The Effects Of Long-Term Air Exposure On The Stability Of The Different Wcontanting TiAl Alloys

The thermal stability of three fully-lamellar castγ-TiAl based alloys (coarse-grained alloy 1, alloy 2, alloy 3, which contains 0, 1.0at.% and 1.4at.% (tungsten, at.%), respectively) has been studied in this thesis. The three coarse-grained cast alloys were exposed to 700℃air for up to 10000 hours. The characteristic changes in microstructure and the corresponding effects on mechanical properties have been investigated in detail. The experimental work in this degree research includes: microstructural investigation using optical microscopy, scanning electron microscopy and transition electron microscopy; mechanical testing for fatigue and tensile properties; energy dispersive x-ray analysis for changed chemistry of microstructures; and image analysis for volume fractions of phases concerned.It was found that the microstructures of the three cast alloys were thermodynamically unstable and readily changed to a more stable condition after the long-term thermal exposure. The change in microstructure was characterized mainly by the decomposition ofα₂ lamellar, in the form ofα₂→α₂+γphasetransformation.It was observed that a "parallel decomposition" occurred on the alloy 1: coarseα₂ lamellar decomposed into alternately arrangedα₂+γfine lamellar. Correspondingly, the fatigue strength and tensile stress were enhanced after the exposure. On the other hand, a "perpendicular decomposition" ofα₂ lamellar occurred insignificantly on alloy 2: some singleα₂ lamellar laths decomposed into severalα₂+γshort sections along the originalα₂ lamellar. In accordance with such a change in microstructure, tensile properties were decreased slightly, while fatigue strength was increased to some degree. The alloy 3 showed a decomposition manner similar to alloy 2. After long term exposure tensile properties remained essentially unchanged, while fatigue strength was increased.It was important to find that the ductility at room temperature was decreased significantly in alloy 1 subjected to 10000 h exposure. This was becauseα₂ lamellar enriched in oxygen oriniginally, released oxygen into the lamellar colonies during the widespread parallel decomposition, therefore causing brittleness of the alloy. In contrast, W-containing alloy 2 and alloy 3 showed relatively unchanged ductility. This was attributed to the slow diffusion of W, hence, causing less degree ofα₂ decomposition.In the course of the long term air exposure the thermal stability ofα₂ lamellar in the three alloys in decreasing order was determined to be: alloy 3 > alloy 2 > alloy 1.