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Intermetallic gamma titanium aluminides display attractive engineering properties at high temperatures of up to 750 °C. To date, they have been used in low-pressure turbine blades and turbocharger rotors in advanced aircraft and automotive engines. This review summarizes the fundamental information of the Ti–Al system. After providing the development of TiAl alloys, typical phases, microstructures and their characteristics in TiAl alloys, the paper focuses on the effects of alloying elements on the phase boundary shifting, stabilizing effects and strengthening mechanism. The relationships between chemical additions, microstructure evolution and mechanical properties of the alloy are discussed. In parallel, the processing technologies and the common heat treatment methods are described in detail, both of which are applied to optimize the mechanical properties via adjusting microstructures. On this basis, the effects from chemical composition, processing technologies and heat treatments on microstructure, which controls the mechanical properties, can be obtained. It has a certain guiding significance for tailoring the microstructures to gain desired mechanical properties.

The wheel flat can cause train and rail system infrastructure damage and endanger the running safety. To monitor the early wheel flat, it is urgent to carry out the theoretical basic research on the relationship between the vibration signal and the wheel flat. Moreover, to extract the characteristics of the wheel flat, an advanced and effective signal processing method need to be studied. A three-dimensional vehicle-track coupled dynamics model verified by field test is established based on the multi-body dynamics at first. The acceleration of the axlebox excited by the different wheel flat length is obtained by the dynamic simulation. The simulation considers the influence of various speeds and the short-wavelength track irregularities. Then, a combined method based on the variational modal decomposition (VMD) and the envelope spectrum (ES) is employed to detect the wheel flat signal. The feasibility of the method is further validated by comparing the co-existence of the wheel flat and the wheel eccentricity. Finally, field test is carried out to detect the wheel flat by using this method. The results indicate that the VMD-ES method accurately extracts the impact characteristics of the wheel flat and can quantitatively identify the wheel flat faults of small sizes.

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