OXIDATION PROTECTION OF TI-ALUMINIDE ORTHORHOMBIC ALLOYS - AN ENGINEERED MULTILAYER APPROACH

The lack of high-temperature environmental resistance is a major issue in the application of orthorhombic-based titanium aluminide alloys (O alloys) and their composites. Improvement in environmental capability can be achieved by applying diffusion barrier coatings to the surface of the orthorhombic matrix alloy. However, since thin coatings are pr one to foreign-object damage, an approach based on thicker multilayer materials may be more prudent for fracture-critical applications. In t he present study, foils of the orthorhombic alloy were diffusion bonde d on either side with an gamma alloy, the latter used in an attempt to provide environmental protection. Mechanical tests suggested that the gamma alloy was successful in preventing degradation of the O alloy d ue to oxidation and interstitial embrittlement under thermal cycling c onditions. Processing below the beta transus of the O alloy provided a n improvement in the stress and strain to failure of the joined materi al compared to materials processed above the transus. However, in eith er case, the strengths of the joined materials were significantly lowe r than that of the uncoated O alloy with similar microstructures. Resu lts suggest that the low strength of the joined materials may be due t o cracking of the gamma alloy, resulting in premature failure of the O alloy. Finite element analysis (FEA) was performed to understand the stress distribution in the joined material and to investigate approach es for reducing the residual stress. Several approaches for improving the stress and strain to failure of the joined material are presented.