Carbide Matrix Research Articles

Qualitative and quantitative analysis of neutron irradiation effects in SiC/SiC composites using X-ray computed tomography

Silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites are candidate materials for cladding of light water reactor (LWR) fuels. Loss of fission product gas retention due to the formation of microcrack networks is considered a potential failure mechanism for SiC/SiC-cladded fuels. In this study, a variety of SiC/SiC composite tubes were irradiated with and without an LWR-relevant radial heat flux in the High Flux Isotope Reactor, followed by detailed characterization with X-ray computed tomography (XCT). This first set of XCT data for neutron-irradiated samples confirmed that the internal stresses arising from a combination of temperature gradients and irradiation-induced swelling act as the primary driver for cracking. While the observed cracking patterns varied depending on the tube architectures, the sharp edges of relatively large pores were found to be the common stress concentrator. These findings are useful to help improve the design and manufacturing of SiC/SiC fuel claddings for reduced failure probability.

Effects of adding GPLs dispersed at different sonication times on the thermal and electrical conductivities of spark plasma sintered silicon carbide

This study has investigated the effects of adding 1 wt % graphene platelets (GPLs) dispersed at different probe-sonication times such as 1, 2, 4 and 6 h on the microstructure, thermal and electrical conductivities of the spark plasma sintered (SPSed) silicon carbide (SiC) matrix. According to scanning electron microscope (SEM), size measurement and Raman analyses, the dispersion of the GPLs increased, the platelet sizes decreased, and the defect concentrations on the structure of GPLs increased as the sonication time increased. While the size reduction was only around 2–5% up to 2 h sonication time, it increased to 25–30% at the 4 and 6 h sonication times. 2 h sonicated GPLs provided the highest contribution among all GPLs, increasing the thermal and electrical conductivity of the SiC matrix by 24 % and ∼106 times, respectively due to having a combination of large lateral size, good dispersion, and low defect concentration characteristics. Anisotropic effect on thermal and electrical conductivities of GPLs-SiC composites decreased with increasing probe-sonication time since the reduction in platelet size of GPLs led to a decrease in their orientation degree within the matrix microstructure.

New Low-Temperature Method of Synthesis of Boron Carbide Matrix Ceramics Ultra-Dispersive Powders and their Spark Plasma Sintering

The ultra-dispersive powders of pre-ceramic precursors for boron carbide based composites were obtained by relatively low-temperature (at 200 – 1000 °C) synthesis from liquid charges containing available compounds such as salts and oxides. Boron carbide matrix ceramics were compacted by their reactive spark plasma sintering (SPS) at 1500 – 1700 °C. It is noted that the X-ray diffraction (XRD) peaks corresponding (m)ZrO2 and WC phases presented in the synthetic pre-ceramic precursors disappear after the SPS is conducted at 1500 °C. It is established that the addition of tungsten and cobalt compounds promotes both the low-temperature synthesis of ceramic components and sintering processes of their powders. Energy dispersive X-ray (EDX) analysis showed that the ceramics contain a small amount of Co (0.8 – 2 wt.%). The density of samples of cobalt-containing ceramics B4C–ZrB2–W2B5–Co is higher compared to that of cobalt-free ceramics B4C–ZrB2–W2B5.

Ordered Particle Packing in Dense TRISO/SiC Fuel Elements and Preliminary Assessment of Neutronic and Thermomechanical Characteristics

Detailed analysis of the particle distribution in Transformational Challenge Reactor fuel elements indicates that particle packing is not random; instead, it follows a relatively ordered structure near fuel element surfaces. Discrete particle neutronic simulations indicate that the core reactivity is not impacted when assuming homogenization of particles with the silicon carbide matrix. However, the neutronic power distribution resulting from the ordered packing structure indicates that the highest-power particles reside at the top and bottom of the fuel elements and nearest the YH1.85 moderator rods. The power distribution results were applied to thermomechanical simulations using mesh-based power distributions. Previous results indicated high stress at the bottom of the fuel element, where packing is most ordered. To reduce this stress concentration, additively manufactured protrusions were added to the bottom of a test fuel element to disrupt dense particle packing. These protrusions reduced the overall power peaking, but the thermomechanical simulations did not indicate a significant change in the fuel element’s maximum stress or failure probability.

