Effect of Nd on the microstructure and corrosion behaviour of Mg-9Li-3Al magnesium alloy in 3.5 wt.% NaCl solution

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ScienceDirect Materials Today: Proceedings 15 (2019) 126–131

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FCCM-2018

Effect of Nd on the microstructure and corrosion behaviour of Mg-9Li-3Al magnesium alloy in 3.5 wt.% NaCl solution P. Dinesha,*, S.Manivannanb, S.P. Kumaresh Babua, S.Natarajana a

Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu, India b Department of Mechanical Engineering, Karpagam Academy of Higher Education, Coimbatore, Tamil Nadu, India

Abstract As-cast Mg-9Li-3Al-xNd (x = 0-2.0 wt.%, with the variation of 0.5wt.%) magnesium alloys were prepared through casting in electrical induction melting furnace under a controlled inert gas atmosphere. Influences of Nd over the metallography changes and corrosion characteristics of Mg-9Li-3Al alloys have been examined by following techniques are OM, XRD, SEM and potentio-dynamic polarization (PDP) test. The results illustrate that adding Nd modifies the continuous β-Li phases into discontinuous phase by the formation of new precipitates such as Al2Nd and Al11Nd3. Meanwhile, the grain sizes of Mg-9Li-3AlxNd alloys reduced with respect to the increasing Nd content. PDP results disclose that the Nd addition reduces the corrosion current density (icorr) of Mg-9Li-3Al alloys which can be essentially produced by grain refinement. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018. Keywords: rare earth; corrosion behavior; polarization

1.

Introduction In recent decades, magnesium alloys have been attracted by the researchers as well as aerospace, military and 3C industries due to increasing demand for lightweight structural components. Magnesium alloys exhibit impressive following characteristics; low density, good ductility, castability, good specific strength, magnificent machinability at high speed, excellent damping capacity, high specific stiffness, strong electromagnetic shielding property [1-4]. Nevertheless, the formability of Mg alloy is insignificant because of the HCP crystal structure of magnesium [5]. To avoid this shortcoming, addition of lithium with density of 0.534g/cm3 has been introduced to Mg alloys. Therefore, adding lithium heads to further reduction in the density of alloys from 1.74 g/cm3 to 1.4-1.65 g/cm3. * Corresponding author. Tel.: +91-9566782564; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018.

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As state in, magnesium-lithium phase diagram, Mg alloys be composed of 5.7 -11 wt.% of Li consist of two phases, those are α-Mg phase with HCP crystal structure and β-Li phase with BCC crystal structure [6-8]. This dual phase magnesium-lithium alloys possess superior properties than the single-phase magnesium-lithium alloys [9-11]. On another hand, the addition of Li in Mg alloys exhibits relatively low tensile properties and strength [12], so new compositions and strengthening methods such as grain refinement have been established to recompense the decrement of the strength. Aluminum is the alloying element commonly used for improving the mechanical strength of magnesium-lithium alloys by means of solid solution strengthening [13]. Tao wang described that the adding of 3wt.% of aluminium enhances the mechanical strength and it also maintains the plasticity of magnesium-lithium alloys [14]. However, magnesium-lithium alloys possess disadvantages such as poor corrosion properties and comparatively poor mechanical strength. Rare earth metals have been introduced to overcome the shortage in corrosion and mechanical properties. These rare earth metals form the precipitates with Al such as Al11RE3[15], Al3RE [16] which improves the thermal stability, corrosion and mechanical characteristics. Hence, Mg-9Li-3Al-xNd (x = 0-2.0 wt.%, with the variation of 0.5wt.%) alloys were prepared through an electrical induction melting furnace with RJ2 preservative flux and the impact of Nd addition on the microstructures and corrosion properties of the Mg9Li-3Al-xNd alloys are examined and reported. 2.

