impact of rare earth elements ce and pr addition on … · (rees = ce and pr) were prepared...

Journal of Chemical Technology and Metallurgy, 53, 5, 2018

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IMPACT OF RARE EARTH ELEMENTS Ce AND Pr ADDITION ON GRAIN REFINEMENT OF AA5083 ALLOY

A. Ali Al-Bakoosh, Jamaliah Idris

ABSTRACT

One of the most common methods for grain refinement (grain size reduction) is the addition of a specific alloying ele-ment (as grain refiner). Through grain refinement, it is possible to obtain significant improvement in the specific properties of materials. The grain refinement is obtained by changing the size of a grain structure. Thus the measurement of grain size is the indicator of performance for grain refinement. In this work, the rare earth elements REEs ( REEs = Ce and Pr ) with five different mass percentages (0.1 %, 0.3 %, 0.5 %, 0.7 %, and 1.0 %) of Ce and Pr, were added to the AA5083 alloy to estimate the grain size of modified AA5083 alloys as a function of type and percentage of REEs additives. The average grain size was estimated by two methods; by intercept method according to the ASTM E 112 standard, and by image processing technique using Image J software.

It was found that the type and percentage of content of rare earth element Ce and Pr additives play a role in control-ling grain refining efficiency. It was also found that the estimation of the grain size by the image processing technique by using image J software gave satisfactory results compared with the results obtained by the intercept method. Thus we can consider the image j software to have a high efficiency for estimation of the grain size.

Keywords: AA5083 alloy, rare earth elements, grain size, image processing, intercept method.

Received 18 October 2017Accepted 23 January 2018

Journal of Chemical Technology and Metallurgy, 53, 5, 2018, 916-923

Department of Materials Engineering Faculty of Mechanical EngineeringUniversiti Teknologi Malaysia (UTM ) Skudai 81310, Johor Bahru, MalaysiaE-mail: [email protected]; [email protected]

INTRODUCTION

In general, the grain refinement plays a critical role in improving the castability, mechanical properties and corrosion behaviour of the cast and wrought Al-alloys [1 - 4].The grain size that has been achieved during grain refinement process depends on many parameters such as the type and mass percentage of grain refiner (additives) and holding time after the addition of a grain refiner to liquid metal before casting process [5].

Grain size boundaries are effective delphinium to the dislocation motion, and therefore small grained materials will have higher grain boundaries density per unit volume. Thus, we find the decreasing of grain size leads to increasing the yield stress [6]. In general, the yield stress of the materials is a function in grain size.

The average grain size d of a material (grain - re-finement) can be controlled by one of these methods: (i) Thermal methods such as increasing cooling rate; (ii) chemical methods such as the addition of some elements that work as grain size refiners promoting nucleation and obstructing growth; (iii) mechanical methods that include mechanical methods performed during casting process like vibration and stirring of the melt during solidifica-tion. Also, mechanical methods implemented after casting process can lead to grain refining such as rolling process, Equal-Channel Angular Pressing, etc. [7 - 9].

The mechanisms of grain refinement are still not clear yet because it is difficult to observe the nuclea-tion process in grain refinement and the complexity of chemical reactions of the elements [9]. However, there are some theories that try to explain this phenomenon


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(grain refinement).These theories have proposed that the grain refinement mechanisms for Al-alloys are based on the following [1, 10]: (i) the refiner acts as nucleation site and becomes active during solidification leading to heterogeneous nucleation of aluminium grains, (ii) grain growth retardation by solute additions and (iii) edge-to-edge matching of the planes in the inoculant particle and nucleating aluminium grain.

The estimation of average grain size can be per-formed through one of these methods: (i) the comparison procedure, (ii) the planimetric (or Jeffries) procedure, and (iii) the intercept procedures such as Heyn’s method. These test methods were covered in the standard speci-fication ASTM E112 [11].

In this paper, chemical method (REEs additions) was utilised for grain refinement. The literature review highlighted the chemical method for grain refinement only. The chemical method (alloying element addition) is regarded as an economical and effective method for the improvement of the morphology and grain size of Al-alloys [12].

