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The effect of Ce on the hydrogen content and liquid structure of Al–16% Si melts
Materials Characterization 51 (2003) 29 – 33
The effect of Ce on the hydrogen content and liquid structure of Al–16% Si melts Hongxia Geng a,*, Haoran Geng b, Xianying Xue a a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China b College of Materials Science and Engineering, Jinan University, Jinan, 250022, PR China Received 27 June 2002; received in revised form 6 August 2003; accepted 8 September 2003
Abstract The effects of small additions of cerium (Ce) on the hydrogen content and the viscosity of Al – 16% Si melts have been studied by using Hyscan II and a viscometer. The experimental results showed that the hydrogen content increased with an increase of temperature, but decreased with an increase of the Ce level. The atomic densities of Al – 16% Si melts containing 0%, 0.05%, and 0.15% Ce were measured and calculated, and the relationship between the hydrogen content and the liquid structure of Al – 16% Si melts with and without Ce additions was investigated. According to the test results and analysis, it was concluded that a melt structural change caused a change of the hydrogen content when Ce was added, and the viscosity change further verified the structural change. D 2003 Elsevier Inc. All rights reserved. Keywords: Cerium; Hydrogen content; Viscosity; Al – 16% Si melts
1. Introduction Hydrogen is the main gas that is appreciably soluble in Al and its alloys [1]. The difference in solubility between liquid and solid Al usually results in the rejection of almost all of the dissolved hydrogen on the solidification of liquid Al and leads to the formation of hydrogen bubbles, which ultimately cause porosity in castings and ingots and blisters on sheets and plates [2]. Gas porosity has a negative effect, not only on the mechanical properties but also on the machinability and the surface properties of Al castings [3,4]. To reduce the hydrogen content in Al alloy melts, the influences of some elements, includ* Corresponding author. Tel.: +86-531-839-6345. E-mail address: [email protected] (H. Geng). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2003.09.007
ing Cu, Si, Zn, Fe, and Mg [5 –10], on the hydrogen content have been investigated. Most of the previous work mainly focused on the Al2O3 film on the melt surface and it was thought that a more compact and dense surface film made due to the effect of the additional elements could prevent water vapor going into the melt, thereby preventing the uptake of hydrogen. Little information concerning the relationship between cerium (Ce) and the hydrogen content in hypereutectic Al alloy melts was available, even less concerning the melt structure. Viscosity is one of the basic properties sensitive to the melt structure and is also a measure of the friction among atoms from the microscopic viewpoint, whose change can reflect a structural change. This present work explored the changes in the hydrogen content and viscosity of hypereutectic Al – 16% Si alloy melts with Ce addi-
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H. Geng et al. / Materials Characterization 51 (2003) 29–33
tions. Simultaneously, the atomic densities of Al – 16% Si melts with different Ce contents were also measured. These results may provide a valuable pointer for engineering practice.
2. Experimental procedure The Al – 16% Si alloys used in this work were made up of 99.5 mass% Al, 99.9 mass% Si, and Al – 15 mass% Ce master alloys. The melts were prepared using a clay-bound graphite crucible in an electric resistance furnace. During the hydrogen content tests, the environmental humidity was about 34% RH. A reduced-pressure test (RPT) method was used to determine the hydrogen in the melts. The instrument applied in our work was a Hyscan I¨ made by Severn Science UK. The measurement accuracy was 0.01 cm3/100 g and the measurement range was 0 –1.99 cm3/100 g. The testing procedure was as follows: A constant mass of the melt (approximately 100 g) was placed in a chamber and the pressure reduced rapidly to a predetermined value (0.1 mbar) by a vacuum pump. The chamber and associated vacuum system was then isolated from the pump and the sample was allowed to solidify. As the melt cooled, hydrogen was released and its pressure was measured by a calibrated Pirani gauge whose output was converted continuously to a digital display of hydrogen content. The hydrogen content value of the melt was displayed and printed after about 5 min. The liquid alloy viscosity was measured using an oscillating vessel viscometer. The experimental sample was placed in a vessel hung by a torsional suspension and heated, the vessel then oscillated around a vertical axis, and the oscillation was gradually dampened due to frictional energy absorption and dissipation within the liquid. The viscosity of the liquid sample was calculated by using the decremental data as well as the period of oscillation. The main parameters of the measurement system were temperature precision (3 jC), measuring precision (5%), and highest temperature (1500 jC). The specimen was placed in an Al2O3 container heated in a vacuum of 0.4 Pa and kept at the testing temperature for 1 h, then the viscosity value was measured as the temperature dropped. The liquid structure of the Al –16% Si melts was studied using a 0-0-type liquid metal X-ray diffrac-
