Elektronenmikroskopische Untersuchung der Mikrostruktur und der lokalen chemischen Zusammensetzung
Final Report Abstract
In this part of the sub-project, the systematic transmission electron microscopic investigation of the Al-rich Ti-Al alloys has been carried out. The microstructure of the alloys, structural and local chemical composition, orientation and interfacial relationships of the phases involved have been studied in the case of Al-rich Ti-Al alloys with and without alloying elements, thus, determining of the influence of selected alloying elements on it. The possibility of the application of Al-rich Ti-Al alloys as light weight, high performance materials at elevated temperatures attracted scientific attention during the past years. While it is possible to produce microstructures in Al-rich Ti-Al alloys, comparable to near-gamma Ti-Al alloys which are nowadays entering technical applications, their mechanical properties remain unsatisfactory. The most important properties for the aforementioned applications are high oxidation resistance, low creep deformation, long fatigue life and good thermal conductivity. During a preliminary study on Al-rich TiAl alloys it was found that duplex or lamellar microstructures of TiAl + r-TiAl2 can be obtained by appropriate heat treatments. However, in as-cast alloys the metastable phases h-TiAl2 and Ti3Al5 are usually observed as precipitates in the TiAl matrix. In-situ heating experiments in a TEM were carried out to analyze the temperature dependent phase transitions in Ti40Al60 and Ti38Al62. In Ti40Al60, metastable Ti3Al5 precipitates have been found at room temperature. These are transformed to TiAl by heating up to 900 °C. Subsequent cooling to 800 °C results in the reappearance of Ti3Al5 accompanied by precipitates of h-TiAl2. Both precipitate from TiAl which becomes supersaturated in Al during cooling. In Ti38Al62, TiAl and h-TiAl2 are preserved during the heat treatment, while Ti3Al5 transforms to TiAl when heated to 900 ° C and precipitates when cooled below 800 °C. The crystallographic interrelations of the phases involved are analyzed using the formalism of group-subgroup relationships. This allows classification of the phase transitions crystallographically and helps in understanding the different transformation kinetics. Generally, the addition of alloying elements like Nb, enhances the room temperature ductility, fracture toughness, elevated temperature strength and oxidation resistance. The refractory elements like Nb improve creep resistance as well as tensile ductility of the Ti-Al alloys. Ti36Al62Nb2 alloy has been studied in this work for example, the as-cast state and after heat-treatment (1000 °C). The effect of the addition of Nb on the microstructure of Al-rich Ti-Al alloys has been examined for Ti36Al62Nb2. Besides the different temperature protocols, similar sample preparation procedures have been employed on both alloys. Since the samples are brittle, great care has been taken to avoid breaking and the production of artifacts by the TEM specimen preparation. Structural and chemical analysis of the samples has been carried out by combining several analytical TEM techniques. In the studies pertaining to the as-cast ternary alloys, the phases h-TiAl2-, Ti3Al5- and gamma-TiAl have been identified, as discussed in the following section. After heat treatment of these as-cast alloys, phase transformations were identified, e.g. the metastable h-TiAl2-type is replaced by the more stable r-TiAl2-type. Besides the phase transformations, changes of the microstructural features were apparent, particularly the formation of interfaces with different orientation relationships. The orientation and interfacial relationships of the phases involved were compared to those of binary Ti-Al alloys rich in Al. In particular, importance under this project was given to the determination of the local sample chemistry in the TEM by both the methods of energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) accompanied by the structural analysis through selected area electron diffraction (SAED) and precession electron diffraction (PED).
Publications
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Characterization of the microstructure of Al-rich TiAl-alloys
by combined TEM imaging techniques; Microscopy and Microanalysis,
Vol. 13. 2007, Suppl. 03, pp. 294-295.
K. Kelm, S. Irsen, M. Paninski, A. Drevermann, G.J. Schmitz, M. Palm, F. Stein, M. Heilmaier, N. Engberding, H. Saage, D. Sturm
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TEM studies of the Ternary Ti36Al62Nb2 alloys; Materials Science and Engineering 2010, 24. - 26. August 2010, Darmstadt, B1-593.
V. S. K. Chakravadhanula, K. Kelm, D. Sturm, M. Heilmaier, J. Aguilar, G. J. Schmitz, A. Drevermann, V. Duppel, A. Lotnyk, L. Kienle, F. Stein, M. Palm
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TEM studies of the ternary Ti36 Al62 Nb2 alloy
Mater. Res. Soc. Symp. Proc. Vol. 1295. 2011, mrsf10-1295-n03-09.
Chakravadhanula, V. S. K.; Kelm, K.; Kienle, L.; Duppel, V.; Lotnyk, A.; Sturm, D.; Heilmaier, M.; Schmitz, G. J.; Drevermann, A.; Stein, F.; Palm, M.
