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The retro-effect in CuAu and its kinetic limitations
Intermetallics 8 (2000) 81±83
Short communication
The retro-eect in CuAu and its kinetic limitations B. SprusÏ il a,*, W. Pfeiler b a
Department of Metal Physics, Charles University, Ke Karlovu 5, CZ-12116 Praha 2, Czech Republic b Institut fuÈr Materialphysik, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria Received 27 July 1999; accepted 27 July 1999
Abstract The retro-eect in CuAu is limited to a certain interval of heat-treatment conditions. A model is proposed to explain this behaviour. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Order/disorder transformations; B. Phase transformations; B. Electrical resistance and other electrical properties
1. Introduction The practical interest in modern intermetallic materials and their investigation with respect to order* )disorder transformations or order* )order relaxations, experimentally as well as theoretically, has brought about a renewed interest in the phase transformations of the very classical ordering prototype material Ð the Au±Cu system. In recent years several works dealed with the socalled retro-eect in the stoichiometric CuAu alloy [1±4], which is observed upon heating of an ordered sample. It consists in lowering the temperature which marks the disappearance of the long-range order with increasing heating velocity. This eect was observed just for certain heat-treatment conditions. It is the aim of this paper to clarify the limiting conditions for its observation. 2. Experimental data and their analysis In Fig. 1 one ®nds an example of the retro-eect as observed by resistometry [3]. From the temperature (T) dependence of resistance R(T) we have calculated the derivative of this function T
1 dR : R T dT
1
* Corresponding author. Tel.: +420-2-2191-1365; fax:+420-2-21911490.
For a sample disordered by annealing 15 min at 480 C and then cooled to room temperature with cooling rate C =2 K/min, the T-curves observed upon heating by dierent heating rate H =0.5, 1, 2, 5, 10 and 20 K/min are shown in Fig. 1. For the high heating rates (e.g. H =20 K/min) a single peak is observed on the T-curve with a maximum at 412 C (1.5 K). For the low heating rates (e.g. H =0.5 K/min) one gets two peaks; the main one is shifted up to 422 C, whereas another peak emerges with a maximum at about 400 C. Similar curves represent the retro-eect in DSC (differential scanning calorimetry) measurements Ð where, however, just the main peak is unequivocal; see ®g. 2 (taken from [2]) or ®g. 2(b) in [4]. The extensive DSC study of this behaviour [4] has revealed, however, that the position of the main peak does depend on the heating rate just for certain values of the rate of the preceding cooling (see Table 1). In a series of experiments the specimen was C =1, 2, 5, 10, 20 K/min, to room temperature; from each of these states it was then heated with three dierent rates H =2, 5, 10 K/min. Table 1 gives the temperature of the maximum of the main peak for dierent combinations of the cooling and the heating rate. For cooling rates c =2 and 5 K/min the peak temperature diminishes with increasing heating rate (the retro-eect). For cooling rates 1, 10 and 20 K/min, however, the peak temperature does not depend on the heating rate and one asks why the retro-eect is absent under these conditions. It seems that there exists a lower limit (4 408 C) and an upper limit (5 415 C) to the peak temperature. One
0966-9795/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00076-X
82
B. SprusÏil, W. Pfeiler / Intermetallics 8 (2000) 81±83
®nds a certain range of heat-treatment conditions (cooling and heating rates) which allows for the observation of the retro-eect. Outside this range the peak remains in its lower or upper position. For resistometric data the lower and the upper limits of the main peak temperature have slightly dierent value (414 and 5422 C, respectively; see Fig. 1). Several reasons might be responsible for this dierence and we are unable to check their actual contribution. With the idea of the lower and the upper limit of the main peak temperature in mind we have analyzed the experimental data published in [1±4] as a function of the rate of (preliminary) cooling and the rate of (subsequent) heating. The result of this analysis is given in Table 2, which was constructed under the observation of following rules: . a letter was ascribed to a given set of data ([A] for [3], Fig. 3, resistometry; [B] for [3], table 4, resistometry; [C] for [1], cyclic resistometry, [Y] for [4], Table 2, DSC [Z] for [4], Fig. 1, DSC and [2], DSC);
. this letter is written as upper-case (lower-case) when the upper (lower) limit of peak temperature was attained for a given combination of cooling/ heating rate. The data in Table 2 seem to indicate the tendency of the upper-case letters to concentrate in the upper-right part, of the lower-case letters to concentrate in the lower-left part of the Table. It is therefore tempting to de®ne the following two ``ideal'' combinations of heat treatment Ð the ``upper'' one (with in®nitely high cooling rate and in®nitely low heating rate) and the ``lower'' one (in®nitely slow cooling followed by in®nitely quick heating). Under the ``ideal upper'' heat treatment combination the main peak observed upon heating should correspond to the disordering transformation of the orthorhombic CuAu II state. A disordering transformation of the tetragonal CuAu I would then correspond to the main peak under the ``ideal lower'' combination of heat treatment. This tentative model of the retroeect does not contradict our indirect experimental evidence and general thermodynamic expectations.