tometer. MoKa radiation (wavelength k = 0.07089 nm) was reflected from the free surface of the specimen and reached the detector through a graphite monochromator in the diffraction beam. Hightemperature X-ray diffraction was carried out in a high-purity helium (99.999%) atmosphere of 1.3 105 Pa and before the chamber was cleaned in vacuum of 2 10 6 Pa. Specimens were placed in an alumina crucible of 30 25 8 mm in size, using Ta sheet as heating elements. The surface of the specimens was fitted to one horizontal position using a laser calibrator. Some other parameters of the diffractometer were as follows: angular precision (0.001j), sampling time precision (0.001 s), temperature precision ( F 5 jC), and uppermost temperature (1800 jC). The data relating to X-ray diffraction are gained from intensity measurements, intensity corrections, normalization, Fourier transformation, and reliability checks, and then the atomic density was calculated [11,12]. The atomic density of Al –16% Si melts containing 0%, 0.05%, and 0.15% Ce was obtained using the above mentioned methods.
3. Results The temperature dependence of the hydrogen content in Al– 16% Si alloy melts containing 0%,
Fig. 1. Hydrogen content of Al – 16% Si melts with different Ce contents as a function of temperature: (1) 0% Ce, (2) 0.05% Ce, and (3) 0.15% Ce.
H. Geng et al. / Materials Characterization 51 (2003) 29–33
Fig. 2. Viscosity of Al – 16% Si melts with different Ce contents as a function of temperature: (1) 0.15% Ce, (2) 0.05% Ce, and (3) 0% Ce.
0.05%, and 0.15% Ce is illustrated in Fig. 1. It can be seen that the hydrogen content rises with an increase of temperature. However, it increases rapidly above a particular temperature, that is, the gradient variation above a particular temperature is a little bit steeper than that at lower temperatures. An anomalous increase in the hydrogen content with an increase of melt temperature was observed for temperatures higher than about 730, 770, and 800 jC for Al– 16% Si– 0% Ce, Al – 16% Si– 0.05% Ce, and Al – 16% Si– 0.15% Ce, respectively. Samples with Ce additions have noticeably less hydrogen, and the higher Ce level, the lower the hydrogen content.
31
The results of viscosity variation by adding Ce are indicated in Fig. 2. The viscosity values reduce with an increase of temperature, in particular, an anomalous temperature dependence of viscosity is observed at about 730, 770, and 800 jC for Al –16% Si –0% Ce, Al –16% Si –0.05% Ce, Al– 16% Si– 0.15% Ce, respectively, which are in agreement with those on the hydrogen content curves. It should be noted that the viscosity increases with Ce content into Al – 16% Si melts. The viscosity increases with an increase of Ce concentration. The macrographs and micrographs of cross sections of the specimens used in the test of hydrogen content are indicated in Figs. 3 and 4. The porosity area fraction in the macrographs is about 40.3%, 35.1%, and 3.95% for Al –16% Si, Al – 16% Si – 0.05% Ce, and Al – 16% Si –0.15% Ce, respectively. In addition, the porosity area fraction in the micrographs is 25.5%, 14.8%, and 4.3% for the three samples, respectively. It is well known that hydrogen pickup is a dynamic process: the melt absorbs hydrogen in several forms. Hydrogen exists in the melt with three forms: hydrogen atoms, hydrogen molecules, and as hydrides. They jointly determine the final hydrogen content in the Al alloy melts. The hydrogen content of Al – 16% Si– 0.15% Ce specimen, with a low porosity area fraction, is low in the melt. By contrast, the Al – 16% Si specimen with a high porosity area fraction has a high hydrogen content. The variation of the atomic density of Al– 16% Si melts as a function of the temperature and Ce level is presented in Fig. 5. The atomic density drops with an
Fig. 3. Macrographs of cross sections of reduced-pressure specimens containing different Ce contents: (a) 0% Ce, (b) 0.05% Ce, (c) 0.15% Ce.
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increase of temperature, but decreases slightly in the high temperature range. Nevertheless, the atomic density rises with an increase of Ce content.