The grain refinement of Al-alloys has been practi-cally employed by adding alloying elements (chemical elements) and it is found to be an effective method for improvement of mechanical properties and corrosion behaviour, where the selection of the proper refiner plays critical role in grain refinement. Generally many chemical elements are used such as Ti, B, Sr, Na, Sb and rare earth elements are used for grain refinement of Al- alloys [12 - 14]. Because the research in this area is relatively recent, and therefore not fully covered yet. Thus, no research has been found that surveyed the effect of REEs (REEs = Ce, Pr) additions on the grain refinement for AA5083 alloy. However, some studies relatively related with the subject that focus on the influence of Ce additions on grain refine-ment of Al-3.2Mg alloys [15], A357 alloy [12], and Al-8 % Si alloys [16] have found that Ce addition is effective in terms of grain refinement. As for the Pr addition, a study on the effect of Pr addition on the microstructure of Al-Si MMCs revealed that Pr additives have positive influence on the grain size refinement [17]. Other rare earth elements were added to pure Al and Al-Mg alloys such as Sc, Nd, Y, Gd and Er. It has been found that they all have an positive effect on grain refinement, where the Er element was the most effective[18].

XU Guo et al. have studied the effect of the addition of trace Er to Al alloys. It was found that this addition

leads to improvement of grain refinement. This is at-tributed to the presence of Al, an Er-rich intermetallic compound that can serve as nucleus for heterogeneous nucleation [19].

EXPERIMENTALMaterial fabrication

AA5083 alloy and modified AA5083 alloy with REEs (REEs = Ce, Pr) have been fabricated by in situ-casting technique under degaussing condition (Argon gas), followed by homogenization process at 450˚C for 24 h. To study the effect of Ce and Pr element percentage content, the modified AA5083 alloys with Ce additions was fabricated with five different amounts (0.1 mass %, 0.3 mass %, 0.5 mass %, 0.7 mass %, 1.0 mass %). Also, the modified AA5083 alloy with Pr additions had five different amounts of Pr additions similar to Ce additions.

Determining average grain size The samples of AA5083 alloy (basic alloy/As-

reference) and modified AA5083 alloys with REEs (REEs = Ce and Pr) were prepared according to ASTM E3-11 (standard guide for preparation of metallography specimens) [20]. After that the samples were subjected to double etching technique, where the purpose of first etching was to remove the Altered Surface Layer (ASL) and the second etching for revealing the grain bounda-ries. It has been reported that the polishing process of Al-alloys creates an ASL Altered Surface Layer that has different microstructure compared with the bulk material[21-23]. This phenomenon was taken into ac-count. Thus, the removal of the ASL is a very necessary issue. The removal of ASL was performed by etching the samples with sodium hydroxide solution (5 mass % NaOH for 10 s at 80°C) followed by immersion in 70 % HNO3 for 3 s followed by rinsing in distilled water, and finally drying in cool air [23]. After that, the samples are etched for prevailing grain boundaries by using etching solution (10 % H3PO4) at 60˚C for 5 min [24]. After the etching process was completed, the samples are cleaned ultrasonically for 5 min at room temperature, where the utilised cleaning solution was A Ticko solution, after which the samples immediately washed under running water, rinsed with acetone and dried with an air dryer. The AA5083 alloy sample and modified AA5083 alloys samples were then immediately taken for microscopic observation for grain size measurements, where five


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representative images were captured from each sample.The etched samples were observed under Light

Optical Microscope (LOM), (Zeiss, Stemi 200-C). The contrast of the microstructure was satisfactory. The im-ages were captured by zooming 50X and then are saved for further image analysis. The specialised hardware and Software (iSolution DT) image processing program is used for grain size measurement. The grain size was determined according to the ASTM E112 standard by utilized quantitative image analysis and employing linear intercept method. In this method, the lines are drawn in the photomicrograph and the number of grain boundaries intercepts (Nl) along the test lines is counted. The mean lineal intercept is then given as

I = L /( Nl M) (1)

where L is the length of the test lines and M is the mag-nification in the photomicrograph of the material.

The mean lineal intercept (I) does not really provide the grain size. The most correct way to express the grain size (d) from lineal intercept measurements is: d= I.

To confirm the average grain size diameter measure-ment, the image processing technique known as Image j software has also been used. The implementation of ImageJ can be summarized as follows: upload the mi-crostructure image, set measurement, convert it to 8 bits, and then adjust its threshold. After that, go to analyse and click on analyse particles to get information about the microstructural grain size. After measuring the grain size of all samples, the correlation between the average grain size diameter and REEs content was established.

RESULTS AND DISCUSSION

The influence of REEs (REEs = Ce and Pr element) addition individually on grain refinement of the AA5083 alloy was studied through grain size estimation accord-ing to ASTM E112 standard by the intercept method as well as by image processing technique using Image J software. Measuring grain size is very important because it is one of the most important factors in controlling the materials properties [25].

