Mechanical Behaviour And Micro-deformation Mechanisms Of The Mg-Y-Zn Alloys Containing Long Period Stacking-ordered Structures

The strengthening and toughening of magnesium alloys are the keys to a wider application of magnesium alloys as light-weight structural materials.Mg-RE-Zn/Cu alloys containing long period stacking ordered(LPSO)structures have received considerable attention due to their potential to achieve excellent mechanical performance at both ambient and elevated temperatures.As polytypes sharing the basal plane of ?-Mg,the strengthening mechanisms of LPSO structures were widely discussed,while the precise stacking structure and the formation and transformation mechanisms were revealed.The plastic deformation mechanisms study of LPSO is the basis of a better understanding of the strengthening mechanism.Previous studies suggest that basal slip and formation of kink band are two mainly deformation mechanisms of LPSO structures.However,it is still lack of systematic study about the deformation mechanism,especially the anisotropy mechanism of plastic deformation caused by the crystal structure.This thesis is about the micro-deformation mechanisms of Mg-Y-Zn based Mg-LPSO alloy investigated using extruded polycrystalline alloys with basal fiber texture and single-crystalline/bi-crystalline micro-pillars of different crystallographic orientations.This thesis started with the microstructure characterization and the mechanical properties of the hot extruded alloys to discuss the effect of LPSO structures on the macro deformation of the alloy.The microstructure of alloys extruded after solid solution treatment was divided into three areas including the recrystallization area,worked grain area containing LPSO precipitates and the bulk LPSO area.The 14 HLPSO precipitates were deformed by kinking and bending to accommodate the strain concentration,which contributed to a good plasticity of extruded alloy.However,the bulk 18R-LPSO structures in the as-cast alloy are prone to the crack initiation after hot extrusion,which is negative to the improvement of strength.Based on the microstructure study of a series of extruded alloys,the one with smallest grain size was selected for heat treatment at 500°C for 48 h.The heat-treated alloy has a uniform recrystallized microstructure that 14H-LPSO plates embedded in most of the grains while basal fiber reserved.Then the tensile test along extrusion direction was carried out,so that the basal slip and kink band formation commonly activated in LPSO structures were inhibited due to a negligible Schmid factor and a reversed loading direction,respectively.EBSD-assisted slip trace analysis and TEM analysis confirmed the prevalently activation of non-basal <a> dislocation slip in the tensile Mg-LPSO alloy of 6% engineering strain,independent of the local strain value.No kink band was observed in the sample,indicating the tension-compression asymmetry of LPSO structure.Under compressive loading along basal plane,the predominant deformation mechanism is the formation of kink band,while under tensile loading non-basal <a> slip is predominantly activated.Furthermore,the plastic anisotropy mechanism of LPSO structure was studied.Oligo-crystals containing bulk 18R-LPSO grains prepared using Bridgman method was used to select grains of three different crystallographic orientations.Micro-pillar compression was conducted at both room temperature(RT)and elevated temperature(215°C)to investigate the micro mechanical behavior and deformation mechanism of micro-pillars containing LPSO structure,?-Mg and Mg/LPSO interface.Basal slip traces were observed in the surface of micro-pillar when the Schmid factor of basal <a> slip is not negligible.According to the results of TEM characterization,the micro-pillars with basal slip traces were classified into two cases.When the crystallographic orientation of micro-pillar is about 45° to basal plane,basal <a> slip was the predominant deformation mechanism for all of the micro-pillars compressed at RT and 215°C,even at a quite low stress.The activation energy of basal slip decreased significantly with the increase of temperature,independent of structures.When the crystallographic orientation of micro-pillar is about 7° to basal plane,the activation of basal slip in the micro-pillar compressed at RT was accompanied with the activation of <c+a> dislocations in ?-Mg and the formation of micro shear bands along secondary pyramidal plane in LPSO structure,leading the highest stress state among three orientations.In the micro-pillars of the same orientation compressed at 215°C,LPSO structure showed only prevenlent basal <a> slip,while the high flow stress of ?-Mg revealed the existence of other deformation mechanisms other than only basal slip.In the micro-pillars with a negligible Schmid factor of basal slip and a high Schmid factor for prismatic <a> slip,the deformation at the initial stage was dominated by a particular prismatic <a> slip and the activation energy of the prismatic <a> slip was dependent on structure.The calculated CRSS of prismatic <a> slip at RT in LPSO was about 2~3 times higher than that in ?-Mg and about 15 times higher than basal <a> slip in LPSO.At higher engineering strain,deformation at RT was governed by multiple deformation mechanism,i.e.deformation twinning in ?-Mg,as well as shear banding and kink banding in ?-Mg and LPSO structure.The deformation of micro-pillar with higher engineering strain at 215°C was attributed to the double slip in prismatic planes.