Titanium aluminides (TiAl) have the potential of substituting nickel-based superalloys (NBSAs) in the aerospace industries owing to their lightweight, good mechanical and oxidation properties. Functional simplicity, control of sintering parameters, exceptional sintering speeds, high reproducibility, consistency and safety are the main benefits of spark plasma sintering (SPS) over conventional methods. Though TiAl exhibit excellent high temperature properties, SPS has been employed to improve on the poor ductility at room temperature. Powder metallurgical processing techniques used to promote the formation of refined, homogeneous and contaminant-free structures, favouring improvements in ductility and other properties are discussed. This article further reviews published work on phase constituents, microstructures, alloy developments and mechanical properties of TiAl alloys produced by SPS. Finally, an overview of challenges in as far as the implementation of TiAl in industries of interest are highlighted.
This research paper summarises the practical relevance of additive manufacturing with particular attention to the latest laser powder bed fusion (L-PBF) technology. L-PBF is a promising processing technique, integrating intelligent and advanced manufacturing systems for aerospace gas turbine components. Some of the added benefits of implementing such technologies compared to traditional processing methods include the freedom to customise high complexity components and rapid prototyping. Titanium aluminide (TiAl) alloys used in harsh environmental settings of turbomachinery, such as low-pressure turbine blades, have gained much interest. TiAl alloys are deemed by researchers as replacement candidates for the heavier Ni-based superalloys due to attractive properties like high strength, creep resistance, excellent resistance to corrosion and wear at elevated temperatures. Several conventional processing technologies such as ingot metallurgy, casting, and solid-state powder sintering can also be utilised to manufacture TiAl alloys employed in high-temperature applications. This chapter focuses on compositional variations, microstructure, and processing of TiAl alloys via L-PBF. Afterward, the hot corrosion aspects of TiAl alloys, including classification, characteristics, mechanisms and preventative measures, are discussed. Oxidation behaviour, kinetics and prevention control measures such as surface and alloy modifications of TiAl alloys at high temperature are assessed. Development trends for improving the hot corrosion and oxidation resistance of TiAl alloys possibly affecting future use of TiAl alloys are identified.
Alumina-silicon carbide (Al2O3–SiC) composites of varying compositions (15, 20, 25 and 30 vol.%)–SiC were produced by the ball milling of Al2O3 and SiC powders, followed by spark plasma sintering. The samples were sintered at a temperature and pressure of 1600 °C and 50 MPa, respectively, thermally etched at 1400 °C and mechanically fractured by hammer impact. The effect of SiC additions to monolithic Al2O3 on the densification response, microstructural and phase evolutions, and fracture morphologies were evaluated. The wear performance of the composites using a ball-on-sample configuration was evaluated and compared to that of monolithic Al2O3. In addition, the corrosion performance of the composites in a 3.5% NaCl solution was examined using open circuit potential and potentiodynamic polarization assessments. SiC additions to monolithic Al2O3 delayed densification due to the powder agglomeration resulting from the powder processing. SiC particles were observed to be located inside Al2O3 grains and some at grain boundaries. Intergranular and transgranular fracture modes were observed on the fractured composite surfaces. The study has shown that the Al2O3–SiC composite is a promising material for improved wear resistance with SiC content increments higher than 15 vol.%. Moreover, the increase in SiC content displayed no improvement in corrosion performance.
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