Effect of Oxide Dispersion on Dendritic Grain Growth Characteristics of Cast Aluminum Alloy

Transcription

1 Materials Transactions, Vol. 51, No. 10 (2010) pp to 1957 #2010 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Effect of Oxide Dispersion on Dendritic Grain Growth Characteristics of Cast Aluminum Alloy Gwang-Ho Kim 1; * 1, Sung-Mo Hong 2, Min-Ku Lee 2, Soon-Ho Kim 3, Ikuo Ioka 4, Byoung-Suhk Kim 1 and Ick-Soo Kim 1; * 2 1 Department of Functional Machinery and Mechanics, Shinshu University, Ueda , Japan 2 Nuclear Materials Research Division, Korea Atomic Energy Research Institute (KAERI), Daejeon , Korea 3 Department of Automotive Mechanical Engineering, College of Engineering, Silla University, Busan , Korea 4 Advanced Reprocessing Materials Development Group, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki , Japan The dispersion characteristics of the nano-sized Y 2 O 3 powders in molten aluminum were investigated from the viewpoint of changes in microstructure and mechanical property as a function of oxide contents. As the solidification structure, the oxide nanoparticles dispersed in the columnar crystal was mainly segregated on the grain boundary, whereas the oxide nanoparticles dispersed in the equiaxed crystal was uniformly dispersed on both grain boundary and inside the crystal. The most uniform dispersion of oxide nanoparticles was observed at Y 2 O 3 content of 2 mass%. As Y 2 O 3 content of 3 mass%, the size of oxide nanoparticles in metal matrix increased due to the particle aggregation, as confirmed by SEM analysis. Moreover, it was found that the mechanical properties such as hardness and tensile strength were improved at Y 2 O 3 content of 2 mass%, indicating the well-dispersion of nano-sized Y 2 O 3 powders in cast aluminum. The hardness was increased by 1.2 times up to 57 H V and tensile strength was increased by 1.55 times up to 80 MPa, compared with the case of pure aluminum. However, at Y 2 O 3 content of 3.0 mass%, tensile strength was sharply decreased by 0.6 times due to aggregation of oxide nanoparticles, while the hardness was increased to 57 H V, which is the same as the case of Y 2 O 3 content of 2.0 mass%. [doi: /matertrans.m ] (Received May 11, 2010; Accepted July 26, 2010; Published September 8, 2010) Keywords: solidification, dendritic growth, stir-casting, oxide dispersion strengthened alloy, nano-particle, aluminum 1. Introduction Oxide Dispersion Strengthened (ODS) alloys have higher strength and excellent thermal stability at higher temperature due to combined characteristics of metal with high-temperature stability and oxide particles with high-strength. ODS alloys are regarded as high-tech material in the fields such as aerospace, automobile and fusion reactor because of the above mentioned characteristics. 1 5) Until now, ODS alloys have been manufactured mostly by using Mechanical Alloying (MA) method. However, such MA method is not suitable for mass-production and large-size product of the material because of a complicated manufacture procedure as well as higher cost. Accordingly, other ODS alloy manufacture methods using casting technique, such as Rheocasting, compo-casting, stircasting etc., 6 11) are being widely studied. The casting method is attractive because the manufacture processing is relatively simple, and the manufacture of large-size components and mass-production is possible. 4,11) However, the problem of this method is that uniform dispersion of oxides is difficult due to the fundamental problems such as incompatibility and poor wettability between metal and oxide particles. Thus, in order to improve the dispersion of the oxide nanoparticles into the metal matrix, two important points should be taken into consideration: the first is the uniform dispersion of oxide nanoparticles into the liquid metal and the second is the dispersion behavior of oxide nanoparticles in the process of its solidification in the liquid metal. Firstly, the oxide nanoparticles generally float on the * 1 Present address: Advanced Reprocessing Materials Development Group, Japan Atomic Energy Agency (JAEA), Ibaraki , Japan * 2 Corresponding author, kim@shinshu-u.ac.jp (I.S. Kim) surface of the liquid metal because of the lower specific gravity. Therefore, various techniques for solving this problem are being suggested, which is the most important key issue. Secondly, it is also important to investigate how the oxide nanoparticles behave during the solidification since the final microstructure of alloy is determined during the solidification process, and thus, should be understood. Nevertheless, currently, the second case is less studied compared to the first case. Recently, it was reported that the agglomeration of oxide nanoparticles dispersed in metal matrix occurs during welding of the ODS alloy, 12 14) which can be correlated with the solidification process