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Role of microstructure and secondary phase on corrosion behavior of heat treated AZ series magnesium alloys
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 175–181
www.materialstoday.com/proceedings
ICAMME-2018
Role of microstructure and secondary phase on corrosion behavior of heat treated AZ series magnesium alloys P. Pradeep Kumara, A. Raj Bharata, B. Sesha Saia, R. J. Phani Saratha, P. Akhila, G. Pradeep Kumar Reddyb, V. V. Kondaiahc, B. Ratna Sunila,* a
Department of Mechanical Engineering, Rajiv Gandhi University of Knowledge Technologies (AP-IIIT), Nuzvid 521202, India b Department of Mechanical Engineering, Vignana Bharathi Institute of Technology, Hyderabad 501301, India c Department of Mechanical Engineering, Tirumala Engineering College, Narasaraopet 522601, Andhra Pradesh, India
Abstract Magnesium (Mg) and its alloys belong to light metals group are now attracting the attention of the manufacturing engineers as promising candidates to develop structures for energy efficient applications. Among the all available Mg alloys, AZ series is the important group of alloys which contains Aluminum (Al) and Zinc (Zn) as the main constituting alloying elements. As the solubility of Al into Mg is limited to ≈ 1 Wt. % at the room temperature, presence of secondary phases is inevitable in AZ series Mg alloys if Al content is more than 1%. In the present study, pure Mg and two AZ series Mg alloys, i) AZ31 which is with 3% Al and ii) AZ91 which is with 9% Al were selected and heat treated (pure Mg and AZ31 at 340 and AZ91 at 410°C) to alter the amount of Al dissolution and to investigate the effect of heat treatment on microstructural modification, microhardness and corrosion behavior. Interestingly, heat treatment severely affected the microstructure of AZ91 Mg alloy as clearly demonstrated by the decreased secondary phase. Hardness was observed as decreased for all the samples due to stress revealing and modified microstructure. From the immersion studies carried out in 3.5% NaCl solution, weight loss measurements showed decreased corrosion rates for all the heat treated samples which can be attributed to the microstructural modification due to heat treatment. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engi- neering, ICAMME-2018. Keywords: Magnesium alloys; AZ31; AZ91; microstructure; hardness; corrosion.
1. Introduction Recently, interest on energy efficient light weight materials is tremendously increasing in the field of manufacturing engineering. Magnesium (Mg) and its alloys are a group of non-ferrous metals exhibit lower density and higher specific strength compared with their close counter parts, aluminum alloys [1]. Among several Mg alloys, AZ series Mg alloys are a specific group of alloys consisting of aluminum (Al) and Zinc (Zn) as main constituting elements [2]. In metals, microstructure plays a very important role to influence the mechanical and corrosion performance [3]. In our earlier study, increased biomineralization was observed in the presence of simulated body fluids as the grain size was decreased in fine grained AZ31 Mg alloy processed by groove pressing ∗ Corresponding author. Tel.: +91-9677119819 ; fax: + 08656 235150. E-mail address: [email protected], [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018.
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[4]. Similarly, several authors reported decreased corrosion rate due to grain refinement in AZ31 Mg alloy [5-8]. Furthermore, in the recent work, we have observed that the machining behavior was influenced by the decreased amount of secondary phase and grain refinement in AZ91Mg alloy processed by friction stir processing (FSP) [9]. Saikrishna et al., [10] also reported the decreased corrosion rate due to the decreased grin size in FSPed AZ31 Mg alloy. From the available literature, it can be understood that along with the grain size, other influencing parameters such as texture, presence of secondary phase and the grain size distribution also affect the mechanical and corrosion behavior [10-12] of AZ series Mg alloys In AZ series Mg alloys, a secondary phase of Mg and Al (Mg17Al12) is appeared at the grain boundaries as the Al content is increased above 1%. Therefore, AZ91 Mg alloy contains more amount of Mg17Al12 compared with AZ31 and AZ61 Mg alloys [2]. The mechanical characteristics of Mg17Al12 are different compared with Mg solid solution grains in AZ series Mg alloys. Modifying the microstructure of metals by heat treatment is common practice in the production of different structures to alter the material properties. Heat treatment of steels is well established subject and wide variety of heat treatment options were proposed for different steels. However, even though Mg alloys are slightly different compared with steels, same heat treatment processes can be adopted to alter the microstructure and the presence of secondary phase to bring modification to the bulk properties. The amount of secondary phase in AZ series Mg alloys can be altered by adopting suitable heat treatment methods. The information on the influence of heat treatment on corrosion behavior of AZ series Mg alloys is insufficient. Hence, in the present work, three AZ series Mg alloys with different Al content i) pure Mg (0% Al), ii) an alloy with lower Al content (AZ31 Mg alloy) and iii) an alloy with higher Al content (AZ91 Mg alloys) were selected and heat treatment was done to alter the microstructure, the secondary phase amount and distribution. Then the effect of heat treatment on hardness and corrosion behavior was investigated. 