Fig. 5. Variation of atomic densities of Al – 16% Si melts with Ce contents and temperature: (1) 0.15% Ce, (2) 0.05% Ce, and (3) 0% Ce.
4. Discussion
Fig. 4. Micrographs of cross sections of reduced-pressure specimens containing different Ce contents: (a) 0% Ce, (b) 0.05% Ce, (c) 0.15% Ce.
We now give a brief account of the mechanism of the fact that the hydrogen content of the specimens reduces with an increase of Ce concentration. The strong Si –Si chemical bonds [13] are destroyed by elemental Ce in Al – 16% Si melts. It is assumed that Al, Ce, and Si atoms might form atom clusters of a larger size with Ce addition into the melts, that is, the existence of local crystal orderings may occur. Moreover, the radius of the Ce atom is bigger than that of Si, Al (Al = 0.143 nm, Ce = 0.183 nm, La = 0.187 nm, Si = 0.11755 nm). Both the bigger Ce atoms and the clusters formed occupy more gaps in the melt compared to the Al – 16% Si melt without Ce, which decreases the amount of free volume and increases the atomic density of the Al –16% Si melt as demonstrated in Fig. 5. From the viewpoint of free volume [14], it is true that the specimens with the addition of Ce have less hydrogen as shown in Fig. 1. As a result, the more the Ce content, the lower the hydrogen content. It is believed that the variation of the hydrogen content is dependent on the liquid structural change of Al – 16% Si alloy with Ce additions. The variation of the atomic structure from an open one to a dense one may reduce the hydrogen content. The energy absorbed by the Al alloy melts increase as the temperature rises, weakening the chemical bond force among atoms and increasing the amount of free volume, which is beneficial to the dissolution of more
H. Geng et al. / Materials Characterization 51 (2003) 29–33
hydrogen [15]. Thus, the hydrogen content increases as a function of temperature in Al – 16% Si melts. The same reason can be employed to explain the results in the Al – 16% Si melts containing Ce. However, it can be seen that the hydrogen content of the melts changes abruptly at certain temperatures. The change rate of hydrogen content with temperature is obviously different between the high temperature region and the low temperature region. The structure of Al – 14% Si melts changed abruptly at about 875 jC, and after this temperature, the hydrogen content increased sharply [16]. In the case of Al – 16% Si melts containing Ce, the rate of decrease of the atomic density slows down in the high temperature range, which indicates that Ce atoms and clusters occupy more gaps in the melt in the high temperature range than they do in the low temperature range. The appearance of anomalous phenomena of the hydrogen content shifts to higher temperature with an increase of Ce content. It is assumed that elemental Ce delays any abrupt change of the liquid structure of the melt. Of course, further study and more work are needed for a detailed explanation. Additionally, the atomic size ratio is considered to have a significant role in the variation of the viscosity with small additions of Ce. It is thought that a smaller atom occupies the gaps between larger atoms, leading to a drop in the atomic diffusivity with a higher degree of dense random packing. Moreover, the bigger clusters obtained in the combination among Ce, Al, and Si have also been responsible for the viscosity increase. In a nutshell, these analyses allow us to conclude that the rise in the viscosity originates from the change in the liquid structure with Ce additions. Similarly, the viscosity – temperature curves have anomalous changes consistent with those on the hydrogen content curves. The change in the viscosity confirms that the structural change in Al– 16% Si melts gives rise to a change in the hydrogen content when Ce is added. Our results thus indicate that the anomalous behavior of different properties occurs simultaneously, reflecting a kind of essential structural variation in the melt.
5. Conclusions The hydrogen content increases with increasing temperature in the Al – 16% Si melts with or without
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Ce and decreases with Ce additions. Viscosity and atomic density all decrease with increasing temperature and increase with Ce additions. But an anomalous change in the hydrogen content, viscosity, and atomic density as a function of temperature is observed at about 730, 770, and 800 jC for Al– 16% Si with 0%, 0.05%, and 0.15% Ce concentration, respectively.
