Interdiffusion kinetics of the intermetallic coatings on AZ91D magnesium alloy formed in molten salts at lower temperatures

Journal of Alloys and Compounds 610 (2014) 173–179

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Journal of Alloys and Compounds

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Interdiffusion kinetics of the intermetallic coatings on AZ91Dmagnesium alloy formed in molten salts at lower temperatures

http://dx.doi.org/10.1016/j.jallcom.2014.04.2090925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 21 34202981.E-mail address: [email protected] (C. Zhong).

Jingjing Le, Lei Liu, Fan Liu, Yida Deng, Cheng Zhong ⇑, Wenbin HuState Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China

a r t i c l e i n f o

Article history:Received 26 February 2014Received in revised form 26 April 2014Accepted 28 April 2014Available online 9 May 2014

Keywords:Magnesium alloyIntermetallic coatingsMolten saltDiffusionInterdiffusion coefficientActivation energy

a b s t r a c t

A continuous Mg–Al diffusion coating can be formed on the AZ91D Mg alloy by the diffusion coatingtreatment in molten salts in the lower temperature range from 280 to 400 �C. The microstructure andcomposition of the diffusion coatings were investigated by scanning electron microscopy and energy dis-persive X-ray analysis. The results showed that the diffusion coating consists of continuous c-Mg17Al12

phase and b-Mg2Al3 phase. The b-Mg2Al3 phase layer grows faster than the c-Mg17Al12 phase layer.The interdiffusion coefficients for each phase were investigated by Heumann’s method. As the tempera-ture increases from 320 to 400 �C, the interdiffusion coefficient in c-Mg17Al12 phase (~Dc) increases from2.2 � 10–12 to 9.6 � 10–11 cm2/s. When the temperature increases from 360 to 400 �C, the interdiffusioncoefficient in b-Mg2Al3 phase (~Db) increases from 3.5 � 10–10 to 7.4 � 10–10 cm2/s. The activation energiesfor the interdiffusion in c-Mg17Al12 and b-Mg2Al3 phases are 155.9 and 66.3 kJ/mol, respectively.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Mg and its alloys are promising light structural and functionalmaterials being increasingly used in the automotive, aerospace,electronics and energy industries, owing to their unique character-istics such as high strength-to-weight ratio, high electrical conduc-tivity and thermal conductivity, and good recycling ability [1,2].However, the further application of Mg alloys is limited by theirpoor corrosion and wear resistance [3]. Thus, various surface mod-ification technologies have been proposed to improve the surfaceproperties of Mg alloys, such as anodizing [4], chemical conversion[5], electro/electroless plating [6], gas-phase deposition processes[7], diffusion coatings [8,9] and laser and ion beams [10,11].

Among these techniques, recently developed diffusion coatingon Mg alloys is of great interest because of the following potentialadvantages [9]: (1) The adhesion strength of the coating is highsince there is a metallurgical diffusion bond between the coatingand the substrate. (2) The diffusion coating consists of intermetal-lic compounds, which may improve not only the corrosion resis-tance but also the wear resistance [12]. (3) The high thermal andelectrical conductivity, as well as the electromagnetic shieldingproperties of the Mg alloys can be maintained by applying a metal-lic diffusion coating. Therefore, considerable research has beendone trying to achieve diffusion coatings on Mg alloys [8,9,13].

For example, Shigematsu et al. [14] have obtained an Al-enricheddiffusion coating by covering the Mg alloys with Al powders at450 �C in 2000. It is found that the surface layer mainly consistedof c-phase Mg17Al12 and its hardness was HV140-160, which wasmuch higher than that of the AZ91D Mg alloy substrate (HV60).A pack cementation process with a powder mixture of Al and Znhas also been applied to obtain a diffusion coating on pure Mg at480 �C [15], ZM5 Mg alloy at 470 �C [16], AZ91E alloy from 350to 413 �C [13] and Mg alloys with various Al and Zn contents at400 �C [8], respectively. Liu et al. [17,18] used a pack cementationprocess under a vacuum environment to form diffusion coatings onpure Mg from 400 to 445 �C. It was found that the microstructureof the diffusion coating was a hypoeutectic structure, which wassimilar to that reported by Shigematsu et al. [14] and Zhu et al.[19] Park et al. [20] carried out powder pack cementation processwith the addition of halide salt activator (i.e., AlCl3). Previous workfrom our group has also prepared Mg–Al and Mg–Zn intermetalliccompounds on the surface of AZ91D Mg alloys at 427 �C by using Alpowder as donor and ZnCl2 as activator [21]. The ZnCl2 reacts withthe Al to form AlCl3 which can react with the Mg, allowing thedeposition of active Al atoms.

