Preparation And Properties Of NbMoTaWSi_x Refractory High Entropy Alloys

The throat lining components of rocket engines are not only subjected to oxidative erosion by high temperature and high velocity gases during service,but also to strong mechanical shock and thermal stress.Therefore,the development of throat lining materials that can serve beyond2000°C has become a research challenge.The NbMoTaW refractory high-entropy alloys（RHEAs）investigates exhibit outstanding ductility at room temperature and excellent high-temperature strength,but their applicability is severely constrained by low oxidation resistance at high temperatures.Based on this problem,NbMoTaW RHEAs was investigated to improve the mechanical properties,high temperature oxidation resistance and high temperature ablation resistance by microalloying Si in induction levitation melting process.The results are as follows:（1）NbMoTaWSi_x RHEAs were prepared by vacuum induction suspension melting method,and the effect of micro-alloying Si on the organization of NbMoTaW RHEAs and their mechanical properties were investigated.It was shows that NbMoTaW RHEAs demonstrated a single-phase BCC structure.With the addition of Si elements,the formation of the interdendritic（Nb,Ta）₅Si₃ phase was induced.The RHEAs matrix was enhanced by the formed improved the strength and hardness of the alloy.With the volume fraction of silicide increases,local deformation becomes disorganized and decreased ductility,which resulting in of alloys their eventual fracture.The alloy exhibited typical brittle cleavage fracture as the fracture surface reveals the river-like cleavage surface of the brittle fracture of the silicide phase and the smooth region of plastic fracture of the substrate phase.The addition of Si increased the strong hardness and oxidation resistance of the alloy,thus improving the wear resistance of RHEAs.The wear mechanisms of NbMoTaWSi_x（x=0,0.25,0.5）RHEAs were adhesive wear and oxidation wear.（2）The effect of different Si contents on the high temperature oxidation resistance of NbMoTaW RHEAs was analyzed using constant temperature oxidation as well as high temperature short time oxidation experiments.The results showed that the oxidation weight gain of NbMoTaWSi_x（x=0,0.25,0.5）RHEAs at 1000°C decreased with increasing Si content,i.e.,from 47.91 mg/cm² to 32.32 mg/cm².at the same time,the oxidation rate constant（K_p）of the alloy is reduced,which in turn slows down the oxidation of the alloy.The NbMoTaW alloy’s oxide layer has a low density and experiences increased internal stress because of the volatilization of W and Mo element oxides at high temperatures as well as the high PBR values of WO₃ and MoO₃.As a result,the oxide layer has obvious fractures and pores,which lowers the alloy’s resistance to oxidation at high temperatures.The Si addition led to the formation of SiO₂oxide film,which filled the pores and cracks in the oxidation reaction and formed a denser protective oxide film on its surface.The thickness of the oxide layer of the alloy decreased significantly with the increase of Si content,and the oxide layer bonded well with the substrate.This shows that Si addition lowers the alloy’s oxidation rate,increasing the alloy’s high temperature oxidation resistance.（3）The effect of micro-alloying Si on the high-temperature ablation resistance of NbMoTaWSi_x（x=0,0.25,0.5）RHEA was investigated by conducting high-temperature ablation tests on this alloy.The results showed that the ablation layer of NbMoTaWSi_x RHEAs could be roughly divided into the ablation center region,transition region,edge region,and ablation outer region after ablation at a heat flow density of 4 MW/m² for 240 s,and a gradient microstructure was formed.The oxide melt has strong fluidity because of the erosion of ultra-high temperature and high heat flux density,and a thick oxide layer was created in the ablation edge zone.The comparison revealed that Si0.25 has the strongest ablation resistance by scrutinizing the linear ablation rate and mass ablation rate of the alloy.Oxygen ions preferentially access the grain boundary during high-temperature ablation,leading to grain boundary oxidation,because Si0.5alloy has a larger interdendritic volume percentage.Larger grain boundary zones in alloys lead to more severe oxidation.Therefore,in ablation environments with high heat flux,Si0.5 alloy exhibits the most severe ablation,which is different from the short-term high-temperature oxidation mechanism of the alloy.