References [1] Dong R. Melting Theory of Molten Alloys. Chongqing: Mechanical Press; 1983. p. 82 – 105. [2] Crepeau PN. Molten aluminum contamination: gas, inclusions and dross. Mod Cast 1997;7:39 – 41. [3] Rading GO, Li J, Berry JT. Fatigue crack growth in Al – Cu alloy A206 with different levels of porosity. AFS Trans 1994; 102:57 – 67. [4] Piwonka TS, Flerning MC. Pore formation in solidification. Trans AIME 1966;236:1157 – 65. [5] Zang Z, Bian X, Qin J, Wang W, Lu X. Hydrogen content and structure of aluminum melt. Acta Metall Sin 2000;36:34 – 8. [6] Caceres CH, Selling BI. Casting defects and the tensile properties of an Al – Si – Mg alloy. Mater Sci Eng A 1996;220: 109 – 16. [7] Anyalebechi PN, Talbot DEJ, Granger DA. Solubility of hydrogen in liquid binary Al – Li alloys. Metall Trans B 1988; 19:227 – 32. [8] Grigoreva AA, Danilkin VA. Effect of magnesium and zinc on the solubility of hydrogen in aluminum alloys. Tsvet Met 1984;1:87 – 9. [9] Li X, Bian X. The effect of Cu element on the hydrogen content of the Al melts. Spec Foundry Non-Ferrous Alloys 2000;3:19 – 20. [10] Whittenberger EJ, Rhines FN. Origin of porosity in castings of magnesium – aluminum and other alloys. J Organomet 1952; 14:409 – 20. [11] Wang WM. Microstructure of Liquid Al – Si Alloys and Evolution of Si Clusters. Jinan: Shandong University of Technology; 1998. p. 21 – 40 [in Chinese, D]. [12] Teixwira J. Structure of liquid metals determined by scattering techniques. Mater Sci Eng A 1994;178A:9 – 14. [13] Li P. Physical properties and structure heredity of Al – Si alloy melt. Haerbin: Haerbin University of Technology; 1995. p. 73 – 114 [in Chinese, D]. [14] Doolittle AK. Studies in Newtonian flow II. The dependence of the viscosity of liquids on free-space. J Appl Phys 1951; 22:1077 – 98. [15] Li X, Bian X. Behavior of hydrogen in superheated aluminum and its alloys melt. Mater Sci Forum 1951;331 – 337: 209 – 14. [16] Meng Q, Bian X, Wang W. Relationships between hydrogen content and silicon content in Al – Si alloy melt. Acta Metall Sin 2001;37:258 – 62 [in Chinese].
The effect of Ce on the hydrogen content and liquid structure of Al–16% Si melts Hongxia Geng a,*, Haoran Geng b, Xianying Xue a a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China b College of Materials Science and Engineering, Jinan University, Jinan, 250022, PR China Received 27 June 2002; received in revised form 6 August 2003; accepted 8 September 2003
Abstract The effects of small additions of cerium (Ce) on the hydrogen content and the viscosity of Al – 16% Si melts have been studied by using Hyscan II and a viscometer. The experimental results showed that the hydrogen content increased with an increase of temperature, but decreased with an increase of the Ce level. The atomic densities of Al – 16% Si melts containing 0%, 0.05%, and 0.15% Ce were measured and calculated, and the relationship between the hydrogen content and the liquid structure of Al – 16% Si melts with and without Ce additions was investigated. According to the test results and analysis, it was concluded that a melt structural change caused a change of the hydrogen content when Ce was added, and the viscosity change further verified the structural change. D 2003 Elsevier Inc. All rights reserved. Keywords: Cerium; Hydrogen content; Viscosity; Al – 16% Si melts
1. Introduction Hydrogen is the main gas that is appreciably soluble in Al and its alloys [1]. The difference in solubility between liquid and solid Al usually results in the rejection of almost all of the dissolved hydrogen on the solidification of liquid Al and leads to the formation of hydrogen bubbles, which ultimately cause porosity in castings and ingots and blisters on sheets and plates [2]. Gas porosity has a negative effect, not only on the mechanical properties but also on the machinability and the surface properties of Al castings [3,4]. To reduce the hydrogen content in Al alloy melts, the influences of some elements, includ* Corresponding author. Tel.: +86-531-839-6345. E-mail address: [email protected] (H. Geng). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2003.09.007
ing Cu, Si, Zn, Fe, and Mg [5 –10], on the hydrogen content have been investigated. Most of the previous work mainly focused on the Al2O3 film on the melt surface and it was thought that a more compact and dense surface film made due to the effect of the additional elements could prevent water vapor going into the melt, thereby preventing the uptake of hydrogen. Little information concerning the relationship between cerium (Ce) and the hydrogen content in hypereutectic Al alloy melts was available, even less concerning the melt structure. Viscosity is one of the basic properties sensitive to the melt structure and is also a measure of the friction among atoms from the microscopic viewpoint, whose change can reflect a structural change. This present work explored the changes in the hydrogen content and viscosity of hypereutectic Al – 16% Si alloy melts with Ce addi-
30
H. Geng et al. / Materials Characterization 51 (2003) 29–33
tions. Simultaneously, the atomic densities of Al – 16% Si melts with different Ce contents were also measured. These results may provide a valuable pointer for engineering practice.
