Modification mechanism of Ti-6Al-4V alloy with pre-coated Ti-Cu-Al multilayer film treated by ion nitriding: Experiments and first-principles calculations

In this paper, the Ti-6Al-4V(TC4) alloy specimens pre-coated with Ti-Cu-Al multilayer films were plasma nitrided at different temperatures (673K-823K), and their microstructure and mechanical properties were studied. Surface hardness changes induced by pre-coating combined with plasma nitriding were also analyzed and correlated with microstructural features and elemental distribution. Ti-Cu-Al multilayer films with clear interfaces on deformed TC4 alloy were fabricated. The multilayer structure survived after nitriding, and a thick gradient nitrided layer with a gradient distribution of N elements was formed. The bonded nitride on the surface provides high hardness, reaching 948HV at 723K. In addition, the nitride TC4 alloy's wear resistance exhibits a significant improvement compared with the untreated specimen, especially at 823K, the mass wear rate decreased by more than 80%. It is mainly attributed to the N/metal element gradient distribution of the thick nitride layer. The nitriding behavior of α-Ti/Cu/Al was investigated by first-principles calculations. According to the thermodynamic calculation results, N atoms preferentially occupy the octahedral interstitial sites of α-Ti and the tetrahedral interstitial sites of Al. It is worth noting that N atoms have poor thermodynamic stability in Cu, which is consistent with the experimental results. The calculated elastic properties agree with the experimental cross-sectional hardness results, showing a smooth decreasing trend of the complex modulus and hardness on the micrometer length scale and explaining the improved structure performance. These results may provide crucial inspirations for future experimental work preparing high-performance nitrided layers.