Table 1 Estimated peak temperatures; accuracy0.3 K
Fig. 1. The temperature dependence of (see text) for dierent heating rates (in K/min) (Fig. 3 of Ref. [3]).
Cooling rate/heating rate [K/min]
Peak temperature [ C]
1/2 1/5 1/10 2/2 2/5 2/10 5/2 5/5 5/10 10/2 10/5 10/10 20/2 20/5 20/10
408.8 407.8 408.1 415.0 414.1 410.6 415.0 413.8 413.8 415.1 415.0 415.0 415.0 415.0 415.2
Table 2 Peak temperature limits attained for dierent combinations of cooling/heating rate (K/min)
Fig. 2. Heat ¯ow vs. temperature for dierent heating rates (in K/min) (Fig. 1 of Ref. [1])
Heating/cooling rate (K/min)
0.2
0.2 0.5 1 2 5 10 20
Z
0.5
1
Z y y c,y
2
5
10
20
A C Y
Y
Y Y B,Y
Y Y Y z
a a
B
B. SprusÏil, W. Pfeiler / Intermetallics 8 (2000) 81±83
3. Conclusions The retro-eect in stoichiometric CuAu takes place in a ``border-region'' between two types of heat treatment conditions, represented by very high cooling rate followed by very low heating rate (the ``upper'' one) on the one hand and of very slow cooling followed by very rapid heating (the ``lower'' one) on the other hand (Table 2). In this region the disordering transformation changes with increasing heating rate from one type (the ``upper'' one) into another type (the ``lower'' one). We propose a tentative model, in which the ``lower'' type is
83
due to CuAu I! disorder, whereas the ``upper'' type is due to CuAu II ! disorder transformation. References [1] SprusÏ il B, SÏõÂma V, Chalupa B, Smola B. Z Metallkde 1993;84:118±23. [2] Chalupa B, ChmelõÂk F, SprusÏ il B, Spanl M, Lang H, Pfeiler W. Mat Res Soc Symp Proc 1996;398:581±6. [3] SprusÏ il B, Pfeiler W. Intermetallics 1997;5:501±5. [4] Spanl M, SprusÏ il B, Pfelier W. In: Nathal MV et al., editors. Structural intermetallics. Warrendale, USA: The Minerals, Metals & Materials Society, 1997, p. 83±90.
Short communication
The retro-eect in CuAu and its kinetic limitations B. SprusÏ il a,*, W. Pfeiler b a
Department of Metal Physics, Charles University, Ke Karlovu 5, CZ-12116 Praha 2, Czech Republic b Institut fuÈr Materialphysik, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria Received 27 July 1999; accepted 27 July 1999
Abstract The retro-eect in CuAu is limited to a certain interval of heat-treatment conditions. A model is proposed to explain this behaviour. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Order/disorder transformations; B. Phase transformations; B. Electrical resistance and other electrical properties
1. Introduction The practical interest in modern intermetallic materials and their investigation with respect to order* )disorder transformations or order* )order relaxations, experimentally as well as theoretically, has brought about a renewed interest in the phase transformations of the very classical ordering prototype material Ð the Au±Cu system. In recent years several works dealed with the socalled retro-eect in the stoichiometric CuAu alloy [1±4], which is observed upon heating of an ordered sample. It consists in lowering the temperature which marks the disappearance of the long-range order with increasing heating velocity. This eect was observed just for certain heat-treatment conditions. It is the aim of this paper to clarify the limiting conditions for its observation. 2. Experimental data and their analysis In Fig. 1 one ®nds an example of the retro-eect as observed by resistometry [3]. From the temperature (T) dependence of resistance R(T) we have calculated the derivative of this function T
1 dR : R T dT
1
* Corresponding author. Tel.: +420-2-2191-1365; fax:+420-2-21911490.
