The Effect Of Cryogenic Treatment On The Microstructure And Mechanical Properties Of Mg-1.5Zn-0.15Gd Alloy

Magnesium alloys are the lightest structural materials for the industry of aircraft,spacecraft and missiles. However, the applications of magnesium alloys are stronglylimited by their low ductility, poor corrosion resistance and weak wear resistance.Thus, an efficient is urgently needed to overcome such shortcoming of magnesiumalloy.Cryogenic treatment (CT) means the treatment of material under cryogenictemperature (-230℃to-190℃). CT is widely applied in the manufacture of steelsince the hardness and wear resistance of steel are significantly improved after CT.The investigation of CT on magnesium alloy is very significant, while rare studies arereported. Therefore, the present work investigates the effect of CT on themicrostructure and mechanical properties of Mg-1.5Zn-0.15Gd alloy. The results arefollows:The microstructure observation of Mg-1.5Zn-0.15Gd alloy before and after CTis presented. The microstructure of as-extruded alloy is composed of α-Mg and Wphase. After CT for1min, there are some secondary phase particles precipitating.With increasing cryogenic duration, the volume fraction of precipitates increases aswell. After the selected area electron diffraction by transmission electron microscope,these precipitates are also identified as W phase. After CT for24h, the volumefraction of W phase increases from0.1%to4.3%. The precipitation mechanism of Wphase is due to the contraction of Mg lattice under cryogenic environment, since itneeds extra space as nucleation kinetics.The tensile tests of Mg-1.5Zn-0.15Gd alloy before and after CT are carried out.The ultimate tensile strength, yield strength and elongation of as-extrude alloys are205MPa，175MPa and8.6%，respectively. After CT for10min, the strength of alloyhardly changes, while the elongation increase to15.5%.When the cryogenic durationfurther increases to24h, the elongation of alloy keeps above14%, while strength stillhardly changes.The polarization curves of Mg-1.5Zn-0.15Gd alloy are given. The result shows that the current density decreases after CT for24h, which means the corrosionresistance of alloy is reduced by CT.Lattice density has been suggested as a criterion on the formation of precipitate.The lattice density means the number of atom in a unit lattice. Cryogenic treatmentcan be efficient only for the formation of precipitate with a much lower lattice densitythan the matrix. The result will provide the guidance to use the process of cryogenictreatment for Mg alloy, in order to maximum the precipitate strengthening effects.Finally, the Mg-9Al and Mg-6Zn alloys are used to verify the effectiveness ofthe lattice density criterion. The result agrees well with the theoretical predictions. Inaddition, it was found that CT can promote the aging precipitation of Mg-9Al alloyby “pre-aging” effect. The hardness of Mg-9Al alloys after “Solution-CT-Aging”treatment is7.5%higher than the one after traditional “Solution-Aging” treatment.As a systematic study to the precipitating behavior, microstructure evolution andmechanical properties of Mg alloy under cryogenic treatment, this work has presentsa new way for the research of Mg alloy. In addition, a new process has also beensuggested for Mg alloy, which may be with great significance and applicationprospects.