| dc.description.abstract | Over the last two decades, a family of ternary carbides and nitrides commonly referred to as MAX phases has gathered tremendous attention of scientists and engineers. Thanks to their layered crystal structure consisting of by ceramic-like M-X layers interleaved with metal-like A layers, MAX phases combine properties of both metals and ceramics. In addition, alloying with other elements on M, A, and X sites provide the extra potential for optimizing and tailoring their properties for various applications.
Among all MAX phases, Nb-Al-C and V-Al-C are of special interest in this dissertation work for two reasons. First, they are considered to be good candidates for high-temperature applications, and secondly, previous computational results suggest that they can accommodate more late transition metals in solid solution, unlike much more studied Ti-Al-C MAX phases. Among all other reaction sintering techniques used in fabricating MAX phases, Pulse Electric Current Sintering (PECS) was selected for synthesis of Nb- and V-based MAX phases in this study, because of rapid heating and cooling rates and easy operational control of the grain size in the sintered material.
This dissertation starts by investigating how excess of Al powder used in synthesis of Nb2AlC affects its final phase composition. Once obtaining the optimal sintering conditions of synthesizing phase-pure Nb2AlC, the recipe was employed to fabricate phase-pure (TixNb1-x)2AlC solid solutions with x = 0-1. The microstructure and room temperature mechanical properties of (TixNb1-x)2AlC solid solutions were also further characterized. Motivated by introducing magnetism into MAX phases, the research expanded to study of the phase stability of the Nb-Al-C system alloyed with Fe, Co, and Ni. Although no quaternary carbides with MAX phase structure were detected in Nb-Fe-Al-C, Nb-Ni-Al-C and Nb-Co-Al-C systems, we report for the first time on formation of new -carbides in those systems.
Since (VxTi1-x)2AlC solid solutions have been successfully synthesized and characterized in the past, the focus of this study is on (i) synthesis of higher order MAX phase solid solutions from V-Ti-Al-C system, and (ii) synthesis and characterization of (VxFe1-x)2AlC. Herein we report for the first time on synthesis of (VxTi1-x)3AlC2 solid solution with x up to 0.9, which is particularly important because V3AlC2 does not exist. On the other hand, unlike in the case of Nb-Fe-Al-C system, we showed formation of (VxFe1-x)2AlC with x=0.03 and characterized its magnetic properties.
High-entropy MAX phase is a new emerging area starting from 2020. In the remainder of this dissertation, the current research progress on this topic and future research directions are discussed in more detail. | |
