Abstract
Although the benefits of titanium aluminides for intermediate service temperature applications were well conceived and significant research and development activities were conducted in the past four decades, they remained as developmental materials due to barriers associated with melting, processing, scale-up, and cost. Demanding requirements of efficient aero-engines and extensive risk reduction demonstrations paved the path for commercial introduction of gamma titanium aluminides. The single most attractive current application is for low pressure turbine blades (LPTBs) in advanced aero-engines replacing conventionally cast nickel superalloys. This paper provides an overview of recent progress, producibility challenges, and opportunities. The successful journey of gamma (γ) TiAl LPTB development from laboratory demonstrations to production insertions in mass-produced commercial jet engines will be described. Collaboration and integrated product development were identified as the most critical needs for rapid maturation and implementation of γ-TiAl into aerospace applications. An integrated computational materials engineering modeling framework and toolsets developed under a collaborative US Air Force Metals Affordability Initiative project between industry, government, and academia will be illustrated. Model-based optimization of material and processing for achieving desired performance goals will be highlighted.
Highlights
Intermetallic compounds formed between light elements Tic and Al are attractive because of their low density and good elevated temperature strength
The formation of intermetallic compounds reduces the symmetry of the parent metal lattice, restricting available deformation modes
The risks associated with reduced ductility and fracture toughness have been viewed as outweighing the benefits of increased strength
Summary
Intermetallic compounds formed between light elements Tic and Al are attractive because of their low density and good elevated temperature strength. A class of γ-TiAl compositions based on 42-48 at% Al, plus additional elements as identified, evolved over the past 40 years that offer attractive specific strengths compared to high-density nickel superalloys/steels up to 750°C service temperatures (Figure 1) These compositions can be broadly classified into two groups – alloys that contain greater than 85 vol.% γ phase (peritectic-solidifying) and alloys that contain less than 75 vol% γ (β solidifying). To meet the property requirements of higher rotational speeds of GTF LPT, Ti-43.5Al-4Nb1Mo-0.1B (TNM), was developed in the 2000s (Figure 4) This β-solidifying γ-TiAl exhibits a re ned as-cast grain size due to trace B additions and a higher volume fraction of high-temperature β-TiAl (disordered body centered cubic structure) phase. The ability to minimize the volume fraction of βo-TiAl (ordered B2 structure) phase via a post-forging heat treatment provides balanced properties for TNM at room and service temperatures [3]
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