Insights into the mechanical properties and thermal transport of Ti₃(Al_1-xA_x)C₂ solid solutions: A comprehensive theoretical study combined with experiment

The solid solution is a widely used and effective approach at a low cost to achieve desirable properties. Here, we performed theoretical investigation on a series of binary-A solid solution MAX (SS-MAX) phases, i.e., Ti₃(Al_1-xA_x)C₂ SS (A = Ga, In, Tl, Si, P, and S; x = 0–1 in 0.25 increments). Under the insight of the mixing enthalpy ΔH_mix, Gibbs free energy ΔG_mix, and formation enthalpy H_cp, S-alloyed MAX phases were evaluated to be unstable and are further corroborated by experiments. Interestingly, the bulk modulus of Ti₃(Al_0.75AGa_0.25)C₂ (Ti₃(Al_0.75In_0.25)C₂) unusually showed an approximate 12.8 % (11.1 %) enhancement than that of Ti₃AlC₂ and 11.0 % (17.1 %) enhancement than that of Ti₃GaC₂ (Ti₃InC₂). Moreover, the elastic moduli were exclusively enhanced by A elements capable of reducing triangular prism distortions. The anisotropic behavior was also revealed and interpreted as that the group IIIA elements (Ga, In, Tl) showed a pronouncedly strengthened effect on the intralayer bonding states, while Si and P acted on the interlayer interaction. Ti₃(Al_1-xSi_x)C₂ presented strong resistance to phonon scattering, resulting in their similar lattice thermal conductivities. The phenomena and the underlying physics driven by the MX-A interlayer interaction will not only provide insights into manipulation on multi-site/lattice materials but also pave the way for accelerating the developments and applications of high-entropy MAX phases.