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Role of heat treatment on order-disorder transformation of some Cu-Ni-Zn Alloys
51
Physica 112B (1982) 51-56 North-Holland Publishing Company
ROLE
OF HEAT
OF SOME Mohamed Department
Ahmed
TREATMENT
Cu-Ni-Zn
ON ORDER-DISORDER
TRANSFORMATION
ALLOYS
I. ISMAIL of Chemical
S.A.
Engineering,
University
of Waterloo,
Waterloo,
Ontario, Canada
Alexandtia
University,
Alexandria,
AMMAR
Math. Phys. Department,
Faculty of Engineering,
Egypt
Received 3 March 1981 Revised 10 June 1981
The specific heats of some Cu-Ni-Zn alloys were measured in the temperature range 350 to 850 K, using a computer differential scanning Perkin-Elmer calorimeter (DSC-2). The heat capacity measurements indicated the regions of atomic ordering in the alloys tested. Alloys were annealed in pure argon at 1073 K for 3 h followed by either water quenching or furnace cooling. The water-quenched specimens showed order-disorder transformation during the heating or the cooling processes while furnace-cooled specimens showed only disordering during heating or cooling. The alloy composition near the CugNiZn indicated a high degree of ordering which is in reasonable agreement with the published data.
1. Introduction As copper, nickel and zinc alloys are important materials for a variety of applications, the data on their thermophysical properties are of great interest [4-lo]. Physical and mechanical properties are directly related to the atomic ordering of the alloys [4, 111. The heat capacity measurements were used to follow ordering in some copper-nickel-zinc alloys as there are few data in the literature regarding these alloys for the temperature and heat-treatment conditions tested [l, 4,5]. The aim of this work was to establish the relationship between the chemical composition and the heat capacity for the copper-nickel-zinc alloys tested in the temperature range 350 to 850 K.
by Perkin-Elmer corporation. A detailed description of the equipment and the procedure are available elsewhere [12, 131. The Cu-Ni-Zn alloys tested were Cu,NiZn, where the x and y ranges are 1.01 to 4.48 and 0.96 to 1.97, respectively. The specimens have the composition shown in table I. The specimens were disks of 4.5 mm diameter and 1 mm thickness. The specimens were heated in a calorimeter at a rate of 40 K/min (sensivity 17 J/s) and the cooling rate was 20 K/min. Heat treatment of the specimens was performed in pure argon at 1073 K for 3 h followed by furnace cooling or water quenching. The heat capacity was determined during the heating and cooling processes.
3. Results and discussion 2. Experimental The apparatus (Perkin-Elmer DSC-2) used for heat capacity measurements was made available 0378-4363/82/0000-0000/$02.75
@ 1982 North-Holland
Typical curves for the temperature dependence of the heat capacity are shown in fig. 1. The heat capacity curves for the different specimens show a smooth variation with the tem-
M.I. Zsmail and A.S.A.
52
Table I Chemical
composition
Ammar
of specimens
I Order-disorder
transformation
of some Cu-Ni-Zn
alloys
used
Specimen No. Symbol
Cu (at.%)
Ni (at.%)
Zn (at.%)
Average atomic weight
Cu/Ni ratio
Zn/Ni ratio
1 2 3 4 5 6
33.7 50.5 60.3 70.1 60.05 60.1
31.3 23.4 20.1 15.2 25.7 13.4
35.0 26.1 19.6 14.7 14.25 26.5
62.667 62.886 62.926 63.073 62.558 63.377
1.07 2.15 3.00 4.61 2.33 4.48
1.11 1.11 0.97 0.96 0.55 1.97
0 0
. A x x
perature. From figs. 1 and 3, the magnitude of C, of the different furnace-cooled alloys at temperatures up to 500 K is fairly close to each other with a maximum about difference of 2.5 J mol-‘K-r while at about 750 K this difference is increased by sixfold. For the waterquenched specimens (figs. 2 and 4) the reverse is true, i.e. the difference in the C, values at 750 K is lower and at 500 K higher than in the case of the corresponding furnace-cooled specimens. The thermal effects (defined as the maximum C, value divided by the temperature, T,,at which this maximum occurs) are also dependent on both heat treatment and chemical composition.