In this work, the grain size was measured by inter-cept method, (or Heyn’s method), because it is a com-mon method used by many researchers. This method is simple and it is recommended for all structures which

do not have uniform equated grain structure (structure consisting of elongated grains) [26].

The grain size of microstructure does not depend on the chemical compositions only, but also depends on the casting process variables. Thus the casting process variables must be carefully controlled, such as the tem-perature of the molten during pouring, environmental temperature when pouring the molten, the taken time for pouring process and the height of crucible on the mould during pouring process, etc. This is to ensure that the change in grain size was due to the effect of REEs ad-dition and not the other factors, and thus a quantitative assessment of grain size for modified AA5083 alloy as a function of REEs type and levels additions is accurate. Accordingly, the samples were prepared by in-situ cast-ing technique for producing the modified alloys in this study. It is worth noting that the casting process variables that have been mentioned can be easily controlled by the in-situ casting process.

Also, it is an important phenomenon, which is the formation of ASL layer. Several studies have indicated that ASL has been created as a result of the polishing process due to plastic deformation [21 - 23].This layer has different microstructure compared with the bulk material [22]. Thus, removing of this layer is a very important issue.

Accordingly, the ASL layer was removed by etching the samples with sodium hydroxide solution as explained in detail in the experimental method.

The impact of REEs ( REEs = Ce and Pr ) addi-tions on grain refinement for AA5083 alloys was as-sessed experimentally by measuring the grain size of the modified AA5083 alloys by implementation of the intercept method (Heyn’s method) by utilising optical microscope and software ( semi-automatic and automatic image analysis) isolation DT-M according to ASTM E112 standard.

Fig. 1 shows an example of the use of the intercept method for estimation the grain size by utilizing isola-tion DT-M software program. The implementation of the intercept method on the samples was presented in Fig. 2. The summarized results have been presented in Table 1.

The average grain size diameter of the base alloy (AA5083) was around 108 μm, and decreases to 105, 91, 87, 80, and 72 μm, respectively, with Ce addition as shown in Fig. 2. It has been shown that as the mass % of Ce increases, the grain size gradually become smaller.


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In other words, the grain size decreased about (refined by 2.3 %, 15.75 %, 19.5 %, 26 % and 33.3 % due to the Ce addition.

For modified AA5083 alloy with Pr additions, the grain size decreased by increasing the amount of Pr additions, the reduction of average grain size diameter

was (107, 96, 90, 83, and 74 μm), respectively, as shown in Fig.3. The most effective was at the 1.0 mass % Pr, which led to a decrease in average grain size diameter by 31.5 %. In the case of Pr additives, the results were similar to the Ce additives but less effective as repre-sented in Table 1.

Fig. 1. An example of the use of the intercept method for estimating grain size by utilizing isolation DTM software program.

Addition Level (mass %)

Average grain size diameter (µm)

Percentage reduction of the Average grain size diameter (∆d/do)

AA5083 alloys 0.0 mass % REEs 108 -

Modified AA5083 alloys with Ce additions 0.1 mass % Ce 105 -2.777% 0.3 mass % Ce 91 -15.740% 0.5 mass % Ce 87 -19.444% 0.7 mass % Ce 80 -25.926% 1.0 mass % Ce 72 -33.333%

Modified AA5083 alloys with Ce additions 0.1 mass % Pr 107 -0.926% 0.3 mass % Pr 96 -11.111% 0.5 mass % Pr 90 -16.666% 0.7 mass % Pr 83 -23.148% 1.0 mass % Pr 74 -31.481%

Table 1. Average grain sizes diameter of the base material (AA5083 alloy) and modified AA5083 alloys.


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Fig. 2. The use of the intercept method for estimating grain size for modified AA5083 alloys by utilizing isolation DTM software program.

Fig. 3. The average grain size diameter vs. various contents (mass %) of Ce for modified AA5083 alloys.

Fig. 4. The average grain size diameter vs. various contents (mass %) of Pr for modified AA5083 alloys.

Addition Level (mass %)

Average grain size diameter (µm)

Average Ferret diameter (µm)

AA5083 alloys 0.0 mass %

REEs 108 111

Modified AA5083 alloys with Ce additions 0.1 mass % Ce 105 106 0.3 mass % Ce 91 93 0.5 mass % Ce 87 89 0.7 mass % Ce 80 81 1.0 mass % Ce 72 73

Modified AA5083 alloys with Pr additions 0.1 mass % Pr 107 109 0.3 mass % Pr 96 99 0.5 mass % Pr 90 92 0.7 mass % Pr 83 84 1.0 mass % Pr 74 76

Table 2. The comparison between two methods for estimation the grain size of the base material (AA5083 alloy) and modified AA5083 alloys. The intercept method (average grain size diameter) and image processing technique (ferret diameter).


