Research On The Relationship Between Boundary Character And Strengthening In Mg Alloys

First of all,the influence of different?0002?distributions on k for twinning and slip was systematically studied using an AZ31 rolled plate?[0002]//ND?and extruded rod?[0002]?ED together with a random distribution around the ED?.The k,ND and ED refer to the Hall-Petch slope,normal direction of the plate and extrusion direction of the rod,respectively.The mechanism for the high dependence of k for twinning and slip was well explained.On the base of the above mechanism,a coarse grained AZ31 plate was refined by{101?2}twin boundaries?TBs?and grain boundaries?GBs?,respectively.A comparative study about the different effects of grain refinements by GBs and by TBs on prismatic slip and{1012}twinning was performed.A study regarding the influence of{101}2twins on hardening of prismatic slip and basal slip was performed.Furthemore,Detwinning behavior of a strongly textured magnesium alloy AZ31 at different temperatures?30°C,100°C,150°C,200°C and 250°C?was investigated,with a focus on the effect of temperature on twin boundaries?TBs?migration.Mechanical behavior and microstructure evolution during detwinning deformation were examined.Then,a comparative study regarding the influence of grain size on hardening against detwinning with solute segregation at twin boundaries?detwinning-s sample?versus detwinning without solute segregation?detwinning sample?in Mg alloys was studied.At last,four alloys with a similar solute atomic percent,Mg-Y,Mg-Gd,Mg-Al and Mg-Zn,were chosen.The effect of solute atomic misfit and migration of solute atoms at grain boundaries?GBs?on thermal stability was systemically studied,and the relevant mechanisms were disclosed.The results in this thesis are shown as follows:?1?A high dependency of k on the?0002?distribution is found,namely,a much lower k for{1012}twinning in a plate(219 MPa?m^1/2)than that in a rod(435 MPa?m^1/2),but a much higher k for slip in a plate(437 MPa?m^1/2)than that in a rod(235MPa?m^1/2).Compound use of the difference in Schmid factor??SF?and geometric compatibility factor?m'?quantitatively explains this orientation effect on k.?SF relates to the extra stress needed for the activation of slip/twinning in a neighboring grain,and m'reflects the efficiency of the stress concentration at the onset of slip/twinning in an adjacent grain.The lower m'for twinning in the rod versus the plate primarily accounts for the higher k for twinning in the rod.A much larger inclination of basal poles away from the ideal texture exists in the plate than in the rod,which induces a higher activity of basal slip during tension.The resultant high fraction of slip transfer from basal slip in one grain to prismatic slip in the neighboring grain largely amplifies?SF and reduces m',both of which yield a higher k for slip in the plate than in the rod.The relationship between the crystallographic orientation and m'was also calculated for different types of deformation transfer,and the main factor that determines m'was revealed.?2?Our results show that both the refinements by GBs and by TBs increase the tensile and compressive yield strengths,but to a different degree.{101?2}TBs are more effective to harden{101?2}twinning,but yield a lower strengthening against prismatic slip,and a much lower tension-compression yield asymmetry is thus obtained.Both the differences in boundary coherence and misorientation between GBs and TBs affect the hardening.The misorientation of TBs provides a lower geometric compatibility factor?a higher hardening?for both prismatic slip and{101?2}twinning than that of GBs,which in detail is the result of the much higher angle between c-axes of the two sides of TBs?about 86°?than GBs?0-50°?.It is found that,for hardening of prismatic slip,boundary coherence plays a more important role than misorientation.With regard to{101?2}twinning,the different misorientation of TBs from GBs mainly accounts for their different hardening effects.The results also show that grain refinement by{101}2twins is more effective to strengthen basal slip?28 MPa increment in yield strength?than prismatic slip?11 MPa increment?.It is shown that a lower geometric compatibility factor?m'?for basal slip transfer than prismatic slip transfer across boundaries exists in the pre-twinned samples,indicating a higher boundary obstacle effect.{101}2twin boundary with a basal pole inclined by 86°generates a lower m'for basal slip transfer than that for prismatic slip transfer.This together with the high fractions of twin boundaries account for the lower m'for basal slip transfer than that for prismatic slip transfer in the pre-twinned sa_-mple.?3?Our results show that the activation stress for{1012}TBs migration decreases with the increasing of temperature from 30°C to 250°C.Even at 250°C,detwinning rather than non-basal slips dominates tension along the ED of pre-strained specimens.All strain hardening curves of detwinning deform_-ation at temperatures of 30-250°C contain three stages that often appear in a{1012}twinn_-ing predominant deformation.The length of stage II is predominantly related to{1012}twin volume fraction.It is found that the peak hardening rate in stage II decreases with increased temperature.Our results also show that grain refinement could harden against detwinning-s sample more effectively than detwinning sample.In detail,with refining grain size from 45?m to 15?m,the gap in yield strength between detwinning-s sample and detwinning sample increases from 31 MPa,37 MPa to 44 MPa.A reduction in grain size serves to an enhanced activity of prismatic slip and accelerates dislocation-twin interaction.The dislocation-twin interaction induced by grain refinement would yield a large number of steps?BPs/PBs?on twin boundaries and be responsible for the higher efficiency in hardening against detwinning-s sample than detwinning sample.?4?On the one hand,despite of the similar solute atomic percent,four alloys show distinct difference in thermal stability.Annealed at a temperature from 250? to450?,the thermal stability of Mg-Y and Mg-Gd alloys is much better than that of Mg-Al and Mg-Zn alloys.This is attributed to the fact that Y and Gd atoms have a higher atomic size misfit than Al and Zn atoms,and to the fact that Y and Gd atoms have a lower diffusion rate than Al and Zn atoms.When annealed at 500? for 8 h,Mg-Y,Mg-Gd,Mg-Al and Mg-Zn alloys have a similar grain size and their thermal stability is similar.This might have some connections with the atomic percent of solute segregation at GBs.Generally,at a higher temperature,the average atom energy level is enhanced significantly and therefore more atoms can overcome the activation energy barrier for grain growth.On the other hand,the thermal stability of Mg-Al and Mg-Zn is also different.At a tempetature from 250? to 400?,the grain size of Mg-Zn alloy is evidently slower than that of Mg-Al alloy.At a tempetature from 350? to 400?,the thermal stability of Mg-Zn alloy is better than that of Mg-Al alloy,and this is attributed to the enhancement of solute segregation at a GB,yielding a higher pinning effect.In contrast,at a temperature of 438? or above,Al atom will have a higher trend to migrate to a GB.At a tempetature from 450? to 500?,Mg-Al alloy has a similar grain size with Mg-Zn.