累积叠轧焊AZ31镁合金微观组织和织构演变的EBSD研究

对AZ31镁合金热轧板在350℃进行了累积叠轧焊(ARB)变形,采用EBSD技术研究了AZ31镁合金的微观组织和织构演变.结果表明,ARB可以显著细化AZ31镁合金的晶粒组织,经过3道次变形后平均晶粒尺寸为2.18μm,后续的ARB变形使AZ31镁合金的微观组织更均匀,但晶粒不会再显著细化,说明存在临界ARB变形道次,使晶粒细化和晶粒长大之间达到动态平衡.AZ31镁合金在ARB变形过程中的晶粒细化机制为连续动态再结晶,尤其还观察到了旋转动态再结晶.动态再结晶的形变储存能来源于多道次累积的剧烈应变和沿厚度方向分布复杂的剪切变形.ARB变形过程中旋转动态再结晶和剪切变形使新晶粒c轴发生旋转,导致基面织构弱化.

AN EBSD STUDY ON THE MICROSTRUCTURE AND TEXTURE EVOLUTION OF AZ31 MAGNESIUM ALLOY DURING ACCUMULATIVE ROLL-BONDING

Accumulative roll-bonding(ARB) process is appropriate to manufacture nanocrystalline and ultrafine grained sheets and plates which are most widely used material shapes in the commercial and industrial fields.The ARB process was proved to be very effective in refining grains and enhancing the strength of aluminum,steels and copper.However,the ARB was used only for cubic materials and rarely for hcp structured metal.The information available in the literatures about the microstructure change of magnesium alloys during the ARB process is still very limited.ARB was applied to AZ31 magnesium alloy sheets in the present work.ARB procedures were repeated for up to 5 cycles.Before each ARB cycle,the stacked sheets were heated at 350℃for 5 min in an electrical furnace near the rolling mill.The microstructure and texture evolution of an AZ31 during ARB were characterized by electron backscatter diffraction(EBSD).The results show that ARB is an effective grain refinement method for producing AZ31 sheets with fine grain structure.Significant grain refinement was achieved after 3 ARB cycles with average size of about 2.18μm.Grain refinement almost occured during the first three cycles and the distributions of grain size became more uniform as the cycle number increased.The results confirm the existence of critical ARB cycles to obtain dynamic balance between grain refinement and grain growth during the ARB process.Fraction of high angle grain boundaries increased with the increase of ARB cycles.Continuous dynamic recrystallization (CDRX) including rotation dynamic recrystallization(RDRX) occured during ARB of AZ31 as the grain refinement mechanisms.The dynamic recrystallization of AZ31 was activated and enhanced by the accumulated severe strain and shear strain across sheet thickness during ARB.In addition, large strain rate during ARB also contributed to grain refinement with the increase of Zener-Hollomon parameter.Microtextures of AZ31 tended to decrease and the average Schmid factor increased during the first three ARB cycles due to the sever and complex distrubution of shear strain,rotation dynamic recrystallization and rotating the new grains during ARB.