2. Experimental procedure The Al – 16% Si alloys used in this work were made up of 99.5 mass% Al, 99.9 mass% Si, and Al – 15 mass% Ce master alloys. The melts were prepared using a clay-bound graphite crucible in an electric resistance furnace. During the hydrogen content tests, the environmental humidity was about 34% RH. A reduced-pressure test (RPT) method was used to determine the hydrogen in the melts. The instrument applied in our work was a Hyscan I¨ made by Severn Science UK. The measurement accuracy was 0.01 cm3/100 g and the measurement range was 0 –1.99 cm3/100 g. The testing procedure was as follows: A constant mass of the melt (approximately 100 g) was placed in a chamber and the pressure reduced rapidly to a predetermined value (0.1 mbar) by a vacuum pump. The chamber and associated vacuum system was then isolated from the pump and the sample was allowed to solidify. As the melt cooled, hydrogen was released and its pressure was measured by a calibrated Pirani gauge whose output was converted continuously to a digital display of hydrogen content. The hydrogen content value of the melt was displayed and printed after about 5 min. The liquid alloy viscosity was measured using an oscillating vessel viscometer. The experimental sample was placed in a vessel hung by a torsional suspension and heated, the vessel then oscillated around a vertical axis, and the oscillation was gradually dampened due to frictional energy absorption and dissipation within the liquid. The viscosity of the liquid sample was calculated by using the decremental data as well as the period of oscillation. The main parameters of the measurement system were temperature precision (3 jC), measuring precision (5%), and highest temperature (1500 jC). The specimen was placed in an Al2O3 container heated in a vacuum of 0.4 Pa and kept at the testing temperature for 1 h, then the viscosity value was measured as the temperature dropped. The liquid structure of the Al –16% Si melts was studied using a 0-0-type liquid metal X-ray diffrac-
tometer. MoKa radiation (wavelength k = 0.07089 nm) was reflected from the free surface of the specimen and reached the detector through a graphite monochromator in the diffraction beam. Hightemperature X-ray diffraction was carried out in a high-purity helium (99.999%) atmosphere of 1.3 105 Pa and before the chamber was cleaned in vacuum of 2 10 6 Pa. Specimens were placed in an alumina crucible of 30 25 8 mm in size, using Ta sheet as heating elements. The surface of the specimens was fitted to one horizontal position using a laser calibrator. Some other parameters of the diffractometer were as follows: angular precision (0.001j), sampling time precision (0.001 s), temperature precision ( F 5 jC), and uppermost temperature (1800 jC). The data relating to X-ray diffraction are gained from intensity measurements, intensity corrections, normalization, Fourier transformation, and reliability checks, and then the atomic density was calculated [11,12]. The atomic density of Al –16% Si melts containing 0%, 0.05%, and 0.15% Ce was obtained using the above mentioned methods.
3. Results The temperature dependence of the hydrogen content in Al– 16% Si alloy melts containing 0%,
Fig. 1. Hydrogen content of Al – 16% Si melts with different Ce contents as a function of temperature: (1) 0% Ce, (2) 0.05% Ce, and (3) 0.15% Ce.
H. Geng et al. / Materials Characterization 51 (2003) 29–33
Fig. 2. Viscosity of Al – 16% Si melts with different Ce contents as a function of temperature: (1) 0.15% Ce, (2) 0.05% Ce, and (3) 0% Ce.
0.05%, and 0.15% Ce is illustrated in Fig. 1. It can be seen that the hydrogen content rises with an increase of temperature. However, it increases rapidly above a particular temperature, that is, the gradient variation above a particular temperature is a little bit steeper than that at lower temperatures. An anomalous increase in the hydrogen content with an increase of melt temperature was observed for temperatures higher than about 730, 770, and 800 jC for Al– 16% Si– 0% Ce, Al – 16% Si– 0.05% Ce, and Al – 16% Si– 0.15% Ce, respectively. Samples with Ce additions have noticeably less hydrogen, and the higher Ce level, the lower the hydrogen content.