Up to date, most of the work has used conventional powder packcementation process which has to be carried out at high tempera-tures (near or even above the Mg–Al eutectic reaction temperatureof 437 �C). However, such high temperatures may lead to the sur-face melting, cracking and distortion of the workpieces, which willlimit its industrial application [13]. Therefore, the major challenge

Fig. 1. (a) SEM image of the AZ91D specimens treated in molten salt at 280 �C for8 h, and (b) EDX elements line scanning results along the white line in Fig. 1 (a).

Table 1EDX point analysis results corresponding to the marked points in Fig. 1(a).

Location Mg (at.%) Al (at.%) Corresponding phase

1 59.88 40.12 c (Mg17Al12)2 90.60 9.40 Mg substrate

174 J. Le et al. / Journal of Alloys and Compounds 610 (2014) 173–179

for diffusion coating of Mg alloys is to lower the treatment temper-ature in order to avoid the negative effects during the coating pro-cess [13,22]. Previous studies have attempted to address thischallenge. For instance, Hirmke et al. [13] have found that the addi-tion of Zn in the powder mixture significantly promotes the forma-tion of diffusion coatings on the surface of the Mg alloy at processtemperatures between 350 and 413 �C, which is more than 50 �Clower than the previously reported powder pack cementation pro-cesses. Recently, Zhang et al. [23] and Sun et al. [22] have success-fully lowered the diffusion coating temperature to as low as 380 �Con an AZ91D Mg alloy with a nanostructured surface layer formedby surface mechanical attrition treatment (SMAT). Unfortunately,current SMAT techniques are not suitable for components withcomplex shapes.

Previous work from our group [9,24] has focused on the diffu-sion coating of Mg alloy in a new system, i.e., AlCl3–NaCl moltensalts. It was found that a continuous Mg–Al intermetallic com-pound layer can be formed in the molten salts at temperaturesas low as 300 �C [24]. However, the growth behaviour especiallythe diffusion kinetics or parameters (e.g., interdiffusion coefficientand activation energy for each phase) of the Mg–Al intermetalliccoatings formed in the molten salts at lower temperatures haveremained unclear in the previous studies. In the present work,the effect of the treatment time and temperature on the formationof the diffusion coating on an AZ91D Mg alloy in the Al-containingmolten salts at the temperature from 280 to 400 �C was investi-gated. In particular, the interdiffusion coefficients of Mg and Al indifferent phases and the corresponding activation energies wereobtained.

2. Experimental

An as-received AZ91D Mg alloy (8.86 wt.% Al, 0.72 wt.% Zn, 0.18 wt.% Mn,<0.01 wt.% other elements and balance Mg) ingot was cut into 15 mm diame-ter � 5 mm thick specimens and ground using successively finer emery papers(up to 1200 grit). The specimens were washed with deionized water and acetone,and then dried. The salt mixture containing 50% NaCl and 50% AlCl3 (molar ratio)was used in the present work. The cleaned AZ91D specimens were embedded inthe mixed salts in a ceramic container. An electric resistance furnace that can auto-matically control the temperature was employed to achieve the diffusion coatingprocess. The diffusion coating process was carried out within a temperature rangefrom 280 to 400 �C. Various holding times (from 4 to 12 h) at these elevated tem-peratures were investigated to study the growth behaviour of the intermetalliccoatings. During the diffusion coating process, protective Ar gas (99.999% purity)was used to prevent oxidation on the surface of the AZ91D specimens.

The microstructure of the surface-alloyed layer on the specimen was character-ized by a Quanta FEG250 field-emission scanning electron microscopy (SEM). Thecomposition analysis of the surface-alloyed layer, and the elemental line scanningalong the surface-alloyed layer were carried out by an energy dispersive X-ray spec-troscopy (EDX) coupled to SEM.

3. Results and discussion

3.1. Microstructure characterization

Fig. 1(a) shows the typical SEM images of the cross-section ofAZ91D specimens, which are treated in molten salts at 280 �C for8 h. The SEM image indicates that a continuous Mg–Al intermetal-lic coating (approximately 2 lm thick) is formed during the diffu-sion coating process. In addition, EDX elemental line scanningresults along the white line in the cross-section of the treatedAZ91D specimens are also given in Fig. 1(b). The concentration ofAl increases while the Mg content decreases with the decreasingdistance from the treated surface of the specimens. Further, theconcentration of Mg and Al remains relatively constant in the sub-strate. For this reason, SEM and EDX analyses reveal that interdif-fusion between Mg and Al occurs during the diffusion coatingtreatment in molten salts. Table 1 lists the quantitative analysisof the chemical composition obtained by EDX corresponding to

