Microstructure And Mechanical Properties Of HfZrTiTa_x High-Entropy Alloys Energetic Structural Materials

As for Energetic structural materials(ESMs),highly efficient damage can be obtained through the combination of mechanical damage derived from the conversion of kinetic energy and extra damage from the release of chemical energy.However,the existing ESMs are not sufficiently strong and ductile to be directly applied in high strength structural appliance since they are unable to satisfy the complex requirements from external loadings.Recently,high-entropy alloys(HEAs)are a new hotspot and frontier in the field of materials.Due to its excellent mechanical properties,high specific weight,good chemical activity and componential flexibility and adaptability,refractory HEA is one kind of potential penetrating and highly damaging materials.In this study,novel HfZrTiTax(x=0.16,0.33,0.53,0.75,1.00 and 1.29)HEAs were designed and fabricated through vacuum arc melting(named shortly as Ta0.16,Ta0.33,Ta0.53,Ta0.75,Ta1.00 and Ta1.29 respectively).The effect of Ta content on the microstructure of HfZrTiTax alloy were studied through XRD,optical microscopy,SEM and TEM.The mechanical properties and deformation behavior of the alloy at low and high strain rates were studied through quasi-static mechanical experiments and Split-Hopkinson pressure bar respectively.The energetic characteristics upon highspeed impact were studied through ballistic experiments.Especially,the study of dynamic mechanical properties and ballistic experiments is of great significance for the development of highly efficient ESMs and new type od armor-piercing materials.Ta0.16 and Ta0.33 are composed of dual-phase microstructure with hexagonal closepacked(HCP)and body-centered cubic(BCC)while only one BCC phase is observed in Ta0.53,Ta0.75,Ta1.00 and Ta1.29.Ta0.16 has irregular basket-like morphology composed of lamellar phase with entangled dislocations and lamellar phase with low density dislocations while Ta0.33 forms equiaxed grains with lamellar precipitation near the grain boundary.Ta0.53,Ta0.75,Ta1.00 and Ta1.29 all form equiaxed grains and have modulated structure composed of nano-lamellars which is typical morphology of spinodal decomposition.With the increase of Ta content,the wavelength of spinodal decomposition decreases and the difference of element composition between two phases increases.The thermal stability of the alloys are poor;after heat treatment,the crystal structure of Ta0.16 and Ta0.33 maintains while that of Ta0.53,Ta0.75,Ta1.00 and Ta1.29 transforms from single BCC phase to dual-phase with HCP and BCC structure respectively.With the Ta content increases,BCC phase fraction increases,exceeds the HCP phase and eventually dominates.Under quasi-static loading,Ta0.16 and Ta0.53 have a good combination of high strength and sufficient ductility.The compressive strength is 1394 MPa and 1314 MPa while the facture strain is 10.6% and 14.1% respectively.High strength of Ta0.16 is ascribed to solid solution strengthening derived from severe lattice distortion and hindrance to dislocations from lamellar phase with high density dislocations while that of Ta0.53 is derived from solid solution strengthening and spinodal decomposition hardening.Sufficient ductility of Ta0.16 is resulted from coordinating deformation from lamellar phases while that of Ta0.53 is ascribed to abundant sub-grain boundaries.As for Ta0.75,Ta1.00 and Ta1.29,due to spinodal decomposition hardening,the higher for Ta content,the higher for yield strength and that of Ta1.29 reaches 1835 MPa.Under dynamic loading,Ta0.16,Ta0.53 and Ta0.75 have a good combination of high strength and sufficient ductility,and the dynamic compressive strength of which are 1723 MPa,1658MPa and 2021 MPa respectively.The facture strain is 12.4%?12.4% and 8.8% respectively.The combination of high strength and ductility of Ta0.75 is superior to that of most Ti-based alloys,bulk metallic glasses(BMGs),HEAs and insitu metallic glass matrix composites(MGMCs).Due to low thermal conductivity of the constituent elements and severe lattice distortions,the thermal conductivity of the alloy is low,which is not conducive to the release of heat during dynamic deformation.Thus it is prone to thermoplastic instability and high adiabatic shear sensitivity in the alloy.The degree of fragmentation and the number of adiabatic shear bands in the matrix of brittle Ta0.33,Ta1.00 and Ta1.29 are both higher than those of ductile Ta0.16,Ta0.53 and Ta0.75,suggesting the formers' higher adiabatic shear sensitivity.As the second phase near the grain boundary in brittle alloys leads to stress concentration,adiabatic shear bands form and the alloys crack along the grain boundaries.Also,the adiabatic shear bands are frequently bifurcated and interwoven.As for ductile Ta0.16,Ta0.53 and Ta0.75,there are a major adiabatic shear band connecting crack forming and expanding in the matrix.They adsorb plastic deformation work through irregular lamellar structures,sub-grain boundary deformation,grain fragmentation and the formation of slip bands respectively.Upon high-speed impact,due to high theoretical combustion energy,the alloy react violently with air and releases a large amount of energy.The worse the plasticity and the higher adiabatic shear sensibility,the more thorough the alloy cracks and the more energy releases.With excellent properties including high strength,good plasticity,low adiabatic shear sensitivity and energetic characteristics,ductile Ta0.16,Ta0.53 and Ta0.75 are able to maintain structural stability before impacting on targets and have great potential as shells of high-strength structures,which significantly enhance the damage of weapons.Due to high adiabatic shear sensitivity,high strength,high density and energetic characteristics,it is conducive to produce self-sharpening effect and enhance the ability of armor piercing of Ta1.00 and Ta1.29,which further leads to highly efficient damage.