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Effect of 0.23 wt.%Si on precipitation in the Al-1.52wt.%Cu-0.74wt.%Mg alloy
Materials Science and Engineering, A 110 (1989) 187-192
187
Effect of 0.23 wt.% Si on Precipitation in the AI-1.52wt.%Cu-0.74wt.% Mg Alloy M. C. CHATURVED1
Department of Mechanical Engineering, UniversiO:of Manitoba, Winnipeg, Manitoba (Canada R3T 2N2)
A. K. GUPTA
National Aeronautical Establishment, National Research Council, Ottawa, Ontario (Canada K IA OR6)
A. K. JENA
Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016 (India) (Received July 2 l, 1988)
Abstract
The precipitation behaviour of the A l 1.52wt. %Cu-O. 74wt. %Mg-O.23wt. %Si alloy has been investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The DSC curves of solution-treated and quenched alloy give peaks like those of the siliconfree alloy as a result of GPB zone precipitation, GPB zone and GPB zone-dislocation complex dissolution, S' precipitation and S' dissolution. TEM studies s'how that the addition of silicon results in the precipitation of rod-shaped particles of the S' phase. The kinetics of GPB zone formation are altered by the presence of silicon. The rate of zone formation is given by dY/dT=f(Y)ko x exp(- Q*/RT), where f(Y) = 1 - Y, Y is the transformed fraction, ko = 2. 7x 104 s t and the activation energy Q*= 45.1 kJ tool -t. The values of the activation energy and the frequency factor are appreciably lower than those observed in the silicon-free alloy.
1.2wt.%Mg alloy is finer dispersion of the S' phase. This is attributed to the strong binding of silicon atoms with vacancies [4]. On the other hand, Suzuki et al. [5] have reported that addition of 0.25 wt.% Si to the A1-2.0wt.%Cu-0.9wt.%Mg alloy induced precipitation of three phases, including the S' phase which was found in the silicon-free alloy. Gupta et al. [6] have observed that in an alloy with a lower total solute content (Al-l.53wt.%Cu-0.79wt.%Mg), rod-shaped particles rather than laths of the S' phase were present, while laths of the S' phase have been reported in the AI-2.5wt.%Cu-l.2wt.%Mg alloy [3]. Thus addition of silicon appears to modify the precipitation behaviour depending upon the total solute content of AI-Cu-Mg alloys with a copper to magnesium ratio close to 2:1. This communication reports the effects of the addition of 0.23wt.%Si to the dilute Al-l.52wt.%Cu-0.74wt.%Mg alloy which was investigated by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC).
1. Introduction
AI-Cu-Mg alloys with a copper to magnesium ratio of 2:1 are capable of developing high strength by age hardening and retaining their strength at relatively high temperatures [1, 2]. Small additions of silicon are reported to increase the strength of these alloys [3-5]. Wilson and Partridge [3] have shown that the only effect of the addition of 0.25 wt.% Si to the AI-2.5wt.%Cu0921-5093/89/$3.50
2. Experimental procedures
As-cast ingots of two high purity A11.53wt.%Cu-0.79wt.%Mg and Al-l.52wt.%Cu0.74wt.%Mg-0.23wt.%Si alloys were supplied by the Aluminium Company of Canada. The ingots of these alloys were homogenized at 505 °C for 48 h. The homogenized specimens © Elsevier Sequoia/Printed in The Netherlands
188
were skimmed, upset forged and hot and cold rolled to 1.2 mm strips• Discs 5 mm in diameter for DSC were prepared from these strips by spark cutting. The discs and the strips were solution treated at 505 °C for 1 h and quenched in water• Aging treatment was carried out at 190 °C in an air furnace for various lengths of time. Thin foils for TEM were prepared by mechanically grinding the strips to foils 0.1 mm thick from which discs 3 mm in diameter were punched and electropolished using the jet electropolishing technique in a 5% perchloric acid-methanol bath at - 6 0 °C and 12.5 V. The electropolished foils were examined in a Philips 300 electron microscope. The Du Pont 910 differential scanning calorimeter with the 9900 programmer recorder was used. The DSC consisted of two pure aluminium pans which were placed on two raised platforms inside a silver-lined chamber capable of being heated at the desired rate. One of the pans was the holder for the reference material and the other was the holder for the sample• The rate of heat flow to the reference relative to the sample was recorded as a function of temperature. Annealed and furnace-cooled pure aluminium discs were used as the reference material. The sample and the reference discs were placed in the DSC cell at room temperature• The cell was cooled to a temperature of 5 °C below the temperature at which data were required, and allowed to equilibrate at that temperature for a few minutes. Heating was carried out at the desired rate. A minimum of two runs were obtained with samples heat treated in the same manner. The thermograms of the two runs were found to be highly reproducible.
