Deformation And Damage Mechanism Of CrCoNi High Entropy Alloy System Under High-strain-rate Loading

As the potential structural materials,CrCoNi system high-entropy alloys（HEAs）have great application potentials in aerospace and defense industries,due to their excellent physical and me-chanical properties,a wide range of adjustable structure-performance and perfect preparation processes.However,current investigations about the deformation and damage mechanisms of the CrCoNi system HEAs always concentrate on low or medium strain rate loadings.In order to understand and predict the service performances of CrCoNi system HEAs under extreme condi-tions,it is necessary to study their deformation and damage mechanisms under high shock stress and high stress rate conditions.In this study,based on the gas gun or dynamic equal channel angular pressing（D-ECAP）,systematic high strain rate loading experiments are conducted on different kinds of HEAs in the CrCoNi system,along with the free surface velocity histories and microstructure characterizations of postmortem samples.We explain how microstructure evolu-tions affect the macroscale mechanical behaviors under high strain rate loading.The innovative achievements are listed as follows:1.Dynamic equal channel angular pressing（D-ECAP）experiments are conduced on the HEA Al_0.1Co Cr Fe Ni.Along with the microstructure characterizations,the deformation mecha-nisms under high strain rate shearing are revealed.Severe shear deformation and ultrahigh strain rate give rise to simultaneous activation of the{111}{112}macro deformation twins and kink bands with the{110}or{112}rotation axes.Secondary kink bands are also observed in primary kink bands.It is the first observation on kink bands within a face-centered cubic HEA under var-ious loading conditions.Mechanical properties of D-ECAP-processed+subsequent annealing samples are measured.The yield strength is increased by～155%solely by D-ECAP compared to the as-cast HEA.A reasonable combination of high strengh（yield strength 346 MPa）and good ductility（uniform elongation 33%）is achieved through annealing twins in recrystallized grains via annealing the D-ECAP-processed alloy at 900?C.A structure-based strength model based on the grain boundary strengthening and dislocation strengthening is proposed to explain the strengthening in these D-ECAP-processed samples before and after annealing.2.The accurate Hugoniot equation of state and spall strength of the HEA Al_0.1Co Cr Fe Ni are obtained via plate impact experiments.Along with the microstructure characterizations,the shock deformation and spallation damage mechanisms under high strain rate loading are revealed.Dislocation glide and stacking faults are the important deformation mechanisms in the Al_0.1Co Cr Fe Ni HEA.Nanotwins are only observed at high shock stress.Damage in the Al_0.1Co Cr Fe Ni HEA is ductile in nature.Voids are nucleated preferentially in grain interiors,and the intragranular voids show strong dependence on grain boundary misorientation and peak stress.3.The accurate Hugoniot equation of state and spall strength of a lamellar eutectic HEA AlCoCrFeNi_2.1are obtained via plate impact experiments.Along with the microstructure char-acterizations,the shock deformation and spallation damage mechanisms under high strain rate loading are revealed.Upon shock compression,both the L1₂ and B₂phases retain their ordered structures.Dense dislocations in the{111}slip planes,stacking faults and deformation twin-s are found in the L1₂phase,along with fewer dislocations in the{110}slip bands in the B₂phase.Shock-induced deformation twinning within the L1₂phase of this HEA is observed as a new deformation mechanism under various loading conditions.For spallation,both ductile and brittle damage modes are observed.The phase boundaries are the predominant nucleation sites for micro voids and cracks,followed by the B₂phase.Under similar shock stress,the spall strength of Al Co Cr Fe Ni_2.1HEA is about 40%higher than those of other reported dual-phase HEAs due to the high stability of its semi-coherent phase boundaries.4.The spall strength of the HEA CrMnFeCoNi processed by different methods（as cast,hot-rolled,hot-rolled+annealing）are obtained via plate impact experiments.Along with the mi-crostructure characterizations,the shock deformation and spallation damage mechanisms under high strain rate loading are revealed.The hot-rolled CrMnFeCoNi has a heterogeneous struc-ture,while the as cast and hot-rolled+annealing CrMnFeCoNi are in a homogeneous structure.Shock-induced deformation twinning is an important deformation mechanisms at high shock stress for as cast and hot-rolled CrMnFeCoNi.Shock-induced deformation twinning is activat-ed firstly in the unrecrystallized stretched（UR）domains.The different initial structures caused by the preparation methods affect the damage nucleation sites.Compared to the homogeneous（annealed）structure,the heterogeneous structure leads to a negligible increase in spall strengths because of grain refinement in the fine recrystallized（FR）domains and relatively severe plastic deformation around the UR-FR domain boundaries.