Research On The Design And Performance Of NbMoTaW-Based High-Entropy Alloys

With the development of aerospace technology,more and more stringent requirements have been put forward for ultra-high temperature structural materials.At present,the metal materials widely used in high temperature are mainly nickel-based alloy,whose temperature is limited below 1150 ?.NbTaMoW refractory high entropy alloy(RHEA),as a novel super-alloy,is composed of high melting point elements such as Mo,Nb,Ta,W.It has the compressive strength of 1211 MPa and 405 MPa at room temperature(RT)and 1600?,and can maintain a stable BCC phase at 1400?.NbMoTaW alloy broke through the serving temperature limit of existing high temperature nickel-base alloy,and is a new type high temperature structural material.However,the RT's plasticity of NbMoTaW alloy(only 2.6%)is very poor,showing brittle fracture behavior,which severely restricts the further development and engineering application.In this work,to improve the RT's plasticity,hardness and strength of NbMoTaW alloy,first-principles calculations conbinated with experiments were employed to invstigate the mechanical performance by means of alloying and composition optimization.First principles calculations were used to calculate and predict the phase structure,electronic structure and mechanical properties,and the effects of alloying elements and composition optimization on the mechanical properties of NbMoTaW alloy were studied.XRD and SEM-EDS were used to characterize the phase structure and microstructure.The hardness and mechanical properties were tested.These results were mutually verified with that of first-principles calculations.Firstly,to enhance RT's plasticity and strength of NbMoTaW alloy,the method of alloying was adopted.In the premise of keeping high melting point,different refractory elements of Ti,V,Cr,Zr and Hf,Re with high melting point were choosed to improve mechanical performance of NbMoTaW alloy.It was found that Ti,V,Zr and Re elements can significantly enhance RT's strength and plasticity of NbMoTaW alloy.Ti-containing and Zr-containing RHEAs increased the strength and ductility up to 2000 MPa and 16%,respectively.First-principles calculations show the strengthening effect was derived from the decrease of covalent bonds in alloy system and the formation of part Ti-Ti and Zr-Zr metallic bonding.However,Cr and Hf have little effect on strength and toughness.Secondly,the effects of brittle Mo and W elements and their contents on the mechanical properties of NbMoTaW alloy were studied by first-principles calculations combinated experiments.It was found that the strength and toughness of NbMoTaW alloy can be improved by decreasing the content of Mo element,but the efect cannot be achieved by changing the content of W element.The strength and plasticity of NbTaW alloy improved to 1460 MPa and 16.2%,respectively.Lastly,to improve the RT's hardness of NbMoTaW alloy,the influences of adding different contents of transition elements such as Fe and Ni on the RHEAs' performance were explored.It was found that the addition of Fe and Ni can significantly improve the hardness of NbMoTaW alloy.With the augment of Fe and Ni content,the hardness of Fe-and Ni-containing RHEAs increased to 1316 HV.