Auswirkungen von Grenzflächen auf die Rissausbreitung in TiAl-Legierungen unterschiedlicher Lamellenbreite bzw. Korngröße
Final Report Abstract
Due to the limited ductility of TiAl-alloys at low temperatures the influence of the microstructure on the damage mechanisms in these alloys is of interest to optimize TiAl-alloys with respect to ductility and damage tolerance. The microstructure of TiAl-alloys was extensively characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) in the past. The Atomic Force Microscope (AFM) which is widely used in this project is seldom employed on TiAl-alloys. Especially the measurement of local mechanical properties of the individual phases in technical TiAl-alloys by nanoindentation in an AFM is a novel and unique approach. Here the mechanical properties of the γ- and α2-phase were measured directly and effects of niobium additions and microstructure on the local hardness were characterized. While SEM was frequently used for the in-situ observation of crack propagation in TiAl-alloys, doing the same in AFM as in this project is new and not reported before for this material class. AFM is suited to image the crack tip and detect traces of plastic deformation at the specimen surface. Nucleation of new cracks was often preceded by the interaction of deformation traces with interfaces and also by strong shear band activity in the γ-TiAl lamellae visible as significant surface topography in the AFM. In a boron and carbon containing alloy (Ti-44.5Al-4.6Nb-0.2B-0.2C) additional sites for crack nucleation were the brittle precipitates at the particle/γ-interfaces where cracks tended to open. While the precipitates were not characterized in this project they are supposed to be either borides or carbides with respect to the alloy composition. The plastic deformation is not equal in both phases. The γ-phase shows strong traces of plastic deformation while the α2-phase is only deformed plastically in the direct crack tip vicinity. Nevertheless, when comparing a niobium free and a niobium containing alloy, it was found that the confinement of the plastic deformation in the γ-phase is more pronounced in the niobium free alloys while in the niobium containing alloy traces of plastic deformation were observable in both phases γ and α2. A novel aspect in this project was the possibility to image also elastic distortions in the crack tip vicinity as elevation or suppression of the surface in the AFM. It was observed by this method that the elastic deformation field in front of the crack tip is strongly altered by the presence of a γ / α2-interface. This is due to a significant confinement of the elastic deformation field inside the γ-phase. This localized elastic deformation field seems to influence the deviation of the crack path from inclined to the lamellar interfaces to a path parallel to the lamellar orientation. These observations of the elastic field are novel as neither SEM nor TEM until now used for in-situ observation of crack propagation in TiAl-alloys can show the elastic deformation field as clearly as possible by AFM. In order to explain the strong confinement of the elastic field inside the γ-phase, the role of internal stresses was considered in both phases. By Finite Element Method (FEM) simulations, it was shown that the internal tensile stresses present in the γ-phase in combination with the compressive stresses present in the α2-phase can lead to the kind of confinement of the elastic deformation field in the γ-phase as observed in this project. The potential of TiAl-alloys for the use in light weight high temperature applications such as turbo charger turbine wheels for cars and low pressure turbine blades for aircraft engines is of prime interest for reducing CO2 emission. But due to the limited ductility of modern TiAl-alloys at low temperature a deeper understanding of the damage mechanisms is necessary for successful and safe operation in these potential applications. Niobium containing TiAl-alloys possess high strength and improved oxidation resistance but it was found in this project that their local damage and deformation behavior is different from other alloys with low or no niobium content. The major future objective of this work is to get a full understanding of the damage mechanisms of niobium containing in comparison with niobium free TiAl-alloys. Due to the growing importance of this alloy type material which often contains the β-phase in the room temperature constitution in future a alloy variant with remaining β-phase should be included in the investigations. TiAl-alloys with additional β-phase are interesting from an application point of view as they can have a better ductility at room temperature and also improved hot workability. The influence of the β-phase on the crack propagation and the interaction of the crack tip with the βphase as well as the resulting elastic and plastic deformation will be investigated. The novel methods namely in-situ tests under AFM, investigation of sub-surface structures with Focused Ion Beam (FIB) and Electron Back Scattered Diffraction (EBSD) characterization of the crack tip region gave valuable insights into the local deformation and damage mechanisms and should be employed in future investigations on crack propagation in TiAl-alloys. Also the link between in-situ tests under AFM, which showed the confinement of the elastic field in the γphase, and the FEM simulations carried out to explain this effect supported the idea that the internal stress state present at the coherent and semi-coherent lamellar interfaces plays a decisive role in the cracking behavior of these alloys. Therefore, the quantitative analysis of internal stresses of the investigated alloys with X-ray and eventually with synchrotron diffraction is an important next step. It could also yield information about the internal stresses to include in future simulations.
Publications
-
International Conference on Strength of Materials, ICSMA 2006, Xian, China, Microstructure, Nanohardness and Crack Propagation in Niobium Containing TiAl Alloys
F. Pyczak. S. Gebhard, M. Göken
-
EUROMAT 2007, Nuremberg, Germany, The Influence of the Mechanical Properties of TiAl Alloys Measured by Nanoindentation
F. Pyczak. S. Gebhard, M. Göken
-
Materials Science & Engineering, 01-04 September 2008, Nuremberg, Germany, In-situ study of crack propagation in TiAl-alloys using atomic force and scanning electron microscopy
F. Iqbal, F. Pyczak, M. Göken
-
TMS Annual Meeting and Exhibition, 9-13 March, 2008, New Orleans, USA, Mechanisms of Crack Propagation in TiAl-Alloys Observed by In- Situ Testing in the Atomic Force Microscope (AFM)
F. Pyczak, F. Iqbal, M. Göken
-
DPG Frühjahrstagung, 22-27 March 2009, Dresden, Germany, In-situ investigation of crack propagation in γ-TiAl alloys using atomic force, focus ion beam and scanning electron microscopy
F. Iqbal, F. Pyczak, M. Göken
-
Microstructural and micromechanical characterisation of TiAl alloys using atomic force microscopy and nanoindentation, Materials Science and Engineering A 523, 235-241 (2009)
S. Gebhard,F. Pyczak, M. Göken