while a disc of the alloy was placed in the sample pan. From these two runs it was possible to isolate the heat effects which occurred in the alloy• It follows from the above that [8]
•
Q=~
(42- 0,)-~-(C~)-
)
CA,(~/)
(1)
where Q is the rate of heat evolution in the alloy per unit mass, M is the mass of the specimen, 4l and q2 are the heat flows to the reference relative to the sample in runs 1 and 2 respectively, • is the constant heating rate, E is the calibration constant of the calorimeter, and C~s / and CA~<~/are the heat capacities of the alloy disc and the aluminium disc respectively which were placed in the sample pan. 4 2 - 4 1 is the difference between the thermograms of runs 2 and 1, and (~/E)(C~s I CAI(s)) is a small correction factor. It follows from eqn. (1) that the correction factor is ( 0 2 - q 1)0= 0. The correction factor corresponds to the difference between the thermograms of runs 2 and 1 when the heat effect in the sample is zero• Thus eqn. ( 1 ) reduces to -
•
E
(2)
Q = ~ {(42 - 01 )-(42 - 01)0=0}
In order to find Q, the term inside the braces was experimentally determined for each run. The calibration constant E was calculated by performing experiments with indium and using the heat effects caused by the fusion of indium. The value of E was 1.054. The quantity within the braces of eqn. (2) is plotted in Fig. 1. for solution-treated and waterquenched samples of silicon-free and silicon-
3. R e s u l t s and d i s c u s s i o n
Optical examination of the specimens of the alloys showed homogeneous microstructure and did not indicate the presence of any insoluble particles. This is consistent with our earlier report [7] that the Al-l.53wt.%Cu-0.79wt.%Mg alloy can retain up to 0.3 wt.% Si in solid solution. DSC thermograms of the specimens of siliconfree and silicon-containing alloys were recorded. For each specimen two runs were performed under identical conditions. In the first run, pure aluminium discs were used in both reference and sample pans; in the second run, the aluminium disc in the reference pan was left undisturbed
V
co)'I°
....t.a
(b) F A •
"r N
• o-
0
50
/
\
x,Z
si ALLOY
100 150 200 250 300 350 400 450 Temperoture (*C)
Fig. 1. DSC thermograms of solution-treated and quenched specimens of the A I - 1.53wt.%Cu-0.79wt.%Mg and Al-l.52wt.%Cu-0.74wt.%Mg-0.23wt.%Si alloys at a heating rate of l 0 °C min '.
189
containing alloys. The principal features of the thermograms of the two alloys appear to be similar. The electron microscopic studies have been used to associate DSC peaks of the silicon-free alloy with various precipitation and dissolution processes [8]. The peaks which appear with increase in temperature correspond to GPB zone precipitation, GPB zone dissolution, GPB zone and GPB zone-dislocation complex dissolution, S' precipitation and S' dissolution respectively. The peaks in the silicon-containing alloy may be attributed to the same processes which occur in the silicon-free alloy. This was confirmed by TEM studies carried out on the silicon-bearing alloy. A typical electron micrograph of the siliconcontaining alloy is given in Fig. 2(a). It shows the structure of a specimen aged at 190 °C for 500 h. The corresponding selected area diffraction (SAD) pattern (Fig. 2(b)) is typical of the S' phase [6]. The precipitates are rod-shaped and lie parallel to the (100) matrix directions. The precipitation characteristics are the same as those observed in the silicon-free alloy [6]. Thus addition of 0.23 wt.% Si does not induce precipitation of any new phase in the Al-l.52wt.%Cu0.74wt.%Mg alloy, although the AI-2wt.%Cu0.9wt.%Mg-0.25wt.%Si alloy is reported to contain precipitates of three phases [5]. The distribution of rod-shaped particles of the S' phase remains unaltered by the presence of silicon, and the laths reported in the A1-2.5wt.%Cu1.2wt.%Mg-0.25wt.%Si alloy [3] are not found. The temperature intervals over which the DSC peaks of the two alloys appear are listed in Table 1. Scrutiny of these data suggests that the kinetics of the GPB zone precipitation may have been appreciably altered by the presence of 0.23 wt.% Si. The GPB zone precipitation peaks obtained at different heating rates are shown in TABLE 1
Fig. 3. The analysis of these peaks can yield information on the kinetics of GPB zone formation. 3.1. Kinetics of GPB zone formation The changes in the GPB zone peak temperatures with heating rate are listed in Table 2. The table shows considerable shifts in the peak temperatures at all heating rates. Systematic shifts of the peak heating rate indicate that the process of
Fig. 2. (a) Bright field image of the Al-l.52wt.%Cu0.74wt.%Mg-0.23wt.%Si alloy aged at room temperature for 48 h and further aged at 190°C for 500 h. (b) The SAD pattern.
Peak characteristics of silicon-containing and silicon-free alloys at a heating rate of 10 °C m i n t
Peak
A I - 1.52%Cu-0. 74%Mg-0.23%Si
A I - 1.53%Cu-0. 79%Mg
Heat effect Q ( J g ')
Heat effi,ct Q ( J g ')
Peak temperature (°C) Initial
Final
Nature of peak
l'eak temperature
(°c) Initial
Final
1st exothermic
6.4
34.5
148.0
6.5
26.0
125.0
GPB zone precipitation
1st endothermic
7.0
175.0
261.0
7.9
151.3
265.(I
GPB zone dissolution and complex dissolution
2nd exothermic
5.6
261.0
350.0
6.5
265.(I
340.0
S' precipitation
2nd endothermic
5.4
350.0
420.0
6.4
34(I.0
425.5
S' dissolution
190 3"
2
E
•
"° F o.sr-
~20*C/min
,...................... ,..~flso
o .,=o
i A cr
,
IN
\\
50
0
...'
