High-entropy alloy (HEA) is a multi-component alloy constituting five or more principal elements in equi- or near equi-atomic percentages. The high configurational entropy in a HEA composition, in contrast to conventional alloys, leads to the stabilisation of the alloying elements in stable solid solutions of face-centred-cubic (FCC), body-centred-cubic (BCC) and/or amorphous structures. The characteristic properties of HEAs are mainly governed by lattice distortion, sluggish diffusion and entropy- and cocktail-effects. Refractory high-entropy alloys (RHEAs), consisting of refractory elements, are considered as a paradigm shift in developing materials for high temperature applications.

The current PhD project investigates four different aspects of RHEAs. First, it involves developing CuMoTaWV RHEA by spark plasma sintering (SPS) and utilising the cocktail effect of HEAs for high temperature tribological application. The use of the cocktail effect, defined as selecting favourable compositions for particular applications, is utilised for RHEA compositions in order to yield adaptive tribological behaviour at changing temperatures or environments. The sintered CuMoTaWV showed formation of BCC solid solution and a composite microstructure. The high temperature tribological investigations showed an adaptive behaviour at different temperatures. At lower temperatures Cu lowered the wear rate through formation of CuO, and at higher temperatures V enhanced the tribological resistance through formation of lubricating V₂O₅ phases.

The second aspect involves studying the effect of lattice distortion on mechanical properties of magnetron sputtered thin film after adding Cu to the refractory elements of Mo, Ta, W and V. A target of CuMoTaWV was developed through partial sintering and used to deposit thin film on different substrates. The deposited film showed formation of BCC solid solution, which was verified through DFT calculations. The lattice distortion in CuMoTaWV film showed high hardness and nano-pillar compressive strength. Furthermore, the tribological properties were enhanced at temperatures up to 400^oC due to the addition of Cu.

The third aspect involves studying the effect of configurational entropy on the formation and high temperature stability of refractory high-entropy thin film metallic glass and its nitrides, by increasing the number of principal elements. A partially sintered target of TiVZrNbMoHfTaW was used to deposit thin films of metallic glass and nitrides through magnetron sputtering. The metallic glass thin films and its nitrides were found to have high hardness of 7.3 GPa and 19–43 GPa, respectively. Furthermore, the metallic glass thin films showed a high nano-pillar compressive strength of up to 3 GPa, almost twice as high as conventional metallic glass films. The phase stability of metallic glass and its nitride thin films were found to be stable at temperatures up to 750^oC and 950^oC, respectively. The exceptionally superior mechanical properties and high temperature stability has been attributed to the presence of high configurational entropy.

The last part of this PhD thesis consists of studying high-entropy-based W-rich alloys for high temperature applications. A W-based alloy of composition W_0.5(TaTiVCr)_0.5 was consolidated using SPS. The resulting alloy revealed a BCC solid solution structure. The microstructure of W-rich alloys consist of a combination of W-rich, high-entropy and TiC phases. The BCC solid solution structure in W-rich alloys was found to be stable with exceptionally high compressional strength up to 1,400^oC. A high compressive yield strength of 1136 ± 40 MPa, 830 ± 60 MPa and 425 ± 15 MPa was found at test temperatures of 1,000^oC, 1,200^oC and 1,400^oC, respectively. The resulting high strength has been related to the formation of high-entropy phases, which in return induces sluggish diffusion at higher temperatures. The high temperature tribology at 400^oC showed an average COF and low wear rate of 0.5 and 1.37 x 10^-5 mm³/Nm, respectively. The high temperature wear resistance at 400^oC was enhanced due to the presence of HEA and TiC phases.

The studies carried out in this thesis suggest the possibility of utilising the full potential of the cocktail effect, lattice distortion and configurational entropy in designing new high-entropy compositions for applications requiring adaptive tribological behaviour, superior mechanical properties and high temperature phase stability.