| The bottlenecks that hinder wide application in magnesium alloys are low absolute strength and poor plastic deformation at room temperature.Slip and twinning are important mechanisms of plastic deformation in magnesium alloys.Because of limited slip systems at room temperature,twinning plays a very important role in the plastic deformation of magnesium alloys.It is found that annealing strengthening in twin-structured magnesium alloys is an interesting phenomenon in recent years,that is,pre-deformation and the intermediate annealing can make the solution atoms segregate in the twin boundaries to produce pinning effects.The pinning effects can hinder the growth of twins in the subsequent deformation,resulting in an annealing strengthening.However,the microstructure evolution of the alloys during the subsequent deformation is not yet studied when the twin boundaries are pinned.The study on the microstructure evolution of twin-structured magnesium alloys will help us to understand the deformation mechanism of twins better and provide a scientific basis for regulating the twinning behavior.In this study,two kinds of Mg-Nd alloys with Nd atomic percentage of 0.03%(1#)and 0.18%(2#)were selected,and the evolution of twinning in the process of further deformation was investigated and compared.For the selected extruded bar,the pre-compression experiment was carried out to produce the pre-straining sample,and then a part of the sample was annealed at 200℃for 6h before re-compression.The other part of the sample was not annealed before the re-compression experiment.In the experiment,the evolution process of the twins was traced by electron back-scattered diffraction(EBSD)analysis.The characteristic parameters of twins are discussed,such as the number of twins,the volume fraction of twinning,the Schmid factor(SF)of twin nucleation and growth,the contribution of twin nucleation and growth to twin processes.The effects of the intermediate annealing and the alloy elements on the twin evolution are also discussed.The major conclusions are summarized as follows:(1)For the twin evolution process of 1# alloy with intermediate annealing,the volume fraction of twins increased by 14% after re-compression,the contribution of nucleation and growth is 14% and 86%,respectively.Twin growth is dominant mechanism during re-compression.The number fraction of twin nucleation and growth is 24% and 38%,respectively.The average Schmid factor(SF)of twin nucleation and growth is 0.35 and 0.47,respectively.The SF of twin nucleation is mainly distributed in the range of 0.4-0.3.The SF rank of twin nucleation is mainly R1 and R2.The SF of twin growth is mainly distributed in the range of 0.5-0.4,and the SF rank of twin growth is mainly R1 and R2.(2)For the twin evolution process of 1# alloy without intermediate annealing,the volume fraction of twins increased by 22% after re-compression,the contribution of nucleation and growth is 59% and 41%,respectively.Twin nucleation is dominant mechanism during re-compression.The number fraction of twin nucleation growth is 22% and 39%,respectively.The average Schmid factor(SF)of twin nucleation and growth is 0.35 and 0.40,respectively.The SF of twin nucleation is mainly distributed in the range of 0.5-0.4.The SF rank of twin nucleation is mainly R1 and R2.The SF of twin growth is mainly distributed in the range of 0.5-0.4,and the SF rank of twin growth is mainly R1 and R2.(3)For the twin evolution process of 2# alloy with intermediate annealing,the volume fraction of twins increased by 3.7% after re-compression,the contribution of nucleation and growth is 35% and 65%,respectively.Twin growth is dominant mechanism during re-compression.The number fraction of twin nucleation and growth is 36% and 32%,respectively.The average Schmid factor(SF)of twin nucleation and growth is 0.19 and 0.29,respectively.The SF of twin nucleation is mainly distributed in the range of 0.1-0.The SF rank of twin nucleation is mainly R1 and R3.The SF of twin growth is mainly distributed in the range of 0.5-0.4,and the SF rank of twin growth is mainly R1 and R2.(4)For the twin evolution process of 2# alloy without intermediate annealing,the volume fraction of twins increased by 4.1% after re-compression,the contribution of nucleation and growth is 60% and 40%,respectively.Twin nucleation is dominant mechanism during re-compression.The number fraction of twin nucleation and growth is 44% and 28%,respectively.The average Schmid factor(SF)of twin nucleation and growth is 0.19 and 0.29,respectively.The SF of twin nucleation is mainly distributed in the range of 0.2-0.1.The SF rank of twin nucleation is mainly R1 and R2.The SF of twin growth is mainly distributed in the range of 0.4-0.3,and the SF rank of twin growth is mainly R1 and R2.(5)For the twin evolution process of 1# and 2# alloy without intermediate annealing,twin nucleation is dominant mechanism during re-compression.However,for the twin evolution process of 1# and 2# alloy with intermediate annealing,twin growth is dominant mechanism during re-compression.This shows that intermediate annealing can change the dominant mechanism of twinning precoss during re-compression in 1# and 2# alloy.(6)For the twin evolution process of 1# and 2# alloy with intermediate annealing,the contribution of nucleation increases with the increase of Nd content during re-compression.However,for the twin evolution process of 1# and 2# alloy without intermediate annealing,the contribution of nucleation decreases with the increase of Nd content during re-compression. |
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