Investigated into high-temperature oxidation behaviors of the coatings fabricated on Ti6Al4V by laser cladding MoNbTiZr with varying contents of TaC

Abstract

In view of poor high-temperature oxidation resistance of Ti6Al4V, the composite coatings were designed and prepared on its surface by cladding the refractory alloy system of MoNbTiZr with varying TaC contents. The effects of TaC content (0 wt.%, 5 wt.%, 10 wt.% and 15 wt.%) on microstructure and high-temperature oxidation behaviors were investigated in detail. The oxidation mechanism was especially highlighted. The coating without TaC was mainly composed of β(Ti) with a BCC structure accompanied with a small quantity of αʹ(Ti) with an HCP structure. The introduced TaC was completely dissolved and then precipitated in the form of TiC dendrites. The high-temperature oxidation tests were conducted at 800 °C for 50 h. The weight gains of the coatings with 5 wt.%, 10 wt.% and 15 wt.% were reduced by approximately 16.78%, 19.79% and 43.32%, respectively when compared with that of the coating without TaC, which proved that the introduction of TaC can effectively improve oxidation resistance of the substrate. Ta₂O₅ was formed prior to the other oxides from the point of thermodynamics, shielding the coating body from serious oxidation. The formation of Ta₂O₅ made the oxidation film more compact due to the generated larger volume expansion than the other oxides, greatly preventing oxygen from intrusion into the coating body. The cross-sectional morphology of the oxidation film also revealed that the volume fraction of the compact oxidation zone rich in Ta, Nb and Zr demonstrated an upward tendency along with the increase in TaC content.