the marked points in Fig. 1(a). According to the Mg–Al binary phasediagram [25], there exist c-Mg17Al12 and b-Mg2Al3 equilibriumphases under the investigated temperature from 280 to 400 �C.As assessed by Murray [25], the c phase Mg17Al12 has a wide com-position range with Al concentration from 39.5 to 55 at.%. The bphase Mg2Al3 has a narrow composition range of about two tothree percent with Al content from 60 to 62.5 at.%. Thus, it canbe concluded that the diffusion coating formed at 280 �C for 8 hcomprises of c-Mg17Al12 phase. It is worth noticing that a diffusioncoating can be formed at a temperature as low as 280 �C by treat-ment in Al-containing molten salts. This temperature is muchlower than that reported for the conventional powder pack cemen-tation process.

Fig. 2(a), (c) and (e) shows the typical SEM images of the cross-section of AZ91D specimens, which are treated in molten salts at320 �C for different times (4–12 h), and the corresponding elemen-tal line scanning results along the white line in the cross-section ofthe treated specimens are also given in Fig. 2(b), (d) and (f). Thethickness of the coating increases with the increase of the treat-ment time. It is again seen that the Al concentration decreaseswhile the Mg concentration increases with the increasing distancefrom the top surface within the coating, indicating the formation ofthe diffusion coating. Table 2(a)–(c) lists the point analysis of the

Fig. 2. SEM images of the AZ91D specimens treated in molten salt at 320 �C for (a) 4 h, (c) 8 h and (e) 12 h, and (b), (d) and (f) are EDX elements line scanning results along thewhite line in Fig. 2(a), (c) and (e).

Table 2EDX point analysis results. (a)–(c) correspond to the marked points in Fig. 2(a), (c) and(e), respectively.

Location Mg (at.%) Al (at.%) Corresponding phase

(a)1 59.38 40.62 c (Mg17Al12)2 60.64 39.36 c (Mg17Al12)3 91.13 8.87 Mg substrate

(b)1 52.18 47.82 c (Mg17Al12)2 57.77 42.23 c (Mg17Al12)3 91.78 8.22 Mg substrate

(c)1 54.18 45.82 c (Mg17Al12)2 59.30 40.70 c (Mg17Al12)3 92.81 7.19 Mg substrate

J. Le et al. / Journal of Alloys and Compounds 610 (2014) 173–179 175

chemical composition obtained by EDX corresponding to themarked points in Fig. 2(a), (c) and (e), respectively. It is found thatthe atomic ratio of Mg/Al in the coating agrees well with the cphase Mg17Al12, suggesting that the diffusion coating is composedof c-Mg17Al12 phase.

Fig. 3(a), (c) and (e) shows the typical SEM images of the cross-section of AZ91D specimens, which are treated in molten salts at360 �C for different times (4–12 h), and the corresponding elemen-tal line scanning results along the white line in the cross-section ofthe treated specimens are also given in Fig. 3(b), (d) and (f). Twosubzones can be observed in the coating, i.e., an inner dark greylayer and an outer light grey-coloured layer, as shown inFig. 3(b), (c) and (e). This suggests that the diffusion coating is com-posed of different phases. Table 3(a)–(c) list the point analysisresults of the chemical composition obtained by EDX correspond-ing to the marked points in Fig. 3(a), (c) and (e), respectively. Theresults indicate the presence of two intermetallic compounds. It

Fig. 3. SEM images of the AZ91D specimens treated in molten salt at 360 �C for (a) 4 h, (c) 8 h and (e) 12 h, and (b), (d) and (f) are EDX elements line scanning results along thewhite line in Fig. 3(a), (c) and (e).

176 J. Le et al. / Journal of Alloys and Compounds 610 (2014) 173–179

is seen that the outer layer consists of Al-rich phase while innerlayer is composed of Mg-rich phase. According to the Mg–Al phasediagram, it can be deduced by the Mg/Al atomic ratio that the outerlayer of the coating is b-Mg2Al3 phase and inner layer is c-Mg17Al12

phase. The presence of the Mg17Al12 and Mg2Al3 intermetallic com-pounds has been confirmed by X-ray diffraction in our previousstudy [26].