Experimental procedures The commercially pure Mg ingot (99.97 wt. %), Li pellets (99 wt. %), aluminium (99.95 wt. %) and Mg25Nd (wt.%) master alloy were used to produce Mg-9Li-3Al-xNd (x = 0-2.0 wt.%, with the variation of 0.5wt.%) magnesium alloys. These alloys were prepared in stir casting electric resistance furnace under a restrained inert gas atmosphere which consists of gases of Ar-SF6 (99%vol. Ar – 1%vol.SF6). RJ2 flux has been used for preventing oxidation of alloys. To avoid thermoelectric voltage arising due to very high-temperature differences between the mold and molten metal, the mold was preheated at 300°C for 1-hr. Stirring has been done by rotatable impeller mounted on its shaft when the melt reached above the 700°C. Stirring process is actually used to attain the homogenization of the melt temperature and composition, and it promotes the removal of some impurities from melt also. Then, the molten metal was discharged into a permanent mold whose dimensions; diameter 30mm and height 500mm. ICP-AES performance was used to discover the chemical composition of Mg-9Li-3Al-xNd alloys, and the outcomes are presented in Table 1. For the metallographic observations, experimental alloys were polished by emery papers for rough polishing followed by fine polishing using diamond cloth. Nital (4%HNO3, 96% ethyl alcohol) was used as the etchant to observe the metallographical changes of polished experimental alloys. Microstructural changes of as-cast alloys were observed under the metallographic DIC Leica OM model No: DM750M and SEM (Japan, Model No. S3000H) EDS. The constituent phase of Mg-9Li-3Al-xNd alloys were observed by XRD. PDP tests of Mg-9Li-3Al-xNd alloys was carried out in 3.5wt.% NaCl aqueous solution at RT by using traditional three-electrode cell setup (ACM- Gill AC2) which consists of a platinum electrode (counter), an Ag/AgCl electrode (reference) and the working electrode (experimental alloys). Mirror finish polished by emery paper and alumina cloth experimental specimens with 1 cm2 surface area was subjected to 3.5% NaCl, Scanning rate; 10 mV/min. Table 1. The elemental composition of Mg-9Li-3Al alloys (weight fraction, %) Weight fraction (%) Alloy Li Al Nd Mn Ni Cu

Be

Mg

Mg-9Li-3Al

8.95

3.05

-

0.132

0.0036

0.003

0.0602

Si

0.00069

Bal.

Mg-9Li-3Al-0.5Nd

8.72

3.22

0.56

0.146

0.0038

0.004

0.0585

0.00077

Bal.

Mg-9Li-3Al-1.0Nd

9.04

2.97

0.95

0.142

0.0042

0.003

0.0607

0.00064

Bal.

Mg-9Li-3Al-1.5Nd

8.86

3.26

1.39

0.139

0.0041

0.004

0.0614

0.00056

Bal.

Mg-9Li-3Al-2.0Nd

8.75

2.91

2.41

0.157

0.0039

0.005

0.0590

0.00059

Bal.

3. Results and discussion 3.1 Metallography observations Fig. 1. represents the metallographical observations of Mg-9Li-3Al alloys with magnifications of 100x and 500x. The Mg-9Li-3Al alloy microstructures predominantly made up of two different phases. Those are continuous

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dark β-Li (solid solution of Mg in bcc Li lattice) phase and gray α-Mg (solid solution of lithium in hcp magnesium lattice) phase which were confirmed by XRD patterns. The random distribution of some lamellar were identified in α-Mg phases, concurrently some granular particles were identified in β-Li phases also. These lamellar particles are determined as AlLi lamellar phase and MgLi2Al granular phase from previous literature [17-18] and these phases were confirmed by XRD.

Fig. 1. Microstructures of Mg-9Li-3Al alloy (a) 100x (b)500x

Fig. 2. Represents the metallographical observations of Mg-9Li-3Al alloys with different Nd additions from 0.5wt.% to 2.0wt.%. It can be confirmed that from Fig 1. &2, Nd addition in Mg-9Li-3Al magnesium alloys changes the continuous β-Li phases into discontinuous by impelling the decrease of the proportion β-Li phases and formation of new precipitates such as Al2Nd, Al11Nd3 which were confirmed by XRD (Fig.3) and EDS (Fig.4) results. Al-Nd precipitates consume the greater number of Al atoms, so formation of other precipitates such as AlLi and MgLi2Al also reduces which leads to further grain refinement.

Fig. 2. Optical micrographs of Mg-9Li-3Al+xNd alloys (a) x= 0.5wt.%, (b) x= 1.0wt.%, (c) x= 1.5wt.%, (d) x= 2.0wt.%

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These new precipitates lead to refinement of microstructures. Adding Nd in Mg-9Li-3Al alloys upto 1.5wt.% Nd addition induces grain size reduction continuously and microstructure of alloys becomes coarser whereas the Nd addition is more than 1.5 wt.%. The mechanism of grain refinement in Mg-9Li-3Al-xNd alloys are decided by following factors; firstly, equilibrium distribution coefficient (k) of Mg-Li alloys during the solidification which is less than 1, so Nd atoms in Mg-Li alloys generally concentrate on the solid-liquid intersection which enhances the nucleation by an increase in the constitutional supercooling which leads to grain refinement [15]. Secondly, Nd has a very low solubility in the α-Mg matrix, it reduces the diffusion of other elements in the α-Mg matrix; so, formation of new precipitation occurs such as Al2Nd, Al11Nd3 which leads to grain refinement.