500
700
Figs. 5 and 6 show that the furnace-cooled specimens mostly have higher values of thermal effects than the water-quenched specimens, particularly for lower Cu/Ni ratio. The thermal effects measured during cooling of quenched alloys increases linearly with the Cu/Ni ratio. The minimum values of these thermal effects increase linearly with an increase of the Cu/Ni ratio of quenched specimens when C, is measured during the heating process, as shown in fig. 6. For lower Zn/Ni values (0.5 in fig. 7) the thermal effects increase with the increase of the Zn/Ni (ratio but no significant change occurs when this ratio exceeds 1.11. The difference between thermal effects measured during heating or cooling of
900
T. K
Fig. 1. Temperature dependence of the heat capacity, C,, furnace-cooled specimens; C, measured during heating: Cu, Ni, Zn:50.5, 23.4, 26.1 at.% (table I, specimen 2-Cu, Ni, Zn:60.3, 20.1, 19.6 at.% (table I, specimen 3- Cu, Ni, Zn :70.1, 15.2, 14.7 at.% (table I, specimen 4 - Cu, Ni, Zn : 60.1, 13.4, 26.5 at.% (table I, specimen 6).
of l2); 3); 4);
ac.0
,
500
700
900
T. K
Fig. 2. Heat capacity-temperature curves for water-quenched specimens; C, measured during heating: 1, 2, 3,4, same as in fig. 1.
M.I. Ismail and A.S.A.
I
Ammar
/ Order-disorder
transformation of some Cu-Ni-Zn
53
alloys
c 0
-_?L _-
---
__c ,g
ii;_
8
35.
L3_--“---\,
a_
/
----a
_T‘+.-+
__-
/
/’
I
Fig. 3. Heat capacity-temperature specimens; C, measured during fig. 1.
/
curves for furnace-cooled cooling: 1, 2, 3, 4, same as in
s
3
specimens for the same heat treatment cooling or water quenching) generally
(furnace increases
c 0(0
2.5 COPPER/NICKEL
with the Cu/Ni ratio (figs. 5 and 6) and with the Zn/Ni ratio (fig. 7). These thermal effects are a result of solid state transformation of phases present or atomic rearrangement or order that occur in the bulk. However, the mechanism of the role played by phases change is complicated. The classification of phases in alloys is based on the factors by Hume-Rothery as well as on the consideration of the space electron correlation [14]. Usually the electron factor and the factor of atomic size difference are meant by the Hume-
.: . _ . . - - .
“r
RATIO
IN ALLOY
Fig. 5. Effect of heat treatment and alloy composition on thermal effects (max CJT, where T, is the temperature which the C, peak occurs): A - furnace-cooled specimens measured during heating; B - water-quenched specimens measured during cooling; C-water-quenched specimens measured during heating; D-furnace-cooled specimens: measured during cooling.
the at : C, : C, : C, C,
I 1
500
700
900
0
5.0
2.5 COPPER/NICKEL
RATIO
IN ALLOY
T. K
Fig. 4. Heat capacity-temperature specimens; C’ measured during fig. 1.
curves of water-quenched cooling: 1, 2, 3, 4, same as
Fig. 6. Minimum CJT, (where TP is the temperature which C, is minimum) vs. the Cu/Ni ratio in the alloy: same as in fig. 5.
at C,
MI.
54
1 0.5
Ismail and A.S.A.
1.0
Ammar
1.5 ZINC/NICKEL
I Order-disorder
2.0
RATIO
Fig. 7. Effect of heat treatment and the Zn/Ni ratio in alloy on the thermal effects (C,lT,): A, B, C, D, same as in fig. 5.