2. Experimental details Pure Mg and AZ series Mg alloys (AZ31 and AZ91) have been purchased from Exclusive Magnesium, Hyderabad, India in the form of billets and rolled sheets. Pure Mg cast billet chemical composition contains 99.9 % Mg and remaining being impurities. AZ31 Mg alloy rolled sheet has a chemical composition of 2.75%Al, 0.91%Zn, 0.001%Fe, 0.01%Mn and remaining being Mg by Wt.%. Die cast AZ91 Mg alloy has a chemical composition of 8.67%Al, 0.85%Zn, 0.002%Fe, 0.03%Mn and remaining being Mg by Wt.%. Specimens of 10×10×4 mm3 were cut from as received materials and heat treated materials at different temperatures. Fig 1 shows the heat treatment cycles for the samples. For pure Mg and AZ31 Mg alloy, the furnace was set to a temperature of 3400 C with a uniform heating rate of 50 C per minute. This temperature was maintained for 6 hour and then the samples were cooled to room temperature along with the furnace. Based on the preliminary studies on heat treatment of AZ91 Mg alloy, soaking time for heat treatment was fixed as 6 h to obtain considerable change in microstructure. Since the AZ91 Mg alloy needs different heat treatment temperature due to its chemical composition, the furnace was set to 410̊̊ C (the temperature corresponding to maximum solubility of Al) and the specimens were allowed to soak for 6 hours. Then the specimens were collected from the furnace and quenched in water. Fig 1 shows the heat treatment cycles for pure Mg, AZ31 and AZ91 Mg alloys.
Fig 1. Heat treatment cycle for pure Mg, AZ31 and AZ91 Mg alloys
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Microstructure observations and micro-hardness measurements were done for unprocessed and heat treated samples of pure Mg, AZ31 and AZ91 Mg alloys respectively. The samples were polished with emery papers ranging from 220 to 2000 grade and then polished using Alumina paste followed by diamond paste (1-3μm) polishing until the surfaces were obtained to mirror finished condition. Then the polished samples were etched with Picric acid reagent solution (0.25 grams of Picric acid, 0.25 ml of acetic acid, 0.25ml of distilled water and 25 ml of ethanol). After etching, the samples were washed with ethanol and dried. Optical microscope images were obtained at different regions by using an inverted polarized optical microscope (200 X and 500 X, Leica, Germany). For hardness measurements (Omnitech, India), a load of 100 grams was applied for a period of 15 sec (dwell time) on each sample. The specimens were characterized by X-ray diffraction method (XRD, D8 Advanced, Bruker, USA) with Cu-Kα radiation (λ = 1.54 Å) between 20 and 80 deg with a scanning rate of 1step/s and step size of 0.1°. For corrosion experiments, the specimens were thoroughly cleaned in deionized water and dried in a hot oven for 10 min. A solution of 3.5% NaCl was prepared as per the ASTM standards for immersion test to measure corrosion rate [13]. The specimens in triplicates (n=3) were immersed in the corroding solution for a period of 7 days. The weight and surface area of the individual specimens were recorded before starting the immersion studies. The specimens were then removed after different intervals of time (1day, 3days and 7days) and gently washed in deionized water. In order to remove the corrosion products, chromium trioxide solution (180 g of CrO3 per one liter of de-ionized water) was prepared and all the immersed samples were kept in the chromate solution maintained at 100°C until all the corrosion products were dissolved. Then the weight of the specimens was measured and weight loss due to the corrosion during the immersion was calculated by considering the weight of the specimen before immersion. The corrosion rate was calculated as per the ASTM-G-32 standard by using the following formula [13]. Corrosion rate(mm/year) = 8.76 *104 .W/A.T.D
(1)
Where, W is the weight loss (g), A is the surface area of the sample before immersion (cm2), T is the immersion time (h) and D is the density of the sample (g/cm3). 3. Results and discussions Fig 2 shows the optical microscope images of pure Mg, AZ31and AZ91 before and after the heat treatment. It is evident from the observations that the heat treatment lead to develop considerable microstructural changes in AZ31 and AZ91 Mg alloys. Pure Mg was observed as unaffected with same grin size before and after the heat treatment. However, any residual stresses which are hidden within the lattice may get released due to the heat treatment in pure Mg. For AZ31 Mg alloy, a slight grain growth from 12 µm to 17 µm was observed after the heat treatment. For AZ91 Mg alloy, significant changes were noticed. Usually, AZ series Mg alloys contains three distinct regions known as α-Mg (solid solution of Mg and Zn), β-Phase (Mg17Al12) and a eutectic region (mixture of α and β) [2]. These regions are indicated in the optical microscope image of unprocessed AZ91 (Fig 2 (d)). The presence of β can be theoretically seen in AZ series Mg alloys if Al content is increased more than 1% as the maximum solubility of Al in Mg at room temperature is 1% [2]. However, in AZ31 Mg alloy the presence of β is invisible at the grain boundaries.
