Ti-Al based Structural Intermetallics-Corrosion

Based on this systematic study of Ti-AI-M ternaries comprising solubility limits, ternary compound formation, structural chemistry of ternary compounds and their phase relations particularly with the duplex alloy system alpha2 Ti 3 AI+gammaTiAI, it is proposed to investigate the corrosion behaviour of these ternary alloys with respect to hydrogen, nitrogen and oxygen incorporation. This research will cover the systems Ti-M-AI, where M is a transition metal from the 5th to the 8th group in the Periodic Chart. The results of this research are designed to provide basic knowledge on the interaction of hydrogen, nitrogen, oxygen with the phase combination of technical interest: alpha2 Ti 3 AI+gammaTiAI+psi in the temperature region from RT to 1000c and for 1 atm N(O) or up to 50 atm H, respectively. Ab-initio methods will be applied to calculate atomistic and thermodynamical parameters particularly to calculate site preferences of ternary additions M in yTiAI and alpha2 Ti 3 AI and influence of interstitials (H,N,O). Calculations are planned of energetics, atomic geometries and electronic structure of yTiAI /alpha 2 Ti 3 AI interfaces without and with ternary additions M and influence of interstitials X. Derivation of suitable parameters for simplified electronic structure models (such as embedded atom method) for treatment of complex systems and their physical quantities.

The results of the project obtained can be grouped in four major categories: (ii) determination of phase equilibria in the ternary systems (system Ti-Nb-Al; Ti-Re-Al, Ti-Ru-Al and partial phase relations in the systems Ti-Co-Al, Ti-Ir-Al, Ti-Ni-Al, Ti-Pt-Al close to the duplex alloy region a 2 Ti 3 Al+TiAl), (iii) details of atom distribution in the crystal structures of the phases in equilibrium with the duplex alloy a 2 Ti 3 Al+TiAl (G-phases, Laves phases, Cu3 Au-type phases), (iv) details on the corrosion behaviour (air corrosion of Ru-alloyed TiAl, hydrogen take up in G-phases and nitrogen corrosion in Ti-Mn alloys) (v) set up of ab-initio calculations for Ti-based Laves phases and alloys and calculation of the a 2 Ti 3 Al+TiAl interface interactions (determination of the heat of formation of Laves phases (Ti, Zr, Hf)Mn 2 as well as for a series of Laves phases. Calculation of ground state properties of Ti-aluminides and of the interface TiAl-Ti 3 Al). These results provide a thorough basis for metallurgical engineers in the design of TiAl based high temperature structural alloys relying on proper phase relations and solid solubility limits. Secondly for thermodynamic calculation of phase equilibria and consequently of solidification paths in the ternary systems investigated the detailed determination of atom site distribution in the ternary compounds and solid solution phases is of essential importance enabling the calculator to set up the proper thermodynamic sublattice models. Details on the corrosion behaviour will help to select proper alloy combinations for high air corrosion resistance. The investigations of hydrogen uptake in the so-called G-phases (no uptake under an H 2 pressure up to 5 MPa) provide data, which may encourage the use of these phases in age-hardening techniques for higher mechanical stability of the Ti-Al structural materials Ab-initio calculations for Ti-based Laves phases and alloys in comparison with experimental data demonstrated the accuracy of the calculations, which in future may replace tedious and time-consuming calorimetric experiments. Besides the calculations equally well supply necessary data on mechanical properties. For the first time a modelling of the electronic interactions in the a 2 Ti 3 Al+TiAl interface has been attempted.