Heavy ion irradiation damage in Zr3(Al0.9Si0.1)C2 MAX phase

© 2020 Elsevier B.V. A Zr3(Al0.9Si0.1)C2 MAX phase-based ceramic with 22 wt.% ZrC and 10 wt.% Zr5Si3 has been irradiated with 52 MeV I9+ ions at room temperature, achieving a maximum dose of 8 displacements per atom (dpa). The response of this MAX phase-rich material to irradiation has been studied using scanning electron microscopy, transmission electron microscopy and X-ray diffraction techniques. Post-irradiation examination of the material revealed a number of crystalline changes to the MAX phase. At low doses, Zr3(Al0.9Si0.1)C2 maintained a high degree of crystallinity, while at the highest doses, its degree of crystallinity was reduced significantly. A number of radiation-induced phase transformations were observed, including the decomposition of Zr3(Al0.9Si0.1)C2 into ZrC and other phases, and the formation of β-Zr3(Al,Si)C2, a MAX phase with a rearranged stacking sequence. Microstructural examination revealed that the majority of the extended defects in Zr3(Al0.9Si0.1)C2 lie in the (0001) basal planes. Analysis of X-ray diffraction profiles after heat treating the 8 dpa-irradiated material for 1 h at 300 °C and at 600 °C showed that there were only subtle changes to the profiles relative to that of the 8 dpa-irradiated material which had not been heat treated. Overall, the experimental results of this study show that the Zr3(Al0.9Si0.1)C2 MAX phase responds less well to irradiation relative to other MAX phases irradiated with high energy heavy ions at room temperature

High temperature creep of 20 vol.% SiC-HfB2 UHTCs up to 2000 °C

High temperature compressive creep of SiC-HfB2 UHTCs up to 2000 °C has been studied. Microstructural analysis after deformation reveals formation of new phases in the Hf-B-Si and Hf-B-Si-C systems, which are responsible for the poor creep resistance. RE oxide additions have a negative effect reducing the creep resistance of SiC-HfB2 UHTCs. A simplistic analysis for the required creep resistance is described, indicating that only SiC-HfB2 UHTCs could withstand re-entry conditions for 5 min in a single use. However, RE oxide addition to SiC-HfB2 UHTCs does not provide the required creep resistance for them to be candidate materials for hypersonic applications

Impact of microwave processing on porcelain microstructure

[EN] Microstructural evolution on sintering of porcelain powder compacts using microwave radiation was compared with that in conventionally sintered samples. Using microwaves sintering temperature was reduced by similar to 75 degrees C and dwell time from 15 min to 5 min while retaining comparable physical properties i.e. apparent bulk density, water absorption to conventionally sintered porcelain. Porcelain powder absorbed microwave energy above 600 degrees C due to a rapid increase in its loss tangent. Mullite and glass were used as indicators of the microwave effect: mullite produced using microwaves had a nanofibre morphology with high aspect ratio (similar to 32 +/- 3:1) believed associated with a vapour-liquid-solid (VLS) formation mechanism not previously reported. Microwaves also produced mullite with different chemistry having similar to 63 mol% alumina content compared to similar to 60 mol% alumina in conventional sintered porcelain. This was likely due to accelerated Al+3 diffusion in mullite under microwave radiation. Liquid glass was observed to form at relatively low temperature (similar to 900-1000 degrees C) using microwaves when compared to conventional sintering which promoted the porcelains ability to absorb them.W. Lerdprom acknowledges Imperial College London funding no. MMRE_PG54200. A. Borrell acknowledges the Spanish Ministry of Economy and Competitiveness for her Juan de la Cierva-Incorporacion contract (IJCI-2014-49839).Lerdprom, W.; Zapata-Solvas, E.; Jayaseelan, DD.; Borrell Tomás, MA.; Salvador Moya, MD.; Lee, WE. (2017). Impact of microwave processing on porcelain microstructure. Ceramics International. 43(16):13765-13771. https://doi.org/10.1016/j.ceramint.2017.07.090S1376513771431

Elastic properties, thermal stability, and thermodynamic parameters of MoAlB

MoAlB is the first and, so far, the only transition-metal boride that forms alumina when heated in air and is thus potentially useful for high-temperature applications. Herein, the thermal stability in argon and vacuum atmospheres and the thermodynamic parameters of bulk polycrystalline MoAlB were investigated experimentally. At temperatures above 1708 K, in vacuum and inert atmospheres, this compound incongruently melts into the binary MoB and liquid aluminum metal as confirmed by differential thermal analysis, quenching experiments, x-ray diffraction, and scanning electron microscopy. Making use of that information together with heat-capacity measurements in the 4–1000-K temperature range—successfully modeled as the sum of lattice, electronic, and dilation contributions—the standard enthalpy, entropy, and free energy of formation are computed and reported for the full temperature range. The standard enthalpy of formation of MoAlB at 298 K was found to be −132±3.2 kJ/mol. Lastly, the thermal conductivity values are computed and modeled using a variation of the Slack model in the 300–1600-K temperature range

UHTC composites for hypersonic applications

A dream for many scientists, engineers and sci-fi enthusiasts is of an aerospace vehicle that can take off from an airport, fly through the atmosphere and travel to the other side of the earth at hypersonic speeds, and then return through the atmosphere to the same or another airport. Thanks to programs like DARPA’s Falcon Hypersonic Technology Vehicle 2 program (Figure 1), the dream is taking form

Flash spark plasma sintering of UHTCs

During the five year XMat research project supported by EPSRC (Engineering and Physical Sciences Research Council, UK) at Queen Mary we developed a novel sintering technique called Flash Spark Plasma Sintering (FSPS[1]) which is particularly suitable for the ultrarapid (a few seconds) consolidation of UHTCs. As in the case of incandescent lamps, flash sintering techniques use localized Joule heating developed within the consolidating particles using typically a die-less configuration. Heating rates are extreme (104–106 °C/min), and the sintering temperature is therefore reached extremely rapidly. The research covered mostly metallic conductors (ZrB2[2], HfB2,TiB2) and semiconductors (B4C, SiC and their composites). The talk will summarize the joint XMat team efforts to: -Identify the FSPS consolidation mechanism using modelling and transmission electron microscopy, -Characterise the structural properties for the bulk materials and redefine the structure-property relationships of FSPSed materials Please click Additional Files below to see the full abstract

Optimal preparation of high-entropy boride-silicon carbide ceramics

High-entropy boride-silicon carbide (HEB-SiC) ceramics were fabricated using boride-based powders prepared from borothermal and boro/carbothermal reduction methods. The effects of processing routes (borothermal reduction and boro/carbothermal reduction) on the HEB powders were examined. HEB-SiC ceramics with > 98% theoretical density were prepared by spark plasma sintering at 2000 °C. It was demonstrated that the addition of SiC led to slight coarsening of the microstructure. The HEB-SiC ceramics prepared from boro/carbothermal reduction powders showed a fine-grained microstructure and higher Vickers’ hardness but lower fracture toughness value as compared with the same composition prepared from borothermal reduction powders. These results indicated that the selection of the powder processing method and the addition of SiC phase could contribute to the optimal preparation of high-entropy boride-based ceramics