Design And Properties Of Medium/High-entropy Alloys For High-Temperature Applications

The development of high-performance heat-resistant materials is crucial in the upgrading of coal-fired and gas-fired power plants.Coal-fired power plants need a huge amount of heatresistant materials.The upper working temperature of austenitic steels is 650℃,and Ni-based alloys are the only candidate materials for above 650℃.However,the price of Ni element is high as a scarce strategic material in China.In addition,the working temperature of Ni-based alloys used in the turbine blades of the gas turbine has approached its melting point,which limits the improvement of the thermal efficiency of the gas power station.Recently,L12strengthened medium-entropy alloys have attracted widespread attention due to their excellent strength and toughness,high-temperature thermal stability,oxidation resistance,and creep resistance,which are expected to be a candidate heat-resistant material for thermal power plants;refractory high-entropy alloys with BCC structure have high melting points and excellent hightemperature properties,which are expected to break through the service temperature limit of Ni-based alloys and become the next generation of high-temperature structural materials.Therefore,it is of great practical significance to develop Fe-rich heat-resistant medium-entropy alloys strengthened by L12 phase that can work above 650℃ and refractory high-entropy alloys with high strength and toughness.In this paper,L12-strengthened Fe70-xNi30Gex and Fe53xNi30Ge17Crx medium-entropy alloys,second-phase strengthened Ti3V2NbAlxNiy and solidsolution strengthened HfZrVTaMoWTixNby refractory high-entropy alloys were designed.Then,the phase formation,microstructure,mechanical properties,and oxidation resistance of the alloys were systematically studied.(1)Combining the phase diagrams and the multi-principal design idea,the L12strengthened Fe-Ni-Ge medium-entropy alloys were developed.The local isothermal sections of the Fe-Ni-Ge ternary phase diagram at 1000℃ and 800℃ were established by experiments.The Fe70-xNi30Gex medium-entropy alloys were designed,and the Fe53Ni30Ge17 alloy with high volume fraction of L12 phase was obtained.For the polycrystalline Fe53Ni30Ge17 alloy,its tensile yield strength,ultimate tensile strength,and elongation after fracture were 455 MPa,803 MPa,and 39%at room temperature,respectively,and its compressive yield strength at 600℃,700℃,and 750℃ were 459 MPa,385 MPa,and 285 MPa,respectively,which was higher than that of Super304H,Inconel 617,and Haynes 230.For the single crystal Fe53Ni30Ge17 alloy,the loading direction was along the growth direction<001>.Its compressive yield strength at 600℃,700℃,and 750℃ were 417 MPa,443 MPa,and 363 MPa,respectively,which exhibited the yield strength anomaly.Under the tensile creep condition of 750℃/150 MPa,the creep fracture time of the single crystal sample was 160 hours,and the steady creep rate was 2×10-5 h-1,whose creep resistance was between that of Super304H and Inconel 617.(2)On the basis of the Fe53Ni30Ge17 alloy,the Fe53-xNi30Ge17Crx alloys were designed by adding Cr to improve its oxidation resistance.The results indicated that the addition of Cr decreased the volume fraction of L12 phase,but improved the stability of high-temperature strength and oxidation resistance.With the increase of Cr content,the decrease of the yield strength of the alloys from 600℃ to 700℃ decreased from 16.1%to 1.0%.The weight gain per unit area of the Fe47Ni30Ge17Cr6 alloy after cyclic air oxidation at 800℃/300 h was only 1.75%of that of the Fe53Ni30Ge17 alloy,which is due to the formation of continuous inner oxidation layer of Cr2O3 on the matrix surface.The results provide a basis for the further optimization of L12-strengthened Fe-rich heat-resistant medium-entropy alloys.(3)Based on the atomic size difference(Δr),atomic packing parameter(γ),enthalpy of mixing(ΔHmix),and average valence electron concentration(VEC),the stability criteria of BCC solid solution phase and Laves phase in refractory high-entropy alloys were proposed.It is expressed that Δr<4.8%and γ<1.20 are sufficient but not necessary conditions for the formation of BCC solid solution phase;when Δr>4.8%or γ>1.20,ΔHmix>-2.5 kJ·mol-1 and VEC<4.6 tend to form BCC solid solution phase,while ΔHmix<-2.5 kJ·mol-1 and VEC>4.6 tend to precipitate Laves phase.(4)According to the stability criteria of BCC solid solution phase and Laves phase in refractory high-entropy alloys,the second-phase strengthened Ti3V2NbAlxNiy and solidsolution strengthened HfZrVTaMoWTixNby refractory high-entropy alloys were designed,respectively.In the Ti3V2NbAlxNiy refractory high-entropy alloy,A1 and Ni elements were added to control the phase structure and precipitation morphology of the second phase to improve its strength and toughness.The Ti3V2NbAl0.5Ni0.5 refractory high-entropy alloy was composed of BCC,C14 Laves,and B19’ twinned martensite phases.The density,yield strength,specific yield strength,and compressive malleability of the Ti3V2NbAl0.5Ni0.5 alloy were 5.55 g·cm-3,1250 MPa,223 kPa·m3·kg-1,and 40%at room temperature,respectively.In addition,its yield strength and specific yield strength at 700℃ were 1100 MPa and 197 kPa·m3·kg-1,respectively.In HfZrVTaMoWTixNby refractory high-entropy alloys,the precipitation of Laves phase was inhibited by increasing the content of Ti and Nb elements,thereby improving its malleability.The HfZrVTaMoWTi2Nb2 alloy had a dual-phase BCC solid solution structure,and its yield strength,specific yield strength and compressive malleability were 1700 MPa,175 kPa·m3·kg-1,and 30%at room temperature,respectively.Its yield strength and specific yield strength at 1200℃ were 292 MPa and 30 kPa·m3·kg-1,respectively.