Study Of High Cycle Fatigue Performance And Strengthen Of ZM61 Wrought Magnesium Alloys

As the lightest structural metallic material,magnesium(Mg)alloys have attracted great interest for its low density,high specific strength and mechanical processing properties.Mg alloys have a wide application prospect in many fields,especially in ground vehicles and aeroplanes.For the using of Mg alloys as engineering components,these Mg parts are used under dynamic load that involves tension and compression loading.Revealing dynamic mechanical properties(high cycle fatigue properties,fracture toughness etc.)is significant for the application of Mg alloys.In present study,we take ZM61 alloys,a newly developed Mg alloy by our group,as the research object.Firstly,we studied the microstructure and static mechanical properties of three ZM61 alloys,and then we tested the high cycle fatigue properties(HCF)of these three alloys using an MTS servo-hydraulic load frame.Fatigue tests were conducted under two typical load types of full reverse tension-compression load(TC load)and zero-tension load(ZT load).The fatigue failure mechanisms of ZM61 alloys have also been discussed;We studied microstructure evolution mechanism in fatigue by EBSD;In this study we put forward a new method to strengthen ZM61 alloys(AE treatment,aging before extrusion).The effect of aging treatment before extrusion on microstructure and mechanism properties was studied,the HCF properties of ZM61 alloys in optimum technical were tested.In the last,we discussed the influence of AE treatment on fatigue failure mechanisms.Based on our work,the conclusions are as following:Extruded ZM61 alloys have few precipitates,the main precipitates in T5 treated alloy are rod-shaped ?1 ?and a few plate-shaped ?2 ?,double-aged alloys have the most precipitates and these precipitates are much finer;The texture type and texture intensity of three alloy are slightly modified;Double-aged alloys have the highest tensile strength.All the three ZM61 alloys show more outstanding fatigue performance in TC load than that in TC load;In ZT load,fatigue limit of extruded,T5 treated and doubleaged alloys are 169 MPa,159 MPa and 179 MPa,respectively.Double-aged alloys have higher fatigue performance than others.In T5 treated alloy,precipitates promote strength by pinning dislocations,and it results in local stress concentrations which accelerates fatigue crack initiation and reduce fatigue performance.In TC load,fatigue limit of extruded,T5 treated and double-aged alloys are 113 MPa,119 MPa and 105 MPa,respectively.Though double-aged alloys have highest fatigue performance in ZT load,they show the most deteriorative fatigue performance in TC load.The deteriorative fatigue performance of double-aged alloys in TC load is related to twining deformation.The fracture surface of three alloys can be divided into three regions,i.e.,crack initiation region,crack propagation region and final rupture region.Crack initiation region is very small and it locates near surface of specimens.Crack propagation region is very flat,fatigue striation can be found in crack propagation region.In double-aged alloys,twinning deformation can be activated easily for its coarse grains,twinning features are found in crack initiation region and crack propagation region.Due to the lack of constraint in grains at free surface,crack initiation regions locate near fracture surface.Based on experimental observation,there are three fatigue crack initiation modes in double-aged alloys: cracks initiate at inclusions,twin boundary cracking and twin boundary cracking.Coarse grains are replaced by fine grains after high cycle fatigue in extruded and T5 treated alloys.In double aged alloy,after fatigue tests grain size didn't change and many twins were found in specimens,with distance increasing from fracture surface the volume fraction of twins decreased.Lots of LAGBs appeared in coarse grains of postfatigued extruded specimen.With the increasing number of loading cycles,the misorientation angle increased and texture weakened.The grain refinement in extruded alloys during HCF is related to CDRX mechanism.The mobile dislocations passed the sub-grains and were trapped by LAGBs lead to an increasing in LAGBs' boundary misorientation.With the millions of accumulation,local lattice rotation can be prominent and coarse grains were subdivided into multiple grains.The pre-exist Mg-Zn precipitates took part in extrusion.After extrusion,robshaped and plate-shaped precipitates were torn up to spherical precipitates,the orientation relationship was disappeared.The grain size decreased with prolonging the aging time before extrusion.It could find some non-recrystallization regions in extruded alloys with a reduction ratio of 16.The orientation relationship between two precipitates was retained in these regions.Aging treatment before extrusion significantly improved tensile strength,tensile elongation,compressive strength,it reduced tension–compression yield asymmetry at the same time.In TC load and in ZT load,the fatigue limits of optimum technical AE alloys were 139 MPa and 199 MPa,respectively.The fatigue limits were improved by 23% and 18% in comparison with ZM61 alloys subjected to former technologies.The fracture surface of AE alloys can also be divided into three regions,i.e.,crack initiation region,crack propagation region and final rupture region.Crack propagation regions in AE alloys were rougher than that in former alloys.Local plastic deformation regions could be found embedded in fatigue striation in crack propagation regions.Many banded structures appeared in plastic deformation regions.In post-fatigued specimens,fatigue zonal structures were found in coarse grains,misorientation varied little in these regions.During cyclic deformation,plastic deformation was restricted in coarse grains.With cyclic loading,irreversible deformation accumulated in coarse grains and resulted in the formation of PSB in coarse grains.