Thermal Stability,Phase Structure And Mechanical Properties Of Low-density Refractory High-entropy Alloys

There is a great demand for metal alloys with excellent high-temperature strength and low density in aerospace and some related fields,while the common Ni-based superalloys are exposed to temperatures approaching their operating limit,and more new high-temperature resistant metal materials need to be developed to meet the relevant applications.Traditional alloys are mainly based on one or two main elements,and high-entropy alloys,as a class of materials that have attracted great attention in recent years,have broken the traditional alloy design strategy.The concept of high-entropy alloy refers to a new method to design materials with three or more metal elements according to equal atomic ratio or near equal atomic ratio,and a large number of new alloys with unique properties have been developed through this strategy.Therefore,it can make a beneficial attempt that the application of high-entropy alloy strategy to the design and preparation of high-temperature alloys.High temperature stability is very vital for the long-term service of the alloy under high temperature.At the same time,some refractory high-entropy alloys have been developed with high melting point but poor plasticity at room temperature.In our study,the Hf_0.5Nb_0.5Ta_0.5Ti_1.5Zr alloy with good room temperature plasticity is selected and gained the starting state by suction cast,homogenized,cold rolled and recrystallized,and then anneal the alloys at 500-900?for 14 days to understand the thermal stability of such RHEA.The results show that the alloy remains its initial single-phase BCC structure above 800?;NbTa-rich precipitates are observed mainly on the GBs and in some intragranular regions at700?,composition analysis suggests such kind of precipitates might be a quinary variant of the binary phase;more complex microstructure consisting of three phases was distinguished after the anneal at 500^oC:an HCP phase enriched in Ti,a BCC1 phase in Hf and Zr,and another BCC2 phase in Nb and Ta,respectively.Generally,a lamellar structure composed of BCC1 and BCC2 phase exists on the GBs;while a convoluted basket-like precipitate structure composed of HCP,BCC1 and BCC2 phases exists in intragranular areas.In light of the density of such RHEA could be lower,therefore,Al is introduced in this study to replace Hf and Ta to reduce the density value of the alloy.At the same time,the atomic radius size of Al is close to that of Ti and Nb,which makes it easier to form single-phase solid solution;and the Zr which is easy to form intermetallic compound is replaced by V with similar density.The AlNb_0.5Ti_1.5V alloy is designed according to the high-entropy alloy solid solution formation criterion,and AlNb_xTiV?x=1.5;2?are obtained by adjusting the atomic ratio of its constituent elements.After test and analysis,the results show that AlNb_0.5Ti_1.5V and AlNb_xTiV?x=1.5;2?are both single-phase BCC solid solutions,and the density of AlNb_0.5Ti_1.5V alloy is about 4940 kg/m³,which is one of the RHEA with the lowest density known at present;The strength and plasticity of AlNb₂TiV alloy at room temperature are obviously better than that of AlNbTiV alloy with equal atomic ratio,meanwhile,both AlNb_1.5TiV and AlNb₂TiV outperform significantly the mechanical behavior of Cr-Nb-Ti-V-Zr system,AlNbTiVZr,HfNbTaTiZr in the high temperatures,and the AlNb_1.5TiV has the highest compress yield strength of 766 MPa at 800?.Above research results show that for those refractory high-entropy alloys considered as single phase at high temperature,the phase decomposition transformation may occur under appropriate annealing conditions,so more work need to do in matters of their high-temperature stability.At the same time,design and develop alloys with the high-entropy alloy solid solution formation criterion in turn to verify the accuracy and feasibility itself,and also apply new strategy to the design of novel low-density refractory high-entropy alloys.By adjusting the proportion of alloying elements,the alloys with excellent performance of plasticity and strength were obtained,which provided a reference for seeking new high-temperature alloys with potential application prospect.