Resistometric study of SRO-kinetics in α-AgAl

AE~ mcru//. Vol. 31. NO. 10. pp. 1543-1547, 1983 printed in Great Britain. All rights reserved

RESISTOMETRIC

Oool-6160/83 $3.00 +O.OO Copyright Q 1983 Pergamon Press Ltd

STUDY OF SRO-KINETICS IN wAgAl

P. MEISTERLE Institut I’iir Fcstkiirpcrphysilj, Univcrsitlt

and W. PFEILER Wicn. Strudlhofgassc 4, A-1090 Vienna, Austria

(Reccirecl 5 March 1982: in n~ri.wd /i)rrn 29 April 1983)

Abstract-Short-range order (SRO) kinetics of Ag-(3.5. 7.5. I IS. 15.5)at.%Al have been investigated by measurement of electrical resistivity during isochronal and isothermal annealing. Resides an increase of the atomic mobility with increasing Al-content the samples show a reduction of resistivity with increasing degree of SRO. After a small and sudden temperature change the new equilibrium state of SRO is adjusted by a simple relaxation process with an activation energy of 1.37 + 0. I eV. The microstructure of short-range ordered a-AgAI is thercforc assumed to be homogeneous. R&urn&-Nous avons Ctudie la cinCtique de I’ordre a courte distance (OCD) dans Ag-(3,5; 7.5; I l,5; 15.5) at.% Al par mcsure de la tisistivitk ilcctrique au tours d’un recuit isochrone et isotherme. En plus d’une augmentation de la mobilit& atomique avec la teneur en Al, les tchantillons p&scntaient une diminution de la rbsistiviti lorsqu’on augmentatit le degr6 d’0.C.D. Aprts un petit et brusque changcment de temgrature, on obtient le nouvel &tat d’0.C.D. B l’iquilibre par un simple phCnom&nc de relaxation, avec une inergie d’activation de I.37 f 0,I eV. Nous supposons done que la microctructurc d’AgA1-u ordonnt a courte distance est homog&re. Zusammenfasaung-Die Nahordnungskinetik von Ag-(3,5; 75; ll,5; 15,s)at.% Al wurde durch Messung des elektrischen Widerstandes bei isochroner und isothermer Auslagerung untersucht. Neben einer Zunahme dcr AtommobilitHt mit zunehmendem Al-Gehalt zeigen die Proben eine Widerstandsabnabme mit zunehmendcm Nahordnungsgrad. Nach ciner pliitzlichen kleinen Tempcratur&&rttng stcllt sich das neue Gleichgewicht in &rem einzigen Relaxationsprozeg mit einer Aktivierungscncrgie von 1.37 f 0.1 eV ein. Die Mikrostruktur von nahgeordnetem a-AgAl kann daher als homogen angcnommen we&n.

INTRODUCTION the Al-rich side of the AgAl phase-diagram was object of many investigations, especially in connection with the study of GP-zones and metastable transition phases, there are only a few papers on the Ag-rich solid-solution of Al in Ag. The only investigations on short-range order (SRO) in a-AgAl are those of diffuse X-ray scattering by Bagdasarian et al. [1] and Rudman [2] showing the occurrence of SRO-microstructure at the concentrations of 11.O and 14.3 at.%Al at 500°C and of 18.5 at.%Al at 460°C respectively. In the past there were also some differences in the location of the phase-boundary to the disordered cubic p-phase between metallographic [3,4] and X-ray methods [5,6]. Since there are very few papers on this system in the literature it seems worth-while to look at a-AgAl by measuring the electrical resistivity after an appropriate temperature treatment. Such measurements in spite of their simplicity have proved to be a useful tool in studying SRO-kinetics, especially in connection with the question of homogeneity or heterogeneity of microstructure [7-91. In addition, important information for the application of other experimental methods (e.g. neutron scattering, calorimetry, stress relaxation) may be supplied. It is the aim of the present paper to report on Whereas

