Effect of initial microstructure on microstructural instability and creep resistance of XD TiAl alloys
- PDF / 665,544 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 33 Downloads / 295 Views
N
THE creep resistance of TiAl alloys is an essential consideration for high-temperature applications in aerospace and automotive structural components, and these properties strongly depend on microstructure.[1] It is well established that a fully lamellar (FL) structure provides superior creep resistance to a duplex structure or an equiaxed g structure. Decreasing the lamellar spacing also greatly improves the creep resistance of the FL structure, suggesting lamellar refinement is an important way to improve creep resistance.[1–7] In addition, the precipitation of b particles along the lamellar interfaces significantly increases the primary creep resistance, and therefore controlling the initial morphology of these precipitates may decrease the minimum creep rate.[8,9,10] Grain (or colony) boundary morphology may be another important microstructural feature influencing creep resistance. However, very limited work has been carried out to study the effect of this feature on the creep behavior of TiAl alloys, especially those with fine grains, for which the role of grain boundary morphology during creep deformation is even less understood. Nevertheless, lamellar grain boundaries are susceptible to cracking and cavity formation during creep.[11] Interlocked lamellar grain boundaries have been suggested to enhance the resistance to intergranular cracking, leading to increased tertiary creep HANLIANG ZHU, COE Fellow, and K. MARUYAMA, Professor, are with the Graduate School of Environmental Studies, Tohoku University, Aobaku, Sendai 980-8579, Japan. Contact e-mail: [email protected] D.Y. SEO, Assistant Research Officer, and P. AU, Senior Research Officer, are with the Structures and Materials Performance Laboratory, Institute for Aerospace Research, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. Manuscript submitted December 26, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
strength and potentially longer creep life.[12,13] Consequently, it has been suggested that grain boundary morphology may have a relatively larger influence on tertiary creep than primary creep or steady-state creep for FL structures.[2] The stability of the lamellar structure also plays a very important role in the creep resistance of TiAl alloys.[14] During creep deformation, deterioration of the lamellar structure occurs via coarsening[14,15,16] and spheroidization,[4,17,18] decreasing the creep resistance. It has been reported that this kind of microstructural instability can cause a reduction in strength and creep resistance by as much as 20 pct.[19] One of the major contributions to the instability was identified to be the decomposition of the excess a2 phase.[19,20] However, the mechanism by which excess a2 phase promotes microstructural degradation is still unclear. Furthermore, the effect of other microstructural features on instability during creep deformation was not explored in these studies. From the results of earlier studies mentioned previously, it is suggested that the optimal microstructure for the best creep resistance req
Data Loading...
