热轧变形Mg-1.5Zn(-0.2~1.0)Gd合金再结晶组织及织构演变规律研究

Recrystallization and Texture Mechanism of Hot Rolled Mg-1.5Zn-(0.2~1.0)Gd Alloy

研究了稀土Gd元素对变形Mg-1.5Zn合金热轧及退火组织及织构的影响规律。结果表明:稀土Gd元素加入Mg-1.5Zn合金中,经过热变形及后续退火处理,织构得到明显弱化并且沿着横向(TD)方向发生分裂,随着合金中Gd元素含量的增加,偏转的角度有所减小。当Gd元素含量为0.2%时,Mg-1.5Zn-0.2Gd合金的基面织构强度最大值为2.3,并且合金的宏观织构特征为c轴沿轧板TD方向偏转大约40°;随着Gd元素含量的增加,合金基面织构强度最大值有所增加,但最大强度仍小于2.8,c轴沿轧板TD方向角度逐渐减小,偏转大约30°。进一步研究表明,含稀土的细小第二相粒子是影响镁合金再结晶过程的主要因素,稀土第二相粒子弥散分布在原始晶界处,拖拽住原始大角度晶界的迁移,抑制了具有基面取向的晶粒形成,有效地控制了合金的基面织构,为具有随机取向的晶粒形核、长大提供了有利条件,从而使合金织构取向随机分布。然而,随着稀土Gd元素含量的增加,导致含稀土的第二相尺寸增大,对合金原始大角度晶界迁移的阻碍作用减弱,因此合金织构强度有所增加。

With the development of automobile,3C electronics and aerospace,the demand of lightweight becomes more and more important.Rare Earth elements can change or weaken the basic surface texture of magnesium alloy,improve the forming ability of magnesium alloy at room temperature and help to expand the application range of magnesium alloy.Gd addition in Mg-Zn alloys can change the recrystallization process and effectively modify the basal plane texture during rolled and subsequently annealed.The main reason affecting the crystallization process is the very fine particles,which include Mg,Zn,Gd,precipitated on grain boundary restricts high angle grain boundaries migration,and inhibit basal plane texture formation.The grain with random orientation nucleation and growth when the very fine particles restrict high angle grain boundaries migration,resulting in the random texture of Mg-Zn-Gd alloys.Four magnesium alloys denoted as Mg-1.5Zn,Mg-1.5Zn-0.2Gd,Mg-1.5Zn-0.5Gd and Mg-1.5Zn-1.0Gd were prepared in this study.The alloys were prepared with high purity Mg(99.9%),Zn(99.9%),and Mg-30%Gd by vacuum melting.Ingots were prepared by pouring the melt into a preheated steel mold.The ingots were homogenized at 450℃for 12 h and then quenched in water.The ingots were subsequently machined to slabs with a dimension of 120 mm×90 mm×10 mm.The slabs were rolled to sheets with a final thickness of 1 mm at a reduction of 15%~20%per pass.After each pass,the sheets were reheated to 450℃and held for 15 min to maintain a consistent rolling temperature.The study of deformation and recrystallization of Mg-Zn-Gd alloy showed that the macrotexture of Mg-Zn alloy could be effectively changed by adding different content of rare earth Gd into Mg-1.5Zn alloy,the strength of the basic plane texture of the original magnesium alloy was weakened,which was greatly different from that of the general magnesium alloy.A large number of studies on the recrystallization behavior of magnesium alloys showed that neither dynamic recrystallization nor static recrystallization could change the basic plane texture of magnesium alloys.In this work,even if 0.2%Gd was added to Mg alloy,the texture of Mg alloy could be changed significantly,which showed that rare earth elements played an important role in the deformation and recrystallization of Mg alloy.The results of the study on the solid solution atoms and the second phase particles of the Mg-1.5Zn-0.2Gd alloy showed that,the deformation mechanism of Mg-Zn-Gd alloy and the nucleation mechanism induced by second phase particles were not the main factors that affected the texture evolution of Mg-Zn-Gd alloy.It was found that the addition of rare earth elements to Mg alloy would produce a large number of small second phase particles dispersed in the grain boundary,the migration of the original large angle grain boundaries(including the original twin boundaries)could be dragged effectively,so that the grain with base plane orientation could not be migrated,thus inhibiting the formation of the grain with base plane texture and effectively controlling the base plane texture of the alloy.And those grains with random orientation nucleated at the original general large angle grain boundary or twin grain boundary.As the recrystallization proceeds,the newly formed grains presented a"necklace-like"distribution,gradually swallowed up the deformed grains,and finally completed recrystallization,the macroscopical texture of Mg alloy tended to random distribution.It should be noted that during the recrystallization of Mg-Zn-Gd Alloy,the(0002)grains with the orientation of 30°~40°were preferentially grown in the direction of TD,so that the macrotexture of the final alloy was not completely random,however,there was a weak texture at the orientation of the grains.Gd element of rare earth had important effect on the weakening of Mg alloy macrotexture.The fine and dispersed rare earth second phase particles at the original grain boundary could effectively pin the migration of the original large-angle grain boundary,inhibit the formation of the grains with basal texture,and provide the conditions for the nucleation and growth of the grains with random orientation,however,with the increase of Gd content,the size of the second phase containing rare earth increased,which weakened the resistance to the migration of the original large angle grain boundary,so the texture strength of the alloy increased.