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Analysis of the effects of alloying elements on hydrogen solubility in liquid aluminum alloys
Scripta Mekllurgica et Materialia,Vol. 33, No. 8, pp. 1209-1216, 1995 Ekevier Science Ltd Copyright01995 Acta MetallurgicaInc. Printedin the USA. AU rights reserved 0956-716X I95 $9.50 + .OO
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ANALYSIS OF THE EFFECTS OF ALLOYING ELEMENTS ON HYDROGEN SOLUBILITY IN LIQUID ALUMINUM ALLOYS Prince N. Anyalebechi Alcoa Technical Center Aluminum Company of America 100 Technical Drive Alcoa Center, PA 150694001, U.S.A. (Received February 20,1995)
Knowledge of hydrogen solubility limits is a fundamental requirement in the development of theories of hydrogen solution and behavior in metals. It is particularly critical to the practical control and amelioration of the profound, and usually deleterious, effects of hydrogen on the physical and mechanical properties of metal products. In the aluminum industry, hydrogen solubility data is required for: (a) modeling and determination of the nature of hydrogen partitioning in aluminum products; (b) modeling and assessment of the propensity of aluminum alloy products to hydrogeninduced defects, such as porosity in castings and hydrogen embrittlement in aqueous environments; (c) mathematical modeling of melt treatment processes; and (d) calibration of instruments used to measure hydrogen concentration in aluminum alloy melts. As discussed in a detailed and critical review elsewhere [l], accuracy and reliability of experimentally determined hydrogen solubility values are dependent on the measurement techniques, methodology, alloy composition, and test conditions. In the review, reported values of hydrogen solubility in liqluid pure aluminum (Al-H) and aluminum (Al-H-X) alloys were critically assessed. The values deemed to have been determined from tests carried out meticulously, with full appreciation and elimination of the sources of systematic errors, were used to develop empirical equations relating hydrogen solubility, alloy composition, and temperature. In this paper, the effects of alloying elements on the solubility and thermodynamic behavior of hydrogen in liquid aluminum are analyzed in terms of Wagner’s interaction parameter. The practical and theoretical significance of these results are discussed.
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Hvdroeen in Pure Aluminum (Al-H Svsteml Dissolution of atomic hydrogen in liquid (or solid) aluminum and its alloys follows the Sieverts’ law; i.e., it is atomistic rather than molecular in nature. It involves: (i) dissociation of the diatomic hydrogen gas into the atomic form at the boundary layer; (ii) dissolution of the atomic hydrogen in the melt boundary layer; and (iii) transport of hydrogen to the bulk of the melt by diffusion. This process in liquid pure aluminum is described by the reaction:
(1) The underlined symbol H denotes hydrogen dissolved in li uid aluminum and its alloys, concentration being expressed as wt.%. (Note: 1 wt.% = 1.12 x lo4 cm 2 (STP) of hydrogen per 100 g of Al). The equilibrium constant for the above reaction is given by : aH K =-=-
(2)
42 $--
If the dissolved hydrogen is sufficiently dilute that Henry’s law is obeyed, equation (2) becomes: K
=
f~XWt.WinAl-H
(3)
d- PH,
In terms of the standard free energy of solution, AGO, equation (3) can be expressed as:
-
fH*(Wt*%!?)inAl-ti
1
(4)
where an is the activity of hydrogen; fH is the Henrian activity coefficient of hydrogen in liquid aluminum with reference to 1 wt.%H solution; PH, is the partial pressure of hydrogen in Pa; R is the gas constant in J/mol.; and T is temperature in Kelvin (K). Based on the following assumptions: (i) 1 wt.%H and 101.3 kPa (i.e., 1 atm.) hydrogen partial pressure are the standard states, and (ii) activity coefficient of hydrogen in aluminum, fH, goes to unity when the concentration of hydrogen goes to zero; equation (4) can be re-rewritten as: AG” = -RTln(W.%&nA_H
(5)
Thus, l”glO(wt*%&Al-H
AH”(l\ =-2\\?;I
lw +G
where AH’ and M” are the enthalpy and entropy, respectively, aluminum.
(6)
for hydrogen solution in pure
HYDROGEN IN LIQUID ALUMINUM ALLOYS
Vol. 33, No. 8
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The values of AZY”and &Jo can be obtained from plots of log (wt.%H) versus the reciprocal of temperature (ID’) assuming that they do not vary significantly within the investigated temperature range [l]. Of the twenty-five sets of results reported in the past seventy-three years (1922-1995) on the Al-H system, only nine were assessed to be reliable [2-lo]. Combined linear regression analysis of these results yielded the following empirical equation for hydrogen solubility, wt.%H, as a function of temperature, T, (K) at 101.3 kPa hydrogen partial pressure, with a correlation coefficient of 0.96: lOg,,(Wt.%H)inAI_H
=
2691.96 -T
I 32 -
.