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Fig 2 Optical microscope images of pure Mg, AZ31and AZ91 before and after heat treatment: a) pure Mg before heat treatment, b) pure Mg after heat treatment, c) AZ31 before the heat treatment, d) AZ31 after the heat treatment, e) AZ91 before the heat treatment and f) AZ91 after the heat treatment
From the XRD analysis (Fig (3)) of the starting materials, presence of β phase was identified in AZ91 Mg alloy but not in the AZ31. Of course, XRD cannot detect the phase if the fraction is lower. Therefore, in the present study, as shown in Fig 2 (c), no β was observed at the grain boundaries of AZ31 Mg alloy. But, in AZ91 Mg alloy, presence of β phase was clearly observed at the grain boundaries and in the eutectic regions. The maximum solubility of Al in Mg is 12% at 410°C. Therefore in the present study, when AZ91 Mg alloy was kept at 410°C for 6 h, higher amount of Al was dissolved from the eutectic regions and from the β regions. As indicated by black arrow in Fig 2(f), the eutectic regions were disappeared adjacent to β regions as indicted with white arrow. This result strongly suggests the dissolution of more Al into Mg and to form supersaturated grains. Developing supersaturated grains certainly influence the material properties as also reported earlier [9, 14]
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Fig 3 XRD patterns of the samples
The hardness measurements show interesting results as graphically presented in Fig 4. It is obvious that the hardness of AZ31 and AZ91 are higher compared with pure Mg due to alloying effect. Compared with unprocessed condition, decreased hardness was observed for all the samples after heat treatment. As observed from the microstructural studies, no much change was observed in pure Mg before and after the heat treatment. However, stress relieving can happen at 340 °C as it is the annealing temperature for Mg. Therefore, due to decreased internal stresses caused to decrease the hardness of pure Mg marginally. For AZ31 Mg alloy and AZ91 Mg alloy, the grain size was observed as increased after the heat treatment. This might be the reason behind the decreased hardness compared with unprocessed condition. In AZ91 Mg alloy, the solubility of more Al as observed from Fig 2 (e) resulted deceased amount of hard and brittle β-Phase (Mg17Al12). As the hardness of β-Phase is higher than αregions [15], the hardness of AZ91 Mg alloy after the heat treatment was decreased due to reduced amount of βPhase. However, the grains have become supersaturated and hence, the hardness should be increased. But as observed from Fig 4, the hardness of AZ91 was also found to be decreased. This can be understood by counting all the factors which influenced the harness of heat treated AZ91. Here, it can be explained that the contribution of supersaturated grains to increase the hardness has been overcome by the contribution of grin size increase and reduction of β-Phase due to heat treatment.
Fig 4 Microhardness measurements of the samples
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Fig 5 shows the corrosion rates of the specimens calculated from the weight loss of the samples before and after the heat treatment. Before the heat treatment, corrosion rate was observed as decreased with increase of immersion time from day 1 to day 7 for AZ31 and AZ91 samples. Interestingly, for pure Mg, the corrosion rate was observed as increased from day 1 to day 7. This can be attributed to poor resistance of pure Mg towards the corroding environment compared with AZ31 and AZ91. Fig 6 shows the XRD patterns of the immersed samples after 1 day. The phases in the corrosion products were identified as magnesium hydroxide (Mg(OH)2) and magnesium oxide (MgO) along with the Mg and Mg17Al12 phase. During the corrosion of Mg, due to anodic and cathodic reactions, magnesium hydroxide (Mg(OH)2) is formed in any aqueous solutions [16]. In the presence of chloride ions (Cl-), the ceramic layer that is formed due to corrosion is unstable. Mg (OH)2 reacts with Cl- and magnesium chloride (MgCl2) salts are formed which are easily soluble crystals [17,18]. Dissolution of these MgCl2 crystals accelerated the corrosion further. When any alloying elements such as Al and Zn are added to Mg, corrosion mechanisms are changed and the formation of MgCl2 is delayed.