./,/:....""
s
/
, ...,
~ ) . - - ; :
0
(o).
..............
I00
150
200
o.,/./;.';.." '1_/ ::',.,,:........ /
TEMPERATURE (°C)
>..
o.z
Fig. 3. GPB zone precipitation peaks in Al-l.52wt.%Cu0.74wt.%Mg-0.23wt.%Si alloy at four heating rates•
/
40
."
/
60
80
I00
TABLE 2
Peak characteristics and heat effects due to GPB zone precipitation at different heating rates
120
140
160
T (°C) 6.0
Heating rate Total heat @ (°C min ') effect Q(T~) 5 l0 15 20
Peak temperature (°C)
(b)
B e g i n n i n g Maximum
End
(Jg-')
T1
Tt
5.71 6.40 6.27 6.38
26.85 27.73 35.01 30.04
71.43 82.76 92.75 99.21
132.75 157.84 166.16 168.93
5.0
:
:
/
4.0 --
/'
3.0 -
/ .~" ~ ~
\~
\',
_o x A .t"0
i
GPB zone formation is kinetically controlled. The kinetics of this process may be written as
~ \
4 2.0 r-
v
dY
,/'~', =t
,/,/ /,'
o ', \!0 k, ,,
\
,.ot- ~ y
20°C/min
~,15° ;
\
/.,,~-"~
\
,,
\
;
,
,, ',,
- Q*)
dt -T(Y) ko exp ~ -
(3)
40
60
80
I00
120
140
160
where f( Y ) is a function of the transformed fraction Y, k0 is a constant and Q* is the activation energy. Also,
Fig. 4. (a) Y vs. temperature plots for GPB zone precipitation at four different heating rates• (b) Variation of d Y/dt with temperature at four heating rates.
Y= n(T) n~(T)
where A( T ) is the area of the peak between T i and T. Calculated values of Q(Tf), where Tf is the final temperature of the peak, are listed in Table 2. The table shows that the total heat effect Q(Tf) changes only slightly with increase in Tf. Therefore n(Tf) may be treated as a constant. Since n(Tf)=ne(Tr)=n~(T), ne(T)is assumed to be independent of temperature. Hence from eqns. (3)-(6),
(4)
where n(T) is the number of moles of the precipitate formed between Ti (the starting temperature of the peak) and T, and n~(T) is the n u m b e r of moles of precipitate that can form at equilibrium at temperature T. n(T) is related to the heat effects observed in the alloy: Q( T ) = Q0 n( T )
(5)
where Q(T) is the heat effect of a DSC peak between T i and T, and Q0 is the heat effect per mole of precipitate. The heat effect is obtained by integrating eqn. (2). Hence
EA(T)
Q(T) = M~
(6)
y=A(T)
(7)
A( Tr) dY_dY ¢ = ¢ dt
dT
0 2 - 0~ A(~)
(8)
The plots in Figs. 4(a) and 4(b) show the variation of the mole fraction Y and the rate of GPB
191
zone formation, dY/dt, with temperature. The Y-T curve is sigmoidal in shape and shifts to higher temperature with increase in heating rate. The maximum rate of transformation also shifts to higher temperature with increase in heating rate. The activation energy Q* of the process was calculated by the differential technique reported earlier [9]. It can be shown [9] that eqn. (3) may be written as In
{() } ~dY ~" cD /
=ln{f(l/) k o } - ~
(9)
where dY/dT is at the same mole fraction Y' at all heating rates. The data in Fig. 4 have been plotted in Fig. 5 after eqn. (9) for Y ' = 0.3 and 0.5. The least-square lines through the data points yield an average value of 45.1 kJ mol for Q*. An integral technique was also used to confirm the value of the activation energy [10]. In the integral technique, the function A is defined such that
where i=f(irexp( - Q*/RT)dT, and ~1 and (I) 2 are two heating rates. TI and T~ are the temperatures at which Y= Y' at heating rates ~1 and ~2 respectively. It can be shown [10] that A in the above equation is zero provided Q* is the activation energy of the process. Using the values of i listed by Greenhow and Guylai [10] as a function of Q* and T, the experimental data at Y ' = 0.5 are plotted in Fig. 6 after eqn. (10). The value of Q* obtained from Fig. 6 at A = 0 is 46.6 + 0.2 kJ mole-I, which is in excellent agreement with the value determined by the differential technique. The function f( Y ) for GPB zone formation in the silicon-free alloy has been found [8] to be 1 - Y. Therefore the same function may be expected to be applicable to the silicon-containing alloy. With f ( Y ) = 1 - Y, eqn. (3)reduces to In ~ ' 1
Y =lnk0-~-
(11)
The data plotted according to this equation, for a heating rate of 10 °C min- ~, show that the form of f ( Y ) is indeed 1 - Y. The least-square line
A ={ln ~ , - l n i(T~, Q*)IY'
i(T2,Q*)},,,
-{ln ~ 2 - l n
(10)
2.0
1.8 -4.6
1.6 -4.B
--
1.4 -50 0 -5.;!
