Study On Annealing Strengthening Behavior And Twin Interaction Of Mg-Nd(Gd)-Zr Alloy

Due to low density,high specific stiffness,and good electrical conductivity,magnesium alloys have been getting more and more attention in the automotive,aircraft,aerospace,and electronics industries.However,further applications are limited in actual production and life by its low strength and poor plasticity.Although the strengthening mechanism of magnesium alloys and the development of new high-strength magnesium alloys have been widely researched,the strength is so far much lower than that of the corresponding aluminum alloy.The latest researches show that both the pre-deformed magnesium alloy followed by the annealing treatment and the twin interaction induced by the continuous deformation can significantly improve the strength of the magnesium alloy and the related research work will certainly provide a theoretical basis for designing high-strength magnesium alloys.In this paper,optical microscope,scanning electron microscope,X-ray diffraction,universal testing machine and transmission electron microscope are used to investigate the microstructure,mechanical properties,annealing strengthening phenomenon and twin-twin interaction of Mg-1.5Nd-x Zr and Mg-2.7Gd-x Zr alloys treated with solution treatment,compression deformation,process annealing and multi-pass rolling and the conclusions are drawn as follows:The microstructures of as-cast Mg-1.5Nd-x Zr and Mg-2.7Gd-x Zr alloys consist of?-Mg matrix and the second phase,Mg₁₂Nd and Mg₅Nd,respectively,mostly distributed along the grain boundaries.With the increase of Zr content,the grain size of significantly decreases from 504.8±136?m to 70.3±21?m for Mg-1.5Nd-x Zr alloy and from 381.1±122?m to 83.4±30?m for Mg-2.7Gd-x Zr alloy decreased.The optical solution treatment process for the two alloys is 550?×9h and 500?×6h,respectively,and the grain size of the two alloys treated by this process does not grow up.Yield strength of pre-deformed Mg-Nd-Zr and Mg-Gd-Zr alloys does not increase during the re-compression,whose further deformation mechanism is the growth of the pre-deformed twins.However there is a obvious annealing strengthening effect during the recompression of the two pre-deformed alloys followed by process annealing and the main coordinating deformation mechanism is the formation of new twins rather than the growth of the pre-deformed twins for the segregation of Nd and Gd in the pre-deformed twin boundary during the annealing process to hinder the twin growth,which lead to the improvement of the alloy strength.Moreover,the pinning strengthening effect of solution atom Nd is stronger than that of solution atom Gd.In addition,as the grain size decreases,the annealing strengthening effects of the two alloys firstly increase and then almost keep at a certain level.It has been found in the twin interaction mechanism of as-rolled Mg-2.7Gd-0.3Zr alloy that the deformation mechanism of the grain with the single twin variant is the growth of the variant during the continuous compression along the TD direction,whereas the continuously compression deformation mechanism of the grain with the interaction between two twin variants is the nucleation of the new twin.When the continuous compression samples are treated by the process annealing,the following compression mechanism is the formation of the new twin rather than the growth of the original twin.It has also been found in the interface evolution during twin interaction of as-rolled Mg-2.7Gd-0.3Zr alloy that when the incident twin approaches to the barrier twin,the barrier twin is actively close to the top of the incident twin and interacts with the incident twin,then with the blunting and thickening of the top of the incident twin,the continuous or discontinuous interaction interface has been formed during the intersection.