Effects of Alloving Elements (Al-H-X svstems1 In the Al-H binaLrysolution, behavior and solubility of hydrogen (at a given temperature and hydrogen partial pressure) is determined by the nature and magnitude of the interactions between hydrogen and aluminum atoms only. In Al-H-X (where X is Cu, Si, Mg, Zn,Li, Fe, or Ti) solutions, however, three types of interactions occur, viz.: aluminum with hydrogen, aluminum with alloying element (X), and hydrogen with alloying element. The nature and relative magnitudes of these three interactions control the thermodynalmic behavior of hydrogen in the Al-H-X system, and consequently the effects of the alloying element on hydrogen solubility in aluminum alloys. As shown in equation (3) actual solubility of hydrogen in aluminum is determined by its activity coefficient. The question arises as to how the alloying elements affect hydrogen solubility in liquid aluminum. This problem is dealt with by the introduction of interaction coefficients, ff, and interaction parameters, ei.
In the AI-H binary solution, the activity of hydrogen, aH, in an infinitely dilute solution with respect to the Henrian standard state is given by a,, = f{(wt.%gin
AI_H. If, at constant concentration
of hydrogen, the addition of a small amount of alloying element, X, alters the activity coefcient of hydrogen to fH,then the difference between fH and f$ is quantified by the expression fH = fi. fi ; where
fi is referred to as the interaction coefficient of alloying element, X, on hydrogen in
aluminum. It is a measure of the effect of a given concentration of the alloying element, X, on the behavior of hydrogen. Experimental determination of the interaction coefficient involves the measurement of the variation of hydrogen solubility as a function of the concentration of alloying element, while Imaintaining the hydrogen in the aluminum melt at constant activity (by equilibrating with fixed hydrogen pressure, PH,). If addition of the alloying element to the Al-H binary system (equilibrated with a fixed (wt.%~,
PH,)alters the dissolved hydrogen concentration from (wt.%kJinAI_u to
AI_H_x,then equation (3) can be rewritten as: K
f~.(Wt’%H)hA,-H I flFfHX(Wt.%!iawf-X - d-PH*
J-Qh In an infinitely dilute solution,
f$ = 1,
so the interaction coefficient,
fi, is obtained experimentally
as:
(wt*%H)in A-H -
(wr.%H). _H-X - mAl
1 ~,p
n2
(9)
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0.0
Wt.% of alloying element, X
Wt.% of alloying element, X
Figure 1. Reported Effects of Alloying Elements on Hydrogen Solubility in Liquid Aluminum at 973 K and 101.3 kPa Partial1 Pressure of Hydrogen.
Published values of hydrogen solubility in Al-H-X alloy systems are limited. As shown graphically in Figure 1 by some of the most reliable reported values [l, 4-6, 11-141, alloying elements have strong (and varying) influence on hydrogen solubility in liquid aluminum. The Taylor series expansion formalism first proposed by Wagner [15] and Chipman [16] for describing the functional relationship between logarithm of activity coefficient and composition of a dilute constituent in an alloy system consisting of three or more components can be applied to quantify the effects of alloying elements on the thermodynamic behavior of hydrogen in aluminum. Lupis and Elliott [17] extended the treatment of Wagner to include the higher orders which are required for higher concentrations of alloying elements. Thus, for an Al-H-X system, the functional relationship between log,, fH and the concentration of alloying elements in wt.% can be described by the equation:
loglo f~ = log10f; + -
-
wf.%H) + ej$ -
n
1 x-2
n
e$wr.%a -
+
1 x=2
r-(wr.%$ -
+ higher order terms
Since the standard and reference states are based on an infinitely dilute solution of hydrogen (1 wt.%H) in liquid aluminum, fi = 1; the self-interaction parameter of hydrogen in liquid aluminum, ei , is zero since hydrogen solution obeys Sieverts’ law over the reported temperature ranges of interest. Other interaction parameters are defined as follows:
where I$- and r$ - are referred to as the first and second order (Wagner’s) interaction parameter, respectively.
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0.3
0.1
0.6 IAs 0 $0.4
+!; -0.1 s” -0.3
-0.5
-0.7
d
0 -2
4 6 8 Wt.% of alloying element,X
35
10
Wt.% of alloying element,X
Figure 2. Logarithmof Activity Coefficient of Hydrogen Versus Wt.%of Alloying Elements in Liquid Aluminum at 973 K and 101.3 kPa PartialPressureof Hydrogen.