4-
--
">- -5.4 --5.s -
0
~
1.2
1.0
Y 030
Y" 0.50
o
0.8
4-5.8
C
- -
o - -
-6,0
--
-6.2
--
-6,4
--
0.6
0.4
o.z
0 u
-6.6 2.70
2.80
([/Tlx
2 90
IOs
Fig. 5. Plots for the determination of activation energy for GPB zone precipitation after eqn. (9).
IO
20
30
40
50
60
70
Activotion E n e r g y Q~, k c a l / m o l e
Fig. 6. Plots for the determination of activation energy for GPB zone precipitation after eqn. (10).
192
- 2.0
,~l~"m'~
I
•
-%0 Ao 2~o 2;o 2~o 2~0 2;o 3o0 3,o 3~o 33o' 0/T)x =O3
GPB zone-dislocation complex dissolution, S' precipitation and S' dissolution. The precipitates were rod-shaped particles of the S' phase. No laths were observed. (3) The kinetics of the GPB zone formation was changed by silicon addition. The rate equation was found to be
Fig. 7. Plot of In {(dY/dT)~/(1 - Y )} against 1/T after eqn.
(11).
dt = f ( Y ) k0 exp -
through the points in Fig. 7 yield a value of 46.1 kJ mol-I for Q*, which is in excellent agreement with the values determined by the other techniques which are independent of eqn. (11). The value of k 0 obtained from the best line drawn through the data points in Fig. 7 with a slope given by Q* = 45.1 kJ mol- 1 is 2.7 x 10 4 s - 1. The values of Q* and ko found for the formation of GPB zones in the silicon-free Al-1.53wt.%Cu-0.79wt.%Mg alloy are 55.6 kJ mol -l and 5.7x 105 s -~ respectively. Thus the activation energy and the frequency factor in the silicon-bearing alloy are appreciably lower than those in the silicon-free alloy. The low values of the activation energies are consistent with the presence of quenched-in excess vacancies which reduce the activation energy and help the formation of GPB zones. But further reduction of activation energy and a lower value of k 0 in the silicon-bearing alloy imply that zone formation is easier in the presence of silicon because of strong interaction of silicon with other solutes and vacancies. Thus the presence of 0.23 wt.% Si in the dilute Al-l.52wt.%Cu-0.74wt.%Mg alloy does not alter the sequence, structure and distribution of the precipitating phase, but reduces the activation energy and the frequency factor for GPB zone formation. These results are consistent with the results of other investigators [5, 11] in showing that the effect of silicon is sensitive to the total solute content of the alloy.
where f ( Y ) = I - Y , k 0 = 2 . 7 x 1 0 4 s 1 and the activation energy Q* = 45.1 kJ tool -1. (4) The activation energy for GPB zone formation in the silicon-containing alloy was appreciably lower than that reported for the silicon-free alloy. It indicated strong interaction of silicon with other solutes and vacancies. However, this did not alter the precipitating phase and its morphology and distribution.
4. Summary and conclusions ( 1 ) The alloys Al-l.52wt.%Cu-0.74wt.%Mg0.23wt.%Si and Al-1.53wt.%Cu-0.79wt.%Mg were solution treated, quenched and examined in DSC. Both alloys showed almost identical peaks. (2) TEM results confirmed the DSC peaks to be due to GPB zone precipitation, GPB zone and
Acknowledgments We are grateful to Dr. E. D. Murray and S. Arntfield for making their DSC facilities available to us. Thanks are also due to the Aluminium Company of Canada, Kingston, Ontario, Canada, for supplying material for this investigation. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada. Support by Association of Universities and Colleges of Canada in the form of a fellowship to one of the authors (Dr. A. K. Jena) is gratefully acknowledged.
References 1 Y.A. Bagaryatskii, Dokl. Akad. Nauk S.S.S.R., 87(1952) 559. 2 H.K. Hardy, J. Inst. Met., 84 (1954-55) 429. 3 R.N. Wilson and P. G. Partridge, A cta. Metall., 13(1965) 1321. 4 R. N. Wilson, J. Inst. Met., 97(1969) 80. 5 H. Suzuki, I. Araki, M. Kanno and K. ltoi, J. Japan Inst. Light Met., 27(1978) 239. 6 A.K. Gupta, P. Gaunt and M. C. Chaturvedi, Phil, Mag., A55 (1987) 325. 7 A. K. Gupta, M. C. Chaturvedi and A. K. Jena, Mater. Sci. Technol., submitted. 8 A. K. Jena, A. K. Gupta and M. C. Chaturvedi, Acta Metall., in the press. 9 A. K. Gupta, A. K. Jena and M. C. Chaturvedi, Scr. Metall., 22 (1988) 369. 10 E.J. Greenhow and G. Guylai, J. ThermalAnal., 6 (1974) 229. 11 D.L.W. Collins, J. Inst. Met., 86 (1953) 325.