For a sample disordered by annealing 15 min at 480 C and then cooled to room temperature with cooling rate C =2 K/min, the T-curves observed upon heating by dierent heating rate H =0.5, 1, 2, 5, 10 and 20 K/min are shown in Fig. 1. For the high heating rates (e.g. H =20 K/min) a single peak is observed on the T-curve with a maximum at 412 C (1.5 K). For the low heating rates (e.g. H =0.5 K/min) one gets two peaks; the main one is shifted up to 422 C, whereas another peak emerges with a maximum at about 400 C. Similar curves represent the retro-eect in DSC (differential scanning calorimetry) measurements Ð where, however, just the main peak is unequivocal; see ®g. 2 (taken from [2]) or ®g. 2(b) in [4]. The extensive DSC study of this behaviour [4] has revealed, however, that the position of the main peak does depend on the heating rate just for certain values of the rate of the preceding cooling (see Table 1). In a series of experiments the specimen was C =1, 2, 5, 10, 20 K/min, to room temperature; from each of these states it was then heated with three dierent rates H =2, 5, 10 K/min. Table 1 gives the temperature of the maximum of the main peak for dierent combinations of the cooling and the heating rate. For cooling rates c =2 and 5 K/min the peak temperature diminishes with increasing heating rate (the retro-eect). For cooling rates 1, 10 and 20 K/min, however, the peak temperature does not depend on the heating rate and one asks why the retro-eect is absent under these conditions. It seems that there exists a lower limit (4 408 C) and an upper limit (5 415 C) to the peak temperature. One
0966-9795/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00076-X
82
B. SprusÏil, W. Pfeiler / Intermetallics 8 (2000) 81±83
®nds a certain range of heat-treatment conditions (cooling and heating rates) which allows for the observation of the retro-eect. Outside this range the peak remains in its lower or upper position. For resistometric data the lower and the upper limits of the main peak temperature have slightly dierent value (414 and 5422 C, respectively; see Fig. 1). Several reasons might be responsible for this dierence and we are unable to check their actual contribution. With the idea of the lower and the upper limit of the main peak temperature in mind we have analyzed the experimental data published in [1±4] as a function of the rate of (preliminary) cooling and the rate of (subsequent) heating. The result of this analysis is given in Table 2, which was constructed under the observation of following rules: . a letter was ascribed to a given set of data ([A] for [3], Fig. 3, resistometry; [B] for [3], table 4, resistometry; [C] for [1], cyclic resistometry, [Y] for [4], Table 2, DSC [Z] for [4], Fig. 1, DSC and [2], DSC);
. this letter is written as upper-case (lower-case) when the upper (lower) limit of peak temperature was attained for a given combination of cooling/ heating rate. The data in Table 2 seem to indicate the tendency of the upper-case letters to concentrate in the upper-right part, of the lower-case letters to concentrate in the lower-left part of the Table. It is therefore tempting to de®ne the following two ``ideal'' combinations of heat treatment Ð the ``upper'' one (with in®nitely high cooling rate and in®nitely low heating rate) and the ``lower'' one (in®nitely slow cooling followed by in®nitely quick heating). Under the ``ideal upper'' heat treatment combination the main peak observed upon heating should correspond to the disordering transformation of the orthorhombic CuAu II state. A disordering transformation of the tetragonal CuAu I would then correspond to the main peak under the ``ideal lower'' combination of heat treatment. This tentative model of the retroeect does not contradict our indirect experimental evidence and general thermodynamic expectations.
Table 1 Estimated peak temperatures; accuracy0.3 K
Fig. 1. The temperature dependence of (see text) for dierent heating rates (in K/min) (Fig. 3 of Ref. [3]).
Cooling rate/heating rate [K/min]
Peak temperature [ C]
1/2 1/5 1/10 2/2 2/5 2/10 5/2 5/5 5/10 10/2 10/5 10/10 20/2 20/5 20/10
408.8 407.8 408.1 415.0 414.1 410.6 415.0 413.8 413.8 415.1 415.0 415.0 415.0 415.0 415.2
Table 2 Peak temperature limits attained for dierent combinations of cooling/heating rate (K/min)
Fig. 2. Heat ¯ow vs. temperature for dierent heating rates (in K/min) (Fig. 1 of Ref. [1])
Heating/cooling rate (K/min)
0.2
0.2 0.5 1 2 5 10 20
Z
0.5
1
Z y y c,y
2
5
10
20
A C Y
Y
Y Y B,Y
Y Y Y z
a a
B
B. SprusÏil, W. Pfeiler / Intermetallics 8 (2000) 81±83
3. Conclusions The retro-eect in stoichiometric CuAu takes place in a ``border-region'' between two types of heat treatment conditions, represented by very high cooling rate followed by very low heating rate (the ``upper'' one) on the one hand and of very slow cooling followed by very rapid heating (the ``lower'' one) on the other hand (Table 2). In this region the disordering transformation changes with increasing heating rate from one type (the ``upper'' one) into another type (the ``lower'' one). We propose a tentative model, in which the ``lower'' type is
83
due to CuAu I! disorder, whereas the ``upper'' type is due to CuAu II ! disorder transformation. References [1] SprusÏ il B, SÏõÂma V, Chalupa B, Smola B. Z Metallkde 1993;84:118±23. [2] Chalupa B, ChmelõÂk F, SprusÏ il B, Spanl M, Lang H, Pfeiler W. Mat Res Soc Symp Proc 1996;398:581±6. [3] SprusÏ il B, Pfeiler W. Intermetallics 1997;5:501±5. [4] Spanl M, SprusÏ il B, Pfelier W. In: Nathal MV et al., editors. Structural intermetallics. Warrendale, USA: The Minerals, Metals & Materials Society, 1997, p. 83±90.