changes in the microstructure of a-AgAl observed by resistivity measurement during isochronal and isothermal annealing after an adequate temperature quench. EXPERIMENTAL Ingots of Ag-3.5 at.% Al, Ag-7.5 at.% Al, Ag-11.5 at.% Al and Ag-15.5 at.% Al were alloyed by high-frequency heating in vacuum below IO-‘mbar in graphite crucibles from 99.999% Ag (Degussa) and 99.99% Al (Vereinigte Aluminiumwerke Ranshofen-Bemdorf). During the whole process of sample preparation great care has been taken to avoid internal oxidation. After rolling sheets of 0.2 mm thickness with several intermediate anneals in vacuum below lo-’ mbar serpentine shaped samples were cut by spark erosion. The thermal treatments were carried out in a bath of silicone oil (Baysilon PN 200) resistivity changes were detected with an accuracy of f3 x lo-’ by a standard potentiometric method relative to a dummy specimen in a bath of liquid nitrogen. To avoid any contact of the samples with water and to prevent the formation of ice on the sample-surface, quenching treatments were performed directly in liquid nitrogen, reducing the quenching rate from about lOOO”C/s during water-quenching to a value of about IOOC/s.

I543

MEISTERLE

I544

;mtl PI~f~II.ER:

SllORT RANGE

ORDER

IN a-&II

I Ihl

(a) T ("C)

0

I

I

50

loo

I

Ix)

200

250

I 300

Tl*C)

Fig. 1. Resistivity change during isochronal annealing of Ag-3.5 at.% Al (0). Ag-7.5 at.% Al (O), Ag-11.5 at.% Al (A) and Ag-15.5 at.% Al (0). normalized to the value at

200°C. The linear increase above 170-200°C is extrapolated to lower temperatures (broken line).

RESULTS AND DISCUSSION After homogenizing the samples for 2 h at 500°C they were quenched in liquid nitrogen, mounted to the sample holder at room temperature and tempered for half an hour at 300°C. Then the samples were isochronally annealed (AT = lO”C, Ar = 15 min) between room temperature and 260°C (280°C for the sample with 15.5 at.% Al). Figure 1 shows the observed change in resistivity normalized to the resistivity value at 200°C for the four concentrations investigated. After an initial decrease of the electrical resistivity, which can be attributed to increasing SRO because of the increasing atomic mobility owing to excess and thermal vacancies, there suddenly begins a regime of almost linear resistivity increase above about 170°C: equilibrium values of resistivity are achieved during the isochronal time intervals of 15 min obviously caused by stable states of microstructure of decreasing degree of SRO. It can be concluded from the isochronal behaviour of resistivity (Fig. 1): (i) A growing degree of SRO causes a reduction of sample-resistivity. (ii) The change of electrical resistivity due to a change in the degree of SRO given by the slope of the straight resistivity curve strongly increases with increasing Al content. (iii) The atomic mobility increases with increasing Al content. For higher Al concentrations the equilibrium curve is reached earlier. Further, above 230°C the equilibrium values of SRO for Ag-(l1.5 and 15.5) at.%Al are altered during the quench to the mea-

r

(h)

W Fig. 2. Sequence of isothermal anneals at various temperatures showing the adjustment of stable equilibrium values of nsistivity. (a) Ag-3.5 at.% Al; (b) Ag-7.5 at.% Al (O), Ag-11.5 at.%Al (A) and Ag-15.5 at.% Al (0).

suring temperature (liquid N2), which leads to a marked curvature of the equilibrium curve. The thermodynamic stability of microstructure favourably is studied by isothermal annealing at various neighbouring temperatures in the range below the minimum of the isochronal curves [7,9]. Figure 2 shows such a sequence of isothermal anneals: as soon as a constant value of resistivity is reached, a small and sudden change of the annealing temperature leads to an altered equilibrium value of resistivity. These stable equilibrium values belong to specific annealing temperatures; they are completely independent of the preceding temperature treatment and of the time interval up to about 50 h. Further, the equilibrium values exactly fit the extrapolation of the linear resistivity change to lower

MEISTERLE and PFEILER:

SHORT RANGE ORDER IN a-AgAi lb)

) -I

-o

-2

a”

-3

.

