Studies On Second Phase Control And Corrosion Behavior Of Low-Alloyed Mg-Al-Ca Series Alloys

Magnesium（Mg）alloys have attracted wide attention by people in the areas such as aerospace and automotive industries owing to their low density and high specific strength.Low-alloyed Mg alloys typically contain less than 3 wt.%of alloying elements and are less dependent on resources,cheaper to produce,and easier to process.Nevertheless,the wide application of Mg alloys is severely restricted by their intrinsic drawbacks,i.e.poor corrosion resistance and low absolute strength.Masses of efforts,such as addition of alloying elements,heat treatment,and mechanical processing,have been carried out in recent years to improve the strength of low-alloyed Mg alloys.Precipitation strengthening significantly improves the strength of Mg alloy by the precipitates with high number density to hinder dislocation slip.However,the Mg matrix often undergoes severe micro-galvanic corrosion as the anode region,because almost all the precipitates exhibit higher corrosion potential than that of Mg matrix.The difficulty in together improving strength and corrosion resistance is one of the key issues that restrict the practical application of low-alloyed Mg alloys.The corrosion resistance of Mg alloys is found to strongly rely on the features of second phases,such as their distributions,types,shapes,and sizes.For the low-alloyed Mg alloys,the aging precipitation process generally exacerbates the micro-galvanic corrosion,due to the precipitation of discrete cathode phases and change of matrix potential.To impressively promote the corrosion resistance of Mg alloys,it is significantly important to regulate the nucleation and growth process of precipitates.Commonly,the refinement of precipitates not only enhances the precipitation strengthening but also substantially reduces the area ratio of cathode to anode,and thus slows down the micro-galvanic corrosion.Inspired by the above reports,regulating the sizes of second phases to nanoscale as much as possible may be one of the potential strategies to overcome the difficulty in together improving strength and corrosion resistance of low-alloyed Mg alloys.However,the existing studies about nano-sized precipitates in Mg alloys mainly focus on the mechanical properties.It is urgent to study the effect of precipitates on the corrosion of Mg alloys and further clarify its influence mechanism,so as to guide the design of high-performance and low-alloyed Mg alloys.In conclusion,the second phases in Mg-1.3Al-0.3Ca-0.4Mn（wt.%,AXM10304）,Mg-1.0Al-0.3Ca（wt.%,AX103）and Mg-0.6Al-0.2Ca-0.5Mn-0.5Ce（wt.%,AXM060205-0.5Ce）alloys are accurately controled by optimizing the heat treatment process in this paper.The influence rules of nano-sized phases such as Guinier-Preston（G.P.）zone,Al₂Ca and Mg₂Ca on the corrosion behavior of low-alloyed Mg alloys are systematically investigated by combining with experiments and first-principles calculations.The activing mechanism is elucidated.The relevant conclusions are as follows:（1）Based on the regulation of solute redistribution during heat treatment,the control of G.P. zone and Al₂Ca phase in AXM10304 dilute alloy is realized.It is found that the monatomic-layer G.P zones with high number density(3.509×10²³ m^-3)are precipitated along the（0001）_Mg base plane in alloy after aging（T6）treatment at 200℃for 1 h.They are riched in Al and Ca atoms and congruented with Mg matrix.At T6 temperature of 250℃,the G.P.zones rapidly evolve into Al₂Ca precipitates,and coarsen with T6 time.The precipitation strengthening effect of G.P.zone（yield strength YS:～240.6 MPa）is significantly higher than that of Al₂Ca（YS:～222.6 MPa）,owing to the smaller size,the higher number density,and the more uniform distribution of G.P.zones.Hence,G.P. zones have the more significant effect on hindering dislocation slip.（2）The influence mechanism of solute redistribution rule on micro-galvanic corrosion of AXM10304 dilute alloy is revealed.It is found that the masses of Al-Ca phases in alloy redissolve after solid solution（T4）treatment,which increases the concentration of Al and Ca solutes in matrix,reduces the potential difference between Al-Mn cathode phase and Mg matrix,and increases the compactness of corrosion product film.It reduces the corrosion rate（P_W）of alloy from 19.93 mm/y to 11.45 mm/y.The subsequent T6 treatment reduces the corrosion resistance of alloy to varying degrees,due to the dissolution of solute atoms and the formation of precipitates.The corrosion resistance of T6-200℃/1 h alloy slightly decreases（P_W:13.67 mm/y）.T6-250℃/8 h alloy consumes the most solute atoms and has the largest precipitates,resulting in the most severe micro-galvanic corrosion and the worst corrosion resistance（PW:26.02 mm/y）.（3）The influence rule of precipitate characteristics on the strength and corrosion resistance of AX103 dilute alloy is studied.Compared with T4 alloy,the aging precipitation of monoatomic-layer G.P.zone improves the strength and corrosion resistance of alloy, increasing YS by～66.4 MPa and decreasing P_W from 11.34 mm/y to 6.62 mm/y.With the extension of T6-250℃treatment time（0.5-8 h）,the precipitates in Mg matrix gradually evolve from Al-Ca G.P.zones to Al₂Ca phases and continuously coarsen.The continuous precipitation of solute atoms decreases the corrosion potential of Mg matrix,and the increase of Al₂Ca cathode phases intensifies the micro-galvanic corrosion of dilute alloy.（4）The synergistic strengthening mechanism for the strength and corrosion resistance of AX103 alloy via G.P.zones was revealed.The G.P.zones with high number density significantly improve the strength of dilute alloy by hindering dislocation slip.The results of first-principles calculations show that the work function of G.P.zone is lower and the electron state density of G.P.zone at Fermi level is higher than those of Mg matrix,which means that G.P.zone is more prone to corrosion.Therefore,the G.P.zone is not an effective cathode phase to accelerate the micro-galvanic corrosion of Mg matrix.Preferential corrosion for G.P.zone not only promotes the homogeneous corrosion on dilute alloy surface,but also facilitates more Al（OH）₃ and Al₂O₃ corrosion products to gather on alloy surface.It improves the structure of corrosion product film and enhancing the integrity and compactness of film.（5）The phase evolution and corrosion behavior of AXM060205-0.5Ce dilute alloy during heat treatment are studied.The mechanism of Mg₂Ca heterogeneous nucleation to improve the corrosion resistance of dilute alloy is revealed.After T4 treatment,masses of Al-Ce phases andβ-Mn phases are dissolved in Mg matrix,accompanied by the formation of Al₁₀Mn₇Ce₂ nanophases.It reduces the corrosion degree of dilute alloy, resulting in the decrease of P_W from 6.27 mm/y to 1.55 mm/y.In the process of T6 treatment,the Mg₂Ca phases heterogeneously nucleate and grow on the surface of Al₁₀Mn₇Ce₂ phase.The micro-electric pair is formed between Mg₂Ca and Al₁₀Mn₇Ce₂ phases.During the corrosion process,Mg₂Ca phase with the lower work function is preferently dissolved as anode,forming more Ca O and Ca（OH）₂ corrosion products to accumulate on the alloy surface.It further promotes the passivation of the corrosion product film,reducing P_W to 0.83 mm/y.