Fig 5. Corrosion rate of the samples after different intervals of immersion times: a) before heat treatment and b) after heat treatment
Fig 6 XRD patterns of the immersed samples after 1 day of immersion
After the heat treatment, as shown in Fig 5 (b), all the samples have shown similar trend as the immersion time was increased to 7days. Compared with pure Mg, AZ31 and AZ91 have shown decreased corrosion rate. Compared with as received condition, heat treated samples exhibited lower corrosion rates in all the cases. Due to heat treatment, any hidden stresses which present in the Mg grain are relieved and the corrosion due to internal lattice strain can be completely eliminated which is the prime reason behind the decreased corrosion in heat treated pure Mg. Presence of secondary phases (β-phase, in the present work) initiate galvanic corrosion in Mg alloys when
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exposed to corroding environment [8,11]. In AZ31 and AZ91 Mg alloys, the amount of secondary phase is decreased after heat treatment which certainly decreases the galvanic corrosion. Therefore, the corrosion rate of heat treated AZ31 and AZ91 samples were observed as decreased compared with unprocessed samples. Particularly in AZ91 Mg alloy, the grains have become supersaturated with more Al. In principle, corrosion rate must be increased due to the more dissolution of Al. However, the effect of disappeared eutectic regions and decreased amount of βPhase (Mg17Al12) was higher compared with the effect of supersaturated grain on the corrosion. From the results, it can be understood that the heat treatment of AZ series Mg alloys may decrease the hardness but enhances the corrosion resistance. 4. Conclusions In the present study, pure Mg, AZ31 and AZ91 Mg alloys were heat treated at 340 and 410°C respectively to alter the microstructure. Pure Mg and AZ31 did not show considerable changes in the microstructure but AZ91 has undergone significant changes. Due to heat treatment, decreased hardness was measured for all the samples. Corrosion rate calculated from the weight loss measurements indicated better performance compared with the unprocessed samples. The decreased corrosion rate after heat treatment can be attributed to the stress relieving and in particular decreased amount of secondary phase (Mg17Al12). Hence form the results, it can be concluded that the corrosion performance of AZ series Mg alloys can be enhanced at the cost of slightly sacrificing the hardness which makes Mg alloys suitable for promising candidates for structural applications where the structures are intended to work in corroding environment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
H.E. Fridrich, B. L. Mordike, Magnesium Technology, Germany, Springer, 2006 M. M. Avedesian, H. Baker, ASM Specialty Handbook, Magnesium and Magnesium Alloys, ASM International, USA, 1999. M. A. Meyers, A. Mishra, D. J. Benson, Prog. Mater. Sci., 51 (2006), 427–556. B. Ratna Sunil, Arun Anil Kumar, T.S. Sampath Kumar, Uday Chakkingal, Mater. Sci. Eng. C, 33 (2013) 1607–1615. H. Wang, Y. Estrin, H. Fu, G.L. Song, Z. Zúberová, The effect of pre-processing and grain structure on the bio-corrosion and fatigue resistance of magnesium alloy AZ31, Adv. Eng. Mater., 9 (2007) 967–972. C. op’tHoog,M. Birbilis, Y. Estrin, Corrosion of pureMg as a function of grain size and processing route, Adv. Eng. Mater.,10 (6) (2008) 579–582. G.B. Hamu, D. Eliezer, L. Wagner, The relation between severe plastic deformation microstructure and corrosion behavior of AZ31 magnesium alloy, J. Alloys Compd., 468 (2009) 222–229. D. Song, A. Ma, J. Jiang, P. Lin, D. Yang, J. Fan, Corrosion behavior of equal-channelangular- pressed pure magnesium in NaCl aqueous solution, Corros. Sci., 52 (2010) 481–490. G. V. V. Surya Kiran, K. Hari Krishna, Sk. Sameer, M. Bhargavi, B. Santosh Kumar, G. Mohana Rao, Y. Naidubabu, Ravikumar Dumpala, B. Ratna Sunil, Trans. Nonferrous Met. Soc. China., 27 (2017) 804−811 N. Saikrishna, G. Pradeep Kumar Reddy, Balakrishnan