187
Effect of 0.23 wt.% Si on Precipitation in the AI-1.52wt.%Cu-0.74wt.% Mg Alloy M. C. CHATURVED1
Department of Mechanical Engineering, UniversiO:of Manitoba, Winnipeg, Manitoba (Canada R3T 2N2)
A. K. GUPTA
National Aeronautical Establishment, National Research Council, Ottawa, Ontario (Canada K IA OR6)
A. K. JENA
Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016 (India) (Received July 2 l, 1988)
Abstract
The precipitation behaviour of the A l 1.52wt. %Cu-O. 74wt. %Mg-O.23wt. %Si alloy has been investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The DSC curves of solution-treated and quenched alloy give peaks like those of the siliconfree alloy as a result of GPB zone precipitation, GPB zone and GPB zone-dislocation complex dissolution, S' precipitation and S' dissolution. TEM studies s'how that the addition of silicon results in the precipitation of rod-shaped particles of the S' phase. The kinetics of GPB zone formation are altered by the presence of silicon. The rate of zone formation is given by dY/dT=f(Y)ko x exp(- Q*/RT), where f(Y) = 1 - Y, Y is the transformed fraction, ko = 2. 7x 104 s t and the activation energy Q*= 45.1 kJ tool -t. The values of the activation energy and the frequency factor are appreciably lower than those observed in the silicon-free alloy.
1.2wt.%Mg alloy is finer dispersion of the S' phase. This is attributed to the strong binding of silicon atoms with vacancies [4]. On the other hand, Suzuki et al. [5] have reported that addition of 0.25 wt.% Si to the A1-2.0wt.%Cu-0.9wt.%Mg alloy induced precipitation of three phases, including the S' phase which was found in the silicon-free alloy. Gupta et al. [6] have observed that in an alloy with a lower total solute content (Al-l.53wt.%Cu-0.79wt.%Mg), rod-shaped particles rather than laths of the S' phase were present, while laths of the S' phase have been reported in the AI-2.5wt.%Cu-l.2wt.%Mg alloy [3]. Thus addition of silicon appears to modify the precipitation behaviour depending upon the total solute content of AI-Cu-Mg alloys with a copper to magnesium ratio close to 2:1. This communication reports the effects of the addition of 0.23wt.%Si to the dilute Al-l.52wt.%Cu-0.74wt.%Mg alloy which was investigated by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC).
1. Introduction
AI-Cu-Mg alloys with a copper to magnesium ratio of 2:1 are capable of developing high strength by age hardening and retaining their strength at relatively high temperatures [1, 2]. Small additions of silicon are reported to increase the strength of these alloys [3-5]. Wilson and Partridge [3] have shown that the only effect of the addition of 0.25 wt.% Si to the AI-2.5wt.%Cu0921-5093/89/$3.50
2. Experimental procedures
As-cast ingots of two high purity A11.53wt.%Cu-0.79wt.%Mg and Al-l.52wt.%Cu0.74wt.%Mg-0.23wt.%Si alloys were supplied by the Aluminium Company of Canada. The ingots of these alloys were homogenized at 505 °C for 48 h. The homogenized specimens © Elsevier Sequoia/Printed in The Netherlands
188
were skimmed, upset forged and hot and cold rolled to 1.2 mm strips• Discs 5 mm in diameter for DSC were prepared from these strips by spark cutting. The discs and the strips were solution treated at 505 °C for 1 h and quenched in water• Aging treatment was carried out at 190 °C in an air furnace for various lengths of time. Thin foils for TEM were prepared by mechanically grinding the strips to foils 0.1 mm thick from which discs 3 mm in diameter were punched and electropolished using the jet electropolishing technique in a 5% perchloric acid-methanol bath at - 6 0 °C and 12.5 V. The electropolished foils were examined in a Philips 300 electron microscope. The Du Pont 910 differential scanning calorimeter with the 9900 programmer recorder was used. The DSC consisted of two pure aluminium pans which were placed on two raised platforms inside a silver-lined chamber capable of being heated at the desired rate. One of the pans was the holder for the reference material and the other was the holder for the sample• The rate of heat flow to the reference relative to the sample was recorded as a function of temperature. Annealed and furnace-cooled pure aluminium discs were used as the reference material. The sample and the reference discs were placed in the DSC cell at room temperature• The cell was cooled to a temperature of 5 °C below the temperature at which data were required, and allowed to equilibrate at that temperature for a few minutes. Heating was carried out at the desired rate. A minimum of two runs were obtained with samples heat treated in the same manner. The thermograms of the two runs were found to be highly reproducible.