Rothery factors. However, the nature of the structural variety of the phases is not simple [l-5,15]. A sharp fall in the C, vs. T curve was observed in the temperature range 450 to 550 K (fig. 2) with a minimum. This minimum increases with the increase in the Cu/Ni ratio, particularly for the measurements of specific heat during heating of water-quenched specimens as shown in fig. 6. This heat effect may be explained by the redistribution of the electrons in the transition element (Ni) at high temperatures. The existence of a maximum C, is associated with the onset of atomic ordering. The largest change in the value of the specific heat takes place at the solid state transformation, beginning at about 500 up to 750 K depending on the chemical composition and the heat treatment. The heat capacity of specimen no. 2 (table I) rises comparatively rapidly with temperature in most of the cases studied, particularly for furnace-cooled specimens. It also shows a sharp fall in the value of C, measured during heating of water-quenched specimens (fig. 2). The observed variation of ordering temperature of water-quenched and of furnace-cooled specimens depends on the composition of the specimens. For specimen no. 6,
transformation of some Cu-Ni-Zn
alloys
there are no significant thermal effects on the C, vs. T up to a temperature of about 850 K (fig. 3) as the increase in C, is less than 20% in the range 400 to 850 K. Fig. 8 shows the variation in transition temperature range which is the maximum width of the C, vs. T peak. This transition range of water-quenched specimens is narrower than that of furnace-cooled specimens. This temperature range decreases with the increase in the Cu/Ni ratio and has its maximum value at the Zn/Ni ratio of 1.11 (table I) as shown in fig. 9. Representation of the composition of the tested alloys in a Gibbs-triangle, as shown in fig. 10, indicates that the temperature ranges of transformation become short as the alloy composition becomes closer to copper. The data available for the binary alloys of Cu, Ni, Zn systems showed the existence of ordered phases in the Cu-Zn system. The well-known superstructures are formed in bee, fee and hcp lattices [16]. One phase structure was claimed to result from annealing for 6 h at 1053 K of Cu-Ni-Fe-Zn spring alloy [17] which is similar to some of the alloys of this work. The experimental data in this work are in reasonable agreement with the phase diagrams for Cu-Ni-Zn published recently [I, 41. Several models of order-disorder kinetics are available in the literature [18-241; e.g. chemical 500
--
A
Y
2.5 COPPER/NICKEL
Fig. 8. Variation of solid state transformation temperature range with the Cu/Ni ratio in alloys and heat treatment: A, B, C, D, same as in fig. 5.
M.I. Ismail and A.S.A.
Ammar
/ Order-disorder
iransformation of some Cu-Ni-Zn
alloys
order/disorder process is more complicated nature and not yet fully understood.
4. Summary
f . 1.0
1.5
2.0
ZINC/NICKEL
Fig. 9. Variation of solid state transformation temperature range with the Zn/Ni ratio in the alloys: A, B, C, D, same as in fig. 5.
reaction rate equations [18], or stochastic equations of Markovian nature, with the rates of atomic interchanges for a simple atomic model written in terms of a set of thermodynamic parameters [19,20], or the phenomenological diffusion equations of Onsager’s type, which is a of local function concentration [21,22]. However, as shown above, the resolution of the heat capacity data is a difficult task and the COPPER
\
/ ZINC
ATOMIC
\ NICKEL
50 %
Fig. 10. Triangular plot of alloy composition indicating areas of minimum and maximum temperature range of transformation and maximum Cr.: I-area of minimum transformation temperature range and minimum C, value; IIarea of maximum tratisformation temperature range and maximum C, value. Dotted lines are approximate phase diagram according to refs. [l, 41.
in its
and conclusions
The heat capacity,
0 0.5
5.5
C,, measurements during heating or cooling of some heat-treated Cu-NiZn alloys was studied. Specimens were annealed at 1073 K for 3 h in argon atmosphere followed by either furnace cooling or water quenching. The role of heat treatment and composition on the specific heat and ordering of alloys was studied. (1) The temperature dependence of the specific heat of Cu-Ni-Zn alloys was determined in the temperature range 350 to 850 K in both heating and cooling processes. (2) The alloys of composition near to CuzNiZn is the most probable ordered phase in the CuNi-Zn alloy system studied as it has the sharpest maxima in the heat capacity-temperature curves (figs. l-4) and the minimum value of the maximum CJT, for all the heat treatments studied (figs. 5-7) where T, is the temperature at which C, has the maximum value. (3) The ordering temperature, T,, of quenched alloys (at which the C, peak occurs) mostly decreases with the increase in the Cu/Ni ratio in the Cu-Ni-Zn alloy system. Quenched alloys usually have higher ordering temperatures but are characterized by lower C, peak values (figs. l-4). The quenched alloys usually have excess vacancies which result in an initiation of ordering process. (4) Heating of quenched alloys showed a minimum value of C, at lower temperatures which is an indication of an exothermic process, i.e. the atomic ordering of the specimen is disordered by quenching (fig. 2). (5) The critical values of CAT, (maximum C, or minimum C,) increase with the increase of Cu/Ni ratio. However, there is a minimum value at the Cu/Ni ratio of 2.15 (fig. 8). (6) For a lower Zn/Ni ratio of 0.55 in the
56
M.I. Zsmail and A.S.A.