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By comparison between the efficiency degree of the two additives Ce and Pr, the results show that the grain refinement efficiency of Ce is higher than that of Pr. The reduction in average grain size diameter of the modified AA5083 alloys recorded the biggest decrease in grain size (≈ 33.3 %) when the mass % of Ce was 1.0 mass %.

In general, it has been observed that increasing the percentage of REEs additions leads to a reduction in the grain size. This is because when the addition level of grain refiner is increased, the number of nucleating particles added will increase. As the number of particles increases, the intraparticle distance decreases. Thus the grain size decreases [5]. These results (influence of REEs on the addition on grain refinement of the AA5083 alloy) are in agreement with the previous findings for adding REEs to other Al-alloys, where the REEs have a positive effect on grain refinement for Al-alloys.

The grain refinement for modified AA5083 alloys due to addition of REEs additives can be attributed to the grain growth restriction by REEs additives in solid solution here the REEs act as nucleation sites leading to grain growth inhibiting. The fine precipitates not only affect the diffusion of the atom in the alloy but also restrict the movement of the interfaces. Thus, it will act as an obstacle for the grain to grow.

In general, the Ce and Pr elements hardly dissolve in the α-Al phase. This is because the atomic radius of the Ce and Pr are much larger than that of α-Al thereby reducing the refined grain size. This is might be attrib-uted to the concentration of REEs elements that lead to restricting the growth of α-Al phase. The growth restriction effect in the grain refinement is important, but the type and size of particles play a vital role in the grain refinement.

The estimation of grain size by image processing technique was performed to confirm the results of in-tercept method. The quantity that has been measured by image processing software (image j) for the examined samples was ferret diameter, which is a very popular measure of the size of various objects [8].

On Fig. 5 an example illustrates the main steps for one of the examined samples processed to estimate the grain size. The results that have been obtained from image processing were in agreement with the results of intercept method, but their values were a little bit greater than the values that have been measured by intercept method as shown in Table 2. This can be attributed to

the difference in principle of measurements, where in this method, the measurement was performed through the longest projection length

CONCLUSIONS

The addition of REEs (Ce and Pr) efficiently refine the grain size of AA5083 alloy because of the capability to enhance the nucleation and growth restriction, where the grain refinement efficiency was influenced by the type and amount of REEs added.

The grain size achieved for modified AA5083 al-loys during grain refinement by adding REEs (REEs (Ce and Pr) was dependent on the type and percentage of REEs additives.

Fig. 5. Example illustrating the main steps for one of the examined samples processed by the imaging process to estimate the size of the grain: (a) the original image; (b) the binary image B/W; (c) the binary image W/B.


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The Ce additives have more efficiency for grain refinement compared with the Pr additions.

The best reduction achieved in average grain size diameter was ≈ 34 %. It was achieved with 1.0 mass % Ce addition.

REFERENCES

1. K. Kashyap, T. Chandrashekar, Effects and mecha-nisms of grain refinement in aluminium alloys, Bulletin of Materials Science, 24, 4, 2001, 345-353.

2. G.K. Sigworth, T.A. Kuhn, Grain refinement of aluminum casting alloys, International Journal of Metalcasting, 1, 1, 2007, 31-40.

3. T. Ramachandran, P. Sharma, K. Balasubramanian, Grain refinement of light alloys, Proceedings of the 68th WFC-World Foundry Congress, Chennai, India, 2008, 189-193.

4. T. Chandrashekar et al., Effect of growth restricting factor on grain refinement of aluminum alloys, The International Journal of Advanced Manufacturing Technology, 40, 3-4, 2009, 234-241.

5. N. Reddy et al., Prediction of grain size of Al–7Si Alloy by neural networks, Materials Science and Engineering: A, 2005. 391, 1, 2005, 131-140.

6. J.P. Schaffer, A. Saxena, S.D. Antolovich, T.H. Sanders, S.B. Warner, The science and design of engineering materials, Irwin, Chicago, 1995.

7. D.R. Herling, M.T. Smith, Improvements in su-perplastic performance of commercial AA5083 aluminium processed by equal channel angular ex-trusion, Materials Science forum, Trans Tech Publ., 357, 2001, 465-470.

