Refractory High Entropy Alloys and Films for High Temperature Applications

Abstract: 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 HEA, in contrast to conventional alloys, leads to stabilization of the alloy to form stable solid solutions of face-centered-cubic (FCC), body-centered-cubic (BCC) and/or amorphous structures. The characteristic properties of HEAs are mainly governed by lattice distortion, sluggish diffusion, entropy- and cocktail- effects. Refractory high entropy alloys (RHEAs), consisting of refractory elements, are considered as a paradigm shift in developing future potential materials for high temperature applications.This project is based on studying four different aspects of RHEAs. The first work involves developing CuMoTaWV RHEA by spark plasma sintering (SPS) that can utilize the cocktail effect of HEAs towards high temperature tribological application. The use of cocktail effect, defined as selecting favorable compositions for particular application, can be utilized towards RHEA compositions to give adaptive tribological behavior at changing temperature or environment. The sintered CuMoTaWV showed formation of BCC solid solution and a composite microstructure. The high temperature tribological investigation showed an adaptive behavior at different temperature. 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 work involved studying the effect of lattice distortion on magnetron sputtered thin film after adding Cu to refractory elements of Mo, Ta, W and V. A target of CuMoTaWV was developed through partial sintering and deposited on different substrates. The resulting film showed formation of BCC solid solution, which was verified with DFT calculations. The lattice distortion in CuMoTaWV film resulted in showing high hardness and nano-pillar compressional strength. Furthermore, the tribological properties were enhanced up to 400 oC due to the addition of Cu.The third work involved studying the effect of configurational entropy on formation- and high temperature stability- of refractory high entropy thin film metallic glasses 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 by magnetron sputtering. The metallic glass thin films and its nitrides were found to have high hardness of 11 GPa and 21-48 GPa, respectively. Furthermore, the metallic glass thin films showed a high nano-pillar compressional strength of up to 2.7 to 5 GPa, almost twice the time of conventional metallic glass films. The phase stability of metallic glass- and its nitride- thin films were found to be stable 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 work consisted of studying high entropy based W-rich alloy for high temperature applications. A W-based alloy of composition W0.5(TaTiVCr)0.5 was consolidated using SPS. The resulting alloy revealed BCC solid solution structure. The microstructure of W-rich alloy consisted a combination of W-rich-, high entropy- and TiC- phases. The BCC solid solution structure in W-rich alloy was found to be stable with exceptionally high compressional strength up to 1400 oC. A high compressional yield strength of 1136 ± 40 MPa, 830 ± 60 MPa and 425 ± 15 MPa was found at test temperatures of 1000 oC, 1200 oC and 1400 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 resistant at 400 oC was enhanced due to the presence of HEA- and TiC- phase.The studies carried out in this work suggests the possibility of utilizing the full potential of cocktail effect, lattice distortion and configurational entropy in designing new high entropy compositions for applications requiring adaptive tribological behavior, superior mechanical properties and high temperature phase stability.