Effect of the Al-Si eutectic on the microstructure and corrosion behavior of the single-phase Mg alloy Mg-4Li

Chng Zhang ,Liang Wu,* ,Zilong Zhao ,Guangshng Huang,d,* ,Bin Jiang,d ,Andrj Atrns ,Fushng Pan,d

a State Key Laboratory of Mechanical Transmission,College of Materials Science and Engineering,174 Shazhengjie,Shapingba,Chongqing University,Chongqing 400044,China

b National Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing 400044,China

c School of Civil Engineering,Chongqing University,Chongqing 400045,China

d Chongqing Research Center for Advanced Materials,Chongqing Academy of Science &Technology,Chongqing 401123,China

e School of Mechanical and Mining Engineering,The University of Queensland,Brisbane,Qld 4072,Australia

Abstract The influenc of alloying with Al-Si eutectic on the microstructure and corrosion resistance of single-phase Mg-4Li was studied in this paper.The microstructure was characterized using an electron microscopy.The corrosion properties were measured by immersion tests and electrochemical impedance spectrum (EIS) measurements.Alloying with Al-Si eutectic caused the formation of Mg2Si and Mg-Al phase.The grain orientation of Mg-4Li was mainly composed of (0001) while the grain orientation of Mg-4Li-6(Al-Si) was mainly consisted of(10-10) and (11-20).Mg-4Li-6(Al-Si) shows worse corrosion resistance than Mg-4Li owing to the galvanic corrosion from the precipitates and texture change.

Keywords: Mg-Li alloy;Al-Si eutectic;Microstructure;Grain orientation;Corrosion.

1.Introduction

Ultralight Mg-Li alloys are potential metallic structural materials owing to their beneficia properties,including lightweight,good magnetic screening capacity and damping capacity [1-3].These good properties provide Mg-Li alloys with great prospects in the wide fiel of manufacturing industry [4-6].Generally,Li element is greatly active electrochemically and chemically.Li can react with oxygen and water vapor.However,Li can impart some special properties to Mg alloys without changing the advantages of Mg alloys.

Many researches [7,8] have reported that Mg-Li alloys possess three different micro-structures,depending on the Li content.Mg-Li alloys exhibitα-Mg single-phase microstructure if the Li concentration is<5.5wt%.Mg-Li alloys exhibit aα-Mg+β-Li dual-phase micro-structure if the Li concentration is between 5.5wt%and 10.3wt%.Mg-Li alloys exhibit aβ-Li single-phase micro-structure if the Li concentration is>10.3wt%.Many researches have focused on Mg-Li alloys with single-phaseβ-Li and dual-phaseα-Mg+β-Li.These alloys possess more slip systems than theα-Mg alloys [9].Thus,there are a large number of investigations about how to strengthen Mg-Li alloys with theα-Mg+β-Li dual-phase structure and theβ-Li single-phase structure[10-13].However,research about Mg-Li alloys withα-Mg single phase is relatively less.These alloys nevertheless deserve study.

Alloying with Al is an effective method to improve the mechanical strength of Mg-Li alloys.Park et al.[14] found that Mg-15Li-xAl alloys (x=1,3 and 5wt%) had excellent mechanical properties,and could be considered as potential candidates for industrial applications.Shi et al.[15] reported that alloying Mg-8Li with the Al-Si eutectic greatly improved the tensile strength up to 390MPa and was more effective than simply alloying with Al.Zhao et al.[9] also found that alloying with the Al-Si eutectic was more beneficia for enhancing the tensile strength of Mg-4Li than alloying with Al alone.However,the influenc of alloying with Al-Si eutectic on the corrosion behavior of Mg-Li alloys is not clear,and needs to be studied,as part of the effort to better understand the corrosion of Mg alloys [16-22].

In this paper,the Mg-4Li and Mg-4Li-6(Al-Si) alloys were prepared to study the effect of the Al-Si addition on the microstructure and corrosion behavior of the Mg-4Li alloy with single-phase.The microstructure was analyzed by an electron microscopy and the corrosion behavior was evaluated through immersion tests and electrochemical tests.

2.Experimental procedures

Fig.1.SEM images of the two annealed alloys:(a) Mg-4Li and (b) Mg-4Li-6(Al-Si).

Fig.2.EBSD microstructure of the two annealed alloys:(a) Mg-4Li and (b) Mg-4Li-6(Al-Si).

Fig.3.(0002) pole figure of the two annealed alloys:(a) Mg-4Li and (b) Mg-4Li-6(Al-Si).