The interaction parameters express the effects of an alloying element on the behavior of hydrogen in the Al-H-X (t.ernary) solution. The values of the first order interaction parameter are obtained from the slopes of the linear portions of the plots of log,, fi versus v&.%X (Figure 2). With relatively dilute solutions and low concentrations of the alloying elements, the behavior of hydrogen usually can be expressed satisfactorily by the use of the first order term, ei, only. However, as apparent in Figure 2, there is a considerable curvature in the log,, two elements, second order coefficients,
ff versus wt.%X plots for Cu and Si. Thus, for these
&“ and rz
were calculated, by curvilinear regression
analyses. The analyses yielded coefficientsthat were statistically significant (with t ratios >3). It is noteworthy that all of the calculations and analyses were limited to the reported alloying element composition and temperature ranges. The interaction parameters obtained from the analysis are given in Table 1 for a range of temperatures. The sources of the data used in the analysis are given in the “Ref. no.” column of the Table. The values ,for Fe and Ti are derived from single alloy composition of 10 wt.% and 4 wt.%, respectively, the only results reported by Shahani [7] for the systems. Of the four sets of reported results for the Al-H-Cu system [2, 4, 10, 141 which showed that addition of Cu decreases hydrogen solubility in liquid aluminum, only those reported by Opie and Grant were used for the calculation. They were cons:idered to be the most reliable because of the reliability of their experimental technique (Sieverts’ direct absorption) and methodology, and the self-consistency of their results [l]. Vaschenko et al.‘s [14] results were considered unreliable because they were calculated with unreliable data on the hydrogen permeability and diffusion in liquid Al-H-Cu alloys. Their reported hydrogen solubility values for the copper content were 20-40% less than Opie and Grant’s results, the disparity increasing with increase in temperature. The most recent hydrogen solubility values for the Al-H-Cu system reported by Liu et al.[lO] are, for a given temperature and Cu composition, 1.5 to 2 times greater than previously reported values, and are considered unreliable. Liu et al.‘s unreliable results can be attributed to their experimental methodology and conditions which are inappropriate for accurate measurement o:f the low hydrogen solubility limits in Al-H-Cu system. Note that addition of Cu significantly reduces the solubility of hydrogen in liquid aluminum and makes accurate measurement of hydrogen solubility limits that much more difficult.
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Vol. 33, No. 8
TABLE 1
CalculatedFirst Order (es ) and Second Order ( T;) Interaction Parameters for Hydrogen Solution in Al-H-X Alloys at Different Temperatures
and 101.3 kPa Hydrogen Partial Pressure
The interaction parameters for Al-H-Zn were calculated by combining the only two sets of published hydrogen solubility values [6, 121 for the alloy system. The interaction parameters at 1073 and 1123 K are shown in parentheses because they are considered to be unreliable. The interaction parameters for the Al-H-Mg and Al-H-Li alloy systems were obtained from limited sets of data based on only two (3.2 and 6 wt.% Mg) levels [12] and three (1,2, and 3 wt.% Li) levels, respectively. Because of containment problems and significant loss by volatilization of the high vapor pressure elements, reliable (and accurate) determination of the hydrogen solubility in liquid Al-H-Zn, Al-HMg, and Al-H-Li alloy systems is difficult and fraught with errors. Results and Discussion The interaction parameters obtained from this analysis compare well with those reported in the literature. For example, the first, 0.0193-0.0181, and second, from -0.00045 to -0.00059, order interaction parameters obtained from this study for Si at 973-1123 K, compare well with S&worth and Engh’s [18] values of 0.03 and -0.8008, respectively, at 973-1073 K and with Shahani’s [7] first order interaction parameter of 0.021-0.024 at 973-1173 K for an Al-7 wt.%Si alloy. Also, for the temperature range of 973-1123 K, the first, 0.0334-0.0266, and second, from -0.00065 to -0.00046, order interaction parameters for Cu obtained from this study compare very well with those reported by Sigworth and Engh [18] of 0.03 and -0.0004, respectively, for a temperature range of 973-1073 K. Not surprising, however, the interaction parameters reported by Liu et al. [lo] and Vaschenko et al. [14] derived from their unreliable data, differ from the values obtained from this study. The first order interaction parameters for Cu, Si, Zn, and Fe are positive, indicating that addition of any of these alloying elements decreases the affinity of liquid aluminum for hydrogen. This implies
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HYDROGEN IN LIQUID ALUMINUM ALLOYS
