Microstructure, Mechanical And Corrosion Properties Of Magnesium Alloys Under High Pressure

Magnesium alloys exhibit the notable advantages to the automotive and aerospaceindustries as structural parts due to their low densities and high specific strength. The useof Mg alloys, however, has been limited because of their low strength and poor corrosionproperties. At present, super-high pressure （SHP） synthesis has attracted extensiveattentions because it is one of the most effective techniques to prepare new compounds orto obtain alloys with unique properties. This article selected three representativemagnesium alloys as the research object, i.e., the most widely used commercial Mg-Alalloy, the lightest Mg-Li binary alloy and biological material Mg-Y alloy （with a goodmechanical performance and a poor corrosion resistance）. The aim of this study was toinvestigate the effects of super-high pressure solidification on phase transformation,microstructures, mechanical behaviors and corrosion properties of these three magnesiumalloy. Results of the study show that:The variety of phases is same in both as-cast and super-high pressure Mg-30wt.%Al.However, the area fraction of Mg matrix is increased, while the area fraction of eutecticphase is reduced with increasing the pressure and temperature. Furthermore, as thepressure is increased, the Al concentration in eutectic phase is increased firstly, and then itremains a constant （44.1±0.5wt.%）. In addition, the lattice parameter a of Mg matrix isdecreased and the c/a value is increased owing to the increased Al concentration in Mgmatrix. The nano-indentation results indicate that both the hardness and Young’s modulusare improved with increasing the pressure, which is partially connected with a higher Alconcentration. The corrosion properties of the SHP-2GPa-800sample are improvedgreatly in contrast to the as-cast sample or other SHP samples, which is mainly attributedto the formation of a fine and continuous oxide film. In contrast to other super-highpressure samples, the Mg matrix is more homogeneous and the net-like eutectic phasebecomes finer after super-high pressure solidification at800℃under2GPa.Hardness and compression property of Mg-7wt.%Li alloy have been significantlyimproved by super-high pressure treatment at700℃. Compared with the as-cast sample, the microstructures of Mg-7Li alloy （SHP-4GPa-650℃, SHP-4GPa-700℃,SHP-4GPa-850℃） have been greatly changed, i.e., the morphology was changed fromdendrites to trigeminal grain. Moreover, the microstructural observation has confirmedthat super-high pressure is of benefit for the phase transformations from bcc-Li₃Mg₇tohcp phases, and from hcp-Li₃Mg₁₇to hcp-Li_0.92Mg_4.08. The improvement of mechanicalproperties is mainly associated with the reduction of bcc-Li₃Mg₇phase and the formationof compression twins in hcp-Li_0.92Mg_4.08phase.High pressure technology was employed to improve the corrosion properties of Mg-Ybased alloys. The corrosion properties are improved owing to the continuous dendriteboundaries and the formation of oxide layer on the surface.