Mg-4Li and Mg-4Li-6(Al-Si)(wt%)alloys were prepared by the magnetic-leviation,vacuum,high-frequency and induction melting method.The pure Mg(Mg ≥99.95wt%),pure Li(Li ≥99.95wt%) and Al-12.6Si eutectic (wt%) were melted in a vacuum induction furnace with the protection of argon atmosphere and were then cooled by water.Before rolling,the alloys were heated at 300°C for 0.5h and were then rolled into sheets by multi-pass rolling process.The reduction of each pass is about 20%.Finally,the thickness of the rolled alloys was 1.6mm.Subsequently,the rolled sheets were annealed at 150°C for 1h.The actual compositions of the two alloys were analyzed using a plasma-atomic emission spectrometer (ICP-AES),which are presented in Table 1.

Table 1 .The chemical compositions of the alloys (wt%).

The microstructure was examined using a JSM-7800F scanning electron microscope (SEM) equipped with an Oxford electron backscattered diffraction (EBSD) system.The specimens for microstructure characterization were mechanically ground using silicon paper from 400# to 2000#,washed with alcohol and dried using fl wing cool air.The microstructure was revealed by etching with a reagent consisting of 2.5g picric acid+3ml acetic acid+21ml ethanol.

Fig.4.Recrystallization and grain size distribution of Mg-4Li-6(Al-Si).

The sheets were cut into 10mm×10mm×1.6mm for corrosion tests and were mechanically ground using silicon paper from 400# to 2000#,washed with alcohol and dried using fl wing cool air.The hydrogen evolution,weight loss and electrochemical tests were carried out in the 3.5wt% NaCl solution saturated with Mg(OH)2for 48h at 25°C.The specimens for hydrogen evolution tests were inlayed by phenolic resin and the exposed plane was 10mm×10mm.A typical three-electrode cell system was adopted to measure electrochemical impedance spectra (EIS).A platinum electrode worked as the counter electrode and a saturated calomel electrode (SCE) worked as the reference electrode,and the specimen (with an area of 1 cm2) was the working electrode.The specimens were immersion for 0.5h in the NaCl solution before the EIS tests.The EIS measurements were performed with an amplitude of 5mV and the scan frequency from 100kHz to 10 mHz.The tested results were fitte by a software of ZSimpWin 3.60.

3.Results and discussion

3.1.Microstructure

Fig.1 shows SEM images of the two annealed alloys.Mg-4Li had a single-phase microstructure without any second phase particles.In contrast,Mg-4Li-6(Al-Si) contained punctiform second phase particles including Mg2Si and Mg-Al phases.Zhao et al.[9] have reported that the brittle phase Mg2Si,which has a secondary strengthening effect on the alloy as it hinders grain growth.In addition,Mg-4Li-6(Al-Si)contained the Al-Li phase particles,which was also confirme by the literature [14,15,23].

Fig.2 shows EBSD results of the two annealed alloys.Mg-4Li had a relatively coarse grain size.Most of the grains were close to the red color,which indicated a strong basal texture.The color of the grains of Mg-4Li-6(Al-Si) was mostly blue and green,indicating that the grain orientation mostly tends to the cone and cylinder direction,indicating mostly conical and cylindrical textures.This was obviously different to the grains in Mg-4Li that mostly tended to the(0001)orientation.Moreover,Mg-4Li-6(Al-Si) had relatively fin grains compared with Mg-4Li,which contributes to the alloying with Al-Si eutectic on the increase of nucleation rate and the inhibition of grain growth by Mg2Si and Mg-Al phase.

Fig.5.Hydrogen evolution and weight loss rate of annealed Mg-4Li and Mg-4Li-6(Al-Si) after immersion in 3.5wt% NaCl solution saturated with Mg(OH)2 for 48h.

Fig.3 shows the pole figure of the two annealed alloys.Fig.3(a) and Fig.3(b) respectively represent the projection of Mg-4Li and Mg-4Li-6(Al-Si)on the(0001),(11-20),and(10-10) crystal surface exponents.Mg-4Li had a strong basal texture with a maximum value of 17.63.Alloying with Al-Si eutectic,the peak value was located at the cylinder surface of(10-10).The maximum value was 15.67.This indicated that the alloying with the Al-Si eutectic weakened and dispersed the basal texture.