of the ODS alloy. In this study, we report the dispersion behavior of the oxide nanoparticles in the structure of the columnar dendrite and inner equiaxed dendrite generated during the solidification process. In order to uniformly disperse the oxide nanoparticles into the liquid metal, aluminum with the lower value of specific gravidity was selected as metal, and Y 2 O 3 particles, which are relatively heavy among the oxide, were used. In addition, oxide nanoparticles were dispersed into the liquid metal by the Stir-casting method. Oxide content was changed to 1.0 mass%, 2.0 mass% and 3.0 mass% in the aluminum. The relationship between the microstructure and the dispersion of oxide nanoparticles, and mechanical property was investigated. 2. Experimental The ODS alloy was prepared from commercial aluminum (99% purity) by directly casting oxide nanoparticles inside the mold. The oxide nanoparticles used in this study were Y 2 O 3 (specific gravity: 5.03 gcm 3 ) with mean diameters of 100 nm. Among various ceramic oxide nanoparticles, the

2 1952 G.-H. Kim et al. Fig. 2 The geometry of the tensile specimen. Fig. 1 Schematic diagram of a stir-casting method, and stirrer impeller design (unit: mm). specific gravity of Y 2 O 3 is rather higher compared to those of other ceramic nanoparticles and may be hence suitable to achieve the well-dispersion of the particles in the melt by stircasting method. An induction furnace was used to melt the aluminum in a small graphite crucible with the size of 100 mm in diameter by 200 mm in height. A charge of 600 g of pure aluminum was placed in a graphite crucible. The furnace temperature was raised to 700 C and removed impurities and oxide film on the surface of the melt. Then the temperature of melt was further increased to 900 C and a graphite impeller was dipped into the melt. The Y 2 O 3 particles were added into the vortex of the melt during stirring from the top of the crucible. The oxide dispersed melt was stirred with 400 rpm for 4 min at 900 C. Then it was poured into a steel mold to form ingot of 150 mm 100 mm 10 mm. In order to obtain the same cooling rate, a steel mold was preheated to 50 C for 60 min in a resistance furnace. The aluminum melt was protected by argon atmosphere during the casting process. The schematic view of the stir casting apparatus and the design of the stirrer impeller are shown in Fig. 1. All metallographic specimens were polished mechanically to a 0.3 mm finish, followed by etching in 10 mass% NaOH solution. The morphological change in as-cast bulk alloy was examined as a function of Y 2 O 3 content by optical microscope (OM) and scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDS) and backscattered electrons (BSE) were conducted to clarify the distribution of Y 2 O 3 particles in the matrix. In order to compare the concentrations of oxide in the cast structure, the volume fraction of oxide nanoparticles was measured by an image analyzer and the composition of as-cast oxide nanoparticlesdispersed aluminum alloy was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. Vickers hardness measurement was performed using a microhardness tester (HMV 2000, Shimazu) at a load of 200gf. The specimens were measured on the columnar zone of pure aluminum and ODS aluminum with the oxide contents of 1 mass%, 2 mass%, and 3 mass%, which was prepared vertically to the grain growth direction at the position of 2 mm from the sidewall of ingot. More than 10 times of hardness measurements at each location were made. The tensile samples were fabricated following ASTM E8M pin ends sheet-type specimen for the oxide dispersed columnar zone. The dimension specifications were 10 mm in gauge length, 3 mm in width and 1 mm in thickness. Details of tensile specimen are shown in Fig. 2. Tensile tests were performed by an Instron-type machine (Shimazu, AG-IS MO) under the strain rate of 10 3 Ls Results and Discussion 3.1 Structure analysis of solidified ODS aluminum In a casting or ingot, three zones of solidification structure can generally be observed. In the case of solidifying inwards from the mold wall, outer equiaxed, columnar and inner equiaxed structures are gradually formed. These structural characteristics are very important because those are closely related to their mechanical performances. Figure 3 shows OM (left) and BSE (right) images of the cast Y 2 O 3 - dispersed aluminum alloy with respect to the different positions of aluminum ingot dispersed with 2.0 mass% Y 2 O 3. As seen in OM images, each microstructure shows an outer equiaxed structure formed in a micro equiaxed dendrite structure outside the ingot, a columnar structure where some grains are grown and solidified in a longitudinal direction from the outer equiaxed zone, and an inner equiaxed structure where solidification is started in the internal supercooled melt. In the BSE images, the gray part indicates the aluminum matrix, and the white part in the grain boundary and inside the crystal indicate the Y 2 O 3 particles including Fe impurity. From Fig. 3, we can find an interesting point that is the quite different dispersion behavior of oxide nanoparticles depending on the solidification structure. In the case of columnar structure, oxide nanoparticles are agglomerated mainly on the grain boundaries, whereas oxide nanoparticles in the equiaxed structure are uniformly dispersed even inside the crystal. These dispersion behaviors of oxide nanoparticles can be explained by different solidification procedures between two solidification structures. 