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that the intemctions of Al atoms with Cu, Si, Zn, and Fe are stronger than Al interaction with hydrogen. It also suggests that the bonding of Al atoms to Cu, Si, Zn, and Fe atoms is so strong that it leaves fewer All atoms to bond with H atoms, and thus, raises the activity coefficient of the hydrogen in liquid aluminum. It is noteworthy, however, that there is a small but significant decrease in the first order interaction parameters for these alloying elements with increase in temperature (Table 1). It is not clear whetlher this is due to errors in the reported experimental data or because of the expected decrease in strength (and the number) of the bonding between atoms of Al and alloying elements with increase in temperature. It could also be due to the fact that all solutions become more ideal (as interactions become more random) with increase in temperature. Some of the practical implications of these observations include the following: (i) removal of hydrogen from aluminum alloys containing either Cu, Si, Zn, or Fe by inert gas bubbling should require less volume of inert gas; (ii) hydrogen removal becomes more difficult with increase in temperature and at higher Cu and Si levels; and (iii) addition of either Cu, Si, Zn, or Fe to Al may reduce its propensity to hydrogen-induced defects. The latter depends on the effects of the alloying elements on hydrogen solubility in solid aluminum and consequently on the partition coefficient for hydrogen in aluminum. Note that the effects of Cu and Si on hydrogen solubility in liquid aluminum decreases in magnitude at above 15 wt.%. This is confirmed by tlhe parabolic relationship between log interaction coefficient and wt.% Cu and Si (Figure 2). The first order interaction parameters for Mg, Li, and Ti are negative; their addition to liquid Al increases the affinity of the melt for hydrogen. This is not entirely surprising since these elements have high affinity for hydrogen. The negative interaction parameters imply that the interactions between atoms of the alloying elements and H atoms are stronger than those between Al and H. Thus, addition of Mg, Li, or Ti to aluminum melt increases the propensity of the melt to hydrogen absorption and solution and consequently makes removal of hydrogen from the aluminum alloy melt by inert gas bubbling difficult. As shown by practical experience, these alloying elements tend to exacerbate the propensity of aluminum to hydrogen-induced defects. This, of course, depends on the nature and magnitude of the effect of the alloying element on the solid state solubility of hydrogen in aluminum. Summary Reported values of hydrogen solubility in liquid Al-H and Al-H-X (where X = Cu, Si, Zn, Fe, Mg, Ti, or Li) alloys ‘were critically reviewed and analyzed. Plots of log,, fi vs.wt.%X revealed that isothermal hydrogen solubility in liquid Al-H-X alloys at 101.3 kPa hydrogen partial pressure decreases with increase in Cu, Si, Zn, and Fe levels, but increases with addition and rising levels of Mg, Li, and Ti. Within the reported temperature and composition ranges, all of the Al-H-X alloys examined in this study are endothermic occluders of hydrogen. That is, hydrogen solubility increases with increase in temperature. The interaction parameters obtained from this study along with those to be obtained from a similar analysis on hydrogen solubility in solid Al-H-X will be useful for an attempt to predict hydrogen solubilities in the more complex, but commercially important, multicomponent aluminum alloys. These values are critical to the understanding of the behavior of hydrogen in ahuminum alloys, and to the preservation of the integrity of cast and fabricated aluminum products against the multifarious, and often detrimental, effects of hydrogen. References 1. P. N. Anyalebechi, To be submitted to Cast Metals Journal for Publication, (1995). 2. W. Baukloh and F. Oesterlen, Z. Metallkunde, 30, 386, (1938). 3. C. E. Ransley and H. Neufeld, J. Inst. of Metals, 74,599, (1947-M).
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4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
HYDROGEN IN LIQUID ALUMINUM ALLOYS
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W. R. Opie and N. J. Grant, J. Metals. Trans. AIME, 188, 1237 (1950). C. E. Ransley and D. E. J. Talbot, Z. Metallkunde, 46,378, (1955). D. Stephenson, Ph.D. Thesis, Brunei University, Middlesex, England, (1978). H. Shahani, Ph.D. Thesis, The Royal Institute of Technology, Stockholm, Sweden, (1984). J. Kocur, K. Tomasek, and L. Rabatin, Hutnicke Listy, 44, 269-275 (1989). D. E. J. TaIbot and P. N. Anyalebechi, Materials Science and Technology J., 4, 1, (1988). H. Liu, L. Zhiang, and B Bouchard, Recent Developments in Light Metals, (edited by M. Gilbert, P. Tremblay and E. Ozberk), p. 257, Canadian Institute of Metallurgists, (1994). P. N. Anyalebechi, D. E. J. Talbot and D. A. Granger, Metal]. Trans. B, 19B, 227, (1988). A. A Grigoreva and V. A. Danilkin, Tsvetnye Metally, 1,90 (1984). M. Sargent, Ph.D. Thesis, Brunei University, Middlesex, England, (1989). K. I. Vaschenko, D. F. Chemega, D. F. Ivanchuk, 0. M. Vyalik, and G. A. Remizov, Izv. Vyssh Ucheb Zaved Tsevet. Metally., 2.48, (1975). C. Wagner, Thermodynamics of Alloys, p. 51, Addison-Wesley, Reading, MA, (1%2). J. Chipman, J. Iron and Steel, 180, 97, (1955). C. H. P. Lupis and J. F. Elliott, Acta Met., 14.526, (1%6). G. K. S&worth and T. A. Engh, Metall. Trans. B, 13B, 447, (1982).
