The Formation And Evolution Of Long Period Stacking Ordered Structure In As-cast Mg-Gd-Zn Alloy

Nowadays,energy saving has become a worldwide topic.The magnesium alloy,known as the“21st century green engineering material”,has gradually become one of the world's most competitive lightweight metal structural materials.In recent years,with the urgent need for energy conservation and higher-strength lightweight materials in automotive and other industries,the research and development of magnesium alloy is focused on improving the material properties of magnesium alloy,reducing processing costs to expand application of magnesium alloy.Mg-Gd alloy is becoming one of the most promising alloy systems due to its excellent characteristics of aging strengthening,and a long period ordered structure which is formed after the addition of Zn provides a new way to strengthen the alloy.However,the study on Mg-Gd-Zn Alloy has just startted,especially the formation conditions,evolution law and regulation means of LPSO structure in Mg-Gd-Zn Alloy.They are worthy of further study.In this paper,the formation and evolution of LPSO structure in as-cast as well as T4 treated Mg-Gd-Zn alloys with different compositions and different cooling rates were investigated.Optical microscope?OM?,scanning electron microscope?SEM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?and energy dispersive spectroscopy?EDS?were used to analyze the microstructures and phase constitution of alloys.The main conclusions are as follows:In Mg-Gd-Zn alloy,there are two different types of LPSO structure,they are 14H-LPSO lamellae structure and 18R-LPSO structure,respectively.The former is present in?-Mg matrix,while the latter is present in the form of the second phase at the grain boundaries.This is the first discovery in the cast under Mg-Gd-Zn alloy grain boundaries exist18R type long-period structure.With the increasing of Gd/Zn ratio,the lattice constant of eutectic second phase?Mg,Zn?₃Gd changes.When the Gd/Zn?1,the LPSO structure in?-Mg matrix is first to appear,and when Gd/Zn=2:1-3:1,the LPSO structure in grain boundary subsequently appear,but the Gd/Zn ratio continue to increase to 4:1 and 5:1,the LPSO structure in grain boundary disappear and the volume fraction of the LPSO structure in?-Mg matrix decrease.It shows that when the cooling rates are the same,there is an optimum Gd/Zn ratio that you can get the most volume fraction of the LPSO structure.With the increasing of cooling rate,the volume fraction of both18R-LPSO structure and 14H-LPSO structure gradually reduce until they disappear.And between this two types of LPSO,18R-LPSO structure in the grain boundary is more sensitive to the cooling rate,namely with the increasing of cooling rate,18R-LPSO structure in grain boundaries is the first to disappear.After solid solution heat treatment,the X-phase existing in Mg–Gd–Zn alloys derives from two variant sources:one is the transition from eutectic?-?Mg,Zn?₃Gd phase;the other is the transition from 18R-LPSO structure in grain boundaries.This also means the thermal stability of18R-LPSO structure is worse than that of 14H-LPSO structure at high temperature.