γ-TiAl (Cr, Mo, Si, B) specimens with two different fine lamellar microstructures were produced by vacuum arc melting followed by a two-stage heat treatment. The average lamellar spacing was determined to be 200 nm and 25-50 nm, respectively. Creep tests at 700°C showed a very strong primary creep for both samples. After annealing for 24 hours at 1000 °C the primary creep for both materials is significantly decreased. The steady-state creep for the specimens with the wider lamellar spacing appears to be similar to the creep behavior prior to annealing while the creep rate of the material with the previously smaller lamellar spacing is significantly higher. Optical microscopy and TEM-studies show that the microstructure of the specimens with the wider lamellar specing is nearly unchanged, whereas the previously finer material was completely recrystallized to a globular microstructure with a low creep resistance. The dissolution of the fine lamellar microstructure was also observed during creep tests at 800°C as manifested in an acceleration of the creep rate. It is concluded that extremely fine lamellar microstructures come along with a very high dislocation density and internal stresses which causes the observed high primary creep. The microstructure has a composition far away from the thermodynamical equilibrium which leads to a dissolution of the structure even at relatively low temperatures close to the intended operating temperature of γ-TiAl structural parts. As a consequence this limits the benefit of fine lamellar microstructures on the creep behavior.
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Schillinger, W., Zhang, D., Dehrn, G. et al. Creep Behavior and Microstructural Stability of Lamellar γ-TiAl (Cr, Mo, Si, B) with Extremely Fine Lamellar Spacing. MRS Online Proceedings Library 646, 125–130 (2000). https://doi.org/10.1557/PROC-646-N1.4.1