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ANALYSIS OF THE EFFECTS OF ALLOYING ELEMENTS ON HYDROGEN SOLUBILITY IN LIQUID ALUMINUM ALLOYS Prince N. Anyalebechi Alcoa Technical Center Aluminum Company of America 100 Technical Drive Alcoa Center, PA 150694001, U.S.A. (Received February 20,1995)
Knowledge of hydrogen solubility limits is a fundamental requirement in the development of theories of hydrogen solution and behavior in metals. It is particularly critical to the practical control and amelioration of the profound, and usually deleterious, effects of hydrogen on the physical and mechanical properties of metal products. In the aluminum industry, hydrogen solubility data is required for: (a) modeling and determination of the nature of hydrogen partitioning in aluminum products; (b) modeling and assessment of the propensity of aluminum alloy products to hydrogeninduced defects, such as porosity in castings and hydrogen embrittlement in aqueous environments; (c) mathematical modeling of melt treatment processes; and (d) calibration of instruments used to measure hydrogen concentration in aluminum alloy melts. As discussed in a detailed and critical review elsewhere [l], accuracy and reliability of experimentally determined hydrogen solubility values are dependent on the measurement techniques, methodology, alloy composition, and test conditions. In the review, reported values of hydrogen solubility in liqluid pure aluminum (Al-H) and aluminum (Al-H-X) alloys were critically assessed. The values deemed to have been determined from tests carried out meticulously, with full appreciation and elimination of the sources of systematic errors, were used to develop empirical equations relating hydrogen solubility, alloy composition, and temperature. In this paper, the effects of alloying elements on the solubility and thermodynamic behavior of hydrogen in liquid aluminum are analyzed in terms of Wagner’s interaction parameter. The practical and theoretical significance of these results are discussed.
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Hvdroeen in Pure Aluminum (Al-H Svsteml Dissolution of atomic hydrogen in liquid (or solid) aluminum and its alloys follows the Sieverts’ law; i.e., it is atomistic rather than molecular in nature. It involves: (i) dissociation of the diatomic hydrogen gas into the atomic form at the boundary layer; (ii) dissolution of the atomic hydrogen in the melt boundary layer; and (iii) transport of hydrogen to the bulk of the melt by diffusion. This process in liquid pure aluminum is described by the reaction:
(1) The underlined symbol H denotes hydrogen dissolved in li uid aluminum and its alloys, concentration being expressed as wt.%. (Note: 1 wt.% = 1.12 x lo4 cm 2 (STP) of hydrogen per 100 g of Al). The equilibrium constant for the above reaction is given by : aH K =-=-
(2)
42 $--
If the dissolved hydrogen is sufficiently dilute that Henry’s law is obeyed, equation (2) becomes: K
=
f~XWt.WinAl-H
(3)
d- PH,
In terms of the standard free energy of solution, AGO, equation (3) can be expressed as:
-
fH*(Wt*%!?)inAl-ti
1
(4)
where an is the activity of hydrogen; fH is the Henrian activity coefficient of hydrogen in liquid aluminum with reference to 1 wt.%H solution; PH, is the partial pressure of hydrogen in Pa; R is the gas constant in J/mol.; and T is temperature in Kelvin (K). Based on the following assumptions: (i) 1 wt.%H and 101.3 kPa (i.e., 1 atm.) hydrogen partial pressure are the standard states, and (ii) activity coefficient of hydrogen in aluminum, fH, goes to unity when the concentration of hydrogen goes to zero; equation (4) can be re-rewritten as: AG” = -RTln(W.%&nA_H
(5)
Thus, l”glO(wt*%&Al-H
AH”(l\ =-2\\?;I
lw +G
where AH’ and M” are the enthalpy and entropy, respectively, aluminum.
(6)
for hydrogen solution in pure
HYDROGEN IN LIQUID ALUMINUM ALLOYS
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The values of AZY”and &Jo can be obtained from plots of log (wt.%H) versus the reciprocal of temperature (ID’) assuming that they do not vary significantly within the investigated temperature range [l]. Of the twenty-five sets of results reported in the past seventy-three years (1922-1995) on the Al-H system, only nine were assessed to be reliable [2-lo]. Combined linear regression analysis of these results yielded the following empirical equation for hydrogen solubility, wt.%H, as a function of temperature, T, (K) at 101.3 kPa hydrogen partial pressure, with a correlation coefficient of 0.96: lOg,,(Wt.%H)inAI_H
=
2691.96 -T
I 32 -
.