Ammar
1 Order-disorder
Cu-Ni-Zn system, the minimum values of CJT, were observed at all the heat-treatment conditions (fig. 7). (7) The difference between critical thermal effects, CJT,,measured during heating or cooling of annealed alloys increases with the Zn/Ni and Cu/Ni ratios. But during cooling, these thermal effects have higher values for the higher ratios of Cu/Ni or Zn/Ni.
Acknowledgements
One of the authors (A.S.A.A.) would like to thank the Hungarian Cultural Centre for offering him a fellowship. The facilities offered by the Central Institute for Physics and Metal Physics Lab (Budapest), where the experimental work was done, are gratefully acknowledged.
References
111A.
de Rooy, E.W. van Royen, P.M. Bronsveld and J.Th.M. de Hosson, Acta Metall. 28 (1980) 1339. 121A. de Rooy, P.M. Bronsveld and J.Th.M. de Hosson, Internal Friction and Ultrasonic Attenuation in Solids, C.C. Smith, ed. (Pergamon, New York, 1980) pp. 149154. to Ordering, Am. [31 J.Th.M. de Hosson, Atomic Approach Inst. of Physics 53 (1979) 146. [41 H. Thomas, Z. Metallk. 63 (1972) 106. and H. Wollenberger, Z. Metallk. 50 I51 A. Kuzmann (1959) 94.
transformation of some Cu-Ni-Zn
1’51M.
alloys
Matsumoto and T. Homma, Tohoku Doigaku Senko Seiren Kenkyusho Iho, 33(2) (1979) 103. G. Gratias, C. Chatillon-Colinet and [71 J.L. Deneuville, J.C. Mathieu, J. Calorim. Anal. Therm. 9-B, B15 (1978) 111. in Thermoelectr. Met. PI R.D. Parks and R. Orbach, Conduct. (Proc. Int. Cof.) Vol. 1 (1977) 281. Intermetallic Compounds (John Wiley, 191 J.H. Westbrook, New York, 1%7). with Unique Pro11011.1. Kornilov, Metallides-Materials perties, Veestnik Akademii, Nauk, USSR 12 (1970) 30. 1111L.A. Shvartsman, B.M. Mogutnov, E.F. Petrova and A.N. Tsareva, Dokl. Akad. Nauk, USSR 284(l) (1979) 158. WI U. Gaur, A. Mehta and B. Wunderlich, J. Therm. Anal. 13 (1978) 71. 1131 M.I. Ismail and A.S.A. Ammar, The Effect of Boron on Order/Disorder Transformation of Some Heat Treated Cu-Ni-Zn-Mn alloys, Int. J. Phys. Clrem. Solids, accepted for publication. K&tall Structuren Zwei Komponentiger P41 K. Schubert, Phasen (Springer-Verlag, Berlin, 1964). 1151 V.E. Panin and V.P. Fadin, Proc. Int. Symp. on OrderDisorder Transformation in Alloys, Sept. 1973, Tubingen, W. Germany, pp. 28-56. U61 N.S. Golosov, L.E. Popov and L.Ya Pudan, J. Phys. Chem. Solids 34 (1973) 1149. Prakt. Metallogr. 16 (1979) 350. [I71 P.K. Chatterjee, [I81 G.J. Dienes, Acta Met. 3 (1955) 549. [I91 W.L. Bragg and E.J. Williams, Proc. Roy. Sot. Al45 (1934) 699. [201 G.H. Vineyard, Phys. Rev. 102 (1956) 981. [211 H.E. Cook, D. de Fontaine and J.E. Hilliard, Acta Met. 17 (1969) 765. [221 D. de Fontaine and H.E. Cook, in Critical Phenomena in Alloys, Magnets and Superconductors, R.E. Mills, E. Ascher and R.I. Jaffee, eds. (McGraw-Hill, New York, 1971) p. 257. [231 R. Kikuchi, Prog. Theor. Phys. 35 (Suppl.) (1966) 1. 1241 A. Paja and K. Kulakowski, J. Phys. F. 9(8) (1979) 1613.
Physica 112B (1982) 51-56 North-Holland Publishing Company
ROLE
OF HEAT
OF SOME Mohamed Department
Ahmed
TREATMENT
Cu-Ni-Zn
ON ORDER-DISORDER
TRANSFORMATION
ALLOYS
I. ISMAIL of Chemical
S.A.