3.2 Dispersion mechanism of oxide nanoparticles in solidification In this study, we investigated the solidification behaviors

3 Effect of Oxide Dispersion on Dendritic Grain Growth Characteristics of Cast Aluminum Alloy 1953 (d) (e) (f) Fig. 3 Optical micrographs and BSE images of solidification structure with 2 mass% Y 2 O 3 input; OM: outer equiaxed zone, columnar zone and (e) inner equiaxed zone, BSE images: the dendritic grain from the central region of, (d) the dendritic grain from the central region of and (f) the dendritic grain from the central region of (e). of round the columnar and inner equiaxed structures, where are occupied almost with the whole of ingot. So, the term the equiaxed structure means inner equiaxed structure. The form of a solidification microstructure depends not only upon the temperature gradient, but also upon the growth rate. In general, columnar and equiaxed structures have different solidification mechanism due to the difference in heat flux during the cooling process. Figure 4 shows the schematic drawing of the agglomeration mechanism of oxide nanoparticles on growing process of the aluminum nucleus at both columnar and inner equiaxed structures. Before the solidification, the oxide nanoparticles were supposed to be uniformly dispersed in the melt. In the case of columnar structure, nuclei are mostly formed on the outer portion, which has lower temperature, and solidification occurs via a growth of dendrite structure, as shown in Fig. 4 and 4. The columnar crystals are in contact with the mold and heat will be transported through them. So, the hottest part of the system is the melt. It constrains a positive temperature gradient in front of the growth tip of the columnar dendrites. Thus, those are always stable in pure metals and grow in an antiparallel direction to the heat flux. 15,16) Meanwhile, usually, dendrite growth in the process of solidification of pure metal tends not to include the impurity into the crystal, but to push the oxide nanoparticles and the impurity into the residual melt. 15,17) As expected, the oxide nanoparticles dispersed in the molten metal are pushed to the boundaries between dendrites, and then grows further (with the impurity) as shown in Fig. 4. Once the primary spacing is established, that will not change before and after solidification ) This is not so for the secondary arms, which undergo a ripening process. 15,18) Therefore, the oxide nanoparticles are trapped among the primary trunks. They are finally agglomerated on the grain boundary as seen in Fig. 4(d). On the other hand, within the equiaxed region, dendritic growth of pure metals occurs under conditions where only heat fluxs from the interface to the surrounding liquid. The

4 1954 G.-H. Kim et al. (e) (f) (g) (d) (h) Fig. 4 Schematic representations of solidification and particle distribution behavior as a function of cast structure; Columnar dendritic growth: nucleation, dendritic grain growth, dendritic grain growth, and (d) the final solidification structure, Equiaxed dendritic growth: (e) nucleation, (f) dendritic grain growth, (g) dendritic grain growth, and (h) the final distribution of particle in the matrix material. hottest part of system is the nucleated crystals. In this case, the growth and heat flux directions are the same, and the temperature gradient is negative at the interface and a thermal undercooling exists. Thus, equiaxed grains of pure metals are inherently unstable when their diameter exceeds a critical value (of the order of some micrometers) ) As it can be seen in Fig. 4(e), 4(f) and 4(g), equiaxed grains are nucleated and further grow in a random manner. Each grain is made up of six orthogonal primary trunks. 15) The space between the trunks is filled with secondary and possibly higher-order branches. Like a columnar growth, a part of remained liquid including oxide nanoparticles is blocked among the branches with various orders, and then preferentially solidified. Generally, equiaxed grains showed finer dendritic structure more than the columnar s one. 19) The reason can be considered to be no sufficient time for a ripening process, which the grain growth is inherently proceeded under the unstable atmosphere.

