Executive Summary : | High-entropy alloys (HEAs) are presently of great research interest in materials science and engineering. Unlike conventional alloys, which contain one and rarely two base elements, HEAs comprise multiple principal elements, with the possible number of HEA compositions extending considerably more than conventional alloys. The concept of high entropy introduces a new path of developing advanced materials with unique properties, which cannot be achieved by the conventional micro-alloying approach based on only one dominant element. Up to date, many HEAs with promising properties have been reported, e.g., high wear-resistant HEAs, Co1.5CrFeNi1.5Ti and Al0.2Co1.5 CrFeNi1.5Ti alloys; high strength bodycentered-cubic (BCC) AlCoCrFeNi HEAs at room temperature, and NbMoTaV HEA at elevated temperatures. Furthermore, the general corrosion resistance of the Cu0.5NiAlCoCrFeSi HEA is much better than that of the conventional 304- stainless steel. In addition, HEAs have excellent specific strength, superior mechanical performance at high temperatures, exceptional ductility and fracture toughness at cryogenic temperatures, superparamagnetism, and superconductivity. Due to their considerable structural and functional potential as well as richness of design, HEAs are promising candidates for coating applications, which warrants further studies. Thermal-spray (TS) technology to fabricate coatings of the NixCo0.6Fe0.2-CrySizAlTi0.2 HEAs was developed. These sprayed particles are accumulated on the substrate by cooling and building up one by one into a cohesive structure. Thus, coatings are formed. The results also indicate that the hardness of the HEAs prepared by the TS in combination with annealing at 1100°C/10 h is significantly increased to that of the as-cast state (1045 HV). These samples exhibited excellent coarsening resistance, resulting from the Cr3Si and several unidentified phases. NixCo0.6Fe0.2CrySizAlTi0.2 alloy system does precipitate during casting, which is quite different from many other HEAs.
The literature survey confirmed that phase formation and final microstructure of HEAs are strongly dependant on the processing conditions. A low-cost HEA coating with a nominal composition of 6FeNiCoCrAlTiSi was prepared by laser cladding. This technique normally results in a metallurgical bond that has the superior bond strength over TS. The resultant coating is dense with no voids or porosity. One of the advantages of the lasercladding process is the laser beam which can be focused and concentrated to a very small area and keeps the heat affected zone of the substrate very hallow. This feature minimizes the chance of cracking, distorting, or changing the metallurgy of the substrate. Additionally, the lower total heat minimizes the dilution of the coating with materials from the substrate. The coating prepared by laser cladding has a simple BCC solid solution with high micro-hardness, high resistance to softening, and large electrical resistivity. After being annealed at T < 750°C, the coating shows high thermal stability, and its resistivity slightly decreases, but the micro-hardness almost remains unchanged. After annealing at T > 750°C, the microhardness of the coating slowly decreases with increasing the decomposition rate of the BCC solid solutions. A suitable high entropy alloy system and coating process need to be developed for the rocket engines to make the HEAs coating more uniform and with high cohesion with substrates. |