Study On The Deformation Twinning Behavior In Magnesium Alloy

Magnesium is an attractive material due to having the lowest density among engineering metal materials.Magnesium alloys also show a good ability to be casted,molded,machined,and welded,and exhibit many characteristics that are very desirable in several industries.So Magnesium alloys show many potential benefits used in transport,communication and daily life,especially in the automobile and aircraft applications.Rolled AZ31 sheet exhibits poor formability due to its strong basal texture,which limits the use of magnesium alloys.Extension twinning behavior and mechanical properties during interrupted compression was studied by means of optical microscopy,X-ray diffraction and electron backscatter diffraction.The results show that:(1)The initial orientation has an important effect on twinning nucleation.Twin nucleates more easily at grain boundaries whose misorientation is lower.The stress state is sample under uniaxial loading and the selection of twin variants follows Schmid law,i.e.the variants with high Schmid factor would be activated first.(2)At the early stage of deformation(strain to ~2%),there was a dramatic increase in the twin boundary due to the nucleation of twins.The fraction was maintained almost constant in the range of 2%–6% strain,arising from the growth of existing twins.However,it subsequently decreased although the strain increased.From the perspective of twinned volume fraction,it increased with the strain until twinning stopped.The twin boundary fraction is decreased owing to the coalescence of growing twin,while the twinned volume fraction would increase as twins grew.(3)At the early deformation stage,generally,extension twinning variants with highest Schmid factor would be activated first and with strain increasing,other low Schmid variants could be activated to accommodate further deformation.Throughout the entire deformation process,twinning could be observed in some matrix grains whose initial orientation deviates the basal texture while twinning variants with high Schmid factors did not appear in several grains even at the late stage of deformation.This is due to the fact that these active twins need small accommodation strain from neighboring grains,which makes it easier to accommodate local deformation.(4)The paired twins appear in adjoined grain boundary whose misorientation angle is low and the lower misorientation of adjacent grain,the greater propensity to generate paired twins.The misorientation between paired twins is almost the same with that of their matrixes,which indicates that their formation is strongly dependent on their parents’ misorientation.Besides,the initiation of both paired twins satisfy the first rank Schmid factor while they do not necessarily share the same twin variant.The “sequential twinning” nucleation and synergetic growth mechanism can be used to explain the nucleation of paired twins at adjoined grain boundary and the following growing process.Continuous twin bands across grain boundaries into neighboring grains is capable of transmitting the localized shear and releasing stress at the former twin-termination interfaces,which plays an important role in accommodating plastic strain and mediating multiple grains’ deformation.(5)Different twin variant could be activated in the same grain and their further growth would be influenced by the crossing or branching intersection between these variants,of which,in particular,with a high misorientation angle around 60° introduced a significant retarding effect on the migration of twin boundaries.This special crossing phenomenon is ascribed to the release of stress by glide of the slip dislocations converted from twinning partials in the intersecting region,which could be well explained by the priori emissary slip theory.(6)After each step of deformation,the yield strength of the re-compression along RD is enhanced for different mechanism concerning the extension twins at various strain levels.When sample is deformed to 6% strain,the yield strength is raised because of grain subdivision of twin boundaries.Beyond this,twins began to coalesce and the sessile dislocation increases dramatically with twin fraction,which leads to the further strengthening of material.Finally,at larger strain,the strengthening mechanism will be controlled by texture hardening resulting from the transition from the soft orientation to the hard one.