Effects Of Trace Elements On Microstructure Of Mg–8Sn Alloys And Its Modification Mechanism

Mg–Sn alloys are considered to be ideal materials forelevated temperature applications of magnesium alloys due to the high eutectic temperature and the good thermal stability of the eutectic Mg2 Sn.However,the size of the eutectic Mg2 Sn phase is rather coarse and its morphology appears as a lamellar network in the conventional gravity casting conditions.Such coarse eutectic Mg2 Sn particles are hard and brittle,which can act as void/crack initiation sites or result in preferential crack propagation,thus decreasing thestrength and ductility of alloys.Besides,the feature of the microstructure of Mg–Sn alloys makes it difficult to conduct subsequent heat treatments and deformations.Therefore,it is necessary to control microstructures of Mg–Sn alloys to achieve ideal mechanical properties.Adding trace elements into melt is a suitable method to control microstructures of Mg–Sn alloys,such method had the characteristics of stable effects,simple operation,and low treatment cost etc.In recent years,researchers have focused on the influence of trace elements on the mechanical properties,aging precipitation and deformation behavior of Mg–Sn alloys,while less studies were conducted on effects of trace elements on controling solidification microstructure and refining and modifing eutectic Mg2 Sn in Mg–Sn alloys.The ideal solidification structure is the basis for the improvement of the strength and toughness of Mg–Sn alloys and contritute to conduct the subsequent heat treatment and deformationsmoothly.Therefore,exploring effective trace elements and revealing the mechanism of trace elements on the microstructure envolution of Mg–Sn alloys have profound implications for optimizing the preparation process and improving the mechanical properties of Mg–Sn alloys.The microstructure of the eutectic Mg–Sn alloys mainly consists of the ?-Mg phase and the eutectic Mg2 Sn phase.Currently,most researches mostly proceed from the two directions of the grain refinement of the ?-Mg and the metamorphism of the second phase respectively.However,little attention has been paid to whether the trace elements can synergistically refine the ?-Mg grains and the eutectic Mg2 Sn.So it is necessary to select suitable trace element types and contents toobtain fined?-Mg grains and eutectic Mg2 Sn,which have important application values for reducing the cost of alloy preparation and achieving the controllable microstructure of Mg–Sn alloy.Effects of types of elements(Sb,Bi,Si,P,Al,Sr,Zr,and Y)and contents on the microstructure of Mg–8Sn alloys were investigated and elements which can effectively refine and modify the microstructure of Mg–8n alloys were selected.Meanwhile,the mechanism of microstructure envolution was discussed detailedly.The results are su??arized as follows:1)Sb,Bi,Si,P,and Sr all have certain refinement and modification effects on the eutectic Mg2 Sn phase in the Mg–8Sn alloy.The modification mechanism of Sb,P and Sr is adsorption and poisoning,Bi and Si promote the nucleation of Mg2 Sn to refine eutectic phase.Y has less influence on the size of eutectic Mg2 Sn in Mg–8Sn alloy,while the morphology of eutectic Mg2 Sn transforms from coarse lamellar to tight seagrass-like,and its modification mechanism is that Y causes changes of the undercooling of Mg2 Sn in solidification process.2)When the content of Sb increases from 0.1 wt.% to 1.0 wt.%,refinement and modification of the eutectic Mg2 Sn phase in Mg–8Sn alloy gradually weakens.The mechanism is that the Mg3Sb2 phase precipitates during the solidification process of the eutectic Mg2 Sn phase and the effective content of Sb is reduced,weakening the adsorption and poisoning for eutectic Mg2 Sn.After the Si content exceeds 0.5 wt.%,the effect of Si on effect of Sb on refining and modifing the Mg2 Sn phase became ineffective.The mechanism is that the eutectic Mg2 Si grows along the preferred orientation and exposed faces are helpful to promote the nucleationof eutectic Mg2 Sn.3)Sb,Bi,and Si can effectively refine ?-Mg grains.The mechanism of grain refinement is that the trace elements(Sb,Bi,Si)can effectively improve the growth restriction of ?-Mg by solute.The complex additionof Zr–Sb successfully achieves the synergetic refinement of ?-Mg grains and eutectic Mg2 Sn phases.The mechanism is that Zr particles can act as a ?-Mg grain heterogenous nuclei,and Sb poisons the growth of eutectic Mg2Sn;the ultimate tensile strength(UTS)of alloys increased from ~ 62 to ~89 MPa,it provides a reference for optimizing the microstructure of Mg–Sn alloys.