5 Effect of Oxide Dispersion on Dendritic Grain Growth Characteristics of Cast Aluminum Alloy Effect of oxide content on oxide dispersion and structures of solidified ODS aluminum Figure 5 shows the dispersion property of oxide nanoparticles in the columnar and the inner equiaxed structures as function of oxide contents dispersed in molten aluminum. The impurity Fe element was segregated on the grain boundary of pure aluminum in Fig. 5 and 5(e). It can be known from the figure where the microstructure of the columnar zone exhibits a three-dimensional form that the oxide nanoparticles are agglomerated mostly on the grain boundary and little exists inside the crystal. As the content of oxide particles increases, the grain size decreases (that is, the space between dendrite arm gets closer.) and the amount of agglomerated oxide nanoparticles on the grain boundary increases. As described above, the reason that the space of dendrite arm gets closer is because nucleation site increases as the concentration of oxide nanoparticles in the molten metal increases resulting in further formation and growth of dendrite. On the other hand, in all case, oxide nanoparticles in the equiaxed structure were more uniformly dispersed than the columnar structure. It was also found that the oxide nanoparticles were most uniformly dispersed at Y 2 O 3 2 mass%. In contrast, the degree of the dispersion of oxide nanoparticles in the grain at Y 2 O 3 3 mass% was relatively decreased because large amount of oxide nanoparticles were agglomerated. Under the conditions of 1 mass% and 2 mass%, oxide nanoparticles dispersed in the grain have particle size distribution of 0:61 mm, while under the condition of 3 mass%, particle size distribution was increased to 0:63 mm. Even though Y 2 O 3 particle of 100 nm is input at the beginning, the reason why the size of Y 2 O 3 particles increases in the final casting structure is because the oxide nanoparticles are agglomerated among the dendrites during the solidification. Figure 6 and Table 1 show the result of EDS analysis of dispersed oxide nanoparticles inside the crystal and on the (d) (e) (f) (g) (h) Fig. 5 Dispersion property of oxide particles as a function of oxide input contents; A three-dimensional microstructure of the columnar zone: pure Al, 1 mass% Y 2 O 3, 2 mass% Y 2 O 3, and (d) 3 mass% Y 2 O 3, A microstructure of the inner equiaxed zone: (e) pure Al, (f) 1 mass% Y 2 O 3, (g) 2 mass% Y 2 O 3, and (h) 3 mass% Y 2 O 3. Fig. 6 EDS analysis of agglomerated oxide particles inside the crystal and on the grain boundary at 3 mass% Y 2 O 3 : inside the crystal, on the grain boundary, and EDS spectra of S1.

6 1956 G.-H. Kim et al. Fig. 7 The mechanical properties as a function of oxide input contents; micro-vickers hardness, UTS, and stress-strain curve. Table 1 The compositions of dispersed oxide particles inside the crystal and on the grain boundary in Fig. 5 and 5. Spectrum Al Fe Y S S grain boundary in order to confirm the existence of oxide nanoparticles in cast structure. It can be seen that the agglomerated particles have a complex structure where oxide nanoparticles, impurities, and metal-matrix are overlapped each other like a eutectic structure in Fig. 6 and 6. From the EDS analysis, as shown in Fig. 6, it can be confirmed Y and Fe elements at the same position, indicating that Y 2 O 3 particles are moved with impurity Fe element in the solidification process. The agglomerated oxide nanoparticles above a critical size can be a huge defect. Accordingly, the amount of oxide content should be properly controlled. Table 2 shows an analysis result on the oxide content dispersed in the final cast structure according to the oxide content by an image analysis and ICP-AES analysis. Even in pure aluminum, approximately 0.1 mass% of Fe exists as impurities. In addition, it can be seen that the amount of oxide nanoparticles dispersed in the cast structure has increased as increasing the amount of added oxide nanoparticles, and that the content of Fe impurity is almost constant in each condition. 