Research On The Deformation Mechanism Of <c+a> Dislocations And Stacking Faults In Rare-earth Free Mg-Ca-Sn Alloys

With a lower density,Mg alloys are important structural materials and have a unique advantage in automotive and aeronautic components.However,the main challenge for these applications of Mg alloys is to further improve the strength and ductility,especially the research and understanding about the plastic deformation mechanism at room-temperature are not enough.Based on this,we studied the deformation mechanism of Mg alloys containing Ca recently developed in the present work.We show that the effect of component and strain-rate on dislocation,stacking fault and other defects,the following statements are obtained:(1)When Mg-2Sn-1Ca(TX21),Mg-2Sn-2Ca(TX22)and Mg-2Ca(X2)alloys are tensioned or compressed,the tension/compression asymmetry is appeared due to the totally different deformation mechanisms.After 3%tensile deformation,only the<a>and<c+a>slipping are activated in above alloys,no obvious twinning is found.And as the content of Ca dissolved in ?-Mg matrix increases,the main deformation mode changes from basal<a>slipping in TX21 alloy to pyramidal<c+a>slipping in TX22 and X2 alloys.Maybe it should be attributed to that Ca element can reduce the SFE of ?-Mg matrix.While 3%compression deformation,twinning becomes the main deformation modes in three kinds of alloys,with a few<a>dislocations accompanied within the deformed grains.(2)When TX21,TX22 and X2 alloys have a 6%compression deformation,most of the grains have been fully twinned with a large number of<c+a>dislocations.Stacking faults are also observed in twinned grains,and the density will increase with higher dissolved Ca content.In contrast,after a 6%compression deformation at strain-rate of 10-3/s,It can be seen that twinnability of above alloys is suppressed by reducing the compression rate.Basal slipping and pyramidal slipping are operated in twinned grains,while a few obvious SFs can only be found in TX22 and X2 alloys.No SFs are observed in the regions where no twinning occurred,it can be seen that the SF activation is closely associated with deformation twinning.Increasing the Ca content and strain rate can promote to activate more SFs.(3)When pure Mg and Mg-0.3Ca alloys are compressed at a strain-rate of 10-4/s or 0.667/s,their working-hardening behaviors are obviously different.Mg-0.3Ca alloys can keep a high work hardening rate at a lower strain rate.The microstructure indicates that it is associated with<c+a>dislocation dissociation.Lots of linear partial<c+a>dislocations along the basal plane are observed.It can contribute to reduce the density of mobile dislocation and need greater stress to accommodate deformation,causing the elongation reducing.The tiny<c+a>dislocations in Mg-0.3Ca alloys compressed at 10-4/s strain-rate are still undissociated.It can provide mobile dislocation for plastic deformation continuously and contribute to a higher elongation.