Study On The Structure And Properties Of High-entropy Alloys Under Extreme Conditions

High-entropy alloys(HEAs)are newly developed alloys with lots of outstanding properties in recent years,which have broken through the design concept of traditional alloys.The structure and stability of alloy have crucial influence on its properties.Therefore,it is necessary and significant to study the structures,the stability of structure,and the plastic deformation mechanism of the different HEAs under different conditions.In this dissertation,through in-situ high-pressure synchrotron x-ray technologies,the stability of different HEAs under high pressures was investigated.The HEAs of Al0.1CoCrFeNi wih fcc structure and Al3 CoCrFeNi with bcc structure show good stability even under 30 GPa.For the HoDyYGdTb HEA with hcp structure,the route of phase transition process is hcp?Sm-type?dhcp?dfcc,and the phase transition pressures are the average pressures of several elements.Two equiatomic CoCrFe CuNi and CoCrFeMnNi HEAs were prepared by mechanical alloying and high pressure sintering(MA-HPS).The bulk alloys both exhibited simple fcc solid solution structure with a grain size of about 100 nm.The structure of HEAs maintained stable up to 31 GPa,and the bulk modulus were 117.5 GPa and 136.1 GPa for CoCrFeCuNi and CoCrFeMnNi,respectively.The CoCrFeCuNi and CoCrFeMnNi HEAs show high hardness of 494 Hv and 587 Hv,respectively,which are higher than their counterparts in as-cast state.Meanwhile,the CoCrFeCuNi HEA exhibits a high saturation magnetization(Ms)value,whereas the value of the CoCrFeMnNi HEA is very low.This study proofs that MA-HPS can be used as an effective way to design HEA materials with the high hardness,high bulk modulus,and controllable magnetic property.The influence of pressure and temperature on the microstructure of Al0.5CoCrCuFeNi and AlCoCrCuFeNi HEAs were also researched.Through the high pressure torsion(HPT),the deformation of HEAs was investigated.The deformation mechanism of fcc HEA includes dislocation slip and twinning at room temperature.The planar dislocation slip is on the normal fcc slip system,{111}<110>,and twin is {111} planes.Some secondary twins appear when the strain is high enough.The deformation of the HEAs with bcc and hcp structure can be achieved by dislocation.Processing by HPT,the hardness of the HEAs can be evidently improved and even reached saturated level.All the HEAs show high work hardening rate.In order to study the influence of cooling rate on the structure of HEAs,the structure of the spun Al0.5CoCr CuFeNi and AlCoCr CuFe Ni HEAs were investigated.The results showed that high cooling rate could refine the grains effectively,which could change from the coarse dendrite structure to small equiaxed grains.Meanwhile,high cooling rate could effectively restrain the phase separation of Cu element,increase the content of Cu-rich nano particle and decrease its size.The Cu-rich nanotwinned fcc phase dispersed uniformly in the B2 phase matrix in the spun AlCoCuFeNi HEA.The fcc phase and B2 phase keep the K-S crystallographic relationship: {111}fcc ? {110}B2,and <110>fcc ? <111>B2.The nanotwins grow to rodlike grains during annealing,and the growth direction is parallel to the {111}fcc twin planes.The hardness and elastic modulus decrease during annealing for the growth of nanotwins.