This study reports on the synthesis and characterization of MAX phases in the (Zr,Ti)AlC system. The MAX phases were synthesized by reactive hot pressing and pressureless sintering in the 1350-1700 °C temperature range. The produced ceramics contained large fractions of 211 and 312 (n = 1, 2) MAX phases, while strong evidence of a 413 (n = 3) stacking was found. Moreover, (Zr,Ti)C, ZrAl, ZrAl, and ZrAl were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegard's law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegard's law around the (Zr,Ti)AlC stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed MX octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase. The decomposition of the secondary (Zr,Ti)C phase was attributed to the miscibility gap in the ZrC-TiC system.

Guided by predictive theory, a new compound with chemical composition (CrZr)AlC was synthesized by hot pressing of Cr, ZrH, Al, and C mixtures at 1300 °C. The crystal structure is monoclinic of space group C2/ c and displays in-plane chemical order in the metal layers, a so-called i-MAX phase. Quantitative chemical composition analyses confirmed that the primary phase had a (CrZr)AlC stoichiometry, with secondary CrAlC, AlZrC, and ZrC phases and a small amount of Al-Cr intermetallics. A theoretical evaluation of the (CrZr)AlC magnetic structure was performed, indicating an antiferromagnetic ground state. Also (CrHf)AlC, of the same structure, was predicted to be stable.

For the first time, MAX phases in the Hf-Al-C system were experimentally synthesized using reactive hot pressing. HfC was observed as the main competing phase. The lattice parameters of HfAlC and HfAlC were determined by Rietveld refinement based on the X-ray diffraction data. The atomic stacking sequence was revealed by high-resolution scanning transmission electron microscopy. Mixtures of 211 and 312 stacking were observed within the same grain, including 523 layers. This transition in atomic structure is discussed.

The addition of Nb and Sn to Zr2AlC is investigated, targeting the synthesis of a Zr-rich bulk MAX phase free of ZrC. The 211 phase formation in the two quaternary Zr-Nb-Al-C and Zr-Al-Sn-C systems is evaluated. Solubility over the entire compositional range in (Zr, Nb)2AlC and Zr2(Al, Sn)C is observed. In terms of effectiveness, the addition of Sn is preferred over the addition of Nb, as the former is selectively incorporated into the 211 structure. A combinatorial approach results in the formation of phase-pure (Zr0.8, Nb0.2)2(Al0.5, Sn0.5)C. The effect of the added solutes on the microstructure and crystallographic parameters is investigated. The addition of Nb and Sn reduces the distortion parameter of the trigonal prism compared to pure Zr2AlC. Therefore, an attempt is made to establish a more general stability criterion for the M2AC structure based on the steric relationship between the atoms in the M6A trigonal prism. Inspired by the Hume-Rothery rules, it is suggested that comparable atomic radii of the M- and A-atoms provide a good starting point to obtain a stable 211 MAX phase.

Quasi phase-pure (>98 wt %) MAX phase solid solution ceramics with the (Zr,Ti)2(Al0.5,Sn0.5)C stoichiometry and variable Zr/Ti ratios were synthesized by both reactive hot pressing and pressureless sintering of ZrH2, TiH2, Al, Sn, and C powder mixtures. The influence of the different processing parameters, such as applied pressure and sintering atmosphere, on phase purity and microstructure of the produced ceramics was investigated. The addition of Sn to the (Zr,Ti)2AlC system was the key to achieve phase purity. Its effect on the crystal structure of a 211-type MAX phase was assessed by calculating the distortions of the octahedral M6C and trigonal M6A prisms due to steric effects. The M6A prismatic distortion values were found to be smaller in Sn-containing double solid solutions than in the (Zr,Ti)2AlC MAX phases. The coefficients of thermal expansion along the ⟨a⟩ and ⟨c⟩ directions were measured by means of Rietveld refinement of high-temperature synchrotron X-ray diffraction data of (Zr1–x ,Ti x )2(Al0.5,Sn0.5)C MAX phase solid solutions with x = 0, 0.3, 0.7, and 1. The thermal expansion coefficient data of the Ti2(Al0.5,Sn0.5)C solid solution were compared with those of the Ti2AlC and Ti2SnC ternary compounds. The thermal expansion anisotropy increased in the (Zr,Ti)2(Al0.5,Sn0.5)C double solid solution MAX phases as compared to the Zr2(Al0.5,Sn0.5)C and Ti2(Al0.5,Sn0.5)C end-members.

Select MAX phase-based ceramics were screened with respect to their potential susceptibility to environmentally-assisted degradation in an oxygen-poor liquid lead-bismuth eutectic (LBE) environment, both under static and fast-flowing exposure conditions. The majority of the MAX phases exposed to oxygen-poor ( 2.210-10 mass%) static liquid LBE for at least 1000 h at 500°C showed exceptional chemical compatibility with the heavy liquid metal, i.e., no evidence of LBE dissolution attack was observed, despite the absence of a continuous oxide scale on the surface of the exposed MAX phase ceramics. The local LBE interaction observed only with the Zr-rich MAX phases consisted in the partial substitution of Al by Pb/Bi in the MAX phase crystal structure and the in-situ formation of Pb/Bi-containing solid solutions. Moreover, the interaction of Zr-based MAX phases with static liquid LBE was accompanied by the dissolution of parasitic intermetallic phases, which facilitated the further LBE ingress into the ceramic bulk. The erosion resistance of select MAX phase ceramics was also assessed in oxygen-poor ( 510-9 mass%) fast-flowing (v ≈ 8 m/s) liquid LBE for 1000 h at 500°C. Despite the moderate LBE oxygen concentration, oxidation was the predominant corrosion mechanism, while no erosion damages were observed in the exposed MAX phase ceramics. The resistance of the MAX phase ceramics to both dissolution corrosion and erosion in contact with oxygen-poor Highlights  Exposing 11 MAX phases to oxygen-poor, static LBE showed little or no interaction.  The LBE/Zr2AlC interaction formed a new Zr2(Al,Pb,Bi)C MAX phase solid solution.  The LBE/(Zr,Ti)n+1AlCn interaction formed (Zr,Ti)n+1(Al,Pb,Bi)Cn solid solutions.  (Nb,Zr)4AlC3 showed the best erosion resistance in oxygen-poor, fast-flowing LBE.  MAX phases outperformed 316L stainless steels exposed to similar LBE conditions. Recently, Bentzel et al. studied the behaviour of several MAX phases in static liquid sodium, another Gen-IV coolant, up to 750°C for 168 h [21]. They reported that Ti3SiC2, Ti2AlC and Cr2AlC were stable, as well as their grain boundary phases. Ti3AlC2, however, showed signs of degradation along the grain boundaries and Al-depletion in the MAX phase grains [21].

The present work describes a synthesis route for bulk Ta4AlC3 MAX phase ceramics with high phase purity. Pressure-assisted densification was achieved by both hot pressing and spark plasma sintering of Ta2H, Al and C powder mixtures in the 1200-1650°C range. The phases present and microstructures were characterized as a function of the sintering temperature by X-ray diffraction and scanning electron microscopy. High-purity -Ta4AlC3 was obtained by hot pressing at 1500°C for 30 min at 30 MPa. The β-Ta4AlC3 allotrope was observed in the samples produced by SPS. The Young's modulus, Vickers hardness, flexural strength and single-edge V-notch beam fracture toughness of the high-purity bulk sample were determined. The thermal decomposition of Ta4AlC3 into TaCx and Al vapour in high (10 -5 mbar) vacuum at 1200°C and 1250°C was also investigated, as a possible processing route to produce porous TaCx components.

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