Structure Models of Massively Transformed High Niobium Containing TiAl Alloys
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0980-II05-01
Structure Models of Massively Transformed High Niobium Containing TiAl Alloys Christina Scheu1, Limei Cha1, Saso Sturm2, Harald F. Chladil1, Paul H. Mayrhofer1, Helmut Clemens1, Walter Wolf3, and Raimund Podloucky4,5 1 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, FranzJosef Strasse 18, Leoben, 8700, Austria 2 Department for Nanostructured Materials, Jozef Stefan Institute, Ljubljana, Slovenia 3 MaterialsDesign s.a.r.l, Le Mans, France 4 Department of Physical Chemistry, University of Vienna, Vienna, Austria 5 Center for Computational Materials Science, Vienna, Austria
ABSTRACT Ab-initio calculations using the Vienna ab-initio simulation package (VASP) were performed for a high Nb bearing γ-TiAl based alloy with a composition of Ti-47at.%Al-9at.%Nb in order to evaluate the effect of Nb on the crystal structure. The calculations revealed that the c/a ratio of the tetragonal γ-TiAl cell is reduced to ~1, compared to the binary phase, if Nb atoms occupy Ti- and Al sites. In contrast, the c/a ratio is increased, in comparison to the pure γ-TiAl, if the Nb atoms occupy solely Ti sites and if Ti antisite defects (i.e. Ti atoms on the Al sublattice) are formed. The relaxed structure models were used to perform high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image simulations. The results showed that the positions of the Nb atoms should be detectable by these high spatial resolution methods, although it might be easier by HAADF-STEM investigations due to the stronger dependence of the signal on the atomic number Z. INTRODUCTION In recent years novel γ-TiAl based alloys have been developed which contain a high amount of Nb ranging from 5 – 10at.% [1,2]. The benefits of these alloys are excellent mechanical properties such as high tensile strength at elevated temperatures, good ductility at room temperature and a good creep resistance when compared to low Nb bearing alloys [1,2]. The high Nb content reduces the diffusivity [1,3], thus facilitating massive transformation during cooling from the α-phase field [4,5]. This ability for massive transformation and the resulting properties have generated great interest in these alloys resulting in a large number of research studies, see for example [6-8]. Recently, we have studied experimentally the microstructure of a massively transformed Ti-46at.%Al-9at.%Nb alloy by scanning electron microscopy (SEM) and X-ray diffraction (XRD) [9,10]. The XRD spectra have shown that the (200)γ- and (002)γ-reflections for this massively transformed sample can not be resolved separately, as found for the equilibrium 9at.% Nb containing γ-TiAl phase, but instead form a radial broad reflex. This is a consequence of a c/a-ratio ~1 which might stem from a partially wrong site occupation of the constituting atoms in the massively transformed γm-TiAl lattice [9,10]. For the equilibrium phases present in Nb containing γ-TiAl alloys it had been found experime
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