Long-term oxidation of orthorhombic alloy Ti-22Al-25Nb in air between 650 and 800°C

Abstract

Titanium alloys based on the orthorhombic phase Ti{sub 2}AlNb are currently under investigation for those aeroengine applications where the maximum service temperatures exceed the capabilities of conventional titanium alloys but are still lower than recommended for {gamma}-TiAl based aluminides. For some orthorhombic alloys, a better fracture toughness, higher ductility, higher specific yield and lower coefficient of thermal expansion for {gamma}-TiAl based alloys as well as for alpha-2 alloys has been reported and creep and fatigue behavior are similar to that of alpha-2 alloys. However, oxidation and embrittlement of orthorhombic alloys are a major concern when applications in the oxidizing atmosphere of an aeroengine and operating temperatures between 650 and 700 C are considered. The goal of the present study was to assess the long-term environmental resistance of high niobium-containing Ti-22Al-25Nb, an alloy presently under consideration for titanium matrix fiber reinforced composites (TMCs). TMCs are needed to introduce blisk (= bladed disk) and bling (= bladed ring) technology into modern aeroengine design. Oxidation tests on cast Ti-22Al-25Nb matrix material in laboratory air were conducted between 650 and 800 C up to 4,000 hr.