while a disc of the alloy was placed in the sample pan. From these two runs it was possible to isolate the heat effects which occurred in the alloy• It follows from the above that [8]
•
Q=~
(42- 0,)-~-(C~)-
)
CA,(~/)
(1)
where Q is the rate of heat evolution in the alloy per unit mass, M is the mass of the specimen, 4l and q2 are the heat flows to the reference relative to the sample in runs 1 and 2 respectively, • is the constant heating rate, E is the calibration constant of the calorimeter, and C~s / and CA~<~/are the heat capacities of the alloy disc and the aluminium disc respectively which were placed in the sample pan. 4 2 - 4 1 is the difference between the thermograms of runs 2 and 1, and (~/E)(C~s I CAI(s)) is a small correction factor. It follows from eqn. (1) that the correction factor is ( 0 2 - q 1)0= 0. The correction factor corresponds to the difference between the thermograms of runs 2 and 1 when the heat effect in the sample is zero• Thus eqn. ( 1 ) reduces to -
•
E
(2)
Q = ~ {(42 - 01 )-(42 - 01)0=0}
In order to find Q, the term inside the braces was experimentally determined for each run. The calibration constant E was calculated by performing experiments with indium and using the heat effects caused by the fusion of indium. The value of E was 1.054. The quantity within the braces of eqn. (2) is plotted in Fig. 1. for solution-treated and waterquenched samples of silicon-free and silicon-
3. R e s u l t s and d i s c u s s i o n
Optical examination of the specimens of the alloys showed homogeneous microstructure and did not indicate the presence of any insoluble particles. This is consistent with our earlier report [7] that the Al-l.53wt.%Cu-0.79wt.%Mg alloy can retain up to 0.3 wt.% Si in solid solution. DSC thermograms of the specimens of siliconfree and silicon-containing alloys were recorded. For each specimen two runs were performed under identical conditions. In the first run, pure aluminium discs were used in both reference and sample pans; in the second run, the aluminium disc in the reference pan was left undisturbed
V
co)'I°
....t.a
(b) F A •
"r N
• o-
0
50
/
\
x,Z
si ALLOY
100 150 200 250 300 350 400 450 Temperoture (*C)
Fig. 1. DSC thermograms of solution-treated and quenched specimens of the A I - 1.53wt.%Cu-0.79wt.%Mg and Al-l.52wt.%Cu-0.74wt.%Mg-0.23wt.%Si alloys at a heating rate of l 0 °C min '.
189
containing alloys. The principal features of the thermograms of the two alloys appear to be similar. The electron microscopic studies have been used to associate DSC peaks of the silicon-free alloy with various precipitation and dissolution processes [8]. The peaks which appear with increase in temperature correspond to GPB zone precipitation, GPB zone dissolution, GPB zone and GPB zone-dislocation complex dissolution, S' precipitation and S' dissolution respectively. The peaks in the silicon-containing alloy may be attributed to the same processes which occur in the silicon-free alloy. This was confirmed by TEM studies carried out on the silicon-bearing alloy. A typical electron micrograph of the siliconcontaining alloy is given in Fig. 2(a). It shows the structure of a specimen aged at 190 °C for 500 h. The corresponding selected area diffraction (SAD) pattern (Fig. 2(b)) is typical of the S' phase [6]. The precipitates are rod-shaped and lie parallel to the (100) matrix directions. The precipitation characteristics are the same as those observed in the silicon-free alloy [6]. Thus addition of 0.23 wt.% Si does not induce precipitation of any new phase in the Al-l.52wt.%Cu0.74wt.%Mg alloy, although the AI-2wt.%Cu0.9wt.%Mg-0.25wt.%Si alloy is reported to contain precipitates of three phases [5]. The distribution of rod-shaped particles of the S' phase remains unaltered by the presence of silicon, and the laths reported in the A1-2.5wt.%Cu1.2wt.%Mg-0.25wt.%Si alloy [3] are not found. The temperature intervals over which the DSC peaks of the two alloys appear are listed in Table 1. Scrutiny of these data suggests that the kinetics of the GPB zone precipitation may have been appreciably altered by the presence of 0.23 wt.% Si. The GPB zone precipitation peaks obtained at different heating rates are shown in TABLE 1
Fig. 3. The analysis of these peaks can yield information on the kinetics of GPB zone formation. 3.1. Kinetics of GPB zone formation The changes in the GPB zone peak temperatures with heating rate are listed in Table 2. The table shows considerable shifts in the peak temperatures at all heating rates. Systematic shifts of the peak heating rate indicate that the process of
Fig. 2. (a) Bright field image of the Al-l.52wt.%Cu0.74wt.%Mg-0.23wt.%Si alloy aged at room temperature for 48 h and further aged at 190°C for 500 h. (b) The SAD pattern.
Peak characteristics of silicon-containing and silicon-free alloys at a heating rate of 10 °C m i n t
Peak
A I - 1.52%Cu-0. 74%Mg-0.23%Si
A I - 1.53%Cu-0. 79%Mg
Heat effect Q ( J g ')
Heat effi,ct Q ( J g ')
Peak temperature (°C) Initial
Final
Nature of peak
l'eak temperature
(°c) Initial
Final
1st exothermic
6.4
34.5
148.0
6.5
26.0
125.0
GPB zone precipitation
1st endothermic
7.0
175.0
261.0
7.9
151.3
265.(I
GPB zone dissolution and complex dissolution
2nd exothermic
5.6
261.0
350.0
6.5
265.(I
340.0
S' precipitation
2nd endothermic
5.4
350.0
420.0
6.4
34(I.0
425.5
S' dissolution
190 3"
2
E
•
"° F o.sr-
~20*C/min
,...................... ,..~flso
o .,=o
i A cr
,
IN
\\
50
0
...'