Munirathinam, B. Ratna Sunil, J. Magnes. Alloys, 4 (2016) 68–76 Song G, Atrens A and Dargusch M, Corros. Sci., 41 (1999) 249. Singh I, Singh M and Das S, J. Magnes. Alloys, 3 (2015) 142. ASTM Standard G31-72. Standard practice for laboratory immersion corrosion testing of metals. West Conshohocken. PA: ASTM International 2004. doi:10.15s20/G0031-72R04. Jing-yuan Li, Jian-xin Xie, Jun-bing Jin, Zhi-xiang Wang, Trans. Nonferrous Met. Soc. China, 22(5) (2012) 1028-1034. B. Ratna Sunil, K. Ganesh, P. Pavan, G. Vadapalli, C. Swarnalatha, P. Swapna, P. Bindukumar, G.P.K. Reddy, J. Magnes. Alloys, 4 (2016) 15-21. Y. Wang, M. Wei, J. Gao, J. Hu, Y. Zhang, Mater. Lett., 62 (2008) 2181–2184. H. Wang, Y. Estrin, Z. Zúberová, Mater. Lett., 62 (2008) 2476–2479. F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Curr. Opin. Solid. State. Mater. Sci., 12 (2008) 63–72.
ScienceDirect Materials Today: Proceedings 18 (2019) 175–181
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ICAMME-2018
Role of microstructure and secondary phase on corrosion behavior of heat treated AZ series magnesium alloys P. Pradeep Kumara, A. Raj Bharata, B. Sesha Saia, R. J. Phani Saratha, P. Akhila, G. Pradeep Kumar Reddyb, V. V. Kondaiahc, B. Ratna Sunila,* a
Department of Mechanical Engineering, Rajiv Gandhi University of Knowledge Technologies (AP-IIIT), Nuzvid 521202, India b Department of Mechanical Engineering, Vignana Bharathi Institute of Technology, Hyderabad 501301, India c Department of Mechanical Engineering, Tirumala Engineering College, Narasaraopet 522601, Andhra Pradesh, India
Abstract Magnesium (Mg) and its alloys belong to light metals group are now attracting the attention of the manufacturing engineers as promising candidates to develop structures for energy efficient applications. Among the all available Mg alloys, AZ series is the important group of alloys which contains Aluminum (Al) and Zinc (Zn) as the main constituting alloying elements. As the solubility of Al into Mg is limited to ≈ 1 Wt. % at the room temperature, presence of secondary phases is inevitable in AZ series Mg alloys if Al content is more than 1%. In the present study, pure Mg and two AZ series Mg alloys, i) AZ31 which is with 3% Al and ii) AZ91 which is with 9% Al were selected and heat treated (pure Mg and AZ31 at 340 and AZ91 at 410°C) to alter the amount of Al dissolution and to investigate the effect of heat treatment on microstructural modification, microhardness and corrosion behavior. Interestingly, heat treatment severely affected the microstructure of AZ91 Mg alloy as clearly demonstrated by the decreased secondary phase. Hardness was observed as decreased for all the samples due to stress revealing and modified microstructure. From the immersion studies carried out in 3.5% NaCl solution, weight loss measurements showed decreased corrosion rates for all the heat treated samples which can be attributed to the microstructural modification due to heat treatment. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engi- neering, ICAMME-2018. Keywords: Magnesium alloys; AZ31; AZ91; microstructure; hardness; corrosion.
1. Introduction Recently, interest on energy efficient light weight materials is tremendously increasing in the field of manufacturing engineering. Magnesium (Mg) and its alloys are a group of non-ferrous metals exhibit lower density and higher specific strength compared with their close counter parts, aluminum alloys [1]. Among several Mg alloys, AZ series Mg alloys are a specific group of alloys consisting of aluminum (Al) and Zinc (Zn) as main constituting elements [2]. In metals, microstructure plays a very important role to influence the mechanical and corrosion performance [3]. In our earlier study, increased biomineralization was observed in the presence of simulated body fluids as the grain size was decreased in fine grained AZ31 Mg alloy processed by groove pressing ∗ Corresponding author. Tel.: +91-9677119819 ; fax: + 08656 235150. E-mail address: [email protected], [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018.
