Takayoshi Iseki

Interfacial reactions between SiC and aluminium during joining

Reactions between SiC and liquid aluminium were studied. Transmission electron microscopy (TEM) showed that aluminium carbide (Al4C3) phase was formed at the interface between pressureless sintered SiC and aluminium. In contrast, the Al4C3 phase was not detected at the reaction sintered SiC-Al interface. This difference in microstructures results in the change in bending strength of the joints. Mixtures of SiC and aluminium powders were heated to react in vacuum in the temperature range 973 to 1473 K and the reaction products were examined using X-ray powder diffraction. It was confirmed that Al4C3 and silicon were formed, and that the extent of reaction between SiC and aluminium was decreased by the addition of silicon into aluminium.

Synthesis and high temperature mechanical properties of Ti3SiC2/SiC composite

A high density Ti3SiC2/20 vol % SiC composite was hot pressed under a uniaxial pressure of 45 MPa for 30 min in an Ar atmosphere at 1600 °C. The grain size of the Ti3SiC2/SiC composite was finer than that of monolithic Ti3SiC2, though the composite was hot pressed at a higher temperature, due to the dispersion of SiC particles in the Ti3SiC2 matrix. Room temperature fracture toughness of the composite and Vickers hardness were measured as 5.4 MPa m1/2 and 1080 kg mm−2, respectively. A higher flexure strength of the composite compared to that of monolithic Ti3SiC2 was measured both at room temperature and up to 1200 °C. At 1000 °C, the composite showed a lower oxidation rate than that of monolithic Ti3SiC2.

Brazing of pressureless-sintered SiC using Ag−Cu−Ti alloy

A pressureless-sintered SiC was brazed to itself using Ag-Cu alloy foil to which titanium had been added. The results obtained revealed the following. (i) Increasing the titanium addition to the base metal from 2 to 8 wt % improved the wettability greatly, but the bonding generally became weaker. (ii) With 2 wt % Ti addition, a reaction layer about 1 μm thick was formed, regardless of which brazing temperature was used, while bond strength reached was over linearly with temperature. The maximum room-temperature bend strength reached was over 350 MPa. (iii) In the case of the alloy with only 2 wt % Ti additive, bonding was greatly influenced not only by improvement of the wettability at high temperatures and longer holding times, but also the composition and thickness of the resultant reaction layer.

High-resolution electron microscopy of neutron-irradiation-induced dislocations in SiC

Abstract Neutron-irradiation-induced dislocations in β- and α-silicon carbide (SiC) were observed using a high-resolution electron microscope. In β-SiC, small planar defects about 20 nm in diameter lying on {111} planes were determined as interstitial Frank loops, having a Burgers vector b=⅓〈111〉. On the basis of image simulation by the multislice method, it was determined that the loops consist of an insertion of a single Si-C layer into {111} stacking to create two rotated layers. These loops were induced by heavy neutron irradiation doses of above approximately 5 × 1026 neutrons m−2 (E>0.1 MeV) in a fast reactor. Defect nuclei a few nanometres in diameter in hexagonal α-SiC were induced by lower doses in a thermal reactor (2 × 1025 neutrons m−2) (E > 0.1 MeV). They are on the (0001) basal plane and have a Burgers vector b=⅙[0001].

High-resolution electron microscopy of a SiC/SiC joint brazed by a Ag-Cu-Ti alloy

A high-resolution electron microscope observation (HREM) was performed on the joined portion of a brazed polycrystalline or single crystal SiC to itself with (Ag-28wt% Cu) + 2wt% Ti alloy foil. The brazing was done under vacuum at temperatures of 800° C to 950° C with a holding period of up to 30 min. Reaction products formed at the joined interface were found to be mainly TiC. In the specimen brazed at 800° C with the holding time of 0 min, reaction product TiC formed itself into small crystallites with a diameter of less than 20 nm, and an amorphous like layer was found between SiC and TiC. On the other hand, TiC was formed as a layer along the joined interface for the specimen brazed at 950° C for the holding time of 30 min. Lattice matching of SiC to TiC crystals appeared to be good so the high bonding strength of the joint was attributed to the formation of this epitaxial interface between SiC and TiC.

Microstructure and mechanical properties of RB-SiC/MoSi2 composite

Microstructure, high temperature strength and oxidation behaviour of reaction bonded silicon carbide, RB-SiC/17 wt% MoSi2 composite prepared by infiltrating a porous RB-SiC bulk (after removal of free silicon) with molten MoSi2 were investigated. There was good bonding between the SiC and MoSi2 particle, without a significant reaction zone and microcracking caused by the thermal mismatch stresses. A thin (∼2 nm) layer, however, was observed at the SiC/MoSi2 interfaces. At room temperature, the composite exhibited a bending strength of 410 MPa, which is ∼20% loss in comparison to that of RB-SiC alone (containing ∼ 10 wt% free silicon). However, the composite strength increased to a maximum of 590 MPa in the temperature range 1100 and 1200° C and dropped to 460 MPa between 1200 to 1400° C, after which the strength remained constant. The passive oxidation of the composite in dry air in the temperature range 1300 to 1400° C was found to follow the parabolic rate law with the formation of a protective layer of cristobalite on the surface.

