Effect of Long-Term Aging and Creep Exposure on the Microstructure of TiAl-Based Alloy for Industrial Applications
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Effect of Long-Term Aging and Creep Exposure on the Microstructure of TiAl-Based Alloy for Industrial Applications Juraj Lapin1, Mohamed Nazmy2, and Marc Staubli2 1 Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Racianska 75, SK-831 02 Bratislava, Slovak Republic 2 ALSTOM Ltd., Department of Materials Technology, CH-5401 Baden, Switzerland ABSTRACT The effect of long-term aging and creep exposure on the microstructure of a cast TiAl-based alloy with nominal chemical composition Ti-46Al-2W-0.5Si (at.%) was studied. The aging experiments were performed at temperatures between 973 and 1073 K for various times ranging from 10 to 14000 h in air. Constant load tensile creep tests were performed at applied stresses ranging from 150 to 400 MPa and at temperatures between 973 and 1123 K up to 25677 h. During aging and creep testing the α2(Ti3Al)-phase in the lamellar and feathery regions transforms to the γ(TiAl)-phase and fine needle-like B2 precipitates. Microstructural instabilities lead to a softening of the alloy. The effect of this softening on long-term creep resistance is negligible at temperatures of 973 and 1023 K.
INTRODUCTION Low density, high melting temperature, good elevated-temperature strength and modulus retention, high resistance to oxidation and excellent creep properties of TiAl-based alloys make them potential candidate structural materials for various applications in the gas turbine and automotive industry. In recent years a particular interest was devoted to an alloy with nominal chemical composition Ti-46Al-2W-0.5Si (at.%), which was designated as ABB-2 [1-6]. As shown in our previous studies [2,4], the microstructure of large cast turbine blades from the ABB-2 alloy is not homogenous and changes from fully or nearly lamellar in the vicinity of the blade surface to duplex one in the central part. Such microstructural variations affect significantly local mechanical properties of the cast components [2,4]. Although several studies were also published on the microstructure of the ABB-2 alloy [4-6], information about microstructural stability of large cast components during long-term aging and creep testing are still lacking in the literature. Therefore, evaluation of long-term microstructural stability of samples prepared from different regions of cast components is of great practical interest. The aim of this paper is to investigate the effect of long-term aging and creep exposure on the microstructure of a cast TiAl-based alloy for industrial applications. Duration of aging and creep experiments was exceptionally long comparing to existing literature data that has been published for emerging class of TiAl-based alloys.
EXPERIMENTAL DETAILS Aging and creep experiments were conducted on the ABB-2 alloy with the chemical composition given in Table I. The as-received material was subjected to a hot isostatic pressing
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Table I. Chemical composition of the ABB2 alloy (at.%). Material Ti Al W Si Cu C Fe N O H Component 1 Bal 46.61 1.88 0.49 0.006 0.034 0.038 0
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