Microstructure And Corrosion Behavior Of Wrought Mg-Ga Alloys

Magnesium and magnesium alloys have the characteristics of low density,high specific strength and specific stiffness,and have a very broad application prospect in aerospace,military,automobile,computer and electronic industries.However,the standard electrode potential of magnesium is only-2.37V,which leads to the relatively poor corrosion resistance.Therefore,it is very important to improve the corrosion resistance of magnesium alloys to promote their mass production and applications.Alloying is an important means to improve the corro sion resistance of magnesium alloys.In this paper,Mg-Ga and Mg-Ga-Zn alloys are used as the research object to prepare fine grain sheets by high strain rate rolling technology.Through immersion weight loss testing,electrochemical measurement and slow strain rate tensile testing,the correlation between corrosion resistance and microstructure of the alloy sheets is studied,and the action mechanism of Zn element is discussed.The main research results are as follows:（1）When the content of Ga is less than 5wt.%,the average grain size of high strain rate rolled binary Mg-Ga alloy is about 2.5μm and it is not affected by the Ga content.The DRX volume fraction of Mg-1Ga alloy is 86%,while the DRX volume fraction of the alloy with Ga content higher than 2wt.%is more than 90%.There are more Mg₅Ga₂dynamic precipitates in Mg-5Ga alloy.（2）The decreasing order of corrosion resistance of Mg-Ga alloy sheets is Mg-3Ga>Mg-2Ga>Mg-5Ga>Mg-1Ga.The corrosion resistance of Mg-3Ga alloy is the best.Its average corrosion rate is 0.52 mg cm^-2d^-1 and that of Mg-5Ga alloy is 1.73mg cm^-2 d^-1.The corrosion resistance of the binary Mg-Ga alloy sheets is mainly affected by the content of solute Ga atoms in theα-Mg matrix,and has little relationship with the volume fraction of DRX,while the Mg₅Ga₂phase is unfavorable to the corrosion resistance of the alloys.The corrosion of the Mg-Ga alloy sheets mainly occurs at the grain boundaries,tri-valent grain boundaries and the second phase particles.The Mg₅Ga₂particles that are not dissolved into the matrix of Mg-5Ga alloy have a greater effect on the corrosion resistance of the alloys.（3）The decreasing order of corrosion resistance of the ternary Mg-Ga-Zn alloy sheets is Mg-5Ga-0.9Zn>Mg-5Ga-0.6Zn>Mg-5Ga-1.2Zn>Mg-5Ga-0.3Zn>Mg-5Ga.The addition of Zn can enhance the corrosion resistance of Mg-5Ga alloy,and the corrosion resistance of Mg-5Ga-0.9Zn alloy is the best.The average corrosion rate of Mg-5Ga-0.9Zn alloy immersed in 3.5%Na Cl solution for 7 days is 0.36 mg cm^-2d^-1,which is 64%lower than that of Mg-5Ga alloy.The best corrosion resistance of Mg-5Ga-0.9Zn alloy is due to the high contents of Ga and Zn in the corrosion product layer,which provides good protection for the matrix.（4）The addition of Zn increases the tensile strength at room temperature and has little effect on the elongation.The elongation of Mg-5Ga alloy and Mg-5Ga-0.9Zn alloy is above 35%.The addition of Zn has little effect on the tensile strength and elongation of the alloy in 3.5%Na Cl solution.The addition of Zn increases the stress corrosion sensitivity of Mg-5Ga alloy sheet to some extent,which is attributed to the increase of grain boundary area when the grain size decreases,thus increasing the capture point of H and enhancing the hydrogen embrittlement effect.The stress corrosion fracture mechanism of the alloy sheets does not change after the addition of Zn element.The stress corrosion fracture mechanism of the Mg-5Ga and Mg-5Ga-0.9Zn alloys is mixed-mode fracture mechanism,that is,transgranular/intergranular mixed fracture.