Neutronic design and evaluation of the solid microencapsulated fuel in LWR

Solid Microencapsulated Fuel (SMF) is a type of solid fuel rod design that disperses TRISO coated fuel particles directly into a kind of matrix. SMF is expected to provide improved performance because of the elimination of cladding tube and associated failure mechanisms. This study focused on the neutronics and some of the fuel cycle characteristics of SMF by using OpenMC. Two kinds of SMFs have been designed and evaluated - fuel particles dispersed into a silicon carbide matrix and fuel particles dispersed into a zirconium matrix. A 7x7 fuel assembly with increased rod diameter transformed from the standard NHR200-II 9x9 array was also introduced to increase the heavy metal inventory. A preliminary study of two kinds of burnable poisons (Erbia & Gadolinia) in two forms (BISO and QUADRISO particles) was also included. This study found that SMF requires about 12% enriched UN TRISO particles to match the cycle length of standard fuel when loaded in NHR200-II, which is about 7% for SMF with increased rod diameter. Feedback coefficients are less negative through the life of SMF than the reference. And it is estimated that the average center temperature of fuel kernel at fuel rod centerline is about 60 K below that of reference in this paper.

High-performance B4C matrix composites fabricated from the B4C–Si–CeO2 system via reactive hot pressing

Boron carbide (B4C) matrix composites had the advantages of high hardness, high melting point and low density. However, due to the low relative density and poor fracture toughness of B4C, its comprehensive properties were limited in engineering applications. In this work, in order to improve the comprehensive properties of B4C composites, B4C–SiC–SiB6–CeB6 composites were designed and fabricated via reactive hot pressing at 2050 °C and 20 MPa with B4C matrix and novel additives (Double doping of Si and CeO2) as raw materials. The effects of additive CeO2 content on the microstructures and mechanical properties of composite were investigated, and reaction mechanisms of B4C, Si and CeO2 at different temperatures were studied in detail. The work showed that liquid phase Si and SiB6 greatly improved the densification of composites. CeB6 played an indispensable role in the formation of SiC–SiB6 agglomerate structure, increasing strength and supplementing toughness. When the content of CeO2 was 6 wt%, the relative density, hardness, flexural strength and fracture toughness reached to 99.7%, 34.9 GPa, 461.46 MPa and 5.57 MPa m1/2, respectively. Our strategy benefited from the formation of two liquid phases and SiC–SiB6 agglomerate structure, showing great potential in promoting sintering and improving fracture toughness.

Effect of Carbide Precipitation on “Precipitation Plasticity” and Residual Stress during Tempering

Materials with residual stress can experience irreversible plastic strains as carbides precipitate during the tempering process, thereby relaxing the residual stress. In previous studies, this plastic behavior is named “precipitation plasticity.” Although precipitation plasticity plays a key role in the relaxation of the residual stress, the micromechanism involved is not yet elucidated. Here in this study, experiments are performed on low‐carbon steel to study precipitation plasticity and the residual stress during tempering. The results show that precipitation plasticity is mainly produced by the generation of interfacial vacancies at the carbide–matrix interface and the diffusion and evolution of these vacancies under temperature and stress fields. Two precipitation plasticity regimes are observed, namely, Precipitation Plasticity I, which is associated with vacancy diffusion, and Precipitation Plasticity II, which is associated with dislocation proliferation. Precipitation Plasticity I is found to be controlled by the volume mismatch and carbide size and is the main cause of residual stress relaxation. The finer and more dispersed the carbide precipitates are, the more significant the precipitation plasticity is. Precipitation Plasticity II causes a slight increase in the residual stress, which, however, is considerably smaller than the stress relaxation corresponding to Precipitation Plasticity I.

