Microstructure And Mechanical Properties Of HfNbTiZrW_x Refractory High-Entropy Alloys

The progress of human society is inseparable from the development of materials,and the development of new alloy materials is on the way.In recent years,researchers have proposed a new design idea of alloys(multi-principal mixing)——High-Entropy Alloys(HEAs).A series of refractory high-entropy alloys(RHEAs)have been developed under this design concept.The advantage of refractory high-entropy alloys is their excellent high-temperature strength,but poor room temperature plasticity and high density have restricted their further development.HfNbTiZr alloy has excellent room temperature tensile plasticity,which makes up for the poor plasticity of refractory high-entropy alloys,but it has the problem of insufficient strength.In order to improve the comprehensive mechanical properties of RHEAs,this paper proposes a new series of alloys——HfNbTiZr Wx.The main content is to study the influence of W element on the phase structure and mechanical properties of HfNbTiZr alloy.And on the basis of HfNbTiZr Wx alloys,it was studied respectively that the influence of Al,Ta,Mo,Cr and V elements on the phase structure and mechanical properties of the alloy.It was confirmed that HfNbTiZr Wx alloys(x = 0.25,0.5,0.75 and 1)are single-phase BCC solid solution structure by two ways: the thermodynamic parameters values of HfNbTiZr Wx alloys and XRD phase analysis.The actual lattice constants calculated by the experimental data were same with theoretical calculation results of lattice constant.The microstructure of HfNbTiZr Wx alloys showed clear dendrite morphology,and the addition of W element has an obvious refinement effect on the alloy structure.It would lead to the segregation and enrichment of Zr and W elements with more addition of W.The composition analysis found that the contents of Zr and Nb in the grains were relatively high,while the content of Hf element near the grain boundaries was much higher than the average composition.The tensile mechanical properties results of alloys showed that the addition of W element improved the strength of the alloy greatly.The yield strength of HfNbTiZr W0.25 alloy was increased by 312 MPa compared with HfNbTiZr alloy,and the fracture strain was also increased by 1.69%,which has the best comprehensive mechanical properties.However,with continued increase of W,the plasticity of alloys deteriorated significantly.Both the HfNbTiZr W0.75 and HfNbTiZr W alloys exhibited brittle failure when fractured.The HfNbTiZr W0.25 and HfNbTiZr W0.5 alloys did not break during the compression test.In addition,the compressive yield strength of alloys increased with the increase of W,which approximately had a linear correlation.The comprehensive mechanical propertie of HfNbTiZr W0.25 alloy were the best,so we added Al element in HfNbTiZr W0.25 alloy to study the influence of Al.The X-ray diffraction pattern showed that its main structure was still BCC phase after adding Al element to the alloy.However,The SEM morphology showed that the alloy was not a single-phase structure,and there were a large number of precipitates on the BCC matrix.These precipitates were needle-shaped in the grains or ingrown dendrites on the grain boundary.The composition analysis results showed that the precipitated phase was(Hf,Ti)-rich phase,while the matrix was(Nb,Zr)-rich phase.The addition of Al element further improved the compressive strength of alloys,and its yield strength increased continuously with the increase of Al content.But the plasticity of the alloy deteriorated sharply.The reason may be that the needle-shaped and dendritic second phases precipitated inside the alloy caused greater stress concentration and thus reduce the plasticity of the alloy.On the basis of the above-mentioned optimal alloy,we calculated the thermodynamic parameter values of the alloys after adding Ta,Mo,Cr or V and predicted the formation phase respectively.The calculation results showed that the alloy still tended to form a disordered solid solution structure after adding a certain amount of Ta,Mo or V respectively,while the alloy was more likely to form a multi-phase structure of BCC & second phase after adding Cr.This theoretical calculation provided guidance for further study.