Engineering,
University
of Waterloo,
Waterloo,
Ontario, Canada
Alexandtia
University,
Alexandria,
AMMAR
Math. Phys. Department,
Faculty of Engineering,
Egypt
Received 3 March 1981 Revised 10 June 1981
The specific heats of some Cu-Ni-Zn alloys were measured in the temperature range 350 to 850 K, using a computer differential scanning Perkin-Elmer calorimeter (DSC-2). The heat capacity measurements indicated the regions of atomic ordering in the alloys tested. Alloys were annealed in pure argon at 1073 K for 3 h followed by either water quenching or furnace cooling. The water-quenched specimens showed order-disorder transformation during the heating or the cooling processes while furnace-cooled specimens showed only disordering during heating or cooling. The alloy composition near the CugNiZn indicated a high degree of ordering which is in reasonable agreement with the published data.
1. Introduction As copper, nickel and zinc alloys are important materials for a variety of applications, the data on their thermophysical properties are of great interest [4-lo]. Physical and mechanical properties are directly related to the atomic ordering of the alloys [4, 111. The heat capacity measurements were used to follow ordering in some copper-nickel-zinc alloys as there are few data in the literature regarding these alloys for the temperature and heat-treatment conditions tested [l, 4,5]. The aim of this work was to establish the relationship between the chemical composition and the heat capacity for the copper-nickel-zinc alloys tested in the temperature range 350 to 850 K.
by Perkin-Elmer corporation. A detailed description of the equipment and the procedure are available elsewhere [12, 131. The Cu-Ni-Zn alloys tested were Cu,NiZn, where the x and y ranges are 1.01 to 4.48 and 0.96 to 1.97, respectively. The specimens have the composition shown in table I. The specimens were disks of 4.5 mm diameter and 1 mm thickness. The specimens were heated in a calorimeter at a rate of 40 K/min (sensivity 17 J/s) and the cooling rate was 20 K/min. Heat treatment of the specimens was performed in pure argon at 1073 K for 3 h followed by furnace cooling or water quenching. The heat capacity was determined during the heating and cooling processes.
3. Results and discussion 2. Experimental The apparatus (Perkin-Elmer DSC-2) used for heat capacity measurements was made available 0378-4363/82/0000-0000/$02.75
@ 1982 North-Holland
Typical curves for the temperature dependence of the heat capacity are shown in fig. 1. The heat capacity curves for the different specimens show a smooth variation with the tem-
M.I. Zsmail and A.S.A.
52
Table I Chemical
composition
Ammar
of specimens
I Order-disorder
transformation
of some Cu-Ni-Zn
alloys
used
Specimen No. Symbol
Cu (at.%)
Ni (at.%)
Zn (at.%)
Average atomic weight
Cu/Ni ratio
Zn/Ni ratio
1 2 3 4 5 6
33.7 50.5 60.3 70.1 60.05 60.1
31.3 23.4 20.1 15.2 25.7 13.4
35.0 26.1 19.6 14.7 14.25 26.5
62.667 62.886 62.926 63.073 62.558 63.377
1.07 2.15 3.00 4.61 2.33 4.48
1.11 1.11 0.97 0.96 0.55 1.97
0 0
. A x x
perature. From figs. 1 and 3, the magnitude of C, of the different furnace-cooled alloys at temperatures up to 500 K is fairly close to each other with a maximum about difference of 2.5 J mol-‘K-r while at about 750 K this difference is increased by sixfold. For the waterquenched specimens (figs. 2 and 4) the reverse is true, i.e. the difference in the C, values at 750 K is lower and at 500 K higher than in the case of the corresponding furnace-cooled specimens. The thermal effects (defined as the maximum C, value divided by the temperature, T,,at which this maximum occurs) are also dependent on both heat treatment and chemical composition.
500
700
Figs. 5 and 6 show that the furnace-cooled specimens mostly have higher values of thermal effects than the water-quenched specimens, particularly for lower Cu/Ni ratio. The thermal effects measured during cooling of quenched alloys increases linearly with the Cu/Ni ratio. The minimum values of these thermal effects increase linearly with an increase of the Cu/Ni ratio of quenched specimens when C, is measured during the heating process, as shown in fig. 6. For lower Zn/Ni values (0.5 in fig. 7) the thermal effects increase with the increase of the Zn/Ni (ratio but no significant change occurs when this ratio exceeds 1.11. The difference between thermal effects measured during heating or cooling of
900
T. K
Fig. 1. Temperature dependence of the heat capacity, C,, furnace-cooled specimens; C, measured during heating: Cu, Ni, Zn:50.5, 23.4, 26.1 at.% (table I, specimen 2-Cu, Ni, Zn:60.3, 20.1, 19.6 at.% (table I, specimen 3- Cu, Ni, Zn :70.1, 15.2, 14.7 at.% (table I, specimen 4 - Cu, Ni, Zn : 60.1, 13.4, 26.5 at.% (table I, specimen 6).
of l2); 3); 4);
ac.0
,
500
700
900
T. K
Fig. 2. Heat capacity-temperature curves for water-quenched specimens; C, measured during heating: 1, 2, 3,4, same as in fig. 1.
