Study Of Nanocrystallization And Twin Boundary Strengthening Mechanisms In Magnesium Alloys Prepared By Surface Mechanical Attrition Treatment

Grain refinement strengthening mechanism is widely applied to magnesium（Mg）alloys by using severe plastic deformation methods.However,only micron scale and submicron scale grains rather than nano scale grains can be obtained in Mg alloys due to the limitation of temperature,strain and strain rate.Surface mechanical attrition treatment（SMAT）is a deformation method to prepare nano grains in the surface of alloys but has not been used for Mg alloy strengthening of tensile properties.In this study,we fabricated nano grains and heterogeneous structure in Mg alloys and investigated the strengthening effect of each structure on tensile properties.The mechanisms of deformation and refinement during SMAT process in extruded Mg-3Gd alloys,cast,solution-treated and aged Mg-7Al-2Sn（AT72）alloys were carefully investigated in order to adjust and control the deformed structure.It was found that a high density of twins can be obtained in second-phase-free Mg alloys after SMAT.Therefore,for further strengthening of Mg alloys via twins,heat treatment was conducted after SMAT process.The twinning strengthening mechanism was elucidated through observing the microstructure and testing the mechanical properties.In this thesis,gradient microstructure was prepared in Mg alloys by SMAT and annealing and thus more than one strengthening mechanisms including refinement strengthening and twinning strengthening were combined into one piece of Mg alloy.The microstructure and mechanical properties of Mg-3Gd and AT72 alloys before and after SMAT process were observed and tested with the conclusions as following:Gradient structure containing nano grains in the surface and twins in the center could be obtained in Mg-3Gd alloys after 4 min SMAT process under 20 kHz.Tensile testing results show that the increase of tensile yield strength（TYS）equals the decrease of elongation in SMATed alloys,since there were too many residual dislocations inside the SMATed alloys and the room for strain hardening during the tensile test was confined.The SMAT deformation mechanism and recrystallization mechanism of Mg-3Gd alloys are obtained by observing microstructure evolution.Twinning is dominant during SMAT process in Mg-3Gd alloy with an average grain size of 43μm.High density of twin lamellae interact with each other and dislocations accumulate inside twin lamellae to form sub structures.With the increase of strain,dislocations rearrange to lower the total energy of the system,forming low angle boundaries which transform into high angle boundaries gradually to complete rotational recrystallization process.This rotational recrystallization mechanism plays an important role in the formation of nano grains during the room-temperature SMAT deformation.The effect of second phases on deformation mechanism and dynamic precipitation were investigated in AT72 alloys.Both large secondary phases and fine dispersive precipitates can suppress twinning during SMAT process in AT72 alloys,while twinning is dominant in singleα-Mg phased AT72 alloy.The difference in crystal structure could impede twinning formation in the alloys consisting of secondary phases and the phase boundary could supply many dislocations to accommodate plastic strain.After SMAT,a large quantity of Mg₂Sn particle-shaped precipitates can be observed in all AT72 alloys.The microstructure and mechanical properties of SMATed Mg-3Gd and AT72 alloys after heat treatment were observed and tested with the conclusions as following:The microstructure of annealed SMAT Mg-3Gd alloy showed that twin density did not decrease and that nano grains grew from about 50–100 nm to around 120–200 nm.The micro-hardness showed that surface layer was almost as hard as before annealing and that the hardness in twinning zone increased due to Gd segregation in the boundary.The increase of TYS is two times of the decrease of elongation in annealed Mg-3Gd alloys whose uniform elongations（¹1–15%）are not bad actually.The increase of TYS comes from four aspects with their proportions:twinning refinement strengthening（44.56%）,solute TB segregation strengthening（31.24%）,and refinement strengthening in nano grain zone（16.54%）,refinement and back-stress hardening in gradient zone（7.66%）.At coherent twin boundary（CTB）,Gd whose radius is larger than Mg was observed periodically segregating at the extension sites to decrease the total energy of the system.Double Gd atoms segregation at the most and second stable positons could induce a twin double boundary for the sake of balance.Three Gd atoms would not change the twin boundary structure and symmetry.However,with the increase of the Gd atom number,the energy reduction becomes less and less,since the introduction of other Gd atoms into twin boundary would sacrifice part of formation energy.Neither parallel to{10（?）2} twinning plane,nor parallel to[1（?）10] direction,a new incoherent twin boundary（ITB’）was firstly found in annealed Mg-3Gd alloy.The ITB’,driven by dislocation rearrangement and total system energy reduction,is supposed to have higher strengthening effect than normal ITB and CTB.Al and Sn segregation was observed in annealed SMAT solution-treated AT72 alloy.The around 8 nm distance of Sn segregation in TB is probably caused by random solution under the control of solutes density.First principle calculation results show that single Al and Sn atoms whose radii are smaller than Mg tend to take the compressed sites in the twin boundary in order to lower the total system energy.In addition,for Al and Sn together,the most stable sites are taken by them according to the solute density of around 10:1,the second stable sites tend to be taken by the same kind of atoms as the most stable site,which means solutes of Al and Sn maintain an aggregation tendency.The strengthening and toughening mechanisms of annealed SMAT Mg-3Gd alloys are concluded as following.The heterogeneous structured alloy yielded layer by layer till surface during tensile testing and then the global yield of the material happened.Since the dislocation tangles and local strain concentration were relieved and eliminated during annealing process.The increase of TYS is due to the structure rather than the residual dislocation hardening.Strain hardening effect exerting itself well during the plastic stage is also because of the elimination of local stress concentration.The hard surface could block the micro-crack propagation owing to the small grain size in addition to the local dislocation hardening near the tip of crack.Therefore,the heterogeneous structure could effectively strengthen and toughen the Mg-3Gd alloy.SMAT followed by annealing is an effective method to obtain gradient structure consisting of nano grains,gradient grains and twins which have refinement strengthening,back-stress hardening and twinning strengthening effects,respectively.Instead of applying one strengthening effect to the homogeneous structured alloys,the combination of several strengthening effects in one material through different structures definitely leads to new insights into the strengthening of Mg alloys.