A good combination ofstrength and ductility for structural materials is always a goal for materialsresearchers’ pursuit. Recently, high entropy alloys (HEAs) with FCC structurehave been studied extensively. However, this type of HEAs usually have goodductility with lower strength, limiting their applications as structuralmaterials. The refractory elements, normally having a larger difference inatomic size from those of the constituent elements of FCC-type HEAs, areexpected to result in solid solution strengthening and/or second phasestrengthening when added to the parent alloy. In this thesis, refractoryelements (Hf/W) were chosen to be added into FCC-structured HEAs, and the microstructureevolutions and mechanical properties were studied in detail with varyingcontents of Hf/W.

CoCrFeNi HEA withsingle-FCC-phase has been widely studied recently. In Chapter 2, resultsrelating to Hf addition into CoCrFeNi HEA was reported. The results showed thatwith increasing Hf contents, the structure of CoCrFeNiHf_x HEAschanged from FCC single phase to Ni₇Hf₂ + FCC, thento Ni₇Hf₂+ C15 Laves + FCC, and finally to C15 Laves +FCC phases when x values changed from 0 to 0.4. Hypoeutectic icrostructureswere observed in CoCrFeNiHf_x (x = 0.1-0.3) alloys. Fully eutecticstructure with lamellas of FCC phase and C15 Laves phase formed in CoCrFeNiHf_0.4 HEA. The Vickers hardness and compressive yield strength ofthe alloys increased while the plastic strain reduced with increasing Hfcontents, which were resulted from solid solution strengthening and secondphase strengthening. Lateral fracture surfaces indicated that the primary FCCphase in CoCrFeNiHf_0.2 could effectively hinder the propagationof micro-cracks to prevent catastrophic failure during compression.

Chapter 3 reported theeffects of minor amount of W (1 at% and 3 at.%) alloying into CoCrFeNi HEA.After cold-rolling and 900℃-1h annealing, fine μ phase precipitated in(CoCrFeNi)₉₇W₃ HEA. No obviousprecipitates were found in (CoCrFeNi)₉₉W₁ alloy. The yield strength of (CoCrFeNi)₉₇W₃ alloy can beas high as ~529 MPa, which wasalmost the same as or even better than those for CoCrFeNiW_x (x = 0.2 and 0.4) HEAs with higher W contents. Meanwhile, theductility for (CoCrFeNi)₉₇W₃ was ~50%, whichwas far superior to that of as-cast CoCrFeNiW_0.4 HEA after annealing (~10.5%). The finer precipitates formedin (CoCrFeNi)₉₇W₃ was helpful forstrengthening this alloy. The strengthening mechanisms in (CoCrFeNi)₉₇W₃ included solid solution strengthening,precipitates strengthening as well as grain refinement strengthening.

The microstructure andmechanical properties of (CoCuFeNi)_100-xW_x (x = 0, 5, 15, x value in at.%) HEAs, which has much higherW contents than the CoCrFeNiW_x studied in Chapter3, were also investigated and reported in Chapter 4. The results revealed that5 at.% W can completely dissolve into CoCuFeNi forming single face-centeredcubic solid solution phase. In-situ W particles formed in CoCuFeNi matrix with15 at.% W addition with semi-coherent interface between W particles andCoCuFeNi matrix. The yield strength of (CoCuFeNi)₈₅W₁₅ was increased while still maintaining goodductility compared with CoCuFeNi. The strengthening mechanisms for (CoCuFeNi)₈₅W₁₅ included solid solution strengthening,grain refinement strengthening as well as second phase strengthening. The goodductility of (CoCuFeNi)₈₅W₁₅ was resulted from grain size refinement and the plasticity of Wparticles and CoCuFeNi.

Lastly, the volumefraction of W particles in CoCuFeNi matrix was enhanced to increase thestrength further and for studying the microstructure and mechanical propertiesof W-CoCuFeNi alloy system. The W-CoCuFeNi alloys with higher volume fractionof W particles were prepared by liquid phase sintering method due to the highermelting point of W. The microstructure, grain growth behavior and mechanicalproperties of W-CoCuFeNi alloy system sintered at 1440 ^oC, 1500 ^oC and 1560 ^oC by infiltration wereinvestigated in detail. The XRD and SEM results showed that the tungsten-heavyalloys (WHAs) consisted of rounded W grains distributed in CoCuFeNi highentropy alloy (HEA). In addition, M₆W₆C (M= Co, Fe)-type η carbides appeared in the interfacial regionsof W grains and CoCuFeNi HEA. The W grain size and W-W contiguity were found toincrease with increasing sintering temperatures. The activation energy for Wgrain coarsening in CoCuFeNi binder matrix was about 367 kJ/mol, which was farhigher than that of traditional W-Ni-Fe and W-Ni-Co WHAs. This suggested that Wgrain coarsening rate in CoCuFeNi HEA was slower than that intraditional binderalloys. For compressive mechanical properties, the yield strength of WHAsintered at 1440 ^oC reached the highest due to the smallest Wgrains. The compressive ductility of W-CoCuFeNi sintered at 1560 ^oC was the worst due to increased W-W contiguity. The bending testsof W-CoCuFeNi WHAs showed that the WHA sintered at 1440 ^oC alsoexhibited the highest bending strength. From the fracture morphologies afterbending tests, W-W intergranular fracture was the mainfailure mode for WHAssintered at 1500 ^oC and 1560 ^oC due to increased W-W contiguity.

The results of this thesis can provide a better understanding about themicrostructure development and its effects on the mechanical properties of HEAswhen Hf/W was added. The approaches on alloy design and processing used in thisthesis may provide options and alternatives on development of HEA as structuralmaterials to expand the potential applications of HEAs.