Synthesis and Characterization of Double Solid Solution (Zr,Ti)₂(Al,Sn)C MAX Phase Ceramics

Quasi phase-pure (>98 wt %) MAX phase solid solution ceramics with the (Zr,Ti)₂(Al_0.5,Sn_0.5)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 (Zr_1–x,Ti_x)₂(Al_0.5,Sn_0.5)C MAX phase solid solutions with x = 0, 0.3, 0.7, and 1. The thermal expansion coefficient data of the Ti₂(Al_0.5,Sn_0.5)C solid solution were compared with those of the Ti₂AlC and Ti₂SnC ternary compounds. The thermal expansion anisotropy increased in the (Zr,Ti)₂(Al_0.5,Sn_0.5)C double solid solution MAX phases as compared to the Zr₂(Al_0.5,Sn_0.5)C and Ti₂(Al_0.5,Sn_0.5)C end-members.