Synthesis and Characterization of Titanium Carbide and/or Alumina Nanoparticle Reinforced Copper Matrix Composites by Spark Plasma Sintering

Synthesis and Characterization of Titanium Carbide and/or Alumina Nanoparticle Reinforced Copper Matrix Composites by Spark Plasma Sintering

High volume packing fraction TRISO-based fuel in light water reactors

For a decade, fully ceramic microencapsulated (FCM) fuel, containing tri-structural isotropic (TRISO) fuel particles in a silicon carbide (SiC) matrix, has been investigated as an accident-tolerant fuel for light water reactors (LWRs). Other examples exist of TRISO-based concepts for LWR fuels with different matrix materials. Previous studies assumed TRISO particle volume packing of approximately 0.44 in SiC (or another) matrix, the highest realistic packing fractions possible with conventional manufacturing. Recent advances in advanced manufacturing have yielded the development and demonstration of a fuel form that consists of conventionally manufactured TRISO particles in a 3D-printed SiC matrix with significantly higher possible TRISO packing fractions (0.5–0.7). This increased uranium loading enhances the viability of using TRISO-based particle fuel forms in LWRs. The viability of high-packing-fraction TRISO-based particle fuel forms in LWRs is assessed from the perspective of fuel cycle length, achievable fuel burnup, reactivity coefficients, and fuel cycle performance. Higher-packing-fraction TRISO-based fuel enables either longer cycle lengths (by ∼25% at a packing fraction of 0.55 relative to 0.44) at a constant enrichment or decreased enrichments (by ∼25% at a packing fraction of 0.55 relative to 0.44) at a constant cycle length. Studies of different fuel kernel types (uranium nitride, uranium oxycarbide, and uranium carbide) yield similar results, although the cycle length of uranium oxycarbide is shorter than for uranium nitride or uranium carbide (due to the lower density of uranium oxide). This work also characterized the production of 14C resulting from neutron absorption in 14N during operation for uranium mononitride fuel kernels; the ratio of 14C/N was 1–2 at. % at discharge. For the fuel cycle evaluation, the activity of spent nuclear fuel and high-level waste at 100 and 100,000 years was lower for high-packing-fraction fuels than for conventional LWR fuel. Environmental impact metrics were similar overall, but higher on the front end of the fuel cycle and lower on the back end of the fuel cycle. Reactivity coefficients of higher-packing-fraction TRISO-based fuel were reasonable compared with those of conventional fuels.

Hierarchical toughening of laminated nanocomposites with three-dimensional graphene/carbon nanotube/SiC nanowire

High-performance reinforcement together with advanced innovative structure design has been attracting much attention as a means of circumventing the conflict between toughness and hardness or strength of ceramic matrix. Herein, we developed the laminated tungsten carbide (WC) matrix composite reinforced by hybrid one-dimensional (1D) carbon nanotubes (CNTs) and SiC nanowire (SiCnw) as well as two-dimensional (2D) graphene (G), with a striking combination of high hardness (25.3 GPa), high fracture toughness (13.7 MPa m1/2), and high strength (1,501.7 MPa), exhibiting superiority in mechanical properties compared with most WC-based materials, such as WC-metal, WC-intermetallic, WC-carbide, WC-oxide, WC-nitride, WC-whisker, WC-CNT, and WC-graphene. The optimum structure was determined as laminate of five symmetrical layers with thickness ratio of golden ratio squared. In addition, the three-dimensional (3D) hybrid G/CNT/SiCnw demonstrated a strong hierarchical lamellar interconnected structure, in which CNT and SiCnw were well sandwiched between G sheets, synergistically facilitating the enhancement in the mechanical properties of the host matrix. This present investigation shows promising future in achieving state-of-the-art multiscale ceramic composites for safety-critical applications as in the transportation and aerospace.

