Study Of Microstructure Adjustment And Low Cycle Fatigue Properties Of AZ80 Magnesium Alloy

As a lightweight structural metal material,magnesium alloy has a good potential use,especially in the automotive industry.People are eager to use this lightweight metal material to achieve the purpose of reducing gravity and thus reduce the energy consumption and emissions.As a additional process of conventional heat treatment,cryogenic treatment can effectively improve the distribution uniformity of microstructure of magnesium alloy,refine the grain size,promote the precipitation of the second phase in the crystal and enhance the comprehensive mechanical properties of the material.The surface mechanical polishing(SMAT)as a novel plastic processing technology can induce the stable gradient nanostructures on the surface of magnesium alloy,which greatly enhance the hardness and strength of the material.At the same time,the impact of steel balls on the surface can lead the residual compressive stress.Therefore,this study selects the extruded AZ80 deformed magnesium alloy as the experimental analysis object and conduct the cryogenic treatment and SMAT treatment,respectively.After the two processes,microstructure evolution,hardness and strength variation of the AZ80 magnesium alloy were analyzed.Moreover,the fatigue performance and fatigue fracture behavior was also researched and the low cycle fatigue deformation mechanism was explained.In this paper,the microstructural grain size and the change of the second phase of AZ80 deformed magnesium alloy before and after cryogenic treatment(CT)were analyzed by metallographic microscope.The surface hardened layer of AZ80 magnesium alloy treated by SMAT was analyzed.The influence of cryogenic treatment on the yield strength,tensile strength and elongation of AZ80 magnesium alloy was investigated.The fracture morphology and fracture mechanism of AZ80 magnesium alloy was studied by scanning electron microscopy(SEM).The changes of grain composition and morphology were analyzed by SEM.The changes of grain orientation and the texture distribution of AZ80 alloy were analyzed by backscattering electron diffraction(EBSD)before and after cryogenic treatment.The fully reverse strain-controlled pull-pressure fatigue tests were proceeded to investigate the low cycle fatigue behavior of AZ80 magnesium alloy before and after cryogenic treatment.The low cycle fatigue properties of AZ80 alloy with and without SMAT were analyzed.Under the condition of different strain amplitude,the deformation mechanism of the wrought magnesium alloy AZ80 with different treatment was discussed.The average grain size of the original extruded AZ80 magnesium alloy is 21.66 μm,while the average grain size of the microstructure after CT for 8h is 19.62 μm,which is 9.42 % lower than that of the original structure.The grain size of AZ80 alloy with CT 24 h decreased to 17.25 μm,which was 20.36 % higher than that of the original grain.The average grain size after CT 48 h decreased to 17.92μm,which by the extent of 17.27% compared to the original size.There is no significant difference between the grain size of CT 24 h and CT 48 h.Compared with the original extruded specimen,the AZ80 Mg alloy with CT increases the yield strength and tensile strength to a certain extent,and the improvement of the strength increases with the expansion of the cryogenic time.The yield strength of the original extruded AZ80 magnesium alloy is 214.23 MPa,the tensile strength is 296.88 MPa and the elongation is 4.42%.After 8 h of cryogenic treatment,the yield strength of AZ80 magnesium alloy is 225.42 MPa,the tensile strength is 305.76 MPa and the elongation is 5.00%.After 24 hours of cryogenic treatment,the yield strength is 217.48 MPa,the tensile strength is 304.14 MPa,the elongation is 5.31% The yield strength of extruded AZ80 magnesium alloy is 219.50 MPa,the tensile strength is 309.19 MPa,the elongation is 5.61%.It is worth noting that the elongation of AZ80 magnesium alloy increases with the extension of the cryogenic time,which increases by 13.12%,20.14% and 26.92% corresponding to CT 8 h,CT 24 h and CT 48 h respectively compared with the original extrusion state.Thus the comprehensive mechanical properties of the AZ80 alloy are improved obviously.At the strain amplitude of 0.42% and 0.50%,the stress-strain hysteresis curve of the original extruded AZ80 magnesium alloy shows a significant pull-pressure symmetry.The stress-strain hysteresis curve of the alloy with CT is similar to that of the original specimen,which also exhibits the symmetry of the pull-pressure.The symmetry of the pull-pressure is mainly ascribed to that dislocation slip dominates the main way of fatigue deformation at the low strain amplitude.When the strain amplitude is 0.60%,the hysteresis curve of the original AZ80 magnesium alloy exhibits obvious pull-pressure asymmetry in the first cycle,and the concave "S" shape appears in the compression reversal stage.What is more,the residual twins were observed on the metallography near the fatigue fracture,indicating that the occurrence of twinning-detwinning phenomena.Twin deformation becomes the main way of fatigue deformation.When the strain amplitude is 0.60%,the large deformation during the compression stage leads to the activation of the {10(?)2}<10(?)1> tension twinning.The twinning ability is not exhausted before the end of compression reversal,resulting in the upward and concave asymmetric shape of the compression reversal.Compared with the original extrusion state,the AZ80 alloy with CT 24 h shows a higher cyclic hardening rate.At the strain amplitude of 0.50% and 0.60%,the significant cyclic hardening was found into the AZ80 Mg alloy before and after CT 24 h,but there was no obvious difference.It is worth noting that the low cycle fatigue life of the CT 24 h sample is obviously improved compared with the original extrusion state under the conditions of strain amplitude 0.50% and 0.60%,which may be attributed by the grain refinement induced by CT.After SMAT 2 min,the wrought AZ80 magnesium alloy produced a plastic deformation layer with a thickness of about 130 μm from the surface to the center,accompanied by a large number of twin boundary.The surface grain size of AZ80 with SMAT was obviously refined and exhibits a gradient structure distribution.After SMAT 4 min,the thickness of the deformed layer on the surface of AZ80 Mg alloy increases to 170μm and the twins density is larger than that of SMAT 2 min.After SMAT 6 min,the thickness of the plastic deformation layer of AZ80 Mg alloy is about 200μm and the twins density is further increased compared to the SMAT 4 min.The grain size of AZ80 magnesium alloy with SMAT showed a gradient distribution and the twin density exhibited a gradient distribution from the surface to the core.Moreover,the density of twin increases with the expansion of SMAT time.At the strain amplitude of 0.64%,the stress amplitude of AZ80 with and without SMAT increases with the increase of the loading cycle,which strongly indicates that the original extruded AZ80 Mg alloy and SMAT specimen both show the cyclic hardening.After the SMAT,the cyclic hardening rate of AZ80 Mg alloy was higher than that of the original extruded Mg alloy.And the fatigue life of AZ80 after SMAT was further improved compared with the extruded AZ80 Mg alloy.