X-ray diffractometry and high-resolution electron microscopy of neutron-irradiated SiC to a fluence of 1.9×1027 n/m2

Abstract Neutron-induced damage in SiC up to a fluence of 1.9×10 27 n/m 2 ( E >0.1 MeV) was examined by means of X-ray diffractometry and high-resolution electron microscopy. Specimens of β-SiC were irradiated in fast breeder reactors at 370 to 650°C. The lattice parameters of all specimens were increased by the irradiation, but above 2×10 26 n/m 2 lattice expansion decreased and was accompanied by significant peak broadening. Electron microscopy revealed a high density of interstitial loops on {111}, where X-ray diffraction peaks showed marked broadening. Peak broadening could be attributed mainly to the crystallite size effect at lower fluences, but a strain contribution was significant above 2×10 26 n/m 2 . Electron diffraction patterns and high-resolution images indicated preservation of crystallinity up to the highest fluence observed. Thermal annealing up to 1000°C did not affect the peak broadening and average loop diameter. Above 1400°C, decrease in lattice strain and increase in crystallite size with increasing annealing temperature were observed.

Physical property change of heavily neutron-irradiated Si3N4 and SiC by thermal annealing

Abstract Changes in macroscopic length, lattice parameter and thermal diffusivity of neutron-irradiated Si3N4 and SiC ceramics up to a fluence of 4.2×10 26 n / m 2 were measured. Macroscopic length increase of Si3N4 was almost one half of that of SiC. Thermal diffusivity of both ceramics was reduced severely by the irradiation at 390–540°C. Slight increase in the a-axis and slight decrease in the c-axis lattice parameter were detected for Si3N4. The amount of lattice parameter change of Si3N4 was very small compared with the macroscopic length change. Changes in these properties due to post-irradiation thermal annealing up to 1500°C were measured. Large part of thermal diffusivity of Si3N4 was recovered by annealing, with small step at ∼1100°C, but macroscopic length did not significantly change by annealing. Change in lattice parameter showed a complicated trend. It is supposed that formation of interstitial loops on the planes parallel to the c-axis, formation of voids during annealing or difficulty of recovery of points defects/loops, or solid solution formation due to glassy grain boundary phase may influence the recovery behavior of Si3N4 ceramics. Changes in macroscopic length, lattice parameter or thermal diffusivity of SiC by annealing coincided with the results of previous works. The critical irradiation conditions for loop formation/XRD line broadening for SiC is discussed based on the present and previous results.

Helium release and microstructure of neutron-irradiated SiC ceramics

Abstract α-SiC ceramics containing 10 B were neutron-irradiated in the JMTR to a fluence of 6.0 × 10 24 n / m 2 ( E > 1 MeV ) at 650° C. Helium release from SiC ceramics and powders was measured up to 2000° C. Microstructural observation by TEM was also carried out on SiC specimens annealed at temperatures in the range 1200–2000 °C. The helium release rate of the powder specimen started to increase at about 800°C and two peaks were observed at 1100 and 1260°C. By contrast, the sintered specimen showed a low gas release rate up to 1800°C and exhibited a rapid increase above this temperature. The helium release mechanism from the sintered specimen was discussed in terms of the mobility of helium atoms and vacancies, and bubbles at grain boundaries.

Fabrication of silicon carbide fiber-reinforced silicon carbide composite by hot-pressing

Abstract Continuous SiC fiber-reinforced SiC composites (SiC/SiCf) are one of the most attractive structural materials for future fusion reactors. To date, the only successful manufacturing process of SiC/SiCf composites is the chemical vapor infiltration technique, which is a very delicate and expensive process. In this study, therefore, a new method to fabricate the composite with higher density is developed using a conventional hot-press procedure. Matrix SiC sheets and Nicalon or Hi-Nicalon two-dimensional cloths without any coatings were stacked alternately and then this stacked green body was hot-pressed at 1750°C under 40 MPa applied stress in Ar. The matrix SiC sheet consists of fine, β-SiC powders and Al, B and C sintering aids. A sheet of SiC was formed using the doctor blade technique. The relative density of sintered bodies was 66–99%. The load–displacement behavior of the composites with Nicalon cloth showed a slightly ductile load–displacement behavior with low maximum strength, whereas the composite with Hi-Nicalon cloth showed brittle fracture with high maximum strength at room temperature.