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MAX phase solid solutions physical and mechanical properties may be tuned via changes in composition, giving them a range of possible technical applications. In the present study, we extend the MAX phase family by synthesizing (Zr1− xTi x)3AlC2 quaternary MAX phases and investigating their mechanical properties using density functional theory (DFT). The experimentally determined lattice parameters are in good agreement with the lattice parameters derived by DFT and deviate <0.5% from Vegard's law. Ti3AlC2 has a higher Vickers hardness as compared to Zr3AlC2, in agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

Using a synergistic combination of experimental and computational methods, we shed light on the unusual solubility of (Cr,Ti)3AlC2MAX phase, showing that it may accommodate Cr only at very low concentrations (<2 at%) or at the exact Cr/(Cr + Ti) ratio of 2/3, even when the ratio of reactants is far from this stoichiometry (1/2 ≤ Cr/(Cr + Ti) ≤ 5/6). In both phases, Cr exclusively occupies the4fsites, bridging carbide layers with the Al layer. Despite this, the peculiar stability of (Cr2/3Ti1/3)3AlC2is attributed to the formation of strong, spin-polarized Cr–C bonds, which result in volume reduction and a marked increase inc/aratio. IMPACT STATEMENT Solubility of Cr and Ti in (Cr,Ti)3AlC2was investigated using experimental and DFT techniques. It was also determined that (Cr2/3Ti1/3)3AlC2owe its remarkable stability to the formation strong Cr–C bonds. [ABSTRACT FROM PUBLISHER]

Despite having never been synthesized, the MAX phase Zr2AlC attracts a lot of interest owing to its foreseen properties. A possible way to circumvent this obstacle is to stabilize Zr2AlC by partially substituting one of its constituting elements. Here we report on attempts to synthesise quaternary MAX phases (Zr,M)2AlC and Zr2(Al,A)C where M = Cr, Ti or Mo and A = S, As, Sn, Sb and Pb. We were notably able to produce Zr2(Al0.2Sn0.8)C, Zr2(Al0.35Pb0.65)C, and Zr2(Al0.3Sb0.7)C, with the latter representing the first antimony-based MAX phase ever reported. Impact Statement: Numerous syntheses of Zr2AlC derived compositions were attempted. Zr2(Al0.2Sn0.8)C, Zr2(Al0.35Pb0.65)C and Zr2(Al0.3Sb0.7)C were notably produced and reported for the first time. [ABSTRACT FROM AUTHOR]

According to the properties determined for the ternary end-members, MAX phases in the quaternary Cr-Ti-Al-C system could be of interest as protective coatings for nuclear fuel cladding in the case of severe accident conditions. In this study, syntheses of 211 and 312 MAX phase compositions were attempted using pressureless reactions starting from Cr, TiH2, Al, and C (graphite) powders. It was observed that both the Ti substitution by Cr in Ti3AlC2 and the mutual solubility of Ti2AlC and Cr2AlC are limited to a few atomic percent. In addition, the remarkable stability of the (Cr2/3Ti1/3)3AlC2 MAX phase composition was confirmed. Due to the low miscibility of MAX phases in the Cr-Ti-Al-C system, most samples contained substantial amounts of TiC x and Al-Cr alloys as secondary phases, thus forming composite materials. After sintering, all samples were submitted to a single oxidation test (12 h at 1400°C in air) to identify compositions potentially offering high-temperature oxidation resistance and so warranting further investigation. In addition to (Cr0.95Ti0.05)2AlC, composite samples containing substantial quantities of Al8Cr5 and AlCr2 formed a stable and passivating Al2O3 scale, whereas the other samples were fully oxidized. [ABSTRACT FROM AUTHOR]

Rare-earth ( RE) monosilicates are promising candidates as environmental barrier coating ( EBC) materials for ceramic matrix composites for aerospace applications. Five rare-earth monosilicate materials have been investigated: Y2SiO5, Gd2SiO5, Er2SiO5, Yb2SiO5, and Lu2SiO5 produced from RE oxides and silica starting materials pressed and sintered at 1580°C under flowing air. Relative densities above 94% were obtained for all samples and ceramics were made containing 85-100 wt% of the RE monosilicate according to X-ray diffraction ( XRD) with RE disilicates as the second phase in the Gd, Yb, and Lu silicate systems. Microstructures were characterized using scanning electron microscopy and XRD, and thermal properties measured including specific heat, thermal expansion, and thermal diffusivity. For the first time, specific heat capacity values are reported for the monosilicates [0.45-0.69 J·(g·K)−1]. Thermal expansion coefficients ( TECs) of the dense samples ranged between 5.9 and 10.3 × 10−6 K−1 measured for 473 to 1473 K. All EBCs have low thermal conductivities [1.8 W·(m·K)−1 or less] making them excellent EBC insulators. [ABSTRACT FROM AUTHOR]