| As the lightest structural metallic materials in engineering applications,magnesium alloys have a broad application prospect in aerospace,automotive,electronic products,biomedicine and other fields.However,compared with other structural materials such as steels,aluminum alloys and titanium alloys,magnesium alloys exhibit the lower strength,and the hexagonal close-packed structure also leads to limit activatable slip systems and poor formability at room temperature,which seriously restricts the wide application of magnesium alloys.In recent years,the mechanical properties（including strength and ductility）of magnesium alloys can be effectively improved by increasing strain rate,micro texture regulation,grain refinement and alloying.However,further systematic,in-depth and targeted studies are demanded to reveal the strengthening and toughening mechanisms,especially for non-basal dislocations,stacking faults configurations,twinning behaviors,dislocation slip mechanism and precipitation strengthening mechanism.The present work mainly focuses on pure magnesium,Mg-Y alloys and Mg-Gd alloys,and their mechanical properties tests（static tension and dynamic compression）were performed.The crystal defects（including twins,dislocations and stacking faults）were systematically studied on the atomic scale by using advanced characterization technologies,and the strengthening and toughening mechanisms induced by these crystal defects were clarified in detail.The intrinsic relationship between deformation mechanism and mechanical properties of magnesium alloys was revealed.The main contributions of this dissertation are as following:（1）Grain refinement can simultaneously improve the strength and ductility of pure magnesium.In the coarse-grained samples（（?）=125 μm）,only basal<a>dislocations were activated.As the grain size was reduced to 51 μm,additional<c>dislocations and I1 stacking faults were formed,co-exited with<a>dislocations.Moreover,numerous pyramidal<c+a>dislocations were activated in the fine-grained samples（（?）=5.5 μm）,and their tensile ductility and work hardening ability were significantly improved,which increased the uniform elongation of pure Mg from 5.3%to 18.3%.Twinning mechanism of pure magnesium was not affected by the grain size,since {10（?）2} deformation twins were observed in the deformed microstructures of above three samples.In addition to the conventional strengthening by grain refinement,high density of nano-spaced stacking faults provided a strong blocking effect on non-basal dislocations slipping,which acted as another strengthening mechanism.Furthermore,the flow stress was increased with decreasing grain size,leading to activation of<c+a>dislocations.However,the<c+a>dislocations were unstable,which were dissociated in two ways:<c>and<a>dislocations,or into I1 stacking faults.（2）The addition of a small amount of rare earth yttrium can significantly improve the tensile ductility of magnesium alloys.Mg-3Y alloy exhibited a four times higher uniform elongation（-21.5%）than that of pure Mg（-5.4%）.Basal<a>slips and {10（?）2} twins were formed during the tensile deformation of pure Mg,resulted in poor ductility at room temperature.In contrast,besides<a>dislocations,numerous stacking faults and non-basal<c>and<c+a>dislocations existed in the deformed Mg-3Y samples.All of the stacking faults observed in Mg-3 Y alloy were I1 and I2 intrinsic faults,while no extrinsic faults were found in this alloy.The I1 and I2 faults were bounded by Frank partials（b=1/6<20（?）3>）and Shockley partials（b=1/3<10（?）0>）,respectively.Stacking fault energy of magnesium alloys was significantly decreased through the addition of rare earth yttrium,which was proposed as the main reason for the formation of high density of stacking faults.In addition,<c+a>screw dislocations conducted double cross-slip activities between the {10（?）1} and {11（?）2}pyramidal planes,which not only increased the potential dislocation nucleation source,but also promoted dislocation multiplication and annihilation processes,leading to additional contributions to maintain superior tensile ductility and work hardening ability of Mg-3Y alloy.（3）With four different high strain rates(800,1000,1300 and 2000 s-1),the compressive strain of Mg-5Y alloys was increased continuously with increasing strain rate,and the corresponding ultimate compressive strength（UCS）was also increased gradually.The sample with strain rate of 2000 s-1 exhibited UCS of 519 MPa and compressive strain of 0.21.No adiabatic shear band was observed in all the deformed samples,because the work hardening effect was much greater than thermal softening effect.The proportions of low angle grain boundaries（LAGBs）in the deformed samples increased significantly,and numerous {10（?）2} deformation twins were activated.The defect density was the highest in the sample with strain rate of 2000 s-1,which exhibited the LAGBs proportion of～73.4%and many activated twin systems with different types.Mg-5Y alloys mainly depended on multiple twinning,basal<a>dislocations,non-basal dislocations and I1 stacking faults to coordinate deformation.All the four samples possessed three-stage work hardening behaviors under dynamic deformation with different strain rate.Strengthening and toughening of deformation twinning,non-basal dislocations and stacking faults were the underlying mechanisms for the high work hardening ability of Mg-5Y alloy.（4）The optimum aging temperature of Mg-10Gd alloy was 200℃ with peak hardness of 102.4 HV.Hardness improvement of the aged samples mainly originated from precipitation strengthening,and β’ nano-precipitates were the main strengthening phase.The samples exhibited a certain age hardening effect at 225℃ with peak hardness of 89.4 HV,due to the "gourd" shaped β’ precipitates with coarse size.In the two Mg-10Gd aged samples of 200℃-20 h and 225℃-40 h,the long axis sizes of β’ precipitates are 7.1 nm and 102.5 nm,respectively.The unaged annealed sample exhibited the yield strength（YS）of 134 MPa and ultimate tensile strength（UTS）of 190 MPa,as well as the uniform elongation（UE）of 9.5%.The YS of both aged samples were increased to about 190 MPa,and the UTS of 200℃-20 h sample was 267 MPa with UE of 4.2%,which were higher than those of 225℃40 h sample（UTS=230 MPa,UE=3.2%）.Basal<a>dislocations,non-basal<c>or<c+a>dislocations and I1 stacking faults were dominated in the tensile deformation of both aged samples,and β’ precipitates with different size could be shear deformed by basal<a>dislocations.The remarkable improved yield strength largely originated from the increase of stress required for dislocations to overcome the stress field of coherent interfaces.Meanwhile,owing to larger critical shear stress increment for dislocation slip,larger applied stress was required to coordinate strain continuously for the 200℃-20 h sample with better strengthening effect,which was the fundamental reason to explain why its tensile strength was higher than that of 225℃-40 h sample.The tensile ductility of Mg-10Gd alloy was significantly reduced by β’ precipitates,but the precipitates size could affect the reduction range of ductility,resulted in smaller ductility reduction for the 200℃-20 h sample with finer precipitates.The slip movements of numerous non-basal dislocations strongly contributed to coordinate the tensile strain,thus allowed both aged samples to maintain a certain tensile ductility. |
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