Atomic-Scale Stem Study On The Precipitation In Gd-Containing Mg Alloys

Gd-containing Mg alloys are a group of promising metallic materials with outstanding ageing response and excellent mechanical properties.Due to the limitation of conventional characterisation methods,although there have been numerous studies focused on the ageing response of Gd-containnig Mg alloys,yet a considerable amount of disputes still exist in this field,especially concerning the early-stage segregation and structural transformation.The present study using high-angle annular dark field-scanning transmission electron microscopy(HAADF-STEM)comprehensively investigates the segregation and precipitation in Mg-15Gd-0.5Zr,Mg-Gd-Y-Zr and Mg-2.5Gd-1Zn-0.5Zr(wt%).The nucleation mechanisms,crystal structures,structural transformation as well as the relationship between hardness and microstructure are discussed in detail.According to the present study on Mg-Gd-Zr,the nucleation mechanism includes the formation of ?H' single RE-enriched hexagons and a small amount of zigzag lines,as observed in the sample aged for 3 hours.During the following ageing,the ?H' hexagons transform into ?'precipitates,as observed in the sample aged for 30 hours.When the size of a piece of ?' grows over a threshold,protrusions are likely to be included in the piece of precipitate.In over-aged condition,such as the case in the sample aged for 350 hours,orientation transformation takes place,during which the precipitates that originally align along three equivalent[1010]equiprobably change to align along only one of the three equivalent directions.In addition,the ?' precipitates are bridged by ?M' and ?T'structures.A small amount of ?1 precipitates are observed between?' precipitates.It is noticeable that no ?" is observed throughout the whole precipitation.In short,the precipitation sequence in Mg-Gd-Zr includes:S.S.S.S(Super Saturated Solid Solution)??H'(major nucleation mechanism)and "Z" lines(minor)??'??'(major)and?'M(minor)??'T(minor)??1??(equilibrate).The nucleation mechanism in Mg-Gd-Y-Zr includes the formation of ?' single zigzag lines and ?H' single hexagons.The independent pieces of ?' precipitates that do not interact with each other align along three equivalent[1010]directions equiprobably.As the ageing time goes by,the?precipitates grow larger and begin to interact with each other.When the interaction occur,?' precipitates may transform into beta'm and beta's.In addition,the precipitates undergo a orientation transformation and align along only one[1010]-equivalent direction in the over-aged condition.The?' precipitates subsequently transform to betal and ultimately to beta.In brief,the precipitation sequence in Mg-Gd-Y-Zr can be described as:S.S.S.S(Super Saturated Solid Solution)?"Z" lines(major nucleation mechanism)and ?H;(minor)??'??'(major)and?M'(minor)??T'(minor)??1??(equilibrate).The present study reveals that the age hardening in Mg-Gd-Zn is resulted from the simultaneous precipitation of ?'??" and ?'.According to our characterisation from two directions,[1010]and[1120],we proposed a new crystal model for ?".In sum,the age hardening in Mg-Gd-Zn is the consequence of two precipitation sequences:?' and S.S.S.S??'??"??'??"??'??'??"??1??'??(equilibrate)...