Microstructure And Properties Of HfZrTiTa(B/Si) High-Entropy Alloys

This study aims to develop a new alloy with high strength,good toughness and oxidation resistance,since the development of modern weapons and equipments requires the high-temperature materials combining higher strength,higher-temperature resistance,good machinability and oxidation resistance.The unique structure and thermal features of equal-molar high-entropy alloy?HEA?bring about their excellent comprehensive properties.The high-entropy alloys consisting of equal-molar refractory elements are expected to have both favorable high temperature performance and high ductility.The oxidation resistance could be further promoted by alloying those oxidation resistant elements.A quaternary HfZrTiTa alloy was designed and prepared based on the concept of high-entropy alloy.HfZrTiTaBx?x=2,4,6 at.%?and HfZrTiTaSiy?y=1,2,3,4,6 at.%?were also prepared with boron and silicon addition,respectively.A systematic study of microstructures,thermal stabilities,mechanical properties and oxidation behaviors of these aloys is conducted by TEM,SEM,XRD,DSC and so on.The HfZrTiTa alloy forms a single phase solid solution with BCC structure because these elements satisfy the principle of nature similarity.While a nano-sized Spinodal-decomposition texture was formed due to the influence of the interaction energy of these elements.High-temperature heat treatment can promote the homogenization of element distribution.The highest decomposition temperature of this structure is about 1000?.The structure becomes coarse after annealing at mid-temperature below 1000?,but becomes continuous solid solution at elevated temperature.Boron or silicon additions doesn't affect the BCC structure of the alloy.Both of them couldn't be the principle elements in this HfZrTiTa alloy.Parts of boron atoms dissolve in the matrix as interstitial atoms resulting in a severe lattice distortion,while the rest react to form the inter-dendrite secondary phases.The silicon forms silicides in the inter-dendrite region.The HfZrTiTa alloy has a melting temperature of 1873±2?,a density of10.166g/cm³and hardness of 47.15HRC.Its flexural stress and strain are 2.4 GPa and4.2%,respectively;the maximum compression stress and strain are 1.46GPa and28.91%,respectively.The alloy has a compression stress higher than those of most refractory HEAs,with a good ductility.The addition of boron leads to a significant decrease in the flexural properties,but an increase in the compression stress with a small sacrifice in the strain.After all,the HfZrTiTaBx alloys still had compression strains higher than 20%.The effect of silicon on the mechanical properties coincided with that of boron,while the compression strains of the HfZrTiTaSiy alloys were lower than 20%.The linear oxidation rate constant of the HfZrTiTa alloy at 1300?is18.96mg·cm^-2·h^-1.The oxide layer consisting of multiple oxides?HfO₂,ZrO₂,TiO₂,Ti₂O₃and Ta₂O₅?could protect matrix from being oxidized.Boron addition could reduce the oxidation rate of the alloy by 21%.At beginning of oxidation,the harmful porous morphology in HfZrTiTa alloy was not observed in HfZrTiTaBx alloy,while once the needle-like phases in HfZrTiTaBx fractured with increasing oxidation time,the oxide layer was broken,resulting a decrease in the oxidation resistance.The oxidation resistance of the HfZrTiTaBx alloys decreases with the increasing content of boron.Silicon addition could reduce the oxidation rate of the alloy by only 15.6%,while the oxidation rate decreased slowly with the extension of oxida tion time,probably due to the formation of compact oxides on the aloy surface.