Effects of Alloving Elements (Al-H-X svstems1 In the Al-H binaLrysolution, behavior and solubility of hydrogen (at a given temperature and hydrogen partial pressure) is determined by the nature and magnitude of the interactions between hydrogen and aluminum atoms only. In Al-H-X (where X is Cu, Si, Mg, Zn,Li, Fe, or Ti) solutions, however, three types of interactions occur, viz.: aluminum with hydrogen, aluminum with alloying element (X), and hydrogen with alloying element. The nature and relative magnitudes of these three interactions control the thermodynalmic behavior of hydrogen in the Al-H-X system, and consequently the effects of the alloying element on hydrogen solubility in aluminum alloys. As shown in equation (3) actual solubility of hydrogen in aluminum is determined by its activity coefficient. The question arises as to how the alloying elements affect hydrogen solubility in liquid aluminum. This problem is dealt with by the introduction of interaction coefficients, ff, and interaction parameters, ei.
In the AI-H binary solution, the activity of hydrogen, aH, in an infinitely dilute solution with respect to the Henrian standard state is given by a,, = f{(wt.%gin
AI_H. If, at constant concentration
of hydrogen, the addition of a small amount of alloying element, X, alters the activity coefcient of hydrogen to fH,then the difference between fH and f$ is quantified by the expression fH = fi. fi ; where
fi is referred to as the interaction coefficient of alloying element, X, on hydrogen in
aluminum. It is a measure of the effect of a given concentration of the alloying element, X, on the behavior of hydrogen. Experimental determination of the interaction coefficient involves the measurement of the variation of hydrogen solubility as a function of the concentration of alloying element, while Imaintaining the hydrogen in the aluminum melt at constant activity (by equilibrating with fixed hydrogen pressure, PH,). If addition of the alloying element to the Al-H binary system (equilibrated with a fixed (wt.%~,
PH,)alters the dissolved hydrogen concentration from (wt.%kJinAI_u to
AI_H_x,then equation (3) can be rewritten as: K
f~.(Wt’%H)hA,-H I flFfHX(Wt.%!iawf-X - d-PH*
J-Qh In an infinitely dilute solution,
f$ = 1,
so the interaction coefficient,
fi, is obtained experimentally
as:
(wt*%H)in A-H -
(wr.%H). _H-X - mAl
1 ~,p
n2
(9)
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Vol. 33, No. 8
0.0
Wt.% of alloying element, X
Wt.% of alloying element, X
Figure 1. Reported Effects of Alloying Elements on Hydrogen Solubility in Liquid Aluminum at 973 K and 101.3 kPa Partial1 Pressure of Hydrogen.
Published values of hydrogen solubility in Al-H-X alloy systems are limited. As shown graphically in Figure 1 by some of the most reliable reported values [l, 4-6, 11-141, alloying elements have strong (and varying) influence on hydrogen solubility in liquid aluminum. The Taylor series expansion formalism first proposed by Wagner [15] and Chipman [16] for describing the functional relationship between logarithm of activity coefficient and composition of a dilute constituent in an alloy system consisting of three or more components can be applied to quantify the effects of alloying elements on the thermodynamic behavior of hydrogen in aluminum. Lupis and Elliott [17] extended the treatment of Wagner to include the higher orders which are required for higher concentrations of alloying elements. Thus, for an Al-H-X system, the functional relationship between log,, fH and the concentration of alloying elements in wt.% can be described by the equation:
loglo f~ = log10f; + -
-
wf.%H) + ej$ -
n
1 x-2
n
e$wr.%a -
+
1 x=2
r-(wr.%$ -
+ higher order terms
Since the standard and reference states are based on an infinitely dilute solution of hydrogen (1 wt.%H) in liquid aluminum, fi = 1; the self-interaction parameter of hydrogen in liquid aluminum, ei , is zero since hydrogen solution obeys Sieverts’ law over the reported temperature ranges of interest. Other interaction parameters are defined as follows:
where I$- and r$ - are referred to as the first and second order (Wagner’s) interaction parameter, respectively.
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0.3
0.1
0.6 IAs 0 $0.4
+!; -0.1 s” -0.3
-0.5
-0.7
d
0 -2
4 6 8 Wt.% of alloying element,X
35
10
Wt.% of alloying element,X
Figure 2. Logarithmof Activity Coefficient of Hydrogen Versus Wt.%of Alloying Elements in Liquid Aluminum at 973 K and 101.3 kPa PartialPressureof Hydrogen.