Understanding the machining characteristic of plain weave ceramic matrix composite in ultrasonic-assisted grinding

The machining of silicon carbide (SiC) ceramic matrix composites is a significant challenge because of the severe material damage that may occur during material removal, which might shorten the service life of the composite parts. In this study, the effects of different fibre orientations of two-dimensional woven carbon-fibre-reinforced silicon carbide matrix composites (2D-C f /SiC) on the grinding force, surface roughness, and surface/subsurface micromorphology were investigated to clarify the material removal and breakage mechanism in ultrasonic-assisted grinding of ceramic matrix composites. The results show that the predominant material removal mode in ultrasonic-assisted grinding is brittle fracture. The forms of material breakage are matrix cracking, fibre fracture, fibre pull-out, interfacial debonding, and interfacial fracture. Compared with conventional grinding, the normal force, tangential force, and surface roughness in ultrasonic-assisted grinding decreased by approximately 20%, 18%, and 9%, respectively. The machining parameters significantly impacted the grinding force and surface roughness. The material removal modes varied for different fibre orientations. Ultrasonic-assisted grinding can effectively decrease the grinding force and improve the surface integrity. The findings of this study can be applied to predict the grinding force and surface integrity of 2D-C f /SiC and contribute to the design and manufacturing of this type of material.

Low-temperature sintering of silicon carbide membrane supports from disks to single- and 19-channel tubes

As the configuration of asymmetric ceramic membranes is determined by the membrane supports, a low temperature sintering process was developed to prepare ceramic membrane supports from disks to single- and 19-channel tubes. The residual of NaA zeolite was introduced into the silicon carbide (SiC) matrix, and porous SiC membrane supports were successfully prepared at 1100 °C. Compared with the disk-like support, the open porosity, average pore size and pure water permeance of the single-channel supports obviously increased, and their bending strength decreased accordingly. These differences were mainly attributed to the incorporation of organic plasticizer and the change in molding process. Notably, the pure water permeance of disks, single- and 19-channel tubes was 70 ± 3, 74 ± 4 and 22 ± 1 m3·m−2·h−1·bar−1, respectively, which was much higher than the previously reported values. Therefore, this work provides important guides to develop of new generation ceramic membranes for practical application.

Effect of Cu foam on the microstructure and strength of the SiCf/SiC-GH536 brazed joint

The AgCuTi filler and Cu foam interlayer were used to braze silicon carbide fiber-reinforced silicon carbide matrix composites (SiCf/SiC) and nickel-based superalloy. Cu foam interlayer could reduce the brittle reaction layer on the SiCf/SiC side by diluting the brazing seam by producing Cu-based solid solution (Cu(s,s)) to inhibit Ti diffusion from the brazing filler. Cu foam did not completely disappear in the brazed joints and still maintained the basic skeleton. Sound SiCf/SiC-GH536 joints were obtained with AgCuTi filler at 890 °C for 10 min. Compared to the brazed joints without Cu foam, introducing Cu foam could enhance the overall shear strength of the brazed joints by 224%. Cu foam buffer layers provided a technique for increasing the mechanical properties of joints, promoting the use of SiCf/SiC composites in a wide range of applications.

Transformational Challenge Reactor Safety Design and Radionuclide Retention Strategy

An integrated safety design and radionuclide (RN) retention strategy is developed to support the Transformational Challenge Reactor (TCR) demonstration. This demonstration aims to showcase viability for rapid deployment of a novel reactor by leveraging the advances in materials, manufacturing, and computational sciences through a highly integrated and agile design and development approach. This strategy provides a logical description and understanding of how RNs are contained within the facility. Rather than discussing fission product barriers individually between separate design and safety basis reports, this paper provides a consistent description and narrative to better facilitate regulatory interactions and focus safety design efforts. The principal barriers credited include the various coating layers in the tristructural isotropic (TRISO) fuel particle, the silicon carbide (SiC) matrix hosting the particles within the fuel element, the helium pressure boundary, and the confinement system. The choice and assumed performance of the credited barriers are highly conservative, which is a direct reflection of the low hazard that the TCR demonstration presents and the need to simplify and focus the safety review process accordingly. However, the strategy and the associated framework are generalized and may be adopted and tailored to support other advanced reactor demonstration efforts.