8. G.F. Vander Voort, S.R. Lampman, B.R. Sanders, G.J. Anton, C. Polakowski, J. Kinson, K. Muldoon, S.D. Henry, W.W. Scott Jr., ASM handbook, Met-allography and Microstructures, Matwerials Park, Ohio, 44073-0002, 9, 2004.

9. R.-G. Guan, D. Tie, A Review on Grain Refinement of Aluminum Alloys: Progresses, Challenges and Prospects, Acta Metallurgica Sinica (English Let-ters), 2017. 30, 5, 2017, 409-432.

10. M.-X. Zhang et al., Crystallographic study of grain refinement in aluminum alloys using the edge-to-edge matching model, Acta Materialia, 53, 5, 2005, 1427-1438.

11. Standard, A., E112, 2010,” Standard Test Methods

for Determining Average Grain Size,” ASTM In-ternational, West Conshohocken, PA, 2010, DOI: 10.1520/E0112-10.

12. W. Jiang et al., Effects of rare earth elements ad-dition on microstructures, tensile properties and fractography of A357 alloy, Materials Science and Engineering: A, 597, 2014, 237-244.

13. D. Emadi, A.P. Rao, M. Mahfoud, Influence of scandium on the microstructure and mechanical properties of A319 alloy, Materials Science and Engineering: A, M.-X 527, 23, 2010, 6123-6132.

14. W. Prukkanon, N. Srisukhumbowornchai, C. Lim-maneevichitr, Modification of hypoeutectic Al–Si alloys with scandium, Journal of Alloys and Com-pounds, 477, 1, 2009, 454-460.

15. X. Zhang, Z.H. Wang, Z.H. Zhou, J.M. Xu, Z.J. Zhong, H.L. Yuan, G.W. Wang, Effects of Rare Earth on Microstructure and Mechanical Properties of Al-3.2 Mg Alloy, Materials Science Forum, Trans Tech Publications Ltd., 1, 817, 2015, 192-193.

16. V. Vijayan, K.N. Prabhu, The effect of simultaneous refinement and modification by cerium on micro-structure and mechanical properties of Al-8% Si alloy, International Journal of Cast Metals Research, 29, 6, 2016, 345-349.

17. J.-H. Peng, W.F. Li, F.L. Huang, D. Jun, Effect of rare earth Pr on microstructure and mechanical properties of Al2O3-SiO2 (sf)/Al-Si composites, Transactions of Nonferrous Metals Society of China, 2009. 19, 5, 2009, 1081-1086.

18. Z.R. Nie, T. Jin, J. Fu, G. Xu, J. Yang, J.X. Zhou, T.Y. Zuo, Research on rare earth in aluminium, Materials Science Forum, Trans Tech Publ., 396, 2002, 1731-1733.

19. G.F. Xu, S.Z. Mou, J.J.Yang, T.N. Jin, Z.R. Nie, Z.M. Yin, Effect of trace rare earth element Er on Al-Zn-Mg alloy. Transactions of Nonferrous Metals Society of China, J.-H 16, 3, 2006, 598-603.

20. Standard AS E3-11, 2011,” Standard Guide for Preparation of Metallographic Specimens”, ASTM International, West Conshohocken, PA, DOI: 10.1520/E0003-11.

21. S.-S. Wang, F. Yang, G. Frankel, Effect of Altered Surface Layer on Localized Corrosion of Aluminum Alloy 2024. Journal of The Electrochemical Society, 164, 6, 2017, 317-323.

22. J. Seong, F. Yang, F. Scheltens, G.S. Frankel, N.


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Sridhar, Influence of the Altered Surface Layer on the Corrosion of AA5083, Journal of the Electro-chemical Society, 2015. 162, 6, 2015, 209-218.

23. J. Seong, Inhibition of Corrosion and Stress Cor-rosion Cracking of Sensitized AA5083, The Ohio State University, 2015.

24. K.A. Unocic, Structure-composition-property re-lationships in 5XXX series aluminum alloys, The

Ohio State University, 2008.25. B. Roebuck, C. Phatak, I. Birks-Agnew, A com-

parison of the linear intercept and equivalent circle methods for grain size measurement in WC. Co hard metals NPL Report MATC (A), 149, 2004, 1-30.

26. D.C. Zipperian, Metallographic handbook, Chief Technical Officier PACE Technologies, Tucson (AR), 2011.

impact of rare earth elements ce and pr addition on … · (rees = ce and pr) were prepared...

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