M.I. Ismail and A.S.A.
I
Ammar
/ Order-disorder
transformation of some Cu-Ni-Zn
53
alloys
c 0
-_?L _-
---
__c ,g
ii;_
8
35.
L3_--“---\,
a_
/
----a
_T‘+.-+
__-
/
/’
I
Fig. 3. Heat capacity-temperature specimens; C, measured during fig. 1.
/
curves for furnace-cooled cooling: 1, 2, 3, 4, same as in
s
3
specimens for the same heat treatment cooling or water quenching) generally
(furnace increases
c 0(0
2.5 COPPER/NICKEL
with the Cu/Ni ratio (figs. 5 and 6) and with the Zn/Ni ratio (fig. 7). These thermal effects are a result of solid state transformation of phases present or atomic rearrangement or order that occur in the bulk. However, the mechanism of the role played by phases change is complicated. The classification of phases in alloys is based on the factors by Hume-Rothery as well as on the consideration of the space electron correlation [14]. Usually the electron factor and the factor of atomic size difference are meant by the Hume-
.: . _ . . - - .
“r
RATIO
IN ALLOY
Fig. 5. Effect of heat treatment and alloy composition on thermal effects (max CJT, where T, is the temperature which the C, peak occurs): A - furnace-cooled specimens measured during heating; B - water-quenched specimens measured during cooling; C-water-quenched specimens measured during heating; D-furnace-cooled specimens: measured during cooling.
the at : C, : C, : C, C,
I 1
500
700
900
0
5.0
2.5 COPPER/NICKEL
RATIO
IN ALLOY
T. K
Fig. 4. Heat capacity-temperature specimens; C’ measured during fig. 1.
curves of water-quenched cooling: 1, 2, 3, 4, same as
Fig. 6. Minimum CJT, (where TP is the temperature which C, is minimum) vs. the Cu/Ni ratio in the alloy: same as in fig. 5.
at C,
MI.
54
1 0.5
Ismail and A.S.A.
1.0
Ammar
1.5 ZINC/NICKEL
I Order-disorder
2.0
RATIO
Fig. 7. Effect of heat treatment and the Zn/Ni ratio in alloy on the thermal effects (C,lT,): A, B, C, D, same as in fig. 5.
Rothery factors. However, the nature of the structural variety of the phases is not simple [l-5,15]. A sharp fall in the C, vs. T curve was observed in the temperature range 450 to 550 K (fig. 2) with a minimum. This minimum increases with the increase in the Cu/Ni ratio, particularly for the measurements of specific heat during heating of water-quenched specimens as shown in fig. 6. This heat effect may be explained by the redistribution of the electrons in the transition element (Ni) at high temperatures. The existence of a maximum C, is associated with the onset of atomic ordering. The largest change in the value of the specific heat takes place at the solid state transformation, beginning at about 500 up to 750 K depending on the chemical composition and the heat treatment. The heat capacity of specimen no. 2 (table I) rises comparatively rapidly with temperature in most of the cases studied, particularly for furnace-cooled specimens. It also shows a sharp fall in the value of C, measured during heating of water-quenched specimens (fig. 2). The observed variation of ordering temperature of water-quenched and of furnace-cooled specimens depends on the composition of the specimens. For specimen no. 6,
transformation of some Cu-Ni-Zn
alloys
there are no significant thermal effects on the C, vs. T up to a temperature of about 850 K (fig. 3) as the increase in C, is less than 20% in the range 400 to 850 K. Fig. 8 shows the variation in transition temperature range which is the maximum width of the C, vs. T peak. This transition range of water-quenched specimens is narrower than that of furnace-cooled specimens. This temperature range decreases with the increase in the Cu/Ni ratio and has its maximum value at the Zn/Ni ratio of 1.11 (table I) as shown in fig. 9. Representation of the composition of the tested alloys in a Gibbs-triangle, as shown in fig. 10, indicates that the temperature ranges of transformation become short as the alloy composition becomes closer to copper. The data available for the binary alloys of Cu, Ni, Zn systems showed the existence of ordered phases in the Cu-Zn system. The well-known superstructures are formed in bee, fee and hcp lattices [16]. One phase structure was claimed to result from annealing for 6 h at 1053 K of Cu-Ni-Fe-Zn spring alloy [17] which is similar to some of the alloys of this work. The experimental data in this work are in reasonable agreement with the phase diagrams for Cu-Ni-Zn published recently [I, 41. Several models of order-disorder kinetics are available in the literature [18-241; e.g. chemical 500
--
A
Y
2.5 COPPER/NICKEL
Fig. 8. Variation of solid state transformation temperature range with the Cu/Ni ratio in alloys and heat treatment: A, B, C, D, same as in fig. 5.