Fig.4 shows the recrystallization and grain size distribution for Mg-4Li-6(Al-Si).Fig.4(a) shows the dynamic recrystallization grain distribution,in which the red areas represent deformed grains,the yellow areas represent substructure grains,and the blue areas represent dynamic recrystallization grains.Fig.4(a) and (b) indicates that the proportion of red deformed grains is the largest,most of which are larger grains without recrystallization.Many sub-grain (low-angle grain)boundaries were inside these grains (Fig.4(c) and (d)),which were caused by many dislocations when the grains underwent deformation,and the dislocations were entangled together to form the low-angle grain boundaries.

3.2.Corrosion

3.2.1.Immersion tests

Fig.5 shows the average hydrogen evolution volume and weight loss rate of the two annealed alloys after immersion for 48h in the NaCl solution at 25°C.The corrosion rate(measured by hydrogen evolution and weight loss) of Mg-4Li-6(Al-Si) was considerable larger than that of Mg-4Li.Alloying with Al-Si eutectic into Mg-4Li had a detrimental influenc on the corrosion resistance.The average hydrogen evolution volumes were 8.0 and 22.1mL cm-2(8.4 and 23.1mm y-1)for Mg-4Li and Mg-4Li-6(Al-Si),respectively.The average weight loss rates were 11.1 and 16.5mg cm-2(11.6 and 17.3mm y-1) for Mg-4Li and Mg-4Li-6(Al-Si),respectively.

Fig.6.EIS plots of the annealed Mg-4Li and Mg-4Li-6(Al-Si) in 3.5wt%NaCl solution saturated with Mg(OH)2.

Fig.7.Equivalent circuit of EIS plots for the annealed Mg-4Li and Mg-4Li-6(Al-Si) in 3.5wt% NaCl solution saturated with Mg(OH)2.

3.2.2.Electrochemical tests

To better analyze the corrosion behavior of the two annealed alloys,EIS spectra were measured.Fig.6(a-c) shows the Nyquist plot,the Bode magnitude plot and the Bode phase plot,respectively,for the two annealed alloys.The Nyquist plots of the two alloys all consist of one capacitive loop throughout the tested frequency range.The capacitive loop is connected with the charge transfer resistance of the electrochemical reaction [24,25].Mg-4Li shows larger capacitive loop than that Mg-4Li-6(Al-Si),indicates that the Al-Si eutectic had a harmful effect of the corrosion resistance of Mg-4Li,consistent with the hydrogen evolution and weight loss data.The Bode magnitude plots of the two alloys show that Mg-4Li possessed higher impedance modulus |Z| than Mg-4Li-6(Al-Si),which was consistent with the results of the above experiments.The Bode phase plots of the two alloys have one time constant owing to its phase-frequency plot composed of one upward peak.

Fig.7 illustrates the equivalent circuit of EIS of the two alloys,which was used to calculate the corresponding electrochemical parameters,which are summarized in Table 2.Rsrepresents the solution resistance,Rtrepresents the charge transfer resistance andRfrepresents the corrosion fil resistance.Qdlis the constant phase element andQfis the constant phase element of corrosion fil [26-28].The higher value ofRtrepresents better corrosion resistance [29,30].Alloying with Al-Si eutectic significantl decreased theRtof Mg-4Li.Therefore,the results of EIS indicate that Mg-4Li shows better corrosion resistance than that of Mg-4Li-6(Al-Si).

Table 2 .The fittin results of EIS curves for the two annealed alloys.

3.2.2.Corrosion morphologies

Fig.8 shows the optical corrosion morphologies of the two annealed alloys after immersion for 48h in the NaCl solution at 25°C.Mg-4Li had relatively less corrosion than Mg-4Li-6(Al-Si),which had suffered severe corrosion with lots of corrosion cavities.Fig.9 shows the microscopic corrosion morphologies of the two annealed alloys after immersion for 48h in the NaCl solution at 25°C (corrosion products on the surface were removed by the solution of 10g/L AgNO3+200g/L CrO3).Mg-4Li showed characteristic fili form corrosion and some shallow pits.Corrosion pits on the surface of Mg-4Li-6(Al-Si) were deep and irregular.This indicates that the corrosion of Mg-4Li-6(Al-Si) was more severe than that of Mg-4Li.

Fig.8.Optical corrosion morphologies of annealed Mg-4Li and Mg-4Li-6(Al-Si) after immersion in 3.5wt% NaCl solution saturated with Mg(OH)2 for 48h.

Fig.9.Microscopic corrosion morphologies of annealed Mg-4Li and Mg-4Li-6(Al-Si) after immersion in 3.5wt% NaCl solution saturated with Mg(OH)2 for 48h.