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[4]. Similarly, several authors reported decreased corrosion rate due to grain refinement in AZ31 Mg alloy [5-8]. Furthermore, in the recent work, we have observed that the machining behavior was influenced by the decreased amount of secondary phase and grain refinement in AZ91Mg alloy processed by friction stir processing (FSP) [9]. Saikrishna et al., [10] also reported the decreased corrosion rate due to the decreased grin size in FSPed AZ31 Mg alloy. From the available literature, it can be understood that along with the grain size, other influencing parameters such as texture, presence of secondary phase and the grain size distribution also affect the mechanical and corrosion behavior [10-12] of AZ series Mg alloys In AZ series Mg alloys, a secondary phase of Mg and Al (Mg17Al12) is appeared at the grain boundaries as the Al content is increased above 1%. Therefore, AZ91 Mg alloy contains more amount of Mg17Al12 compared with AZ31 and AZ61 Mg alloys [2]. The mechanical characteristics of Mg17Al12 are different compared with Mg solid solution grains in AZ series Mg alloys. Modifying the microstructure of metals by heat treatment is common practice in the production of different structures to alter the material properties. Heat treatment of steels is well established subject and wide variety of heat treatment options were proposed for different steels. However, even though Mg alloys are slightly different compared with steels, same heat treatment processes can be adopted to alter the microstructure and the presence of secondary phase to bring modification to the bulk properties. The amount of secondary phase in AZ series Mg alloys can be altered by adopting suitable heat treatment methods. The information on the influence of heat treatment on corrosion behavior of AZ series Mg alloys is insufficient. Hence, in the present work, three AZ series Mg alloys with different Al content i) pure Mg (0% Al), ii) an alloy with lower Al content (AZ31 Mg alloy) and iii) an alloy with higher Al content (AZ91 Mg alloys) were selected and heat treatment was done to alter the microstructure, the secondary phase amount and distribution. Then the effect of heat treatment on hardness and corrosion behavior was investigated. 2. Experimental details Pure Mg and AZ series Mg alloys (AZ31 and AZ91) have been purchased from Exclusive Magnesium, Hyderabad, India in the form of billets and rolled sheets. Pure Mg cast billet chemical composition contains 99.9 % Mg and remaining being impurities. AZ31 Mg alloy rolled sheet has a chemical composition of 2.75%Al, 0.91%Zn, 0.001%Fe, 0.01%Mn and remaining being Mg by Wt.%. Die cast AZ91 Mg alloy has a chemical composition of 8.67%Al, 0.85%Zn, 0.002%Fe, 0.03%Mn and remaining being Mg by Wt.%. Specimens of 10×10×4 mm3 were cut from as received materials and heat treated materials at different temperatures. Fig 1 shows the heat treatment cycles for the samples. For pure Mg and AZ31 Mg alloy, the furnace was set to a temperature of 3400 C with a uniform heating rate of 50 C per minute. This temperature was maintained for 6 hour and then the samples were cooled to room temperature along with the furnace. Based on the preliminary studies on heat treatment of AZ91 Mg alloy, soaking time for heat treatment was fixed as 6 h to obtain considerable change in microstructure. Since the AZ91 Mg alloy needs different heat treatment temperature due to its chemical composition, the furnace was set to 410̊̊ C (the temperature corresponding to maximum solubility of Al) and the specimens were allowed to soak for 6 hours. Then the specimens were collected from the furnace and quenched in water. Fig 1 shows the heat treatment cycles for pure Mg, AZ31 and AZ91 Mg alloys.
