Microstructure evolution of AZ series magnesium alloys during cyclic extrusion compression

Abstract

Electron Backscatter Diffraction (EBSD), transmission electron microscopy (TEM) and optical microscopy (OM) are employed to characterize the microstructure evolution of AZ31, AZ61 and AZ91 Mg alloys during cyclic extrusion compression (CEC). The results show that CEC is an efficient grain refinement method for magnesium alloys. The most effective CEC pass is the first pass. There exists a critical CEC pass for three Mg alloys and beyond this critical pass the grain size and the homogeneity of microstructure retains almost constant. With increasing CEC passes, the fraction of low angle grain boundaries (LAGBs) tends to decrease and the average misorientation angle increases. Typical network shape structure of fine grains is obtained in the microstructure of AZ31 Mg alloy with fewer Mg 17Al 12 particles after CEC 7 passes. The microstructure of the different Mg alloys at the same number of passes becomes more homogeneous as Mg 17Al 12 particles increase from AZ31, AZ61 to AZ91. The Mg 17Al 12 particles promote increases of the fraction of high angle grain boundaries (HAGBs), the dislocation density, the refinement process of coarse grains at initial CEC pass, and the texture randomization and delay the formation of network shape structures.