Wear and high-temperature oxidation resistances of AlNbTaZrx high-entropy alloys coatings fabricated on Ti6Al4V by laser cladding

Abstract

With respect to poor wear and high-temperature oxidation resistance of Ti6Al4V, the AlNbTaZrx high-entropy alloys (HEAs) coatings were successfully fabricated on Ti6Al4V by laser cladding. The microstructural evolution of the coatings with x was investigated. The changes in wear and high-temperature oxidation behaviors with x were highlighted. The results indicated that the coatings with x ranging from 0.2 to 1.0 were composed of two simple solid solutions in terms of β with a BCC structure and α′ with a HCP structure, which was well in accordance with the modified criterions proposed by us. The increase in x promoted the transformation from β into α′, resulting in the corresponding improvement in microhardness of the coatings (increased by about 17% from x = 0.2 to x = 1.0). The average wear volume and wear rate (0.72 mm3, 1.66 × 10−4 mm3 N−1 m−1) of the coatings were both reduced by about 31% when compared with those of the substrate (1.04 mm3, 2.41 × 10−4 mm3 N−1 m−1). The two indexes evaluating wear resistance firstly presented the downward tendency from x = 0.2 to x = 0.8, then were increased when x was further enhanced to 1.0. The phenomenon was closely related to the evolution in wear mechanism, which was determined to be microcutting (x = 0.2), and converted to the combination of microcutting and oxidation (x = 0.4, 0.6 and 0.8), finally identified as the mixture of microcutting, oxidation and brittle debonding (x = 1.0). The coatings also demonstrated the excellent resistance to high-temperature oxidation (1000 °C for 50 h) due to the average mass gain significantly reduced by about 76% when compared with that of the substrate. The oxidation rate of the coatings was gradually reduced with the increase in x (reduced by about 68% from x = 0.2 to x = 1.0), resulting in about 64% decrease in mass gain after 50 h. The improvement in oxidation resistance of the coatings with x should be attributed to the change in formation difficulty and stability of oxidation layers. Regarding to wear resistance and high-temperature oxidation resistance, the suitable value of x can be determined as 0.8.