Research On Microstructure Evolution And Strengthening-toughening Of Mg-Gd-Y-Zn-Zr Alloy By Thermomechanical Treatment

As the lightest metal structure and functional material,magnesium(Mg)and Mg alloys have attracted scholars owing to their low density,high specific strength and specific rigidity,excellent heat resistance,toughness,and easy recycling.Mg alloys have been widely applied in aerospace,national defense and military industry,transportation and communications.However,due to the poor plasticity and poor metal fluidity of Mg alloys at room temperature(RT),they are prone to cracking during the deformation process.In addition,the mechanical strength of Mg alloys especially the yield strength(YS)is low.And the mechanical properties are generally improved through multi-pass deformation,which hinders its application prospects to a certain extent.Therefore,the use of plastic processing technology has become the one of important means to improve its mechanical properties.Thus,it is essential to promote the development of Mg alloys and it is of great significance.Severe plastic deformation(SPD)technology can achieve the goal of fine grain,uniform microstructure,weaken texture,eliminating anisotropy and improving the mechanical properties of the alloy by accumulating higher strain.In this paper,Mg-13Gd-4Y-2Zn-0.5Zr(wt.%)alloy was used as the experimental material.Based on the traditional multi-directional forging(MDF)process on the condition of decreasing temperature,MDF process on the condition of isothermal temperature and the thermo-mechanical treatment(ITMT)method were conducted.The new method was aiming to improve the mechanical properties of the deformed alloy by adjusting the microstructure of the deformed alloy,especially to significantly enhance the YS of the alloy.This study were focused on the analysis of the change of grain size,the distribution law of the second phase particles,the law of texture evolution and the trend of the mechanical properties during the multi-pass MDF process.Through comparative analysis,clarifying the strengthening mechanism of Mg alloys.Meanwhile,it helped us understand the main contribution of grain boundary strengthening,dislocation strengthening,solid solution strengthening,precipitation strengthening and other strengthening methods to YS of deformed alloys via ITMT method.The specific research content and results were as follows:(1)The MDF experiment on the condition of decreasing temperature with the size of 30mm×50 mm×100 mm was conducted,and the analysis of microstructure and mechanical performance test of the alloy after each pass were also proposed.The deformation temperature of each pass was 480°C,460°C,440°C,420°C,and the average strain amount of each pass is nearly 3.6.Compared with the initial alloy,MDF method on the condition of decreasing temperature can significantly refine the grains.With the increase of the deformation passes and the decrease of the deformation temperature,the massive LPSO phases were broken and gradually formed deformation bands,and the grains were gradually refined.The average size of the alloy after each pass was 10.5?m,9.5?m,and 3.3?m,and2.4?m respectively.At the same time,as the deformation passes increased,the microstructure gradually became uniform.Except for the fist pass during the deformation process,a small amount of needle-like Mg₅(Gd,Y)phases were precipitated inside the grain.The spherical precipitation phases was formed at the grain boundaries.The network structure expanded to the inside of the grains,and the volume fraction of the precipitation phases gradually increased.It showed that a large number of second phase particles precipitated had the pinning effect on the grain boundary movement during the MDF process on decreasing temperature condition,inhibit the growth of dynamic recrystallized(DRXed)grains,and obtain uniform fine-grained microstructure.In addition,the ultimate tensile strength(UTS)of the alloy after each pass of deformation was 309 MPa,334 MPa,341 MPa and 357 MPa,and the YS was 255 MPa,292 MPa,298 MPa and 304 MPa,respectively.It can be found that during the MDF process,the YS of the alloy gradually increased with the decrease of the average grain size,which conformed to the Hall-Petch law.The increase of YS was mainly attributed to the grain refinement.(2)When other deformation conditions remain unchanged,only the deformation temperature of each pass was changed,that is,the MDF experiment was performed on the block-shaped Mg alloy by the isothermal MDF scheme,and the deformation temperature of each pass was 430?.It could be found that as the number of deformation passes increased,the bulk phase was gradually broken,and the grain size decreased accordingly.The average size of the alloy after each pass was 12.9?m,10.4?m,4.7?m,and 2.7?m,respectively.The UTS of the alloy after each pass of deformation was 316 MPa,333 MPa,351 MPa and 385MPa,and the YS was 262 MPa,291 MPa,289 MPa and 360 MPa,respectively.It can be found that during the isothermal MDF process,the YS of the alloy increased significantly as the number of deformation passes increased.When the deformation passes were the same,compared with the MDF process on the decreasing temperature condition,the alloy via MDF on the condition of isothermal temperature had a larger grain size,but YS value was equivalent.However,the 4MDFed alloy after isothermal MDF had a higher ratio of(YS/UTS),and the YS was significantly higher than the 4MDFed alloy on the decreasing temperature condition.At the same time,the 4MDFed alloy also had higher microhardness values.The microhardness values of the?-Mg matrix and the precipitation phases were 132HV and 164 HV,respectively.(3)The intermediate thermo treatment(ITT)systems were respectively introduced into the traditional MDF process on the decreasing temperature condition,the new method was named ad intermediate thermo-mechanical treatment(ITMT).The ITT system were the natural aging after solid solution(T4)and the artificial aging after solid solution(T6),respectively.It can be found that the average grain size of the MDFed alloys showed a significant decrease first and then increase slightly with the increase of the deformation passes.When the ITMT system was T4,the average grain size of the alloy after 2 passes was 3.7?m.The Mg₅(Gd,Y)phases existed withe the spherical shape in the 2MDFed alloy.When the ITMT was T6,the average grain size after 2 passes was 3.1?m.At the same time,the Mg₅(Gd,Y)phase of the alloy had two shapes of spherical and needle-like.With the increase of the MDF passes,the needle-like Mg₅(Gd,Y)phases gradually aggregated and precipitation free zone(PFZ)was formed.It can be found that when the ITMT process was introduced into the traditional MDF process on the decreasing temperature condition,the mechanical properties of the alloy were relatively stable and did not change significantly with the change of grain size.In addition,when the deformation pass was up to 2,the UTS and YS were 341MPa and 320 MPa of the alloy after T4 ITMT,and the UTS and YS were 356 MPa and 332MPa of the alloy after T6 ITMT.Compared with the traditional MDF on the decreasing temperature condition,ITMT method can refine grains and enhance YS significantly.