Studies On Microstructure And Properties Of Mg-Ho-Zn Alloys

Magnesium alloy has many advantages such as low density,high specific stiffness,good damping performance,etc.It is an engineering material with broad application prospects in the fields of automobile,transportation,aerospace and other fields.However,cast magnesium alloy materials are still difficult to meet industrial requirements due to low mechanical properties and poor corrosion resistance.Therefore,the development of high-performance new cast magnesium alloys has received extensive attention at home and abroad.At present,the rare earth alloying method is used to form a special structure in the magnesium alloy,such as long-period ordered stacking structure（LPSO）,quasicrystal,etc.,which can improve the strength of the magnesium alloy.The Mg-RE-Zn based alloy is considered as high strength magnesium alloy system with a good development prospect.Research at home and abroad mainly focuses on the addition of rare earth elements such as Y,Gd,Dy,and Er.The research report on the addition of Ho elements is limited.Therefore,the composition of Mg-RE-Zn alloy is systematically designed,and the intrinsic relationship between its structure and properties is studied on this basis.It has important theoretical and practical value for the development and application of high-performance magnesium alloy.In this paper,Mg-xHo-2Zn（x=4,6,8 wt.%）alloys with different Ho contents were designed.The as-cast alloys were studied by optical microscopy（OM）,X-ray diffractometry（XRD）,scanning.electron microscopy（SEM）,transmission electron microscopy（TEM）,tensile testing equipment,nanoindentation equipment and electrochemical analysis systems,such as microstructure,phase composition,tensile mechanical properties and resistance at room temperature and high temperature corrosion properties and the like were studied.The evolution of the microstructure of the alloys with different Ho contents,the laws of mechanical properties and the development and changes of corrosion processes in corrosive media were systematically observed and analyzed.The following research results were obtained:（1）As the Ho content increases,the grain size of the alloy gradually decreases.When the Ho content is 4 wt.%or 6 wt.%,the microstructure of the alloy mainly consists ofα-Mg and W-phases.And when the Ho content is increased to 8 wt.%.The microstructure of the alloy includesα-Mg,W-phase and LPSO phase;when the Ho content is 4 wt.%or 6 wt.%,the second phase is mainly a network structure distributed along the grain boundary.When the Ho element is increased to 8 wt.%,a lamellar 18R-LPSO distributed along the grain boundary is formed in the alloy,and the W-phase is refined during solidification.（2）In the indentation performance test,the elastic modulus and hardness of Mg-xHo-2Zn（x=4,6,8 wt.%）alloy matrix increased with the increase of Ho element content;while when it comes to the main phases in the alloys,the elastic modulus and hardness values of the W-phase are higher than that of the LPSO phase,respectively.（3）With the increase of Ho content,the strength and elongation of the alloy increase gradually at room temperature;but at 300℃,the increase of Ho content increases the yield strength and tensile strength of the alloy,and the elongation decreases slightly;With the increase of Ho content,the strain hardening ability of the alloy decreases,and at 300℃,the strain hardening ability increases with the increase of Ho content;at room temperature,Mg-4Ho-2Zn and Mg-6Ho-2Zn alloy both exhibit the characteristics of mixed fracture,and the main fracture mechanism of Mg-8Ho-2Zn is the ductile fracture mode.At high temperatures,Mg-4Ho-2Zn and Mg-6Ho-2Zn alloys are mainly ductile fractures,while Mg-8Ho-2Zn alloys exhibit mixed fracture characteristics.（4）The corrosion current density of the three alloys decreased from 5.0×10^-5 A/cm² to1.6×10^-5 A/cm² as the Ho content increased,and the corrosion potential increased from-1.574 V_SCE to-1.548 V_SCE.As the Ho content increases,the degree of corrosion of the alloy decreases.The three alloys have the same corrosion mechanism but different corrosion resistance.The R_ct values of Mg-8Ho-2Zn alloy are 2.3 times and 1.4 times that of Mg-4Ho-2Zn and Mg-6Ho-2Zn alloy,respectively,which is consistent with the change trend of R_f.As the Ho content increases,the corrosion resistance of the alloy increases.（5）The corrosion of the three alloys is mainly local pitting.The small corrosion pits gradually increase with the increase of the soaking time and spread to the surrounding,and finally merge into a large corrosion pit.Due to the increase of Ho content,the LPSO phase is formed in the Mg-8Ho-2Zn alloy.The LPSO phase will delay the corrosion process and play a role in improving the corrosion resistance of the alloy to some extent.