An investigation on the microstructure of an AM50 magnesium alloy

The microstructure and the dislocation arrangement in the die cast AM50 magnesium alloy as well as in the stressed states have been investigated using conventional transmission electron microscopy (TEM), high-resolution TEM and energy dispersive X-ray analysis. The microstructure of the die cast AM50 alloy is found to mainly consist of α-Mg, β-Mg17Al12 and Al8Mn5 phases. Two kinds of β-Mg17Al12 particles with different sizes have been found in the stressed AM50 magnesium alloy. Besides the normal β-Mg17Al12 particles with size around several micrometers as in the die-cast AM50 magnesium alloy, some finer β-Mg17Al12 particles in the stressed states of the AM50 alloy have also been found. The finer β-Mg17Al12 particles are found to be only about tens of nanometers with oval or rod-like morphology, which may be formed during deformation. Dislocation pile-ups have been found in the stressed AM50 alloy for the first time. The spacing between each parallel dislocation in the pile-ups is only several nanometers. The dislocations are confined in the slip planes and piled up against grain boundaries. Dislocations in the networks are found to increase with deformation of the alloy. Also, dislocation networks have been found in the β-Mg17Al12 and Al8Mn5 phases as well as in the matrix in the deformed AM50 magnesium alloy. However, the dislocation pile-ups are found to be almost identical from 1.3% deformation to rupture, which explains the stable tensile yield strength of the AM50 magnesium alloy during the deformation.