M.I. Ismail and A.S.A.
Ammar
/ Order-disorder
iransformation of some Cu-Ni-Zn
alloys
order/disorder process is more complicated nature and not yet fully understood.
4. Summary
f . 1.0
1.5
2.0
ZINC/NICKEL
Fig. 9. Variation of solid state transformation temperature range with the Zn/Ni ratio in the alloys: A, B, C, D, same as in fig. 5.
reaction rate equations [18], or stochastic equations of Markovian nature, with the rates of atomic interchanges for a simple atomic model written in terms of a set of thermodynamic parameters [19,20], or the phenomenological diffusion equations of Onsager’s type, which is a of local function concentration [21,22]. However, as shown above, the resolution of the heat capacity data is a difficult task and the COPPER
\
/ ZINC
ATOMIC
\ NICKEL
50 %
Fig. 10. Triangular plot of alloy composition indicating areas of minimum and maximum temperature range of transformation and maximum Cr.: I-area of minimum transformation temperature range and minimum C, value; IIarea of maximum tratisformation temperature range and maximum C, value. Dotted lines are approximate phase diagram according to refs. [l, 41.
in its
and conclusions
The heat capacity,
0 0.5
5.5
C,, measurements during heating or cooling of some heat-treated Cu-NiZn alloys was studied. Specimens were annealed at 1073 K for 3 h in argon atmosphere followed by either furnace cooling or water quenching. The role of heat treatment and composition on the specific heat and ordering of alloys was studied. (1) The temperature dependence of the specific heat of Cu-Ni-Zn alloys was determined in the temperature range 350 to 850 K in both heating and cooling processes. (2) The alloys of composition near to CuzNiZn is the most probable ordered phase in the CuNi-Zn alloy system studied as it has the sharpest maxima in the heat capacity-temperature curves (figs. l-4) and the minimum value of the maximum CJT, for all the heat treatments studied (figs. 5-7) where T, is the temperature at which C, has the maximum value. (3) The ordering temperature, T,, of quenched alloys (at which the C, peak occurs) mostly decreases with the increase in the Cu/Ni ratio in the Cu-Ni-Zn alloy system. Quenched alloys usually have higher ordering temperatures but are characterized by lower C, peak values (figs. l-4). The quenched alloys usually have excess vacancies which result in an initiation of ordering process. (4) Heating of quenched alloys showed a minimum value of C, at lower temperatures which is an indication of an exothermic process, i.e. the atomic ordering of the specimen is disordered by quenching (fig. 2). (5) The critical values of CAT, (maximum C, or minimum C,) increase with the increase of Cu/Ni ratio. However, there is a minimum value at the Cu/Ni ratio of 2.15 (fig. 8). (6) For a lower Zn/Ni ratio of 0.55 in the
56
M.I. Zsmail and A.S.A.
Ammar
1 Order-disorder
Cu-Ni-Zn system, the minimum values of CJT, were observed at all the heat-treatment conditions (fig. 7). (7) The difference between critical thermal effects, CJT,,measured during heating or cooling of annealed alloys increases with the Zn/Ni and Cu/Ni ratios. But during cooling, these thermal effects have higher values for the higher ratios of Cu/Ni or Zn/Ni.
Acknowledgements
One of the authors (A.S.A.A.) would like to thank the Hungarian Cultural Centre for offering him a fellowship. The facilities offered by the Central Institute for Physics and Metal Physics Lab (Budapest), where the experimental work was done, are gratefully acknowledged.
References
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