Oxidation mechanism and mechanical properties of substitutional transition metal modified Nb₄AlC₃: A first-principles density functional theory study

The MAX phase, Nb₄AlC₃, has been identified as a potential candidate for high-temperature structural materials, but its inherent limitations, including poor oxidation resistance, low Young's modulus, and low hardness, have hindered its widespread application. To address these fundamental questions, in this paper, the oxidation mechanism of Nb₄AlC₃ with and without the influence of transition metal (TM) elements is thoroughly investigated through systematic quantum mechanical calculations. The investigation considers six possible O doping sites in the Nb–Al/C layer, and the calculation results reveal a tendency for O to occupy the hexahedral Site-Ⅱ. This can be mainly attributed to the strong charge interaction between O and Nb, Al. Furthermore, the study explores the impact of all “M" site TM elements on the compound's oxidation resistance. Zr and Mo are identified as representatives that can enhance or weaken the oxidation resistance, respectively. Subsequently, the effects of Zr doping on the phase stability, mechanical properties, lattice thermal conductivity, and electronic properties of Nb₄AlC₃ were examined. The findings reveal that all (Nb_1-xZr_x) ₄AlC₃ compounds are thermodynamically stable, elastically stable, and brittle materials with good thermal conductivity. Among them i, Young's modulus and hardness increase the most when the Zr content is low (0.125) compared to the parent phase, which aligns with the available experimental results. And the system with 0.125 Zr content has the highest high-temperature stability (1300 K). Therefore, as expected, Zr doping can improve the comprehensive properties of Nb₄AlC₃ to some extent.