Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)2AlC MAX phase solid solution by co-sputtering from a composite Cr-Al-C and elemental Y target, at room temperature, followed by annealing. While direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room temperature depositions, followed by annealing to 760 {\deg}C led to the formation of phase pure (Cr,Y)2AlC by diffusion. Higher annealing temperatures caused decomposition of the metastable phase into Cr2AlC, Y5Al3 , and Cr-carbides. In contrast to pure Cr2AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ~200 {\deg}C for 5 at.% Y compared to Cr2AlC. Calculations predicting the metastable phase formation of (Cr,Y)2AlC and its decomposition are in excellent agreement with the experimental findings.