Microstructure And Properties Of ZrNbTaHf-based Refractory High Entropy Alloys

The refractory high-entropy alloys（RHEAs）composed of high-melting-points elements from groups IVB,VB,and VIB,exhibit excellent properties such as high softening points,high strength,outstanding corrosion resistance,wear resistance,and high-temperature oxidation resistance.These alloys have the potential to overcome the limitations of existing high-temperature alloys and are expected to become important structural materials for high-temperature applications in the future.RHEAs hold significant theoretical research value and have a broad range of industrial application prospects.They can be used as materials for components in hypersonic engines,radiation-resistant nuclear components,and other high-temperature applications in key industrial fields such as aerospace and nuclear reactors.However,the poor room-temperature ductility of RHEAs,primarily based on the body-centered cubic（BCC）phase,has limited their industrial processing and practical application.Additionally,there is a lack of systematic research on the correlation between alloy design,microstructure control,heat treatment processes,and mechanical properties.Given the aforementioned challenges,this work selects ZrNbTaHf RHEAs as the research subject.Utilizing a research method involving phase composition regulation,thermodynamic parameter regulation,heat treatment process regulation,combined with the high entropy alloy phase formation criteria and phase diagram calculation,through the alloy composition and heat treatment process of the integrated control strategy,to achieve the series of alloys microstructure,phase structure and mechanical properties of the modulation and get the corresponding mechanism of action.The main results of this paper are as follows:Firstly,the hexagonal close-packed（HCP）phase in biphasic ZrNbTaHf_xRHEAs is modulated by the content of the Hf element,and the mechanism and influence law of HCP phase composition on the optimization of mechanical properties in biphasic RHEAs are obtained.Based on thermodynamic phase composition criteria and phase diagram calculations,the effects of Hf elemental composition changes and high-temperature（1773 K）heat treatment on the structure and properties of ZrNbTaHf_x RHEAs are systematically investigated through the exploration of modulation in the BCC-HCP dual-phase structure.It was found that high-temperature heat treatment transforms ZrNbTaHf_x alloys from a dendritic polarized structure in the as-cast state to a BCC+HCP dual-phase structure with homogeneous composition.The as-cast alloy exhibits high plastic deformation but low strength,whereas the annealed BCC grains with uniform composition enhance the strength of the alloy while reducing its plasticity.The fracture surface of the annealed alloy displays the typical brittle morphology of fracture along the crystal,yet the HCP phase precipitated at the grain boundary generates numerous slip bands during alloy deformation,thereby maintaining a certain degree of plasticity in the annealed alloy.Secondly,Based on the results of biphasic modulation of ZrNbTaHf_xalloys,the Laves phase is introduced into the alloys by adding Cr.This progression-from exploring biphasic modulation to systematically studying multi-phase modulation of the alloys through composition and heat treatment-aims to achieve RHEAs with diverse structures and properties.It is observed that as the introduction of the Laves phase increases,the alloy’s strength substantially rises,albeit at the cost of decreased plasticity.This results in brittle fracture with a notably large fracture energy,with instances where the alloy appears to locally melt during the fracture process.High-temperature heat treatment induces the decomposition of the HCP phase in the ZrNbTaHf_0.2Cr_xalloy,forming both the BCC phase and the Laves phase,in line with the predictions from phase diagram calculations.This process further enhances the hardness and strength of the alloy.However,the alloy with the highest strength in the annealed state,ZrNbTaHf_0.2Cr_1.0,also exhibits the poorest plasticity,with compression deformation amounting to less than 10%.Thirdly,Based on the alloy design idea of changing the thermodynamic basic parameters by modulating the alloying elements to influence the alloy phase composition and organization state and thus realize the control of mechanical properties,the effects of the BCC-stabilizing element Mo on the difference in atomic radius,electronegativity,organization and properties of the alloys have been systematically investigated.The mechanism of the influence of Mo element content and heat treatment on ZrNbTaHf_0.2 alloy was explored by combining thermodynamic evidence,phase diagram calculation and heat treatment experiments.It is found that the addition of Mo element promotes the amplitude modulation decomposition of the BCC phase of the alloy,forming the BCC#1 phase dominated by Ta,Nb and Mo and the BCC#2 phase dominated by Zr and Hf,while a small amount of the HCP phase precipitates due to the phase transformation of the short-range atomic clusters of Zr Hf.The uphill diffusion of the constituent elements occurs after heat treatment of the alloy,the phase separation is more significant,and the distribution of the elements tends to be more concentrated.The smaller atomic radius of the Mo element causes an increase in the lattice distortion of the alloy,which improves the local orientation difference of the crystals,and results in the improvement of the strength and hardness of the alloy compared to that of the ZrNbTaHf_0.2 alloy.The difference in atomic radius,electronegativity and melting point of the elements determines the phase decomposition and elemental distribution tendency of RHEAs in this system.Fourth,to address the lack of research on systematic heat treatment process exploration in RHEAs,different heat treatment parameters（time and temperature）were utilized to study the evolution of microstructure and the influence law of mechanical properties of ZrNbTaHf alloys with V addition,and the modulation mechanism of the mechanical properties of the alloys by the V content and heat treatment process was obtained.It is found that the microstructure and elemental distribution of the alloy are related to the temperature and time of heat treatment,and a higher temperature provides a stronger diffusion driving force for the alloying elements,which affects the diffusion rate of the alloying elements,and at the same time,the combination with the phase diagram calculation confirms the temperature-induced phase transition of the HCP substable phase in the refractory high-entropy alloy of this system.As for the selection for heat treatment parameters of the alloy,the importance of heat treatment time on the phase transformation of the alloy was confirmed.With the extension of the annealing time of the ZrNbTaHf_0.2V_0.75alloy at 1673 K,the structural transformation sequence of the alloy was as follows:BCC+a small amount of dispersed HCP→single-phase BCC solid solution→BCC+grain boundary continuous HCP phase.The ZrNbTaHf_0.2V_0.75alloy with single-phase BCC solid solution structure possesses excellent properties of hardness of 622 Hv,compressive strength of 1950 MPa,and compressive deformation of 21.5%.The refractory high entropy alloys with different mechanical properties at room temperature were prepared by means of dual-phase and multiphase regulation,basic thermodynamic parameter adjustment and heat treatment process optimization.This study offers both path references and experimental support for optimizing performance and advancing the development of BCC refractory high-entropy alloys.