31
The results of viscosity variation by adding Ce are indicated in Fig. 2. The viscosity values reduce with an increase of temperature, in particular, an anomalous temperature dependence of viscosity is observed at about 730, 770, and 800 jC for Al –16% Si –0% Ce, Al –16% Si –0.05% Ce, Al– 16% Si– 0.15% Ce, respectively, which are in agreement with those on the hydrogen content curves. It should be noted that the viscosity increases with Ce content into Al – 16% Si melts. The viscosity increases with an increase of Ce concentration. The macrographs and micrographs of cross sections of the specimens used in the test of hydrogen content are indicated in Figs. 3 and 4. The porosity area fraction in the macrographs is about 40.3%, 35.1%, and 3.95% for Al –16% Si, Al – 16% Si – 0.05% Ce, and Al – 16% Si –0.15% Ce, respectively. In addition, the porosity area fraction in the micrographs is 25.5%, 14.8%, and 4.3% for the three samples, respectively. It is well known that hydrogen pickup is a dynamic process: the melt absorbs hydrogen in several forms. Hydrogen exists in the melt with three forms: hydrogen atoms, hydrogen molecules, and as hydrides. They jointly determine the final hydrogen content in the Al alloy melts. The hydrogen content of Al – 16% Si– 0.15% Ce specimen, with a low porosity area fraction, is low in the melt. By contrast, the Al – 16% Si specimen with a high porosity area fraction has a high hydrogen content. The variation of the atomic density of Al– 16% Si melts as a function of the temperature and Ce level is presented in Fig. 5. The atomic density drops with an
Fig. 3. Macrographs of cross sections of reduced-pressure specimens containing different Ce contents: (a) 0% Ce, (b) 0.05% Ce, (c) 0.15% Ce.
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H. Geng et al. / Materials Characterization 51 (2003) 29–33
increase of temperature, but decreases slightly in the high temperature range. Nevertheless, the atomic density rises with an increase of Ce content.
Fig. 5. Variation of atomic densities of Al – 16% Si melts with Ce contents and temperature: (1) 0.15% Ce, (2) 0.05% Ce, and (3) 0% Ce.
4. Discussion
Fig. 4. Micrographs of cross sections of reduced-pressure specimens containing different Ce contents: (a) 0% Ce, (b) 0.05% Ce, (c) 0.15% Ce.
We now give a brief account of the mechanism of the fact that the hydrogen content of the specimens reduces with an increase of Ce concentration. The strong Si –Si chemical bonds [13] are destroyed by elemental Ce in Al – 16% Si melts. It is assumed that Al, Ce, and Si atoms might form atom clusters of a larger size with Ce addition into the melts, that is, the existence of local crystal orderings may occur. Moreover, the radius of the Ce atom is bigger than that of Si, Al (Al = 0.143 nm, Ce = 0.183 nm, La = 0.187 nm, Si = 0.11755 nm). Both the bigger Ce atoms and the clusters formed occupy more gaps in the melt compared to the Al – 16% Si melt without Ce, which decreases the amount of free volume and increases the atomic density of the Al –16% Si melt as demonstrated in Fig. 5. From the viewpoint of free volume [14], it is true that the specimens with the addition of Ce have less hydrogen as shown in Fig. 1. As a result, the more the Ce content, the lower the hydrogen content. It is believed that the variation of the hydrogen content is dependent on the liquid structural change of Al – 16% Si alloy with Ce additions. The variation of the atomic structure from an open one to a dense one may reduce the hydrogen content. The energy absorbed by the Al alloy melts increase as the temperature rises, weakening the chemical bond force among atoms and increasing the amount of free volume, which is beneficial to the dissolution of more
H. Geng et al. / Materials Characterization 51 (2003) 29–33