Effects of wet oxidation on residual stress variation of SiCf/SiC composites

Continuous silicon carbide fiber-reinforced silicon carbide matrix (SiCf/SiC) composites, used as thermal structures for gas turbines, are prone to degrade under the gas stream (mainly composed of O2 and H2O). Here, the degradation of the composites in wet oxygen environment was investigated by Raman spectroscopy. Results show that the composites exhibit significant residual stress as the oxidation proceeds. SiC matrix in the as-received condition exhibits average hydrostatic tensile stresses of approximately 1350 MPa, while SiC fiber exhibits compression stress of approximately 1450 MPa. Both the residual stresses in matrix and fiber decrease with the increasing oxidation temperature. Moreover, the residual stress evolution is found to be related to the formation of cristobalite and the volatilization of Si(OH)4, CO, H2. This work can open new alternatives for understanding the degradation mechanism of SiCf/SiC composites in gas turbine environment, and for developing mathematical model to predict their service life.

Microstructure and mechanical properties of b4c-tib2 composites reactive sintered from B4C + TiO2 precursors

Ceramic composites consisting of a boron carbide (B4C) matrix and titanium diboride (TiB2) secondary phase were obtained by reactive sintering from boron carbide powder with 40 and 50wt.% of titanium dioxide (TiO2) additive. The same sintering temperature of 1850?C and pressure of 35MPa, but different sintering times from 15 to 60min, were applied during reactive hot pressing of the composites in vacuum. The effects of TiO2 content and sintering time on phase compositions, microstructures and mechanical properties of the composites were studied. The TiO2 additive enhanced densification of the B4C-TiB2 ceramic composites. Both Vickers hardness and the fracture toughness of the composites increased with prolongation of sintering time. The highest hardness of 29.8GPa was achieved for the composite with 29.6 vol.% of TiB2 obtained by sintering of the precursor with 40wt.% of TiO2 additive for 60min. The fracture toughness reached a maximum value of 7.5MPa?m1/2 for the composite containing 40.2 vol.% of TiB2, which was fabricated by reactive sintering of the precursor with 50wt.% of TiO2 additive for 60min.

Macro-hardness and corrosion behavior of silicon carbide or boron carbide reinforced AA5052 MMC

Metal Matrix Composites (MMCs) enhance the mechanical and tribological properties according to the composition of the particulate reinforcement of Silicon Carbide (SiC) or Boron Carbide (B4C) and AA5052 aluminum alloy matrix. The MMC samples were fabricated by stir casting method of the liquid processing route. The same weight percentages (5 wt. %) and same particle size (63μm) of both SiC and B4C particulates are used to develop two different MMC samples. The hardness and corrosion tests were conducted to estimate the enhanced properties of the fabricated composites. The macro-hardness test and the immersion corrosion test were carried out by using a Vickers hardness tester at an applied load of 5kgf in accordance with ASTM E92 and a 3.5% NaCl solution in accordance with ASTM G31, respectively. With the help of optical microscopy, the corroded surfaces were analyzed. The results obtained from the investigation show that AA5052/B4C MMC gives better hardness and AA5052/SiC MMC shows higher corrosion rate compared to the other two samples.

