Thermal stability and oxidation resistance of laser clad TiVCrAlSi high entropy alloy coatings on Ti–6Al–4V alloy

TiVCrAlSi high entropy alloy coatings were deposited on Ti–6Al–4V alloy by laser cladding. SEM, XRD and EDS analyses show that, the as-clad coating is composed of (Ti,V)5Si3 and a BCC solid solution. After annealing at 800 °C for 24 h under vacuum, the coating is composed of (Ti,V)5Si3, Al8(V,Cr)5, and a BCC solid solution. The temperature-dependent phase equilibrium for the coating material calculated by using the CALPHAD method, indicates that above 880 °C the stable phases existing in the coating material are a BCC solid-solution and (Ti,V)5Si3. When the temperature is below 880 °C, the stable phases are (Ti,V)5Si3, Al8(V,Cr)5, and a BCC solid solution. In order to validate the calculation results, they were compared with TiVCrAlSi alloy samples prepared by arc melting, encapsulated in quartz tubes under vacuum, annealed at 400–1100 °C for 3 days and water-quenched. XRD analysis shows that the experimental phase composition agrees with the thermodynamic calculations. After vacuum annealing, there is a small increase of hardness for the laser clad TiVCrAlSi coating, which is due to the formation of Al8(V,Cr)5. The oxidation tests show that the TiVCrAlSi coating effectively improves the oxidation resistance of Ti–6Al–4V at 800 °C in air. The formation of a dense and adherent scale consisting of SiO2, Cr2O3, TiO2, Al2O3 and a small amount of V2O5 is supposed to be responsible for the observed improvement of the oxidation resistance.