high temperature thermal stability and the metallurgical compatibility of Fe-15.4B-0.3C-0.89Si-0.17Al-0.006S-0.004P-0.003O (wt.%) alloy with SS 304L material. In addition, the high temperature specific heat and lattice thermal expansion characteristics of this alloy have also been investigated as a part of the present comprehensive characterisation program. The Fe-15 wt.%B alloy is constituted of principally of two boride phases, namely tetragonal Fe₂B and orthorhombic FeB phases, which in addition to boron also contains some amount of C and Si dissolved in solid solution form. This Fe-B alloy undergoes a series of phase transformation as a function of increasing temperature; the major ones among them are the dissolution of Fe₂B-lower boride in the matrix through a eutectic type reaction, which results in the formation of the first traces of liquid at 1500 K/1227 °C. This is then followed by the dissolution of the major FeB boride phase in liquid and the melting process is completed at 1723 K/1450 °C. In a similar manner, the thermal stability studies performed on combined Fe-B + 304L steel reaction couples revealed that a pronounced pre-melting or liquid phase formation occurs at a temperature of 1471 K/1198 °C, which is lower than the melting onset of both Fe-B and SS 304L. It is found that within the limits of experimental uncertainty, this pre-melting phenomenon occurred at the same fixed temperature of 1471 K/1198 °C, irrespective of the mass ratios of Fe-B and 304L steel. Further, it is also found that SS 304L is completely soluble in Fe-B alloy and the fused product upon solidification formed a mixture of complex intermetallic borides, such as (Fe,Cr)(B,C), (Fe,Cr)₂(B,C) and (Fe,Ni)₃B. In the temperature range 823-1073 K (550-800 °C), the SS 304L clad is found to interact strongly with the Fe-B alloy. The diffusion layer thickness or the attack layer depth (x) is found to vary with time (t) up to about 5000 h, according to the empirical rate law, x² = k(T)t. The temperature sensitivity of the rate constant, k(T) is found to obey the Arrhenius law, k(T) = k_o exp(−Q/RT), with Q = 57 kJ mol⁻¹, being the effective activation energy for the overall diffusional interaction of Fe-B and SS 304L. The room temperature specific heat capacity of Fe-B alloy is found to be 538 kJ kg⁻¹ K⁻¹. The C_P values measured over 300-1350 K, is found vary smoothly with temperature according to the expression, C_P/kJ kg⁻¹ K⁻¹ = 0.62094 + 0.00012T + 10685.81T⁻². The lattice thermal expansion of both FeB and Fe₂B phases are found to be anisotropic in that the c-axis expansion is found to be more than that along a and b axes. The room temperature volume thermal expansivity of FeB and Fe₂B phases are found to be of the order of 48 × 10⁻⁶ K⁻¹ and 28 × 10⁻⁶ K⁻¹, respectively. The thermal expansion of FeB is found to be more temperature sensitive than that of Fe₂B.     

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.     

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.     

Study of electron beam curing process using epoxy resin system

Polymeric matrix composite (PMC) has been used in engineering applications instead of metal in the last few years, due to its corrosion resistance and excellent relation between tensile strength/density and elastic modulus/density. However, PMC materials cured by thermal process require high temperature and are time-consuming. The electron beam (EB) curing technology allows its use at room temperature and reduced curing times, and this is one of the main advantages over thermal technology. The aim of this work is to investigate electron beam curable epoxy formulations to use in filament winding processes to produce composite material with similar or better properties than thermal curable composites. The study has been made with commercial epoxy resins and cationic initiators. The epoxy resin samples were irradiated for few minutes with total dose of 150 kGy. The glass transition temperatures (T_g) were determined by dynamic mechanical analyzer (DMA) and the result was 137 °C. The thermal process was carried out in a furnace following three steps: 4 h at 90 °C, increasing temperature from 90 °C to 130 °C during 4 h and 12 h at 130 °C. The total process time was 20 h. The T_g of this sample was 102 °C.     

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.     

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.     

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.     

