Abstract
Challenging issues concerning energy efficiency and environmental politics require novel approaches to materials design. A recent example with regard to structural materials is the emergence of lightweight intermetallic TiAl alloys. Their excellent high-temperature mechanical properties, low density and high stiffness constitute a profile perfectly suitable for their application as advanced aero-engine turbine blades or as turbocharger turbine wheels in next-generation automotive engines. As the properties of TiAl alloys during processing as well as during service are dependent on the phases occurring, detailed knowledge of their volume fractions and distribution within the microstructure is of paramount importance. Furthermore, the behavior of the individual phases during hot deformation and subsequent heat treatments is of interest to define reliable and cost-effective industrial production processes. In situ high-energy X-ray diffraction methods allow tracing the evolution of phase fractions over a large temperature range. Neutron diffraction unveils information on order-disorder transformations in TiAl alloys. Small-angle scattering experiments offer insights into the materials’ precipitation behavior. This review attempts to shine a light on selected in situ diffraction and scattering techniques and the ways in which they promoted the development of an advanced engineering TiAl alloy.
Highlights
Jet engines are the prevalent form of propulsion systems in commercial airplanes.Increasing economic and environmental pressure, has triggered an ever-rising awarenessMetals 2016, 6, 10; doi:10.3390/met6010010 www.mdpi.com/journal/metalsMetals 2016, 6, 10 of the issues associated with the consumption of fossil fuels
Intermetallic titanium aluminides based on the ordered γ-TiAl phase provide a set of properties well suited for structural high-temperature applications
In situ diffraction and scattering techniques based on synchrotron radiation and neutrons offer unique opportunities for the development of advanced structural materials
Summary
Jet engines are the prevalent form of propulsion systems in commercial airplanes. Intermetallic titanium aluminides based on the ordered γ-TiAl phase provide a set of properties well suited for structural high-temperature applications. The topics addressed range from fundamental research questions, such as the establishment of phase diagrams or the investigation of transformation and precipitation kinetics, to processing- or application-related problems, such as the hot deformation behavior of TNM alloys or the adjustment of optimized microstructures during heat treatments (Figure 2). If different elements possess different scattering lengths, the structure factor for the formerly forbidden reflections attains a finite value This case defines so-called superstructure reflections, which can be observed in the diffraction pattern. Superstructure reflections exhibit low intensities, whereas fundamental reflections appear strong Since these statements are valid for all prevalent phases, neutron and X-ray diffraction are most complementary techniques if applied to intermetallic TiAl alloys. While neutron diffraction offers distinct advantages for the investigation of order/disorder transitions, X-ray diffraction can be employed to study phase transformations and phase fraction evolutions, in which the fundamental reflections play a major role
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