Microstructure Evolution And High Speed Impact Behavior Of Mg-Gd-Zn-Zr By Surface Mechanical Attrition Treatment

Magnesium alloy is the lightest structure material in actual application.It has the advantage of high specific strength and specific modulus,which is known as“environmental engineering material in the 21st century”.With the decreasing of many metal mineral resources,Mg alloy has been highly regarded because of rich resources.Especially,Mg alloy has a widespread application prospect in the large projects field with high requirement for structural lightweight such as defense.However,the low temperature deformation ability of Mg alloy limits development and application in industrial.The most effective strengthening methods for Mg alloy are solution strengthening,refined crystalline strengthening and precipitation strength.Rare earth alloying and heat treatment can effectively enhance the mechanical properties of Mg alloy at room temperature,which can be improved the limitation of Mg alloy in industry and other fields.Surface mechanical attrition treatment（SMAT）can effectively resist the failure of materials by making the microstructure of materials form gradient structure,so as to improve the physical and other mechanical properties of the whole material,so as to extend the service life of materials or meet the requirements of special environment.In this paper,the experiment material selected Mg-15Gd-1Zn-0.4Zr（GZ151K）alloy.Heat treatment and mechanical process treatment method are selected to optimize the process parameters and improve the mechanical properties of GZ151K alloy.In this paper,we investigated microstructure and composition of the alloy by optical microscopy,scanning electron microscope and X-ray Powder diffractometer.The mechanical properties of GZ151K alloy was tested and investigated by nano indentation machine and traditional tensile testing apparatus.The effects of solution treatment（T4）,solution treatment+aging treatment（T6）and solution treatment+surface mechanical grinding treatment（SMAT）+aging treatment（T6）on the strength and other mechanical properties of GZ151K alloy were studied respectively.The main conclusions are as follows:1.The surface layer of as-cast GZ151K alloy was slightly refined after SMAT at different times.After 6 min of SMAT,the average surface grain size of the alloy was refined to 25.65?m,and when the SMAT time increased to 9 min and 12 min,the grain size increased to 30.32?m and 30.88?m,respectively.This indicates that SMAT processing for a certain time can refine the surface grain,and with the increase of time,mechanical energy is converted into heat energy to make the grain size grow.After 6min of SMAT,the average surface hardness of the alloy changed from 92.4 HV to 101.3HV,and its tensile strength increased to 249 MPa by 7.8%,while the fracture elongation decreased to 2.4%.After SMAT,the surface of tensile fracture is brittle fracture,while the core is still ductile fracture.After SMAT treatment,the grain orientation in the surface microstructure of the alloy was random,and the texture of each phase of the alloy was significantly strengthened.2.Solid solution state GZ151K alloy after SMAT 6 min at 200^oC under the aging treatment,aging to 100 h,alloy hardness peak,is 183 HV,tensile strength and yield strength of samples are 155 MPa and 201 MPa,breaking elongation rate is 0.8%.Compared with the samples without SMAT,the hardness and fracture elongation of the alloy were significantly improved.After SMAT,dislocation defects introduced into the alloy contribute to the precipitation of the second phase,and the second phase distribution is more dispersed,which significantly improves the hardness and strength.The fracture morphology of the alloy was observed by SEM.It can be seen that the fracture form from the surface to the core of the sample gradually transited from brittleness to toughness,and the shaping of the alloy mainly depended on the coarse crystal of the core.In order to study the SMAT under different aging temperature on the alloy influence law of precipitation,the alloy further limitation under the 150^oC and250^oC,respectively in 40 h,120 h and the average hardness of the alloy surface peak,205 HV respectively and 157 HV.This indicates that the alloy will show lower hardness due to aging phenomenon in the aging process when the alloy is close to its melting temperature.3.When the strain rate of as-cast GZ151K magnesium alloy is 600s^-1,the yield strength and tensile strength are 246 MPa and 296 MPa respectively,and the ductility rate is 3.8%.After SMAT treatment,yield strength and ductility increased by 2.4%and7.9%respectively,and the change of tensile strength was not obvious.The increase of strength is subject to the hall page relation,which indicates that the surface grain refinement of SMAT can improve the properties of the alloy.When the strain rate increased to 1500 s^-1,the yield strength and tensile strength increased by 1.2%and 13.7%respectively,and the elongation rate decreased by 19.3%after SMAT.When the strain rate was 600 s^-1,the yield strength and tensile strength of the solid solution GZ151K magnesium alloy were 398 MPa and 459 MPa,respectively,and the ductility was 8.4%after aging treatment.After SMAT,yield strength and tensile strength were increased by 3.8%and 3.3%,respectively.As the middle grain decreases after SMAT,the original coarse grain will be split by twins in the process of impact deformation.Multiple twins appear in the grain,which can activate more non-base plane slip system and can rotate by self-regulation,so as to release stress concentration and achieve the purpose of coordinating plastic deformation.