Fig. 3. The XRD Patterns of Mg-9Li-3Al-xNd alloys: (a) x= 0wt.%, (b) x= 0.5wt.%, (c) x= 1.0wt.%, (d) x= 1.5wt.%, (e) x= 2.0wt.%

Fig. 4. EDS of Mg-9Li-3Al-1.5Nd alloys

To identify that the chemical composition of new precipitates formed by Nd addition in Mg-9Li-3Al alloys, EDS was used, and the results are shown in the Fig. 4. It reveals that the pointed compounds in the Fig. 4. are enriched with Mg, Al, Nd and the elemental composition indicates that the atom ratio between Al and Nd is 2:1. Magnesium might be appeared from the matrix, with consideration of this it is confirmed that the new precipitates are Al2Nd. Li can be easily oxidized at ambient temperatures, so it cannot be detected by EDS [19]. 3.2. Potentiodynamic polarization curves Mg-Li alloys are chemically very active. So, these alloys easily form the oxide layer or film on the surface when it gets exposed to the moist conditions, the oxide film or layer will protect the substrate of the alloy up to some extend only. Despite, severe corrosion occurs on the substrate of Mg-Li alloys when it gets exposed in the corrosive

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medium which contains the Cl- ions such as sea water (3.5wt.% NaCl). These, Cl- ions continually impinges the metal oxide layer, so easily reaches the surface of metal and causes the severe corrosion. The PDP data of Mg-9Li-3Al+xNd alloys in 3.5wt.% NaCl solution are shown in Fig. 5. From these PDP curves, it can be confirmed that Nd addition shifts the polarization curves to smaller corrosion current densities (icorr) from 0.24 to 0.10mA/cm2 and increase the corrosion potential (Ecorr) from -1601 to -1525mV which means Nd addition reduces the electrochemical activity of Mg-9Li-3Al, especially 1.5 wt.% of Nd containing alloys exhibits smaller current densities (0.10mA/cm2) and high corrosion potential (-1525mV) compared to base alloys which were confirmed by Tafel plots. Corresponding Ecorr and icorr, values are given in Table 2. The high corrosion resistance of Nd added Mg-9Li-3Al alloys is mainly caused by new precipitates such as Al2Nd and Al11Nd3. According to the Fig. 5. and Table 2. Mg-9Li-3Al-1.5Nd magnesium alloys shows noticeable decrease in the cathodic Tafel slope (bc) value, so it can be concluded that the Nd addition into Mg-9Li-3Al alloy enhances the corrosion resistance of the alloy. In Mg-9Li-3Al+1.5Nd alloy, passivation was started with current density of 3.16 mA/cm2, means this alloy get early passivation as well as become more stable than compare with base alloy. Addition of Nd to the Mg-9Li-3Al alloy, refined the eutectic structure of Mg-Li alloys significantly by forming new precipitates such as Al2Nd, Al11Nd3. Aluminium is also one of the most favorable elements for grain refinement in Mg-Li alloys; these precipitates consumes more number of Al atoms during the solidification it leads to reduction in grain refining effect of Al. This is the reason for microstructure of alloys becomes coarser when the addition of Nd is more than 1.5 wt.%. so, it can be conculded that, dissloution of Mg-9Li-3Al+1.5Nd alloy is great lower than the other combination of Nd added alloys. Table 2. The corrosion parameters of Mg-9Li-3Al-xNd alloys Specimen

Ecorr (mV)

Icorr (mA/cm2)

Ip(mA/cm2)

bc (mV/dec)

ba (mV/dec)

Mg-9Li-3Al

-1601

0.24

19.94

-206.4

38.1

Mg-9Li-3Al-0.5Nd

-1584

0.19

11.74

-253.5

43.4

Mg-9Li-3Al-1.0Nd

-1544

0.14

4.89

-253.1

45.2

Mg-9Li-3Al-1.5Nd

-1525

0.10

3.16

-275.5

50.4

Mg-9Li-3Al-2.0Nd

-1564

0.16

8.32

-249.8

45.2

Fig. 5. PDP curves of Mg-9Li-3Al-xNd Magnesium alloys.

4.

Conclusion 1. Mg-9Li-3Al alloy predominantly made up of two phases; dark β-Li phase, gray α-Mg phase and random distribution of some AlLi lamellar phase and some MgLi2Al granular phase. Addition of Nd in Mg-9Li-3Al magnesium alloys changes the β-Li phases from continuous to discontinuous by impelling the decrease of

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the proportion β-Li phases and formation of new precipitates such as Al2Nd, Al11Nd3. These Al-Nd precipitates consume a greater number of Al atoms, so formation of other precipitates such as AlLi and MgLi2Al also reduces which leads to grain refinement. Nd addition in Mg-9Li-3Al alloys reduces corrosion current densities and greatly shifts the corrosion potential to a more positive value which means Nd addition reduces the electrochemical activity. Mg-9Li3Al-1.5Nd alloy exhibits better corrosion resistance with the smaller current density of 0.10mA/cm2 and more positive corrosion potential -1525mV.

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