Abstract
Intermetallic titanium aluminides based on the γ-TiAl phase, such as the β-solidifying TNM alloy, have found application in aerospace and automotive industries. In order to adjust balanced mechanical properties in the TNM alloy a two-step heat treatment is subjected to a hot-die forged material to provide a fine nearly fully lamellar microstructure. However, during the second heat treatment step, beyond a critical temperature, discontinuous precipitation can occur which represents a coarsening process of the fine lamellar α2/γ-colonies. This microstructural instability is a limiting factor for the TNM alloy, also regarding the maximum operation temperature. In order to investigate the influence of microstructural instability on creep behavior, microstructures with various progress of discontinuous precipitation and average lamellar spacing were adjusted, creep tested and changes in microstructure examined. The creep tests were conducted at 800 °C and 150 MPa. It is demonstrated that the creep resistance is decreased when discontinuous precipitation exceeds a certain volume fraction, thus, revoking the advantages of a narrow lamellar spacing.
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