The interaction parameters express the effects of an alloying element on the behavior of hydrogen in the Al-H-X (t.ernary) solution. The values of the first order interaction parameter are obtained from the slopes of the linear portions of the plots of log,, fi versus v&.%X (Figure 2). With relatively dilute solutions and low concentrations of the alloying elements, the behavior of hydrogen usually can be expressed satisfactorily by the use of the first order term, ei, only. However, as apparent in Figure 2, there is a considerable curvature in the log,, two elements, second order coefficients,
ff versus wt.%X plots for Cu and Si. Thus, for these
&“ and rz
were calculated, by curvilinear regression
analyses. The analyses yielded coefficientsthat were statistically significant (with t ratios >3). It is noteworthy that all of the calculations and analyses were limited to the reported alloying element composition and temperature ranges. The interaction parameters obtained from the analysis are given in Table 1 for a range of temperatures. The sources of the data used in the analysis are given in the “Ref. no.” column of the Table. The values ,for Fe and Ti are derived from single alloy composition of 10 wt.% and 4 wt.%, respectively, the only results reported by Shahani [7] for the systems. Of the four sets of reported results for the Al-H-Cu system [2, 4, 10, 141 which showed that addition of Cu decreases hydrogen solubility in liquid aluminum, only those reported by Opie and Grant were used for the calculation. They were cons:idered to be the most reliable because of the reliability of their experimental technique (Sieverts’ direct absorption) and methodology, and the self-consistency of their results [l]. Vaschenko et al.‘s [14] results were considered unreliable because they were calculated with unreliable data on the hydrogen permeability and diffusion in liquid Al-H-Cu alloys. Their reported hydrogen solubility values for the copper content were 20-40% less than Opie and Grant’s results, the disparity increasing with increase in temperature. The most recent hydrogen solubility values for the Al-H-Cu system reported by Liu et al.[lO] are, for a given temperature and Cu composition, 1.5 to 2 times greater than previously reported values, and are considered unreliable. Liu et al.‘s unreliable results can be attributed to their experimental methodology and conditions which are inappropriate for accurate measurement o:f the low hydrogen solubility limits in Al-H-Cu system. Note that addition of Cu significantly reduces the solubility of hydrogen in liquid aluminum and makes accurate measurement of hydrogen solubility limits that much more difficult.
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TABLE 1
CalculatedFirst Order (es ) and Second Order ( T;) Interaction Parameters for Hydrogen Solution in Al-H-X Alloys at Different Temperatures
and 101.3 kPa Hydrogen Partial Pressure
The interaction parameters for Al-H-Zn were calculated by combining the only two sets of published hydrogen solubility values [6, 121 for the alloy system. The interaction parameters at 1073 and 1123 K are shown in parentheses because they are considered to be unreliable. The interaction parameters for the Al-H-Mg and Al-H-Li alloy systems were obtained from limited sets of data based on only two (3.2 and 6 wt.% Mg) levels [12] and three (1,2, and 3 wt.% Li) levels, respectively. Because of containment problems and significant loss by volatilization of the high vapor pressure elements, reliable (and accurate) determination of the hydrogen solubility in liquid Al-H-Zn, Al-HMg, and Al-H-Li alloy systems is difficult and fraught with errors. Results and Discussion The interaction parameters obtained from this analysis compare well with those reported in the literature. For example, the first, 0.0193-0.0181, and second, from -0.00045 to -0.00059, order interaction parameters obtained from this study for Si at 973-1123 K, compare well with S&worth and Engh’s [18] values of 0.03 and -0.8008, respectively, at 973-1073 K and with Shahani’s [7] first order interaction parameter of 0.021-0.024 at 973-1173 K for an Al-7 wt.%Si alloy. Also, for the temperature range of 973-1123 K, the first, 0.0334-0.0266, and second, from -0.00065 to -0.00046, order interaction parameters for Cu obtained from this study compare very well with those reported by Sigworth and Engh [18] of 0.03 and -0.0004, respectively, for a temperature range of 973-1073 K. Not surprising, however, the interaction parameters reported by Liu et al. [lo] and Vaschenko et al. [14] derived from their unreliable data, differ from the values obtained from this study. The first order interaction parameters for Cu, Si, Zn, and Fe are positive, indicating that addition of any of these alloying elements decreases the affinity of liquid aluminum for hydrogen. This implies
