Microstructure Evolution And Energetic Structural Properties Of NbZrTiTa High-entropy Alloy

High-entropy alloy?HEA?composed of four or more metallic elements forms simple solid solution with unordered atom occupation.The unique structure of HEA results in excellent properties.The refractory high-entropy alloy?RHEA?is a promising energetic structural material?ESM?due to its high strength and high energy density.However,the poor plasticity and low energy release ration of RHEA restrict its application.To resolve these problems,the metastability engineering strategy was exploited in this work.The transformation induced plasticity was introduced in NbZrTiTa HEA.The formation of metastable single-phase solid solution?SSS?,and the evolution of microstructure,mechanical and energy-releasing properties at different loading rates were investigated.Some achievements were obtained.The microstructure of NbZrTiTa alloy is effectively controled by changing the arc-extinguishing current during arc melting.The time of element diffusion and rearrangement time is reduced by increasing the cooling rate and solidification supercooling resulting from Increased arc-extinguishing current.On the one hand,the high-temperature structure could be remained at room temperature.The alloy forms a SSS structure with uniform element distribution and uniform structure.on the other hand,the alloy with a large number of dislocations is in a thermodynamic metastable state.Under static loading,the TiZr-rich region and TaNb-rich region forms through elements diffusion induced by dislocation pipe diffusion effect and strong separation driving force between the components.The thermodynamic stability and solid solubility of various regions are also changed.As a result,the TRIP effect,interfacial strengthening and precipitation strengthening were triggered by structure transformation and formation of supersaturated solid solution.The static fracture stress and strain reaches 1361 MPa and 18.5%,respectively.The plasticity is higher than all the reported values of high strength RHEAs.Under dynamic loading,the uniform deformation and grain refinement were promoted through TRIP effect induced by structure transformation.The two together lead to the increases of strain rate threshold when adiabatic shear band generate,interface strengthening and fine grain strengthening.Finally,the as-cast Nb ZrTiTa high-entropy alloy exhibits both high strength and high plasticity at high strain rates,and its fracture strain at strain rates of 6500 s^-1 is greater than 103%.The dynamic high plasticity and high strain rate threshold of adiabatic shear band,which avoids premature fracture during high-speed penetration and accumulates a large amount of strain energy,result in high fracture rate and high energy release.At the penetration speed of 1200 m/s,the quasi-static overpressure is0.18MPa in a 27L closed target box,which can effectively kill people.The addition of Al aggravates the incompatibility between components.There is spinodal decomposition in as-cast NbZrTiTaAl_x?x=0.1-0.4?alloy,which reduces the plasticity.However,the NbZrTiTaAl_x alloy did not exhibit higher energy release efficiency due to the decrease in the strain rate threshold of adiabatic shear band and the formation of the oxygen barrier alumina layer.