Fig. 4(a), (c) and (e) shows the typical SEM images of the cross-section of AZ91D specimens, which are treated in molten salts at400 �C for different times (4–12 h), and the corresponding ele-ments line scanning results along the white line in the cross-section of the treated specimens are also given in Fig. 4(b), (d)and (f). It is also seen that a diffusion coating with a two-layerstructure is formed. Table 4(a)–(c) list the chemical compositionanalysis corresponding to the marked points in Fig. 4(a), (c) and(e), respectively. The elemental point analysis and line scanninganalysis indicate that the inner layer of the coating is Mg17Al12

(c phase) and the outer layer is Mg2Al3 (b phase). Moreover, ele-mental line scan analysis shows that the c-Mg17Al12 phase has alarger composition range compared to the b-Mg2Al3 phase(Fig. 4(b) and (d)). It is worth noticing that the surface-alloyedlayer formed in the present work is featured by continuous inter-metallic compound layer. This is quite different to the coatingformed by the powder pack cementation process at high tempera-tures [14–19]. When the diffusion coating process is carried outabove the Mg–Al eutectic temperature of 437 �C, Mg/Mg17Al12

eutectic reaction occurs (L M a-Mg + c-Mg17Al12). Therefore, localmelting around the interface between the packed powder andthe substrate occurs during the surface alloying process, resultingin the formation of a two-phase structure (i.e., a-Mg + c-Mg17Al12)or discontinuous distribution of the Mg17Al12 intermetallic phases[14–19,27]. However, the diffusion coating process can be per-formed at lower temperatures from 280 to 400 �C in the presentwork, and consequently the formation of the surface-alloyed layer

Table 3EDX point analysis results. (a)–(c) correspond to the marked points in Fig. 3(a), (c) and(e), respectively.

Location Mg (at.%) Al (at.%) Corresponding phase

(a)1 40.82 59.18 b (Mg2Al3)2 41.79 58.21 b (Mg2Al3)3 54.58 45.42 c (Mg17Al12)4 59.81 40.19 c (Mg17Al12)5 90.83 9.17 Mg substrate

(b)1 40.08 59.92 b (Mg2Al3)2 41.49 58.51 b (Mg2Al3)3 54.06 45.94 c (Mg17Al12)4 58.49 41.51 c (Mg17Al12)5 91.12 8.88 Mg substrate

(c)1 40.45 59.55 b (Mg2Al3)2 41.08 58.92 b (Mg2Al3)3 52.08 47.92 c (Mg17Al12)4 59.80 40.20 c (Mg17Al12)5 90.95 9.05 Mg substrate

Fig. 4. SEM images of the AZ91D specimens treated in molten salt at 400 �C for (a) 4 h, (c) 8 h and (e) 12 h, and (b), (d) and (f) are EDX elements line scanning results along thewhite line in Fig. 4 (a), (c) and (e).

Table 4EDX point analysis results. (a), (b) and (c) correspond to the marked points in Fig. 4(a),(c) and (e), respectively.

Location Mg (at.%) Al (at.%) Corresponding phase

(a)1 39.45 60.55 b (Mg2Al3)2 41.02 58.98 b (Mg2Al3)3 53.89 46.11 c (Mg17Al12)4 57.70 42.30 c (Mg17Al12)5 91.73 8.27 Mg substrate

(b)1 39.43 60.57 b (Mg2Al3)2 41.11 58.89 b (Mg2Al3)3 53.81 46.19 c (Mg17Al12)4 57.63 42.37 c (Mg17Al12)5 91.27 8.73 Mg substrate

(c)1 38.29 61.71 b (Mg2Al3)2 41.59 58.41 b (Mg2Al3)3 53.42 46.58 c (Mg17Al12)4 56.23 43.77 c (Mg17Al12)5 91.06 8.94 Mg substrate

J. Le et al. / Journal of Alloys and Compounds 610 (2014) 173–179 177

178 J. Le et al. / Journal of Alloys and Compounds 610 (2014) 173–179

follows reaction diffusion rules, which has been discussed in theprevious work from our group [9]. Therefore, in this case, the com-position of the diffusion coating depends on the binary equilibriumdiagram and conforms to the phase rule [28].

3.2. Diffusion kinetics

Fig. 5 shows the variation of the thickness of the intermetalliccompounds with the heat treatment time, which shows the growthof the c-Mg17Al12 phase layer, b-Mg2Al3 phase layer and total layer.It is clearly seen that the thickness of the individual Mg17Al12 and

Fig. 5. Variations of the thickness of the Mg–Al intermetallic layers formed on thesurface of AZ91D specimens treated in molten salt at (a) 320 �C, (b) 360 �C and (c)400 �C for different times.

