Elemental Segregation and O-Phase Formation in a Gamma-TiAl Alloy

Abstract:

Titanium aluminides based on the L1₀ ordered g-phase are promising structural light-weight materials for applications in aircraft engines. Typical compositions for γ-TiAl alloys lie in the range Ti-(44-48)Al (at.-%). For high creep resistance, a two-phase microstructure based on lamellar (α₂+γ)-colonies is desirable that may be tuned towards better ductility by introducing pure γ-grains (near lamellar or duplex microstructure).γ-TiAl alloys are often alloyed with niobium for increased oxidation resistance and improved mechanical properties. HEXRD and TEM studies of the alloy Ti-42Al-8.5Nb revealed that the orthorhombic O-phase forms during annealing at 500-650°C. This orthorhombic phase has been known in Nb-rich, Al-lean, α₂-based Ti-aluminides since the late 1980ies (Nb> 12.5 at.-%, Al< 31 at.-%) but the finding in γ-based alloys is new.TEM imaging showed that the O-phase is located within α₂ lamellae of lamellar (α₂+γ)-colonies. O-phase domains and α₂ phase form small columnar crystallites based in the α₂/γ interface. The columnar crystallites grow parallel to the [0001] direction of the α₂ phase and appear as facets when observed along this direction. The evolution of domains and facets with annealing time and the chemical homogeneity of the phases are investigated.The results of STEM imaging show that O-phase domains form during annealing at 550 °C for 8hours or 168 hours. After 168 hours of annealing Nb segregations are observed by EDX mapping within O-phase domains. In comparison, no segregation of niobium is detected after 8 hours of annealing.

• Periodical: Materials Science Forum (Volume 941)
• Pages: 741-746
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.941.741
• Online since: December 2018
• Authors: Heike Gabrisch*, Tobias Krekeler, Uwe Lorenz, Marcus Willi Rackel, Martin Ritter, Florian Pyczak, Andreas Stark
• Keywords: EDX Mapping, Gamma-TiAl, O-Phase, Segregation, Transmission Electron Microscopy

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