hydrogen [15]. Thus, the hydrogen content increases as a function of temperature in Al – 16% Si melts. The same reason can be employed to explain the results in the Al – 16% Si melts containing Ce. However, it can be seen that the hydrogen content of the melts changes abruptly at certain temperatures. The change rate of hydrogen content with temperature is obviously different between the high temperature region and the low temperature region. The structure of Al – 14% Si melts changed abruptly at about 875 jC, and after this temperature, the hydrogen content increased sharply [16]. In the case of Al – 16% Si melts containing Ce, the rate of decrease of the atomic density slows down in the high temperature range, which indicates that Ce atoms and clusters occupy more gaps in the melt in the high temperature range than they do in the low temperature range. The appearance of anomalous phenomena of the hydrogen content shifts to higher temperature with an increase of Ce content. It is assumed that elemental Ce delays any abrupt change of the liquid structure of the melt. Of course, further study and more work are needed for a detailed explanation. Additionally, the atomic size ratio is considered to have a significant role in the variation of the viscosity with small additions of Ce. It is thought that a smaller atom occupies the gaps between larger atoms, leading to a drop in the atomic diffusivity with a higher degree of dense random packing. Moreover, the bigger clusters obtained in the combination among Ce, Al, and Si have also been responsible for the viscosity increase. In a nutshell, these analyses allow us to conclude that the rise in the viscosity originates from the change in the liquid structure with Ce additions. Similarly, the viscosity – temperature curves have anomalous changes consistent with those on the hydrogen content curves. The change in the viscosity confirms that the structural change in Al– 16% Si melts gives rise to a change in the hydrogen content when Ce is added. Our results thus indicate that the anomalous behavior of different properties occurs simultaneously, reflecting a kind of essential structural variation in the melt.
5. Conclusions The hydrogen content increases with increasing temperature in the Al – 16% Si melts with or without
33
Ce and decreases with Ce additions. Viscosity and atomic density all decrease with increasing temperature and increase with Ce additions. But an anomalous change in the hydrogen content, viscosity, and atomic density as a function of temperature is observed at about 730, 770, and 800 jC for Al– 16% Si with 0%, 0.05%, and 0.15% Ce concentration, respectively.
References [1] Dong R. Melting Theory of Molten Alloys. Chongqing: Mechanical Press; 1983. p. 82 – 105. [2] Crepeau PN. Molten aluminum contamination: gas, inclusions and dross. Mod Cast 1997;7:39 – 41. [3] Rading GO, Li J, Berry JT. Fatigue crack growth in Al – Cu alloy A206 with different levels of porosity. AFS Trans 1994; 102:57 – 67. [4] Piwonka TS, Flerning MC. Pore formation in solidification. Trans AIME 1966;236:1157 – 65. [5] Zang Z, Bian X, Qin J, Wang W, Lu X. Hydrogen content and structure of aluminum melt. Acta Metall Sin 2000;36:34 – 8. [6] Caceres CH, Selling BI. Casting defects and the tensile properties of an Al – Si – Mg alloy. Mater Sci Eng A 1996;220: 109 – 16. [7] Anyalebechi PN, Talbot DEJ, Granger DA. Solubility of hydrogen in liquid binary Al – Li alloys. Metall Trans B 1988; 19:227 – 32. [8] Grigoreva AA, Danilkin VA. Effect of magnesium and zinc on the solubility of hydrogen in aluminum alloys. Tsvet Met 1984;1:87 – 9. [9] Li X, Bian X. The effect of Cu element on the hydrogen content of the Al melts. Spec Foundry Non-Ferrous Alloys 2000;3:19 – 20. [10] Whittenberger EJ, Rhines FN. Origin of porosity in castings of magnesium – aluminum and other alloys. J Organomet 1952; 14:409 – 20. [11] Wang WM. Microstructure of Liquid Al – Si Alloys and Evolution of Si Clusters. Jinan: Shandong University of Technology; 1998. p. 21 – 40 [in Chinese, D]. [12] Teixwira J. Structure of liquid metals determined by scattering techniques. Mater Sci Eng A 1994;178A:9 – 14. [13] Li P. Physical properties and structure heredity of Al – Si alloy melt. Haerbin: Haerbin University of Technology; 1995. p. 73 – 114 [in Chinese, D]. [14] Doolittle AK. Studies in Newtonian flow II. The dependence of the viscosity of liquids on free-space. J Appl Phys 1951; 22:1077 – 98. [15] Li X, Bian X. Behavior of hydrogen in superheated aluminum and its alloys melt. Mater Sci Forum 1951;331 – 337: 209 – 14. [16] Meng Q, Bian X, Wang W. Relationships between hydrogen content and silicon content in Al – Si alloy melt. Acta Metall Sin 2001;37:258 – 62 [in Chinese].