Fig 1. Heat treatment cycle for pure Mg, AZ31 and AZ91 Mg alloys
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Microstructure observations and micro-hardness measurements were done for unprocessed and heat treated samples of pure Mg, AZ31 and AZ91 Mg alloys respectively. The samples were polished with emery papers ranging from 220 to 2000 grade and then polished using Alumina paste followed by diamond paste (1-3μm) polishing until the surfaces were obtained to mirror finished condition. Then the polished samples were etched with Picric acid reagent solution (0.25 grams of Picric acid, 0.25 ml of acetic acid, 0.25ml of distilled water and 25 ml of ethanol). After etching, the samples were washed with ethanol and dried. Optical microscope images were obtained at different regions by using an inverted polarized optical microscope (200 X and 500 X, Leica, Germany). For hardness measurements (Omnitech, India), a load of 100 grams was applied for a period of 15 sec (dwell time) on each sample. The specimens were characterized by X-ray diffraction method (XRD, D8 Advanced, Bruker, USA) with Cu-Kα radiation (λ = 1.54 Å) between 20 and 80 deg with a scanning rate of 1step/s and step size of 0.1°. For corrosion experiments, the specimens were thoroughly cleaned in deionized water and dried in a hot oven for 10 min. A solution of 3.5% NaCl was prepared as per the ASTM standards for immersion test to measure corrosion rate [13]. The specimens in triplicates (n=3) were immersed in the corroding solution for a period of 7 days. The weight and surface area of the individual specimens were recorded before starting the immersion studies. The specimens were then removed after different intervals of time (1day, 3days and 7days) and gently washed in deionized water. In order to remove the corrosion products, chromium trioxide solution (180 g of CrO3 per one liter of de-ionized water) was prepared and all the immersed samples were kept in the chromate solution maintained at 100°C until all the corrosion products were dissolved. Then the weight of the specimens was measured and weight loss due to the corrosion during the immersion was calculated by considering the weight of the specimen before immersion. The corrosion rate was calculated as per the ASTM-G-32 standard by using the following formula [13]. Corrosion rate(mm/year) = 8.76 *104 .W/A.T.D
(1)
Where, W is the weight loss (g), A is the surface area of the sample before immersion (cm2), T is the immersion time (h) and D is the density of the sample (g/cm3). 3. Results and discussions Fig 2 shows the optical microscope images of pure Mg, AZ31and AZ91 before and after the heat treatment. It is evident from the observations that the heat treatment lead to develop considerable microstructural changes in AZ31 and AZ91 Mg alloys. Pure Mg was observed as unaffected with same grin size before and after the heat treatment. However, any residual stresses which are hidden within the lattice may get released due to the heat treatment in pure Mg. For AZ31 Mg alloy, a slight grain growth from 12 µm to 17 µm was observed after the heat treatment. For AZ91 Mg alloy, significant changes were noticed. Usually, AZ series Mg alloys contains three distinct regions known as α-Mg (solid solution of Mg and Zn), β-Phase (Mg17Al12) and a eutectic region (mixture of α and β) [2]. These regions are indicated in the optical microscope image of unprocessed AZ91 (Fig 2 (d)). The presence of β can be theoretically seen in AZ series Mg alloys if Al content is increased more than 1% as the maximum solubility of Al in Mg at room temperature is 1% [2]. However, in AZ31 Mg alloy the presence of β is invisible at the grain boundaries.
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Fig 2 Optical microscope images of pure Mg, AZ31and AZ91 before and after heat treatment: a) pure Mg before heat treatment, b) pure Mg after heat treatment, c) AZ31 before the heat treatment, d) AZ31 after the heat treatment, e) AZ91 before the heat treatment and f) AZ91 after the heat treatment
From the XRD analysis (Fig (3)) of the starting materials, presence of β phase was identified in AZ91 Mg alloy but not in the AZ31. Of course, XRD cannot detect the phase if the fraction is lower. Therefore, in the present study, as shown in Fig 2 (c), no β was observed at the grain boundaries of AZ31 Mg alloy. But, in AZ91 Mg alloy, presence of β phase was clearly observed at the grain boundaries and in the eutectic regions. The maximum solubility of Al in Mg is 12% at 410°C. Therefore in the present study, when AZ91 Mg alloy was kept at 410°C for 6 h, higher amount of Al was dissolved from the eutectic regions and from the β regions. As indicated by black arrow in Fig 2(f), the eutectic regions were disappeared adjacent to β regions as indicted with white arrow. This result strongly suggests the dissolution of more Al into Mg and to form supersaturated grains. Developing supersaturated grains certainly influence the material properties as also reported earlier [9, 14]
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Fig 3 XRD patterns of the samples
The hardness measurements show interesting results as graphically presented in Fig 4. It is obvious that the hardness of AZ31 and AZ91 are higher compared with pure Mg due to alloying effect. Compared with unprocessed condition, decreased hardness was observed for all the samples after heat treatment. As observed from the microstructural studies, no much change was observed in pure Mg before and after the heat treatment. However, stress relieving can happen at 340 °C as it is the annealing temperature for Mg. Therefore, due to decreased internal stresses caused to decrease the hardness of pure Mg marginally. For AZ31 Mg alloy and AZ91 Mg alloy, the grain size was observed as increased after the heat treatment. This might be the reason behind the decreased hardness compared with unprocessed condition. In AZ91 Mg alloy, the solubility of more Al as observed from Fig 2 (e) resulted deceased amount of hard and brittle β-Phase (Mg17Al12). As the hardness of β-Phase is higher than αregions [15], the hardness of AZ91 Mg alloy after the heat treatment was decreased due to reduced amount of βPhase. However, the grains have become supersaturated and hence, the hardness should be increased. But as observed from Fig 4, the hardness of AZ91 was also found to be decreased. This can be understood by counting all the factors which influenced the harness of heat treated AZ91. Here, it can be explained that the contribution of supersaturated grains to increase the hardness has been overcome by the contribution of grin size increase and reduction of β-Phase due to heat treatment.
