Thermal stability of an α2+γ titanium aluminide coating on a Ti-22Al-23Nb orthorhombic alloy

The lack of high temperature environmental resistance is a major concern in the application of orthorhombic alloys and their composites. In an attempt to mitigate this issue, in the present study, foils of the orthorhombic alloy were vacuum diffusion bonded on either side with Ti-8Al-2Cr-2Mn foils. The thermal stability of the joint was examined using microscopic analysis and mechanical testing. Microscopic analysis indicates that the reaction product is mainly niobium rich alpha(2) The ultimate tensile strength of the joint was not noticeably affected by thermaI cycling in air. These results suggest that the alpha(2)+gamma-alloy was successful in preventing degradation of the O-alloy due to interstitial embrittlement. However, the strength of the "as-fabricated" material was significantly lower than that of the uncoated O-alloy with a similar microstructure. Therefore, it appears that under the present processing conditions, this coating may not be suitable. However, joining below the beta-transus or incorporating compliant layers may have potential.