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Lee Hsun Lecture Series

Topic: Advanced Engineering Intermetallic Titanium Aluminides

Speaker: Prof. Helmut Clemens

Department of Materials Science，Chair of Physical Metallurgy and Metallic Materials，

Montanuniversitaet Leoben, A-8700 Leoben, Austria

Time: 10:00-11:30, (Mon.) May.6th, 2019

Venue: Room 468，Lee Hsun Building, IMR CAS

Abstract:

After almost four decades of intensive fundamental research and development activities intermetallic titanium aluminides have found application in automotive and aircraft engines. Present applications are, for example, blades in the low-pressure turbine of advanced aero-engines, turbine wheels for turbocharger systems of car diesel engines as well as engine parts used in racing cars. The advantage of this class of innovative high-temperature materials is their low density in combination with good strength and creep properties up to 800°C. A drawback, however, is their limited ductility at room temperature, which is reflected in a low plastic fracture strain. Advanced engineering TiAl alloys are complex multi-phase materials which can be processed by ingot or powder metallurgy, precision casting methods, hot-working as well as additive manufacturing, e.g. electron beam melting. Each production process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat-treatments. The aim of these heat-treatments is to provide balanced mechanical properties, i.e. sufficient ductility at room temperature as well as creep strength at elevated temperature. In order to achieve this goal, the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict the phase diagram of engineering TiAl alloys. After experimental verification, these phase diagrams provided the basis for the development of heat-treatments. To account the influence of deformation and kinetic aspects sophisticated ex- and in-situ methods have been employed to investigate the evolution of the microstructure during thermo-mechanical processing and subsequent heat-treatments. For example, in-situ high-energy X-ray diffraction was conducted to study dynamic recovery and recrystallization processes during hot-deformation tests. The obtained results were helpful to establish hot-forging of turbine blades as well as preforms for turbocharger wheels on an industrial scale. In order to study structure and chemical composition of the individual phases, high-resolution techniques such as transmission electron microscopy and atom probe tomography were employed. Summarizing all results a consistent picture regarding processing and mechanical properties of advanced engineering intermetallic TiAl alloys can be given. Finally, future alloy design strategies will be outlined, for example, how creep strength can be improved by tailoring microstructure and chemical composition.