Effect Of Grain Size On Dynamic Fracture Behavior And Spallation Characteristics Of High-purity Aluminum

The macro-mechanical properties of metal materials are closely related to the microstructure,and the grain size is considered to be one of the most important factors.With the decrease of grain size,the number of grain boundaries increases and a large number of grain boundaries impede the dislocation movement of the plastic deformation of the metal which results in an increase in the yield strength of the metal.In order to improve the strength of metal materials,people have developed a variety of severe plastic deformation(Severe plastic deformation,SPD)technique to prepare ultrafine crystal or nanocrystalline metal materials.The mechanical property test under the quasi-static loading conditions reveals that the relationship between strength and grain size basically meets the Hall-Petch relationship.That is to say,the strength increases significantly with the decrease of the grain size.Selecting high-purity aluminum(purity greater than 99.99%)rolled sheet as the research object,this paper changes the grain size through different heat treatment processes,and characterizes the microstructure of the material.The result shows that the average grain size of high-purity aluminum samples after heat treatment are 60,100,500?m respectively.Studies on the mechanical properties of high-purity aluminum in uniaxial and triaxial tension experiments by means of the Zwick-HTM-5020 hydraulic high-speed tensile testing machine and the first-level light gas gun loading technology,focusing on the effect of gr ain size on dynamic tensile mechanics and spallation behaviors in a state of different strain rates and stress states.Based on findings of studies,the following conclusions could be drawned:(1)With the aid of the Zwick-HTM5020 high-speed tensile testing machine,an experimental study on the axial tensile mechanical properties of high-purity aluminum with different grain sizes was carried out combined with the digital image correlation of strain measurement technology,and tensile stress-strain curve of the material at different strain rates were obtained.The samples were analytically detected which shows that when high-purity aluminum is subjected to tensile stress,the crystal grains will twist,and elongated along the tensile direction,and also the micro holes are in Nucleation at the grain boundary and the normal growth along the stretching direction penetrate;the grain size has a certain effect on the strain strengthening effect of high-purity aluminum material,and the strength of high-purity aluminum materials at the medium strain rate makes the difference to high correlation of grainsize.The elongation of high-purity aluminum material also decreases with the increase of the grain size.High-purity aluminum material qualitatively satisfies the Hall-Petch relationship in the mid-strain rate range.(2)Using the plate impact technique,an experimental study on the spallation behavior of three high-purity aluminum sheets with grain sizes was carried out.By changing the target speed of flying plate,the initial spallation state and the complete spallation state are achieved in the target.Based on the free surface velocity time-history curve,micro-damage evolution and fracture micromorphology analysis,the effect of grain size on the spalling charact eristics of high-purity aluminum sheet was discussed.The experimental results show that:1)The effect of grain size on the spalling strength of high-purity aluminum sheet strongly depends on the amplitude of impact loading stress.Under low stress condit ions,the spalling strength and the grain size shows an inverse Hall-Petch relationship,while the grain size has almost no effect on the spalling strength under high-stress conditions;2)Through the comparative analysis with the experimental results of h igh-purity aluminum rods,it is found that the relationship between the impact direction and the grain boundary orientation has a significant effect on the spalling characteristics.When the impact direction is perpendicular to the grain boundary orientati on,the resistance of materials to spallation is significantly reduced;3)With the increase of the grain size,the distribution range of micropores in the target damage area increases with the reduced number and enlarged size,and it is found that the grai n refinement phenomenon is caused by the impact compression process;4)As the grain size increases,the micro-mechanism of spallation changes from ductile intergranular fracture to quasi-brittle intergranular fracture,and a small number of small random distributions are observed on the fracture.The round ball indicates that the micro-holes grow up and the thermal effect during the aggregation process cannot be ignored.(3)Based on the research results of Jacobi,Zhang Fengguo and others,it is assumed that the effect of grain size is reflected on the total number of potential pore nucleation inside the material.And also according to the research results of Molinarui and Wright,the number of potential nucleation pores per unit volume of material,the relationship with the grain size is N?b^-3.The effect of grain size on pore nucleation is coupled to the NAG model(Nucleation And Growth)damage and fracture model proposed by Curren and others by statistically analyzing the microscopic pore data of soft recovered samples,the modified NAG damage model is adopted in the form of user subprograms.Embedded in the commercial finite element calculation software Abaqus,the high-purity aluminum spallation experiment was numerically calculated.The calculation res ults show that the free surface velocity curves of high-purity aluminum with different grain sizes are in good agreement with the experimental results,which indicates that the model can better describe the effect of grain size on the damage and fracture o f high-purity aluminum.