Cyclic oxidation behavior of a beta gamma powder metallurgy TiAl–4Nb–3Mn alloy coated with a NiCrAlY coating

A new beta gamma TiAl–4Nb–3Mn alloy was synthesized by powder metallurgy (PM) and coated with NiCrAlY powder by Air Plasma Spray (APS). The cyclic oxidation behavior of the coated beta gamma alloy was investigated between room temperature and 1000 °C for up to 800 cycles. During cyclic oxidation testing, a TiN layer formed at the interface between NiCrAlY and TiAl–4Nb–3Mn and the thickness of the TiN layer increased with the number of cycles. The outward Ti diffusion also resulted in the formation of titanium oxides on the coating top surface after 630 cycles. Inward diffusion of Ni led to the formation of an inner diffusion zone containing NiAlTi and a mixture of NiAlTi + TiAl. Due to the spallation of coating scales on the circumferential surface, weight loss was observed after 120 cycles although both the top and bottom surfaces of the specimens remain in contact with the substrate until 800 cycles. Due to inward and outward diffusion, half of the NiCrAlY coating was consumed after 800 cycles suggesting the need for diffusion barrier coating at the interface between the NiCrAlY coating and the TiAl–4Nb–3Mn alloy.

► NiCrAlY remained in contact with TiAl–4Nb–3Mn until 800 oxidation cycles at 1000 °C.
► Interdiffusion was severe due to inward Ni and outward Ti diffusion.
► Diffusion barrier is required between the NiCrAlY and TiAl–4Nb–3Mn for application at 1000 °C.