Microstructure Evolution And Strengthening Mechanism Of ZK60 And GW102K Alloys Fabricated By Cyclic Extrusion And Compression

Cyclic extrusion and compression (CEC) was used to produce ultra-fine grain size ZK60 and GW102K Mg alloys, in order to improve their mechanical properties and plastic deformation ability at room temperature (RT). The emphases were placed on the microstructure evolution, grain refinement mechanism, plastic deformation behavior, and the strengthening mechanism of CEC processed magnesium alloys.In order to optimize and provide an insight into the mechanics of the CEC processing, the flow field, stress field, temperature field and strain field of ZK60 alloy during CEC was simulated using finite element method (FEM). The effects of process parameters on the distribution of strain were investigated. Physical modeling (PM) experiment with same material was carried out to verify the results of the numerical simulations. Results show that the die geometry and process parameters have a significant effect on the strain distribution. A bigger corner radius and a lower extrusion angle are useful to improve the strain homogeneity. A little friction between die and billet is beneficial to strain homogeneity, but the high friction is detrimental. Result of FEM and PM shows that two vortex flow regions with opposite flow direction are formed inside the cylindrical billet during CEC deformation. Although the deformation is inhomogeneous in both end regions of billet, a uniform region of equivalent strain exists, and the extent of uniform deformation increased with the increase of billet length.The effects of CEC pass and CEC temperature on the grain size, grain boundaries structure and texture of ZK60 and GW102K Mg alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The results show that magnesium alloys can be refined effectively by CEC, and the grain refining efficiency decreases with the increase of CEC pass number. Low CEC temperature and high CEC pass should be used to produce ultra-fine grain size Mg alloys. With the increase of the pass number or decrease of the deformation temperature, the number fraction of low angle grain boundaries (LAGBs) tends to decrease while the average misorientation tends to increase for both of ZK60 and GW102K alloys.Texture evolution of ZK60 and GW102K alloy during CEC processing was studied using X-ray diffraction (XRD) and EBSD. Results shows that the texture of Mg alloys are affected by CEC pass number, CEC temperature and the second phase. The initial ED // < 10 10> fiber texture of extruded Mg alloys was disintegrated after CEC processing and developed a new < 2 201> fiber texture, in which the basal plane inclined 20～30o to extrusion direction. The texture intensity tends to decrease with the increase of accumulated strains or with the decrease of deformation temperature or with the increase of second phase. Thus, compared to pure Mg and ZK60 alloy, the intensity of texture of GW102K alloy is lowest.The grain refining mechanism of Mg alloys during CEC was studied. Results show that the primary refining mechanism is dynamic recrystallization (DRX). In the condition of low deformation temperature and low accumulated strains, the refining mechanism is dominated by discontinuous dynamic recrystallization (DDRX) and assisted by continuous dynamic recrystallization (CDRX) and rotation dynamic recrystallization (RDRX). In the case of high temperature and high CEC pass, the refining mechanism is combined both the CDRX and RDRX, while assisted by the DDRX. For the second phase, it is mechanical cracking refining mechanism.The effect of CEC processing on the mechanical properties of ZK60 and GW102K alloys were investigated by tensile testing at RT. Results show that the yield strength (YS) and ultimate tensile strength (UTS) of ZK60 alloy decreased with the increase of CEC pass, although the grain size were refined dramatically. On the other hand, the elongation of ZK60 alloy increased obviously after CEC processing. Different from ZK60 alloy, the YS and UTS of GW102K alloy increased with the increase of CEC pass. The grain size and YS of GW102K alloy is consistent with Hall-Petch relationship. Similar to ZK60, the elongation of GW102K alloy was dramatically increased after CEC. The temperature has similar effect to both of ZK60 and GW102K alloys, i.e., the YS and UTS decreased and elongation increased with the increased of the CEC temperature. Moreover, the intensity of strength-differential effect (SDE) of as-extruded ZK60 alloy was decreased noticeably.The plastic deformation behavior and fracture mode of CEC processed ZK60 and GW102K alloys at RT were investigated by OM, SEM and TEM. Results show that the plastic deformation of as-extruded Mg alloys tends to be rather heterogeneous. In the coarse grains the twinning is predominant deformation mechanism and in the fine grains it is dislocation. Twin boundaries are main crack nucleation sites. After CEC the non-basal dislocation activated and the fraction of twinning decreased, and the deformation became more homogeneous. Furthermore, the main fracture mode changed from quasi cleavage to ductile dimple.The effect of second phase on the microstructure and mechanical properties of ZK60 and GW102K alloys were studied. Results show that the volume of Mg24(Gd, Y)5, i.e. the main second phase in GW102K alloy, is about 5～9%. It is much more than that of MgZn2 in ZK60 (2～4%). The MgZn2 is fine and homogeneously distributed in matrix. Whereas, the Mg24(Gd, Y)5 mainly distributed near the grain boundaries. During CEC processing, the grain boundary sliding (GBS) in GW102K alloy was retarded. As a result, GW102K alloy has a higher strength and a lower elongation and a lower texture intensity than that of ZK60 alloy. StudyThe mechanical properties of CEC processed Mg alloy depends on its grain size, texture type, texture intensity, second phase volume and distribution, dislocation density, grain boundary structure, and so on. The strengthening mechanism can be described as a compound strengthening mechanism, which predominated by grain refinement strengthening, texture strengthening and second phase strengthening.