Research On Microstructure And Mechanical Properties Of Mg-Li-Zn-Gd Alloys

Magnesium-lithium alloy is the lightest metal structure material so far,with low density,high specific strength,good plastic toughness,etc.It has a very wide range of application prospects in the automotive,electronics,aerospace and medical equipment and other fields.However,the low absolute strength of magnesium-lithium alloys and the poor corrosion resistance greatly limit the wider application.Alloying is a very important strengthening method in magnesium-lithium alloys.Zn is a commonly used alloying element in magnesium alloys and plays a role of solid solution strengthening.In this paper,Mg-7Li-3Zn-xGd?x=1,5?and Mg-5Li-3Zn-xGd?x=3,7?were prepared by doping different amounts of rare earth elements Gd on the basis of Mg-Li-Zn alloys.The four component alloys were analyzed by optical microscopy,scanning electron microscopy,X-ray diffractometry,and EDS spectrometer for the as-cast and as-extruded microstructures and phase compositions of the four component alloys.The normal temperature tensile properties of as-cast and as-extruded alloys were tested and the fractures were analyzed.The results of this study are as follows:?1?The Mg-7Li-3Zn-xGd?x=1,5?alloys are mainly composed of four phases:island-like?-Mg phase,?-Li phase,granular and lamellar W-phase,thin lines and network Mg5?Zn,Gd?phases.The Mg-5Li-3Zn-3Gd mainly consists of three phases:?-Mg phase,W phase and Mg5?Zn,Gd?phase.The Mg-5Li-3Zn-7Gd alloy mainly consists of four phases:?-Mg phase,W phase,Mg₅?Zn,Gd?phase and X phase?LPSO phase?.?2?After extrusion,the grains of the Mg-Li-Zn-Gd alloys are refined,the second phase is crushed and the morphology changes greatly,but the distribution position is basically unchanged.?-Mg and?-Li phase of the as-extruded Mg-7Li-3Zn-xGd?x=1,5?alloys are stretched after extrusion and the alloys undergo dynamic recrystallization,resulting in a number of small grains appeared in the?-Li phase.?3?The tensile strength of the as-extruded Mg-7Li-3Zn-1Gd alloy is 211 MPa and the elongation is 25.6%.The tensile strength of the as-extruded Mg-7Li-3Zn-5Gd alloy is 225 MPa and the elongation is 22.6%.When the Gd content increased from 1 wt.%to5 wt.%,the tensile strength increased by 14 MPa and the elongation decreased by 3%.?4?The tensile strength of the as-extruded Mg-5Li-3Zn-3Gd alloy is 214MPa and the elongation is 16.0%.The tensile strength of the as-extruded Mg-5Li-3Zn-7Gd alloy is 251MPa and the elongation is 15.2%.When the Gd content increased from 3 wt.%to7 wt.%,the tensile strength increased by 37 MPa,and the elongation remained basically unchanged.?5?The tensile fracture morphology of the as-cast four-component alloys exhibit typical brittle fracture characteristics.After extrusion processing,a large number of equiaxed dimples appear in the fracture morphology,and second phase particles appear at the bottom of the dimple.The fracture mode is ductile fracture.