Investigation Of The Diffusion Behaviour And Mechanical Properties In The Partial Systems Zr-Nb-Ta-Hf

Refractory high entropy alloys are attracting attention for high melting points,high strength,outstanding corrosion resistance,wear resistance properties,etc.,especially at high temperatures to show excellent performance,which are expected to break the working temperature limits of traditional high-temperature alloys applied to special areas.The current focus of research on refractory high entropy alloys are mainly on comprehensive mechanical properties and applications,while the study of fundamental properties have been neglected.Diffusion properties as one of the fundamental properties of materials,the study of diffusion behaviour between alloy elements are of great importance for the design,preparation and service of materials.Due to the paucity of parameters such as diffusion coefficients for Zr-Nb-Ta-Hf systems,there is a serious lack of research on the diffusion properties of ternary systems in particular.Therefore,it is imperative to establish a corresponding diffusion kinetic database for refractory high-entropy alloys in the Zr-Nb-Ta-Hf system.In the present work,Zr-Nb,Zr-Ta,Zr-Hf,Zr-Nb-Ta and Zr-Nb-Hf systems in the Zr-Nb-Ta-Hf system were investigated.And their diffusion behavior at high temperatures were studied using the diffusion couple technique.The composition and mechanical properties of the diffusion layer in the diffusion couple were tested and analyzed by means of Electronic Probe Micro Analyzer（EPMA）micro-area composition analysis technology and nano-indentation microscope tester.The main findings of the research are as follows:（1）For the Zr-Nb,Zr-Ta,Zr-Hf binary diffusion systems:using EPMA quantitative analysis to obtain the composition-distance curve of the diffusion layer.The interdiffusion coefficients and impurity diffusion coefficients of the three systems were extracted by combining the Sauer-Freise and Hall methods,and the corresponding parameters such as diffusion activation energy and preexponential factor were calculated.The results show that:increasing the content of Nb,Ta and Hf decrease the interdiffusion coefficients of each of the three systems;the content of Nb has the greatest influence on the interdiffusion coefficients;diffusion rate in Zr alloy were Hf>Nb>Ta;maximum activation energy for the Zr-Ta system.The results of nanoindentation test show that Young’s modulus and hardness of the Zr-Nb alloy decrease as the Nb content increases;while the variation in Ta content seems to have little effect on the overall trend of Young’s modulus and hardness of alloys;increasing Hf content causes hardness and Young’s modulus in the Zr-Hf alloy to increase slowly.（2）For the Zr-Nb-Ta ternary system:the diffusion region of the ternary diffusion couple was tested by EPMA and nanoindentation.Interdiffusion coefficients and impurity diffusion coefficients of the Zr-Nb-Ta ternary system after annealing at 1523 K and 1423 K were extracted by using the Whittle-Green and Hall methods.The results show that:the main interdiffusion coefficients D^Zr_{Nb Nb} andD_Ta^Zr _Ta differ from the cross diffusion coefficientsD^Zr_{Nb Ta} andD_Ta^Zr _Nbby 1 to 4 orders of magnitude,and the main interdiffusion coefficientD^Zr_{Nb Nb} is greater than D_Ta^Zr _Ta.This indicates that the elements Nb and Ta interact less in the diffusion process in the Zr-Nb-Ta alloy and that the diffusion rate of Nb is greater than the diffusion rate of Ta.The main interdiffusion coefficients decrease with the increase of Nb and Ta elements,and the impurity diffusion coefficients D*_{Ta（Zr-Nb）}and D*_{Nb（Zr-Ta）}decrease with the increasing elemental composition of Nb and Ta,respectively.The results of the nanoindentation tests show that the elemental concentration has a strong influence on the mechanical properties of the Zr-Nb-Ta alloy.Increasing the content of Nb can play a role in reducing Young’s modulus and hardness of the alloy,while the content of Ta has the opposite effect.（3）For the Zr-Nb-Hf ternary system:the diffusion region of the ternary diffusion couple was tested by EPMA and nanoindentation.The interdiffusion coefficients and impurity diffusion coefficients at 1523 K were extracted by the Whittle-Green and Hall methods.It is known that the main interdiffusion coefficients D_Hf^Zr _Hf are greater thanD^Zr_{Nb Nb} and the main interdiffusion coefficients are more than 1 order of magnitude larger than the cross diffusion coefficients.This indicates that the diffusion rate in the Zr-Nb-Hf alloy is greater for the Hf element than for the Nb element,and that the Nb and Hf elements interact less when diffusing in the alloy.Increasing elemental composition of Nb and Hf reduces the diffusion rate in the diffusion of the alloy and the magnitude of the change in the interdiffusion coefficients are influenced by the elemental content of Hf in the alloy.The impurity diffusion coefficients D*_{Nb（Zr-Hf）}increases with increasing Hf content and D*_{Hf（Zr-Nb）}decreases with increasing Nb content.It is shown by nanoindentation tests that the variation of Young’s modulus and hardness in the Zr-Nb-Hf system are strongly influenced by the Nb element and are inversely proportional to the Nb element composition.The results of this paper provide important empirical data to improve the diffusion kinetic database and mechanical properties of the Zr-Nb-Ta-Hf system,and also provide relevant data references for the design and preparation of related alloys.