Microstructure And Mechanical Properties Of Magnesium Alloys Fabricated By Cyclic Extrusion Compression

The purpose of this paper is to study the microstructure and mechanical properties of AZ31, AZ91 and AZ31-1Si alloys after cyclic extrusion compression (CEC). The effects of CEC pass and CEC temperature on the grain size, grain boundaries structure and texture of AZ31 and AZ91 Mg alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The effect of the second phase with different volume and properties on CEC microstructure and texture was studied. The grain refining mechanism for Mg alloys during CEC was discussed. The mechanical properties at room temperature of as-extruded and CECed AZ31, AZ31-1Si and AZ91 were measured. The fracture mechanism of as-extruded and CEC Mg alloys were investigated by SEM and in-situ EBSD tension. At last the strength and ductility of Mg alloys after CEC were discussed. The main results can be summarized as follows,The microstructure of AZ31 alloy after CEC passes of 1-25 at 300℃was studied. The results show that magnesium alloys can be refined effectively by CEC. The most effective CEC pass is the first pass and then decrease. There will be a critical CEC pass to obtain the final grain size. There is no difference in microstructure of middle area between cross section and longitudinal section. However, significant difference in microstructure between central and peripherial areas of longitudinal section can be obtained at initial strains, which decreases and almost disappearance with increasing strains.The grain refines continuously and low angle grain boundaries (LAGBs) tend to decrease while the average misorientation tends to increase with the increase of CEC pass. The grain size of 25μm, LAGBs of 28.7% and average misorientation of 34.6 are obtained in as-extruded AZ31 alloy and the mean grain size of 1.77μm with fine grains of 150±50nm, LAGBs of 7% and average misorientation of 54.8 can be obtained in AZ31 alloy after CEC 7 passes and 300℃. The fine grains in CEC microstructure tend to form network structure. The original network structure is subdivided to more even network due to more fine grains formed with the increase of accumulated strains. The gather level of fine grains decreases with increasing second phase.The microstructure of AZ31 alloy after CEC 3 passes at temperature of 225℃-400℃was investigated. The results show that it is good for the decrease of grain size and LAGBs and the increase of average misorientation and grain boundary line length/ area as CEC temperature increase. The relationship between grain size and Z parameter can be described as ln d = -0.076 ln z+2.571The microstructure of AZ31, AZ31-1Si and AZ91 alloy after CEC 7 passes at 225℃was compared. The results show that the Mg17Al12 phase can be refined and redistributed by CEC and tends to present network distribution in microstructure. Fine Mg17Al12 phase can help the refinement of coarse grains, the increase of misorientation and the formation of high angle grain boundaries (HAGBs) but little affect on fine grains with the size less than Mg17Al12. The efficiency of Mg17Al12 to contribute grain refinement is better at lower CEC temperature. The coarse mass Mg2Si phase can also be refined but can not be redistributed by CEC. The grain size and grain boundary structure are little affected by Mg2Si.The texture components of Mg alloys are affected by CEC pass, CEC temperature and the second phase. Among them, CEC pass is the most important factor. The texture intensity decreases as CEC pass increases. The texture intensity tends to increase as CEC temperature increases. The texture intensity decreases as the second phase increases. Most grains are difficult to slip at as-extruded and CEC AZ31 1 pass while most grains with available misorientation to slip after CEC 3 and 7 passes.The grain refining mechanism of Mg alloys during CEC can be described as a compound grain refining mechanism, which combined both the Continuous Dynamic Recovery and Recrystallization (CDRR) and Rotation dynamic recrystallization (RDRX) assistanted by Discontinuous Dynamic Recrystallization (DDRX).The mechanical properties of AZ31 and AZ91 alloys after CEC at 300℃and AZ31-1Si alloy after CEC at 225℃were studied. The results show that the elongation of Mg alloys increases as CEC pass increases. The yield strength increases obviously at CEC 1 pass. However, which decreases sharply and presents inverse Hall-Petch relationship with increasing strains. The elongation of AZ31 after CEC 7 passes at 300℃reaches 35.52%, which is 2.2 times of as-extruded AZ31 alloy. The yield strength of AZ31 after CEC 1 pass increases 20MPa, up to 209.69MPa and next decreases to 140.48MPa after CEC 7 passes. The mechanical properties of AZ31, AZ31-1Si and AZ91 alloy after CEC 3 passes at 225℃-400℃were investigated. The results show that the yield strength of Mg alloys after CEC decreases and the elongation tends to increase as CEC temperature increases. When CEC temperature increases from 225℃to 400℃, the yield strength of AZ31 alloy continuously decreases from 166.61 MPa to 103.89 MPa. The elongation is always above 30%. The grain size a1nd yield strength is consistent with Hall-Petch relationship, that is,σs = 48.88 + 280.48d?2The improvement of ductility for Mg alloys after CEC depends on the change of fracture mode. AZ31 alloy with coarse grains is intracrystalline and shear fracture. The fine grained AZ31 alloy fractures along grain boundaries and the boundaries between matrix and the second phase. The Mg17Al12 and Mg2Si phases are the main crack source during deformation.Grain rotation and the formation of new fine grains appear during tensile deformation of fine grained AZ31 Mg alloy. HAGBs, grain number and the average misorientation increases but the texture intensity decreases during tension. The grains having {0001}or {2-1-10} plane being parallel to tensile direction tend to be stable, while grains having other planes are unstable and tend to rotate to {0001} and {2-1-10} during tension.The effects of dislocation intensity, grain boundary structure, texture, grain size and the second phase et, al on the strength and ductility of Mg alloys after CEC were studied. The results show that the yield strength improves as the increase of dislocation intensity, LAGBs and the fine second phase (≦ 1μm). The improvement of ductility depends on grain refinement and texture optimization with high value of Schmid factor.