| Magnesium（Mg）alloys have been widely used in aerospace field and other industries.Components in these applications would inevitably suffer from high-speed impact.Therefore,to study the macro/micro damage behavior of materials under highspeed impact is of great significance to the selection of materials,structural design,and evaluation of the impact resistance performance of materials.Under this background,the macro-and micro-damage characteristics of AZ31 B and AM60 B Mg alloys were systematically studied using computer simulation and experimental analysis methods.Based on these results,the rules and mechanisms of macro-and micro-damage for AZ31 B and AM60 B Mg alloys under high speed impact were analyzed by the microstructure evolution in the vicinity of the crater and the location/time sequence during the impact process,and the corresponding microstructure evolution and damage rules are clarified.These results can provides theoretical guidance and experimental basis for the performance evolution of Mg components.The crater morphology and the evolution of crater parameters as the increase of impact velocities when the GCr15 steel projectile impact the AZ31 B and AM60 B Mg alloy targets were revealed based on the experimental observations and results from the Ansys/Autodyn simulations,and the stress,strain and temperature parameters at come typical moments were obtained.Results show that columnar craters are formed under high-speed impact,the depth of the crater increases significantly with the increase of impact velocities,while the diameter of the crater shows less increment.The diameter of the top of the crater is larger than that of the bottom and this difference increases as the increase of impact velocity.The depth and diameter of crater show a linear relationship with the kinetic energy of projectile.With the distance away from the crater increase,the strain,strain rate and temperature drop sharply first,then slowly decrease;the stress increases first and then decreases,the stress increases first and then decreases.The strain rate and temperature in the three directions increase,while the stress decreases in the order of 0°,45° and 90°.The strain in the direction of 45° is the largest,and the smallest in the 0 ° direction.The stress,strain,strain rate and temperature increase with increasing impact velocity.The microstructure analysis shows that the deformed microstructure around the crater of AZ31 B Mg alloy is as follows in order: zone composed of recrystallized grains→zone composed of adiabatic shear bands and high-density twins→zone composed of high density twins→zone composed of low density twins.The AM60 B Mg alloy shows a similar microstructure composition around the crater,while it does not have the zone composed of recrystallized grains.With the increase of impact velocity,the distribution area of deformed microstructure near the crater decreases first and then increases,the density of twins decreases gradually,and the density of the adiabatic shear bands decreases first and then increases.Results show that the deformed microstructure formed at the bottom of the crater transformed to that in the direction of 45 ° due to the projectile penetration,and finally become the deformed microstructure on the side wall of the crater.As a result,the density of adiabatic shear bands on the side wall of the crater is the largest,and the distance between adjacent adiabatic shear bands is the smallest,the plastic deformation region at the bottom of the crater is the largest.The degree of the localized plastic deformation near the crater increases with the impact velocity.Twining is the main form of plastic deformation for the AZ31 B and AM60 B Mg alloys under high impact velocity.For the AZ31 B Mg alloy,the {10（?）2} tensile twin is the main type of deformation twin during impact,and it gradually consume the entire grain,rotating the grain by 86 °.After the grain orientation changes,it is still the {10（?）2} tensile twin tensile twins coordinate the plastic deformation.With the increase of impact velocity,the type of deformation twin remains unchanged,while the deformation area increases and the degree of localized deformation increases.For the AM60 B Mg alloy,the twin boundary undergoes severe shear deformation under high-speed impact,forming adiabatic shear bands in the boundary regions.As the plastic deformation continues,the adiabatic shear band expands in the width direction,consuming the entire twin,and recrystallized grains are formed throughout the twin.Eventually,the entire twin transforms into an adiabatic shear band.In the AZ31 B Mg alloy,high-density twins near the crater intersect with each other,making the grains are cut and shattered,and induce the recrystallization process.Then the grain mis-orientation increases with the help of subsequent plastic deformation and temperature rising,forming fine recrystallized grains finally.With the increase of impact velocity,the width of the recrystallized grain increases,and the size of the recrystallized grain does not change much.No continuous recrystallized grain zone is observed near the crater of AM60 B Mg alloy.The adiabatic shear zone formed at the bottom of the crater first.The farthest adiabatic shear band at the crater bottom is about 60°to the impact direction.As the distance from the crater bottom decrease,the angle between the adiabatic shear band and the impact direction first increases to 90 °,then gradually decreases.As the impact continues,the adiabatic shear bands at the bottom of the crater transform into that at the region with 45 ° direction,and then the side wall of crater.The angle between the adiabatic shear band and the impact direction decreases,and the distance between the adiabatic shear bands decreases.The length of the adiabatic shear band increases,while the number density increases first and then decreases as the increase of impact velocity.At the bottom of the crater,the length and number density of the adiabatic shear band increases as the increase of impact velocity.When the impact velocity is high,the cracks around the crater release a large amount of the plastic deformation,resulting in weakened localized plastic deformation near the crater,thus the number density of adiabatic shear bands decreases.The microstructure inside the adiabatic shear bands near the crater for both AZ31 B and AM60 B Mg alloys is mainly recrystallized grains.For the AM60 B Mg alloy,some Mg17Al12 precipitates formed on the boundaries of the recrystallized grains in the adiabatic shear bands after a long period natural aging at room period after impact.Results show that the subgrains firstly formed in the adiabatic shear bands during impact,and some of them gradually transform into recrystallized grains.When the adiabatic shear deformation becomes severer,recrystallized grains formed throughout the whole adiabatic shear band,and the size of the recrystallized grains increase.There exists some microstructure caused by non-uniform plastic deformation in the adiabatic shear bands,such as the elliptical none precipitation zone,groove and no precipitation zone,etc.The no precipitation zone in the AM60 B Mg alloy and the dissolution of precipitates in the center region of adiabatic shear band in the AZ31 B Mg alloy under an impact velocity of 1648 m/s indicate that the central regions suffered more serious plastic deformation than the boundary regions of the adiabatic shear bands. |