The ternary system: silicon-uranium-vanadium

Phase equilibria in the system Si-U-V were established at 1100 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed, U₂V₃Si₄ and (U_1−xV_x)₅Si₃, for which the crystal structures were elucidated by X-ray powder data refinement and found to be isotypic with the monoclinic U₂Mo₃Si₄-type (space group P2₁/c; a = 0.6821(3), b = 0.6820(4), c = 0.6735(3) nm, β = 109.77(1)°) and the tetragonal W₅Si₃-type (space group I4/mcm, a = 1.06825(2), c = 0.52764(2) nm), respectively. (U_1−xV_x)₅Si₃ appears at 1100 °C without any significant homogeneity region at x ∼ 0.2 resulting in a formula U₄VSi₃ which corresponds to a fully ordered atom arrangement. DTA experiments clearly show decomposition of this phase above 1206 °C revealing a two-phase region U₃Si₂ + V₃Si. At 1100 °C U₄VSi₃ is in equilibrium with V₃Si, V₅Si₃, U₃Si₂ and U(V). At 800 °C U₄VSi₃ forms one vertex of the tie-triangle to U₃Si and V₃Si. Due to the rather high thermodynamic stability of V₃Si and the corresponding tie-lines V₃Si + liquid at 1100 °C and V₃Si + U(V) below 925 °C, no compatibility exists between U₃Si or U₃Si₂ and vanadium metal.     

Evolution of the nanostructure of VVER-1000 RPV materials under neutron irradiation and post irradiation annealing

A high nickel VVER-1000 (15Kh2NMFAA) base metal (1.34 wt% Ni, 0.47% Mn, 0.29% Si and 0.05% Cu), and a high nickel (12Kh2N2MAA) weld metal (1.77 wt% Ni, 0.74% Mn, 0.26% Si and 0.07% Cu) have been characterized by atom probe tomography to determine the changes in the microstructure during neutron irradiation to high fluences. The base metal was studied in the unirradiated condition and after neutron irradiation to fluences between 2.4 and 14.9 × 10²³ m⁻² (E > 0.5 MeV), and the weld metal was studied in the unirradiated condition and after neutron irradiation to fluences between 2.4 and 11.5 × 10²³ m⁻² (E > 0.5 MeV). High number densities of ∼2-nm-diameter Ni-, Si- and Mn-enriched nanoclusters were found in the neutron irradiated base and weld metals. No significant copper enrichment was associated with these nanoclusters and no copper-enriched precipitates were observed. The number densities of these nanoclusters correlate with the shifts in the ΔT_{41 J} ductile-to-brittle transition temperature. These nanoclusters were present after a post irradiation anneal of 2 h at 450 °C, but had dissolved into the matrix after 24 h at 450 °C. Phosphorus, nickel, silicon and to a lesser extent manganese were found to be segregated to the dislocations.     

Phase relations in the U-Mo-Al ternary system

The phase relations in the U-Mo-Al system of quenched samples annealed at 800 °C for 2 weeks and at 400 °C for 2 months have been established using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis performed at room temperature. Two ternary Al-rich phases, UMo_2−xAl_20+x and U₆Mo_4+xAl_43−x are found stable at 800 °C and 400 °C. They show significant homogeneity ranges resulting from Mo/Al substitution mechanism on various mixed crystallographic sites, as evidenced by single-crystal structure refinements. Substitution of up to 25 at.% of Al by Mo atoms is also observed for UAl₂ (cubic MgCu₂-type) giving a quite large extension (UAl_2−xMo_x, 0 < x < 0.5) into the ternary system. Larger substitution (0.6 < x < 0.7 at T = 800 °C) stabilizes another ternary Laves phase, UAl_2−xMo_x with the hexagonal MgZn₂-type. There is no detectable solubility of Mo in UAl₄, and it is of the order of 1 at.% in UAl₃. The interaction layers between the γU-Mo alloys and the Al matrix in nuclear fuel plates can be successively estimated as composed of the two- and three-phase fields equilibrium indicated on the assessment of the phase relations drawn for samples heat-treated at 400 °C.