Study On The Precipitation Strengthening Mechanism Of Rare Earth Magnesium Alloys

Magnesium and its alloys are the lightest metal materials in engineering applications.They have received extensive attention and broad application prospects in aerospace,transportation,defense military,and “3C”.Unfortunately,magnesium alloys have the disadvantages of poor strength and low working temperature compared with other metal materials.To surmount them,a variety of magnesium alloys containing different alloying elements have been developed.Among them,the Rare Earth magnesium alloys(Mg-RE)with significant precipitation hardening are considered to be the most promising high-strength magnesium alloys,and have been extensively studied.The performance of the alloys is often determined by its microstructure.Therefore,the study of the precipitation strengthening mechanism that basing on the microstructure of Mg-RE alloys,has became the key issues for the development of MgRE alloys.However,at present,the investigation into the precipitation strengthening mechanism of Mg-RE alloys mainly focus on the effects of alloying elements,heat treatment conditions,precipitate phase and other factors on the mechanical properties,and the influence of heat treatment conditions on the type and morphology of precipitates.The study of the precipitation strengthening behavior in the deformation process of Mg-RE alloys is still insufficient.Therefore,this article will study the precipitation strengthening mechanism of representative Mg-10 Gd alloys using in-situ deformation experiment.The interaction between dislocations and precipitates in deformation process will be visually observed in transmission electron microscope(TEM).In addition,the high angle circular dark field scanning transmission electron microscope(HAADF-STEM)will be used to study the atomic mechanism of precipitation strengthening.The detailed researches are as followed:1.Through improving the existing in-situ deformation devices and combining with the focused ion beam system,a method for the preparation of in situ deformation samples with specified crystallographic orientation is developed.This method can greatly simplify the preparation process of in-situ deformed samples with high quality.2.The solid solution Mg-10 Gd samples were aged at 200°C,225°C and 250°C respectively.The hardness of the samples with different aging time were measured using microhardness tester.By comparison,200°C is considered as a suitable aging temperature,in which Mg-10 Gd samples would have peak-hardness 119.35 HV aged for 70 h.The effect of aging time on the average grain size of Mg-10 Gd alloys was also studied by optical microscope.The grain size of the solid solution sample was ~131.45 ?m,and the average grain size of the sample aged for 250 h was ~147?.Therefore,the effect of grain boundary strengthening on the change of the sample strength during aging is excluded.3.Based on the result of aging hardening,the Mg-10 Gd samples aged for different time were selected to implement the uniaxial tensile mechanical properties tests.The yield strength,ultimate strength and the elongation of the solid solution samples are ~126.4MPa,~185.2MPa,and 6.47%,respectively.The yield strength and ultimate strength of the peak-aged samples are ~216.3MPa and ~293.4MPa respectively.The change of tensile mechanical properties of the samples have obvious stages,include initial underaging stage,underaging stage and over aging stage.4.The microstructure of Mg-10 Gd samples aged for different time were observed using HAADF-STEM.The change of the microstructures in Mg-10 Gd samples also have obvious stages.In initial underaging stage,the microstructure evolution involved “Supersaturated solid solution?G.P zone??'' nucleation??' nucleation induced by ?''?the growth of ?' ”.In underaging stage,the main microstructure evolution was the coarsening of ?' phase.In over aging stage,the general phase transition appeared on the main strengthening phase ?'.5.Using in-situ deformation device,the tensile deformation of the Mg-10 Gd samples were studied along [0001] and [12 10] zone axis in transmission electron microscopy.The phenomenons that dislocations slip was impeded by ?' phase,and the dislocations pass through ?' phase were clear observed.In addition,the non-basal slip of pyramidal dislocations,which is difficultly to activate in room temperature,is also observed when tensile direction is closed to [0001] zone axis.6.The ubiquitous,unreported deformation structures were found on the main strengthening phase ?' in deformed Mg-10 Gd samples by HAADF-STEM.Studies in [0001] and [12 10] directions had proved that these deformation structures were formed when the ?' phase was sheared by basal dislocations.This means that the precipitation strengthening of the Mg-10 Gd alloys is caused by “shearing mechanism”.According to the projection along the [0001] zone axis,all the deformation structures can be divided into five types,corresponding to the different dislocations who sheared through the ?' phase.