Transformation of chemical ordering and configuration entropy in quaternary Cr_xTi_0.75Mo_0.75V_1.5−xAlC₂ MAX system

Two new quaternary Cr_xTi_0.75Mo_0.75V_1.5−xAlC₂ (x = 1.25 and 1) MAXs and Cr_0.75Ti_0.75Mo_0.75V_0.75AlC₂ are synthesized via hot pressing. An unprecedented transition in M-site atomic occupancy from out-of-plane order to solid solution is observed along with composition variation, which also increases the configurational entropy from medium- to high-entropy. Through experimental observations and theoretical calculations, the influence of the atomic distribution on the material properties is analyzed. Eventually, an approximately 40% increase in the Vickers hardness compared with that of Cr₂TiAlC₂ and a low thermal conductivity are detected for the three MAXs, which can be attributed to the solid solution strengthening effects and the enhanced scattering of both electrons and phonons from the high-entropy structure.