Transformation of chemical ordering and configuration entropy in quaternary Cr _<i>x</i>Ti _0.75Mo _0.75V _{1.5− <i>x</i>}AlC ₂ MAX system

Two new quaternary Cr<em>_x</em>Ti_0.75Mo_0.75V_{1.5−<em>x</em>}AlC₂ (<em>x</em> = 1.25, 1) MAXs and Cr_0.75Ti_0.75Mo_0.75V_0.75AlC₂ are synthesized by hot pressing. Interestingly, an unprecedented transition in M-site atomic occupancy from out-of-plane order to solid solution is observed along with the composition variation, which also increases the configurational entropy from medium- to high-entropy. Through experimental observation and theoretical calculation, the influence of the atomic distribution on their properties is analyzed. Eventually, about 40% increment on the Vickers hardness than that of the Cr₂TiAlC₂ and low thermal conductivities are detected from the three MAXs, which can be ascribed to the solid solution strengthening effects and the enhanced scattering of both electrons and phonons from the high-entropy structure.&nbsp;