./,/:....""
s
/
, ...,
~ ) . - - ; :
0
(o).
..............
I00
150
200
o.,/./;.';.." '1_/ ::',.,,:........ /
TEMPERATURE (°C)
>..
o.z
Fig. 3. GPB zone precipitation peaks in Al-l.52wt.%Cu0.74wt.%Mg-0.23wt.%Si alloy at four heating rates•
/
40
."
/
60
80
I00
TABLE 2
Peak characteristics and heat effects due to GPB zone precipitation at different heating rates
120
140
160
T (°C) 6.0
Heating rate Total heat @ (°C min ') effect Q(T~) 5 l0 15 20
Peak temperature (°C)
(b)
B e g i n n i n g Maximum
End
(Jg-')
T1
Tt
5.71 6.40 6.27 6.38
26.85 27.73 35.01 30.04
71.43 82.76 92.75 99.21
132.75 157.84 166.16 168.93
5.0
:
:
/
4.0 --
/'
3.0 -
/ .~" ~ ~
\~
\',
_o x A .t"0
i
GPB zone formation is kinetically controlled. The kinetics of this process may be written as
~ \
4 2.0 r-
v
dY
,/'~', =t
,/,/ /,'
o ', \!0 k, ,,
\
,.ot- ~ y
20°C/min
~,15° ;
\
/.,,~-"~
\
,,
\
;
,
,, ',,
- Q*)
dt -T(Y) ko exp ~ -
(3)
40
60
80
I00
120
140
160
where f( Y ) is a function of the transformed fraction Y, k0 is a constant and Q* is the activation energy. Also,
Fig. 4. (a) Y vs. temperature plots for GPB zone precipitation at four different heating rates• (b) Variation of d Y/dt with temperature at four heating rates.
Y= n(T) n~(T)
where A( T ) is the area of the peak between T i and T. Calculated values of Q(Tf), where Tf is the final temperature of the peak, are listed in Table 2. The table shows that the total heat effect Q(Tf) changes only slightly with increase in Tf. Therefore n(Tf) may be treated as a constant. Since n(Tf)=ne(Tr)=n~(T), ne(T)is assumed to be independent of temperature. Hence from eqns. (3)-(6),
(4)
where n(T) is the number of moles of the precipitate formed between Ti (the starting temperature of the peak) and T, and n~(T) is the n u m b e r of moles of precipitate that can form at equilibrium at temperature T. n(T) is related to the heat effects observed in the alloy: Q( T ) = Q0 n( T )
(5)
where Q(T) is the heat effect of a DSC peak between T i and T, and Q0 is the heat effect per mole of precipitate. The heat effect is obtained by integrating eqn. (2). Hence
EA(T)
Q(T) = M~
(6)
y=A(T)
(7)
A( Tr) dY_dY ¢ = ¢ dt
dT
0 2 - 0~ A(~)
(8)
The plots in Figs. 4(a) and 4(b) show the variation of the mole fraction Y and the rate of GPB
191
zone formation, dY/dt, with temperature. The Y-T curve is sigmoidal in shape and shifts to higher temperature with increase in heating rate. The maximum rate of transformation also shifts to higher temperature with increase in heating rate. The activation energy Q* of the process was calculated by the differential technique reported earlier [9]. It can be shown [9] that eqn. (3) may be written as In
{() } ~dY ~" cD /
=ln{f(l/) k o } - ~
(9)
where dY/dT is at the same mole fraction Y' at all heating rates. The data in Fig. 4 have been plotted in Fig. 5 after eqn. (9) for Y ' = 0.3 and 0.5. The least-square lines through the data points yield an average value of 45.1 kJ mol for Q*. An integral technique was also used to confirm the value of the activation energy [10]. In the integral technique, the function A is defined such that
where i=f(irexp( - Q*/RT)dT, and ~1 and (I) 2 are two heating rates. TI and T~ are the temperatures at which Y= Y' at heating rates ~1 and ~2 respectively. It can be shown [10] that A in the above equation is zero provided Q* is the activation energy of the process. Using the values of i listed by Greenhow and Guylai [10] as a function of Q* and T, the experimental data at Y ' = 0.5 are plotted in Fig. 6 after eqn. (10). The value of Q* obtained from Fig. 6 at A = 0 is 46.6 + 0.2 kJ mole-I, which is in excellent agreement with the value determined by the differential technique. The function f( Y ) for GPB zone formation in the silicon-free alloy has been found [8] to be 1 - Y. Therefore the same function may be expected to be applicable to the silicon-containing alloy. With f ( Y ) = 1 - Y, eqn. (3)reduces to In ~ ' 1
Y =lnk0-~-
(11)
The data plotted according to this equation, for a heating rate of 10 °C min- ~, show that the form of f ( Y ) is indeed 1 - Y. The least-square line
A ={ln ~ , - l n i(T~, Q*)IY'
i(T2,Q*)},,,
-{ln ~ 2 - l n
(10)
2.0
1.8 -4.6
1.6 -4.B
--
1.4 -50 0 -5.;!