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that the intemctions of Al atoms with Cu, Si, Zn, and Fe are stronger than Al interaction with hydrogen. It also suggests that the bonding of Al atoms to Cu, Si, Zn, and Fe atoms is so strong that it leaves fewer All atoms to bond with H atoms, and thus, raises the activity coefficient of the hydrogen in liquid aluminum. It is noteworthy, however, that there is a small but significant decrease in the first order interaction parameters for these alloying elements with increase in temperature (Table 1). It is not clear whetlher this is due to errors in the reported experimental data or because of the expected decrease in strength (and the number) of the bonding between atoms of Al and alloying elements with increase in temperature. It could also be due to the fact that all solutions become more ideal (as interactions become more random) with increase in temperature. Some of the practical implications of these observations include the following: (i) removal of hydrogen from aluminum alloys containing either Cu, Si, Zn, or Fe by inert gas bubbling should require less volume of inert gas; (ii) hydrogen removal becomes more difficult with increase in temperature and at higher Cu and Si levels; and (iii) addition of either Cu, Si, Zn, or Fe to Al may reduce its propensity to hydrogen-induced defects. The latter depends on the effects of the alloying elements on hydrogen solubility in solid aluminum and consequently on the partition coefficient for hydrogen in aluminum. Note that the effects of Cu and Si on hydrogen solubility in liquid aluminum decreases in magnitude at above 15 wt.%. This is confirmed by tlhe parabolic relationship between log interaction coefficient and wt.% Cu and Si (Figure 2). The first order interaction parameters for Mg, Li, and Ti are negative; their addition to liquid Al increases the affinity of the melt for hydrogen. This is not entirely surprising since these elements have high affinity for hydrogen. The negative interaction parameters imply that the interactions between atoms of the alloying elements and H atoms are stronger than those between Al and H. Thus, addition of Mg, Li, or Ti to aluminum melt increases the propensity of the melt to hydrogen absorption and solution and consequently makes removal of hydrogen from the aluminum alloy melt by inert gas bubbling difficult. As shown by practical experience, these alloying elements tend to exacerbate the propensity of aluminum to hydrogen-induced defects. This, of course, depends on the nature and magnitude of the effect of the alloying element on the solid state solubility of hydrogen in aluminum. Summary Reported values of hydrogen solubility in liquid Al-H and Al-H-X (where X = Cu, Si, Zn, Fe, Mg, Ti, or Li) alloys ‘were critically reviewed and analyzed. Plots of log,, fi vs.wt.%X revealed that isothermal hydrogen solubility in liquid Al-H-X alloys at 101.3 kPa hydrogen partial pressure decreases with increase in Cu, Si, Zn, and Fe levels, but increases with addition and rising levels of Mg, Li, and Ti. Within the reported temperature and composition ranges, all of the Al-H-X alloys examined in this study are endothermic occluders of hydrogen. That is, hydrogen solubility increases with increase in temperature. The interaction parameters obtained from this study along with those to be obtained from a similar analysis on hydrogen solubility in solid Al-H-X will be useful for an attempt to predict hydrogen solubilities in the more complex, but commercially important, multicomponent aluminum alloys. These values are critical to the understanding of the behavior of hydrogen in ahuminum alloys, and to the preservation of the integrity of cast and fabricated aluminum products against the multifarious, and often detrimental, effects of hydrogen. References 1. P. N. Anyalebechi, To be submitted to Cast Metals Journal for Publication, (1995). 2. W. Baukloh and F. Oesterlen, Z. Metallkunde, 30, 386, (1938). 3. C. E. Ransley and H. Neufeld, J. Inst. of Metals, 74,599, (1947-M).
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4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
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W. R. Opie and N. J. Grant, J. Metals. Trans. AIME, 188, 1237 (1950). C. E. Ransley and D. E. J. Talbot, Z. Metallkunde, 46,378, (1955). D. Stephenson, Ph.D. Thesis, Brunei University, Middlesex, England, (1978). H. Shahani, Ph.D. Thesis, The Royal Institute of Technology, Stockholm, Sweden, (1984). J. Kocur, K. Tomasek, and L. Rabatin, Hutnicke Listy, 44, 269-275 (1989). D. E. J. TaIbot and P. N. Anyalebechi, Materials Science and Technology J., 4, 1, (1988). H. Liu, L. Zhiang, and B Bouchard, Recent Developments in Light Metals, (edited by M. Gilbert, P. Tremblay and E. Ozberk), p. 257, Canadian Institute of Metallurgists, (1994). P. N. Anyalebechi, D. E. J. Talbot and D. A. Granger, Metal]. Trans. B, 19B, 227, (1988). A. A Grigoreva and V. A. Danilkin, Tsvetnye Metally, 1,90 (1984). M. Sargent, Ph.D. Thesis, Brunei University, Middlesex, England, (1989). K. I. Vaschenko, D. F. Chemega, D. F. Ivanchuk, 0. M. Vyalik, and G. A. Remizov, Izv. Vyssh Ucheb Zaved Tsevet. Metally., 2.48, (1975). C. Wagner, Thermodynamics of Alloys, p. 51, Addison-Wesley, Reading, MA, (1%2). J. Chipman, J. Iron and Steel, 180, 97, (1955). C. H. P. Lupis and J. F. Elliott, Acta Met., 14.526, (1%6). G. K. S&worth and T. A. Engh, Metall. Trans. B, 13B, 447, (1982).