Microstructure And Properties Characterization Of Znic Diffusion Coating On Pure Magnesium And Magnesium Alloys

Magnesium alloys,which are the lightest metal in the engineering materials,have been widely used in the filed of communication,electronic industry,automotive industry,aerospace,national defence and military,et al,So they are known as the 21st century green engineering materials.However,the utilization fields of magnesium alloys have been limited because of thelower corrosion resistance and lower surface hardness.Due to the continuous and uniform coating microstructure,better combination,fine wear resistance and good corrosion resistance performance,the solid diffusion process become one of the important methods for surface protection on magnesium alloys.In this paper,pure Zn powder is used as the source of diffusion element.The solid diffusion process is used to form coating on the surface of pure magnesium,Mg-8Gd-4Y-1Zn-0.5Zr and Mg-8Gd-4Y-0Zn-0.5Zr alloys in different protective atmosphere,time and temperature are investigated.The thickness,microstructure,composition,phase constitutes of coating formed at different processing parameter are analyzed.The diffusion coating forming mechanism is used to study the effect of corrosion resistance,the conclusions were obtained as follows:(1)The uniform and continuous coating is formed on the surface of pure magnesium with zinc treatment with 410-450? for 4?20 h under the atmospheric and sealed environment.The coating consists of ?-Mg,Mg0.97Zn0.03 and Mg7Zn3 intermatallic compounds.The thickness and hardness of costing increases with the treatment time extending.And hardness can reach 168 HV0.01 with 430? for 16h,which is related to the content of intermetallic compounds in the coating.In addition,compared with the untreated spcimen,the corrosion resistance of coating which is improved in the 3.5 wt%NaCl solution,and the pure magnesium can be protected by the coating effectively.(2)The uniformity and continuous coating is formed by heating Mg-8Gd-4Y-1Zn-0.5-Zr alloy with Zn poeder under the argon gas with 410?,430? and 450?.The thickness of coating can reach 206?1784 ?m.The coating consists of Mg7Zn3,Mg3Y2Zn3 and Mg3(Y,Gd)Zn6 intermatallic compounds and a magnesium solid solution of zinc.The microhardness of quasicrystal(Mg3(Y,Gd)Zn6)reaches 322 HV0.01,which is four times as hard as matrix.The corrosion resistance of the Mg-8Gd-4Y-1Zn-0.5Zr alloy is improved by the coating.(3)The coating is formed on the Mg-8Gd-4Y-0Zn-0.5Zr alloy by heating in zinc powder under the argon gas at 430?.The Mg12(Y,Gd)Zn-LPSO structure is formed at 480? with zinc treatment.Between the LPOS structure and the Mg matrix,there is a transition zone of Mg3(Y,Gd)Zn6 quasicrystal with 480?,its microhardness reaches 148 HV0.01.Therefore,the corrosion resistance of the alloy is enhanced by the coating.