Mg2Al3 layers increases with increasing treatment time for eachprocess temperature. Furthermore, the higher process tempera-tures accelerate the growth rate of each individual layer due toincreased interdiffusion coefficient. Depending on the treatmenttemperature and time, the thickness of the total layer ranges from2.9 to 32.7 lm. Furthermore, it can be found that the b phase layergrows faster than the c phase layer. Similar results that b-Mg2Al3

phase grows faster than the c-Mg17Al12 phase have been reportedby previous studies including the post-spray annealing of Al coldspray coatings on Mg alloy [29] and heat treatment of the diffusioncouples of pure Mg and Al [30,31]. When two or more phases areformed in a diffusion zone, Heumann [32] derived an equationwhich is valid for determination of the interdiffusion coefficientfor each phase under the condition that the concentration changesapproximately linearly from one end to the other of the phase. Theequation is given by

~Di ¼ �Wi

2t � DCi

Z C1=2i

0xdc; ð1Þ

where ~Di is the interdiffusion coefficient, Wi the layer width, DCi theconcentration difference between both ends, C1=2

i the average con-centration within the phase and the suffix i means the ith interme-diate layer in the diffusion zone. It is seen that the concentrationprofiles within b and c phases are almost linear. Therefore, the Heu-mann’s method was used to determine the interdiffusion coeffi-cients for these phases.

Fig. 6 shows the temperature dependence of the obtained inter-diffusion coefficients for c-Mg17Al12 phase (~Dc) and b-Mg2Al3 phase(~Db), respectively. The activation energy (Q ~D) for the interdiffusionis obtained by the linear fitment between the interdiffusion coeffi-cient (~D) and 1/T. The calculated results are listed in Table 5. Theinterdiffusion coefficient for the b-Mg2Al3 phase (~Db) is aboutone order of magnitude larger than that for the c-Mg17Al12 (~Dc)phase in the investigated temperature range. Previous study onthe diffusion couple of pure Mg and Al also found that ~Db is largerthan ~Dc [30]. For the b-Mg2Al3 phase, the activation energy of the

Fig. 6. Temperature dependence of interdiffusion coefficients for c phase and bphase.

Table 5Calculated values of interdiffusion coefficients and activation energies for c phase andb phase.

Temperature ~Dc (cm2/s) ~Db (cm2/s) Qc (kJ/mol) Qb (kJ/mol)

320 �C 2.2 � 10�12 155.9 66.3360 �C 1.3 � 10�11 3.5 � 10�10

380 �C 3.7 � 10�11 5.1 � 10�10

400 �C 9.6 � 10�11 7.4 � 10�10

J. Le et al. / Journal of Alloys and Compounds 610 (2014) 173–179 179

interdiffusion is 66.3 kJ/mol, which is apparently lower than thatfor the c-Mg17Al12 phase (155.9 kJ/mol). The higher growth rateof the Mg2Al3 phase could be attributed to the more rapid diffusionof Al atoms than that of Mg atoms. Funamizu and Watanabe’s work[30] has found that Al atoms diffuse more rapidly than Mg atomsthrough a marker study in the Mg–Al diffusion couple. As a resultof the faster diffusion of Al, within the diffusion coating, the inter-metallic compound with a higher fraction of Al (Mg2Al3 phase)grows faster than the intermetallic compound rich in Mg (Mg17Al12

phase) [29,30].

4. Conclusions

Continuous Mg–Al diffusion coatings are formed on the AZ91DMg alloy by the diffusion coating treatment in Al-containing mol-ten salts at lower temperatures ranging from 280 to 400 �C. Thediffusion coating consists of c-Mg2Al3 and b-Mg17Al12 phases. Theinterdiffusion in the Mg–Al system during the diffusion coatingtreatment in molten salts was investigated by Heumann’s method.As the temperature increases from 320 to 400 �C, the interdiffusioncoefficient for c-Mg17Al12 phase (~Dc) increases from 2.2 � 10–12 to9.6 � 10–11 cm2/s. When the temperature increases from 360 to400 �C, the interdiffusion coefficient for b-Mg2Al3 phase (~Db)increases from 3.5 � 10–10 to 7.4 � 10–10 cm2/s, which is aboutone magnitude larger than ~Dc. The activation energies for theinterdiffusion in c-Mg17Al12 and b-Mg2Al3 phases are 155.9 and66.3 kJ/mol, respectively.

Acknowledgments

Authors thank Prof. J. Dong and Dr. P.H. Fu for their assistancewith the supplying of the AZ91D Mg alloys. This work was sup-ported by the National Nature Science Foundation of China(51374005 and 51004070), Research Fund for the Doctoral Pro-gram of Higher Education of China (20100073120109), ‘‘ChenGuang’’ project supported by Shanghai Municipal Education Com-mission and Education Development Foundation - China (11CG12),National Key Technology R&D Program (2011BAE13B08), and theInstrumental Analysis Center of Shanghai Jiao Tong University(IAC-SJTU).

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