3.4 Mechanical properties of solidified ODS aluminum Figure 7 shows the hardness and tensile strength of pure aluminum and ODS aluminum with the oxide contents of Table 2 The result of an image analysis and ICP-AES analysis on the amount of oxide particles in the casting structure as a function of oxide particles input contents. Content Image analysis (Y 2 O 3 +Fe, per area) ICP-AES analysis (Y, mass%) ICP-AES analysis (Fe, mass%) Pure Al 1 mass% Y 2 O 3 2 mass% Y 2 O 3 3 mass% Y 2 O mass%, 2 mass%, and 3 mass%. As the oxide dispersion is improved in comparison with pure aluminum, the hardness and ultimate tensile strength (UTS) increased approximately 1.2 times up to 57 H V and 1.55 times up to 80 MPa (2 mass% input), respectively. The reason of increasing in hardness and UTS is considered to be due to the oxide dispersion and grain refinement effects caused by the oxide nanoparticles. However, at oxide nanoparticles 3 mass%, the hardness is the same as those of the oxide nanoparticles of 1 mass% and 2 mass%, but the tensile strength is sharply decreased by about 0.6 times up to 32 MPa compared to pure aluminum. This is because, as can be known in the microstructure, a large amount of oxide nanoparticles are agglomerated into the crystal and onto the grain boundary which acted as a defect. As can be seen in stress-strain curves (Fig. 7), degree of the strain is increased in the case of content of oxide nanoparticles (1 mass% and 2 mass%) compared to pure aluminum. Generally, degree of a strain is closely related to the grain size. Since it was confirmed in the above microstructure that the grains get

7 Effect of Oxide Dispersion on Dendritic Grain Growth Characteristics of Cast Aluminum Alloy 1957 finer as oxide nanoparticles are dispersed, it can be said that the refinement effect of grains is the cause for the increase of the strain. 4. Conclusion Dispersion characteristics and mechanical properties of nano-sized Y 2 O 3 particles in the cast of aluminum have been investigated from the viewpoint of changes in microstructure, hardness and tensile strength as a function of particle input contents. The oxide dispersion strengthened aluminum was fabricated by stir-casting method. (1) It was found that, among the solidification structures, oxide nanoparticles in the columnar structure are mainly agglomerated on the grain boundaries, whereas oxide nanoparticles in the equaxied structure are uniformly dispersed even inside the crystal. (2) As a function of oxide input contents, the degree of dispersion of oxide nanoparticles in the equaxied structure showed the best property at Y 2 O 3 2 mass%. (3) Compared to those for the pure aluminum, mechanical properties of the oxide content of 2 mass% showed an excellent improvement. The hardness and the tensile strength increased approximately 1.2 times up to 57 H V and 1.55 times up to 80 MPa, respectively. In contrast, at oxide particles 3 mass%, the hardness is the same as that of the oxide particles of 2 mass%, but the tensile strength is sharply decreased approximately 0.6 times up to 32 MPa compared to pure aluminum. This is because a large amount of oxide nanoparticles are agglomerated into the crystal and onto the grain boundary which acted as a defect. (4) Under the conditions of 1 mass% and 2 mass%, oxide nanoparticles dispersed in the grain have particle size distribution of 0:61 mm, while under the condition of 3 mass%, particle size distribution is 0:6 3 mm. Acknowledgements This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD), KRF D REFERENCES 1) P. Busse, F. Deuerler and J. Pötschke: J. Cryst. Growth 193 (1998) ) M. S. El-Genk and J. M. Tournier: J. Nucl. Mater. 340 (2005) ) H. Chang, C. H. Pitt and G. B. Alexander: J. Mater. Sci. Lett. 12 (1993) ) J. J. Park, G. H. Kim, S. M. Hong, S. H. Lee, M. K. Lee and C. K. Rhee: J. Mater. Sci. Technol. 24 (2008) ) K. Klimiankou, R. Lindau and A. Möslang: J. Cryst. Growth 249 (2003) ) A. Velhinho, P. D. Sequeira, F. B. Fernandes, J. D. Botas and L. A. Rocha: Mater. Sci. Forum (2003) ) S. C. Lim, M. Gupta, L. Ren and J. K. M. Kwok: J. Mater. Proc. Technol (1999) ) K. Ichikawa and M. Achikita: ISIJ Int. 31 (1991) ) N. P. Hung, F. Y. C. Boey, K. A. Khor, C. A. Oh and H. F. Lee: J. Mater. Proc. Technol. 48 (1995) ) N. Aniban, R. M. Pillai and B. C. Pai: Mater. Design 23 (2002) ) J. Hashim, L. Looney and M. S. J. Hashmi: J. Mater. Proc. Technol (1999) ) B. J. Inkson and P. L. Threadgill: Mater. Sci. Eng. A 258 (1998) ) T. Uwaba, S. Ukai, T. Nakai and M. Fujiwara: J. Nucl. Mater (2007) ) P. D. Sketchley, P. L. Threadgill and I. G. Wright: Mater. Sci. Eng. A (2002) ) W. Kurz and D. J. Fisher: Fundamentals of solidification, Chapter 4 and Appendix 7, (Trans. Tech. Publications, Switzerland-Germany-UK- USA, 1984). 16) M. E. Glicksman: Mater. Sci. Eng. 65 (1984) ) C. A. Gandin: Acta Mater. 48 (2000) ) S. C. Huang and M. E. Glicksman: Acta Mater. 29 (1981) ) V. Pines, A. Chait, M. Zlatkowski and C. Beckermann: J. Cryst. Growth 197 (1999)