Microstructure and mechanical properties of quasi-isotropic chopped fiber-reinforced PyC–SiC matrix composite prepared by novel nondamaging method

In this work, quasi-isotropic chopped carbon fiber-reinforced pyrolytic carbon and silicon carbide matrix (Cf/C–SiC) composites and chopped silicon carbide fiber-reinforced silicon carbide matrix (SiCf/SiC) composites were prepared via novel nondamaging method, namely airlaid process combined with chemical vapor infiltration. Both composites exhibit random fiber distribution and homogeneous pore size. Young's modulus of highly textured pyrolytic carbon (PyC) matrix is 23.01 ± 1.43 GPa, and that of SiC matrix composed of columnar crystals is 305.8 ± 9.49 GPa in Cf/C–SiC composites. Tensile strength and interlaminar shear strength of Cf/C–SiC composites are 52.56 ± 4.81 and 98.16 ± 24.62 MPa, respectively, which are both higher than those of SiCf/SiC composites because of appropriate interfacial shear strength and introduction of low-modulus and highly textured PyC matrix. Excellent mechanical properties of Cf/C–SiC composites, particularly regarding interlaminar shear strength, are due to their quasi-isotropic structure, interfacial debonding, interfacial sliding, and crack deflection. In addition to the occurrence of crack deflection at the fiber/matrix interface, crack deflection in Cf/C–SiC composites takes also place at the interface between PyC–SiC composite matrix and the interlamination of multilayered PyC matrix. Outstanding mechanical properties of as-prepared Cf/C–SiC composites render them potential candidates for application as thermal structure materials under complex stress conditions.

Microstructural and mechanical evolution of SiCf/SiC composites in wet oxygen atmosphere above 1000 °C

As thermal-structural materials used in gas turbine hot sections, continuous silicon carbide fiber reinforced silicon carbide matrix (SiCf/SiC) composites serve under high temperature gas steam corrosion. Therefore the long-term durability of the composites is strongly dependent on the microstructural and mechanical stability under the corrosion. Here, the microstructural and mechanical evolution of SiCf/SiC were investigated in a simulated steam environment (22 vol% H2O + 78 vol% O2, 1100–1300 °C). The results indicate that the temperature plays a major role in the corrosion of the composites. As the temperature rises up to 1200 °C, the fiber/matrix interphase (boron nitride) is completely dissipated due to oxidation. Meanwhile, a dense SiO2 scale forms on the composite surface and cristobalite phase tends to precipitate with the increase of temperature. The dissipation of interphase can lead to significant mechanical degradation on the composites. Furthermore, the degradation mechanism was also proposed in terms of stress distribution and fracture morphology. This study can offer new insight for understanding the wet-oxygen corrosion of SiCf/SiC and thus constructing SiCf/SiC with high stability in gas steam.

Corrosion characteristics of plasma spray, arc spray, high velocity oxygen fuel, and diamond jet coated 30MnB5 boron alloyed steel in 3.5 wt.% NaCl solution

Abstract 30MnB5 boron alloyed steel surface is coated using different coating techniques, namely 60(Ni-15Cr-4.4Si-3.5Fe-3.2B 0.7C)-40(WC 12Co) metallic powder plasma spray, Fe-28Cr-5C-1Mn alloy wire arc spray, WC-10Co-4Cr (thick) powder high velocity oxy-fuel (HVOF), and WC-10Co-4Cr (fine) diamond jet HVOF. The microstructure of the crude steel sample consists of ferrite and pearlite matrices and iron carbide structures. The intermediate binders are well bonded to the substrate for all coated surfaces. The arc spray coated surface shows the formation of lamellae. The cross-section of HVOF and diamond jet HVOF coated surfaces indicates the formation of WC, W2C Cr, and W parent matrix carbide structures. The corrosion characteristic of the coated steel has been investigated in 3.5 wt.% NaCl solution using electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDAX) techniques. The results reveal that the steel corroded in the medium despite the coatings. However, the extent of corrosion varies. HVOF coated sample demonstrated the highest corrosion resistance while arc spray coated sample exhibited the least. EDAX mapping reveals that the elements in the coatings corroded in the order of their standard electrode potential (SEP). Higher corrosion resistance of HVOF coated sample is linked to the low SEP of tungsten.