Fig 4 Microhardness measurements of the samples
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Fig 5 shows the corrosion rates of the specimens calculated from the weight loss of the samples before and after the heat treatment. Before the heat treatment, corrosion rate was observed as decreased with increase of immersion time from day 1 to day 7 for AZ31 and AZ91 samples. Interestingly, for pure Mg, the corrosion rate was observed as increased from day 1 to day 7. This can be attributed to poor resistance of pure Mg towards the corroding environment compared with AZ31 and AZ91. Fig 6 shows the XRD patterns of the immersed samples after 1 day. The phases in the corrosion products were identified as magnesium hydroxide (Mg(OH)2) and magnesium oxide (MgO) along with the Mg and Mg17Al12 phase. During the corrosion of Mg, due to anodic and cathodic reactions, magnesium hydroxide (Mg(OH)2) is formed in any aqueous solutions [16]. In the presence of chloride ions (Cl-), the ceramic layer that is formed due to corrosion is unstable. Mg (OH)2 reacts with Cl- and magnesium chloride (MgCl2) salts are formed which are easily soluble crystals [17,18]. Dissolution of these MgCl2 crystals accelerated the corrosion further. When any alloying elements such as Al and Zn are added to Mg, corrosion mechanisms are changed and the formation of MgCl2 is delayed.
Fig 5. Corrosion rate of the samples after different intervals of immersion times: a) before heat treatment and b) after heat treatment
Fig 6 XRD patterns of the immersed samples after 1 day of immersion
After the heat treatment, as shown in Fig 5 (b), all the samples have shown similar trend as the immersion time was increased to 7days. Compared with pure Mg, AZ31 and AZ91 have shown decreased corrosion rate. Compared with as received condition, heat treated samples exhibited lower corrosion rates in all the cases. Due to heat treatment, any hidden stresses which present in the Mg grain are relieved and the corrosion due to internal lattice strain can be completely eliminated which is the prime reason behind the decreased corrosion in heat treated pure Mg. Presence of secondary phases (β-phase, in the present work) initiate galvanic corrosion in Mg alloys when
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exposed to corroding environment [8,11]. In AZ31 and AZ91 Mg alloys, the amount of secondary phase is decreased after heat treatment which certainly decreases the galvanic corrosion. Therefore, the corrosion rate of heat treated AZ31 and AZ91 samples were observed as decreased compared with unprocessed samples. Particularly in AZ91 Mg alloy, the grains have become supersaturated with more Al. In principle, corrosion rate must be increased due to the more dissolution of Al. However, the effect of disappeared eutectic regions and decreased amount of βPhase (Mg17Al12) was higher compared with the effect of supersaturated grain on the corrosion. From the results, it can be understood that the heat treatment of AZ series Mg alloys may decrease the hardness but enhances the corrosion resistance. 4. Conclusions In the present study, pure Mg, AZ31 and AZ91 Mg alloys were heat treated at 340 and 410°C respectively to alter the microstructure. Pure Mg and AZ31 did not show considerable changes in the microstructure but AZ91 has undergone significant changes. Due to heat treatment, decreased hardness was measured for all the samples. Corrosion rate calculated from the weight loss measurements indicated better performance compared with the unprocessed samples. The decreased corrosion rate after heat treatment can be attributed to the stress relieving and in particular decreased amount of secondary phase (Mg17Al12). Hence form the results, it can be concluded that the corrosion performance of AZ series Mg alloys can be enhanced at the cost of slightly sacrificing the hardness which makes Mg alloys suitable for promising candidates for structural applications where the structures are intended to work in corroding environment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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