Study On Composition Design And Mechanical Property Of WTaNbHf System Refractory High-entropy Alloys

With the development of science and technology,the demand for high-emperature structural materials with good room temperature plasticity and excellent high-temperature strength is becoming more and more urgent in many fields such as aerospace,nuclear power and petrochemical industry.High-temperature alloys represented by Ni-based superalloys and traditional single-principal-element superalloys based on refractory metal elements no longer meet the service requirements in the temperature range above 1300°C.Therefore,there is an urgent need to develop new high-temperature structural materials.Refractory high entropy alloys（RHEAs）are based on a unique design concept of multi-component alloys,which greatly expands the design space.Due to unique four core effects,high entropy alloys show great potential in high-temperature field.Based on the above,this paper focuses on the study of RHEAs with good room temperature plasticity and excellent high-temperature strength,and explain their strengthening and toughening mechanisms at different temperatures to provide critical materials that can serve in extreme high-temperature environments.In this paper,WTaNb Hf series RHEAs were prepared by vacuum arc melting.By adjusting the content of Hf and introducing non-metallic C elements in the alloys,(WTaNb100-x)Hf_x and[（WTaNb）₅₅Hf₄₅]_100-xC_x alloys with good room temperature plasticity and excellent high-temperature strength were developed by reasonably regulating the composition and distribution of alloy phases.A variety of characterization tools were used to analyse the phase,microstructure,and mechanical properties of alloys.We systematically study the structural evolution and their strengthening and toughening mechanisms of alloys.The main results are shown below:（1）With the increase of Hf content,the（WTaNb）_100-xHf_x（x=0,15,30,45,60）alloys gradually generate nano lamellar HCP phase on the initial single disordered BCC matrix phase,and the HCP phase shows coherent relationship with the matrix BCC phase.The（WTaNb）₅₅Hf₄₅ alloy has the best mechanical properties at room temperature,with yield strength,compressive strength and fracture strain of 2228 MPa,3135 MPa and 25.6%respectively,an increase of 74.7%,107.3%and 91.04%compared to the matrix WTaNb alloy,which breaks the dilemma that strength-plasticity has trade-off in conventional alloy The improvement of strength is caused by the significant lattice distortion and second-phase strengthening.The plasticity enhancement is mainly attributed to the combined effect of the coherent relationship of BCC matrix phase and HCP reinforced phase,the nano lamellar morphology of HCP and the introduction of Hf elements that significantly reduce the bulk modulus of the alloy.（2）Introducing C element into the（WTaNb）₅₅Hf₄₅ alloy,the crystal structure of[（WTaNb）₅₅Hf₄₅]_100-xC_x（x=0,5,10,15）alloys changed from BCC+HCP to BCC+FCC.With the increase of C content,the FCC ceramic phase gradually changed from nano lammellar to granular.The room temperature yield strength,compressive strength and fracture strain of[（WTaNb）₅₅Hf₄₅]₉₅C₅ alloy are 2351 MPa,3276 MPa and 28.0%,with a slightly increasing compared to（WTaNb）₅₅Hf₄₅.This is mainly attributed to the preservation of the coherent phase relationship between the nano lamellar structure and matrix phase.Furthermore,the strengthening effect of the FCC phase is higher than HCP phase.Excessive introduction of C causes only granular FCC ceramic phases in the alloy,which cause stress concentration around them and lead to failure,resulting in an increase in strength but a sharp decrease in plasticity.（3）About high-temperature deformation behaviour,both the nano lamellar and the granular ceramic FCC phases can significantly increase the high-temperature strength of the alloys,while the HCP phase deteriorates the high-temperature mechanical properties of the alloys.At 1473 K,the yield strengths of[（WTaNb）₅₅Hf₄₅]₉₅C₅ and[（WTaNb）₅₅Hf₄₅]₈₅C₁₅ alloys are 588 MPa and 738 MP,respectively.[（WTaNb）₅₅Hf₄₅]₉₅C₅ alloy exhibites a typical three-stage characteristic curve,while[（WTaNb）₅₅Hf₄₅]₈₅C₁₅ alloy exhibits brittle fracture.It can be seen that its ultimate serving temperature can be higher.At 1873 K,the yield strengths of[（WTaNb）₅₅Hf₄₅]₉₅C₅ and[（WTaNb）₅₅Hf₄₅]₈₅C₁₅ alloys are 150 MPa and 141 MPa,respectively.Both alloys exhibite typical three-stage characteristic curve.In addition,[（WTaNb）₅₅Hf₄₅]₉₅C₅and[（WTaNb）₅₅Hf₄₅]₈₅C₁₅ alloy both have dynamic recrystallization and phase transformation which changes from BCC matrix phase to W-rich FCC phase.