As shown in Fig.11,the corrosion mechanism of annealed Mg-4Li and Mg-4Li-6(Al-Si) alloys was explained including galvanic corrosion and the texture change.Song and Atrens [17] considered that second phase particles have a significan impact on the corrosion behavior of Mg alloys,and there are two main aspects.On the one hand,continuous and net-like second phases are beneficia for Mg alloys,as they can provide a barrier to the corrosion of the Mg matrix.On the other hand,the potential of Mg is more negative than all second phases,which causes galvanic corrosion between second phase particles and Mg matrix.Thus,the Mg matrix acts as the anode and corrodes preferentially in the micro-galvanic corrosion.Li et al.[31] reported that duplex Mg-7.5Li alloy had worse corrosion resistance than single-phase Mg-4Li,and Mg-14Li alloys resulted from selective dissolution ofα-Mg phases,which was caused by the galvanic corrosion.In this current paper,Mg-4Li had a single phase structure,while Mg-4Li-6(Al-Si) contained Mg2Si phase particles and Mg-Al phase particles.Fig.10 shows the initial corrosion morphologies of annealed Mg-4Li-6(Al-Si) after immersion in the NaCl solution at 25°C for 30min.It indicates that corrosion preferentially occurred around the Mg2Si phase and Mg-Al phase particles.The Mg2Si phase andα-Mg formed a galvanic couple andα-Mg corroded preferentially.The Mg2Si phase accelerated the corrosion of the alloy and thus Mg-4Li-6(Al-Si) shows a higher corrosion rate than Mg-4Li.Similarly,the Mg-Al phase also accelerated the corrosion of Mg-4Li-6(Al-Si) due to microgalvanic corrosion.

Fig.10.SEM images of (a) annealed Mg-4Li-6(Al-Si) and (b) high magnification portions of the images in (a) after immersion in 3.5wt% NaCl solution saturated with Mg(OH)2 for 30min.

Previous literature [32-35] reported that texture has an important impact on the corrosion of Mg alloys.Liu et al.[36] was the firs to investigate the effect of crystallographic orientation on the corrosion of Mg.They confirme that the corrosion rate of basal planes was the lowest,attributed to the highest packing density.Xin et al.[37] also found that the (0001) surface shows the lowest surface energy compared with (10-10) and (11-20) surfaces.Song et al.[38] reported that the rolling surface of AZ31 alloy mainly composed of(0001) crystallographic planes,has better corrosion resistance than its cross-section surface mainly consisting of crystallographic planes (10-10) and (11-20).Fig.2 showed that the grain orientation of Mg-4Li was mainly composed of (0001)while the grain orientation of Mg-4Li-6(Al-Si) was mainly consisted of (10-10) and (11-20).Fig.3 also indicated that Mg-4Li showed strong basal texture,and alloying with Al-Si eutectic weakened the basal texture and turned it to a nonbasal texture.Therefore,the Al-Si eutectic addition has a harmful influenc on corrosion resistance of Mg-4Li.

Fig.11.Schematic illustration of annealed Mg-4Li and Mg-4Li-6(Al-Si) alloys during corrosion processes.

4.Conclusions

1.Mg-4Li had a microstructure consisting of a single phase HCP structure while Mg-4Li-6(Al-Si) contained Mg2Si phase and Mg-Al phase particles.

2.The grain orientation of the Mg-4Li surface was mainly composed of (0001) crystal planes while the grain orientation of Mg-4Li-6(Al-Si) surface mainly corresponded to(10-10) and (11-20).Mg-4Li showed strong basal texture,and alloying with Al-Si eutectic weakened the basal texture and turned it to a non-basal texture.

3.The results of hydrogen evolution,weight loss and electrochemical tests indicated that alloying with Al-Si eutectic decreased the corrosion resistance of Mg-4Li.The increased corrosion rate was attributed mainly to the increased micro-galvanic corrosion due to the second phase particles and the change of the texture from a basal texture to non-basal texture.

Declaration of Competing Interest

None.

Acknowledgments

This work was supported by projects of the National Natural Science Foundation of China (No.51701029,51671041,51531002),the National Key Research and Development Program of China (2016YFB0301100),the China Postdoctoral Science Foundation Funded Project (2018T110943,2017M620410),the Chongqing Postdoctoral Scientifi Research Foundation (Xm2017010) and Fundamental Research Funds for the Central Universities (2018CDGFCL005).