4-
--
">- -5.4 --5.s -
0
~
1.2
1.0
Y 030
Y" 0.50
o
0.8
4-5.8
C
- -
o - -
-6,0
--
-6.2
--
-6,4
--
0.6
0.4
o.z
0 u
-6.6 2.70
2.80
([/Tlx
2 90
IOs
Fig. 5. Plots for the determination of activation energy for GPB zone precipitation after eqn. (9).
IO
20
30
40
50
60
70
Activotion E n e r g y Q~, k c a l / m o l e
Fig. 6. Plots for the determination of activation energy for GPB zone precipitation after eqn. (10).
192
- 2.0
,~l~"m'~
I
•
-%0 Ao 2~o 2;o 2~o 2~0 2;o 3o0 3,o 3~o 33o' 0/T)x =O3
GPB zone-dislocation complex dissolution, S' precipitation and S' dissolution. The precipitates were rod-shaped particles of the S' phase. No laths were observed. (3) The kinetics of the GPB zone formation was changed by silicon addition. The rate equation was found to be
Fig. 7. Plot of In {(dY/dT)~/(1 - Y )} against 1/T after eqn.
(11).
dt = f ( Y ) k0 exp -
through the points in Fig. 7 yield a value of 46.1 kJ mol-I for Q*, which is in excellent agreement with the values determined by the other techniques which are independent of eqn. (11). The value of k 0 obtained from the best line drawn through the data points in Fig. 7 with a slope given by Q* = 45.1 kJ mol- 1 is 2.7 x 10 4 s - 1. The values of Q* and ko found for the formation of GPB zones in the silicon-free Al-1.53wt.%Cu-0.79wt.%Mg alloy are 55.6 kJ mol -l and 5.7x 105 s -~ respectively. Thus the activation energy and the frequency factor in the silicon-bearing alloy are appreciably lower than those in the silicon-free alloy. The low values of the activation energies are consistent with the presence of quenched-in excess vacancies which reduce the activation energy and help the formation of GPB zones. But further reduction of activation energy and a lower value of k 0 in the silicon-bearing alloy imply that zone formation is easier in the presence of silicon because of strong interaction of silicon with other solutes and vacancies. Thus the presence of 0.23 wt.% Si in the dilute Al-l.52wt.%Cu-0.74wt.%Mg alloy does not alter the sequence, structure and distribution of the precipitating phase, but reduces the activation energy and the frequency factor for GPB zone formation. These results are consistent with the results of other investigators [5, 11] in showing that the effect of silicon is sensitive to the total solute content of the alloy.
where f ( Y ) = I - Y , k 0 = 2 . 7 x 1 0 4 s 1 and the activation energy Q* = 45.1 kJ tool -1. (4) The activation energy for GPB zone formation in the silicon-containing alloy was appreciably lower than that reported for the silicon-free alloy. It indicated strong interaction of silicon with other solutes and vacancies. However, this did not alter the precipitating phase and its morphology and distribution.
4. Summary and conclusions ( 1 ) The alloys Al-l.52wt.%Cu-0.74wt.%Mg0.23wt.%Si and Al-1.53wt.%Cu-0.79wt.%Mg were solution treated, quenched and examined in DSC. Both alloys showed almost identical peaks. (2) TEM results confirmed the DSC peaks to be due to GPB zone precipitation, GPB zone and
Acknowledgments We are grateful to Dr. E. D. Murray and S. Arntfield for making their DSC facilities available to us. Thanks are also due to the Aluminium Company of Canada, Kingston, Ontario, Canada, for supplying material for this investigation. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada. Support by Association of Universities and Colleges of Canada in the form of a fellowship to one of the authors (Dr. A. K. Jena) is gratefully acknowledged.
References 1 Y.A. Bagaryatskii, Dokl. Akad. Nauk S.S.S.R., 87(1952) 559. 2 H.K. Hardy, J. Inst. Met., 84 (1954-55) 429. 3 R.N. Wilson and P. G. Partridge, A cta. Metall., 13(1965) 1321. 4 R. N. Wilson, J. Inst. Met., 97(1969) 80. 5 H. Suzuki, I. Araki, M. Kanno and K. ltoi, J. Japan Inst. Light Met., 27(1978) 239. 6 A.K. Gupta, P. Gaunt and M. C. Chaturvedi, Phil, Mag., A55 (1987) 325. 7 A. K. Gupta, M. C. Chaturvedi and A. K. Jena, Mater. Sci. Technol., submitted. 8 A. K. Jena, A. K. Gupta and M. C. Chaturvedi, Acta Metall., in the press. 9 A. K. Gupta, A. K. Jena and M. C. Chaturvedi, Scr. Metall., 22 (1988) 369. 10 E.J. Greenhow and G. Guylai, J. ThermalAnal., 6 (1974) 229. 11 D.L.W. Collins, J. Inst. Met., 86 (1953) 325.