4a” -5 z

r----- -

- -

- -

(74.2

170

b t k

oAp-(7.5~t.Y.IAt dAg-lll.5ot.%lAl c,Ap-1155at.V.lAl

--L-J 05

Fig. 3. Resistivity change during isothermal annealing at 170°C for a previous temperature decrease of 10°C (ordering). The broken lines give a semi-logarithmic plot of resistivity change.

during isochronal annealing (broken lines in Fig. 1). In contrast to prior measurements by the same method applied to a-CuAI [A, a-FeAl [8] and a-CuZn [9], a semi- logarithmic plot of the resistivity kinetics of all investigated samples yields a single ex~o~ent~~ dependence on time for the establishment of a new SRO~~~b~~ pig. 31.A leastmean-square fit gives the characteristic times of Table 1. The small resistivity changes observed for the lowest Al-content (3.5 at.%) did not allow a careful analysis; therefore only higher concentrations are considered. To prove the single-process kinetics a “crossover” experiment has been carried out (Fig. 4). Isothermal annealing is interrupted at t = f,, before the resistivity has adjusted to its new equilibrium value. Annealing is now continued at an intermediate temperature, for which the resistivity value at the inte~ption represents an ~~tib~urn value. The constant resistivity for t > f,, shows that the system conrinuot*rlyruns through states, which are possible equilibrium states for intermediate temperatures and

temperatures

Table

1. Characteristic times for adjustm~t, accuracy: 10%

cAl (at.%) 7.5

11.5

15.5

Ordering (min)

190 180 170 160

14 30 69 156

;: 57

132

150

-

307

190 180 170 160 150

13 30 64 157

180 170 160 150 140

4 9.5 19 47 -

10

15

160

f(h) to ‘c Fig. 4. “Cross-over” experiment on Ag-I 1.5 at.% Al as an example of one-process kinetics for all concentrations investigated. (a) Continuous ordering isotherm at 180°C. At t = to annealing at (b) 180°C and (c) 160°C is interrupted and continued at an intermediate temperature of(b) 1742°C and (c) 166.7”C. For a detailed description of the method see (7).

that no second process with a different relaxation time acts simultaneously. Approaching SR~equilib~~ values in a single relaxation process suggests a homogeneous microstructure, that means there are no concentration variations in addition to thermal fluctuations. This accordance with the quasichemical concept of solutions [lO-121may be a consequence of the very small difference in the atomic volume between Ag- and Al-atoms. A further hint at the hQrno~~~ of the present SRO-microstructure arises from comparing the isochronal annealing of AgAl-samples (present work) with a-CuZn [9], a material, where a new SRO-equilibrium is adjusted via two exponential processes pig. 5J. For a-CuZn the isochronal curve shows a rather broad rni~rn~ and there is a tem-

SRG-

Disordering (mm)

T W)

‘

5 :: 49 117 6.5 17 ;:

Fig. 5. Resistivity change during isochronal annealing of Cu-20at.%Zn, a material where the establishment of a new SRGequilibrium is reached via two exponential processes (9).

MEISTERLE and PFEILER:

SHORT RANGE ORDER IN z-AgAI

TlV1

190 160 f

1

ord. diwrd 0 . A9-17.5 . A9-III.5 A . A9-IIS. D

2.15

2.20

160 1

170 I

150 I

14

small changes of annealing

temperature

(1CPC) no

from a previous quenching process are present during adjustment of SRO equilibrium. The thermal activation of SRO-kinetics with equilibrium vacancy concentration therefore seems to be something easier than of self-diffusion of the alloy atoms. This is in good agreement with the value of I .25 + 0.1 eV for the activation energy of the twoprocess kinetics in C&(20 and 30) at.% Zn [9]. The increasing mobility with increasing Al-content shown by isochronal (Fig. 1) as well as by isothermal annealing (Fig. 2) similar to a-CuZn can be attributed to changes in the pre-exponential factor of the diffusion coefficient, but may also be caused by a very weak dependence of the activation energy on concentration. An interesting aspect on the miscibility of Ag and Al arises from the ‘“stability” of the Ag-15.5 at.% Al alloy up to at least 190°C: the phase-boundary of the a-p two-phase region seems to lie beyond 15.5 at.% Al, which is in accordance with metallographic [3,4], but in ~ntradiction to meanwhile generally accepted X-ray me~urements [S, 61. This might indicate the metastability of the investigated microstructure at high Al concentrations, simulating thermodynamic stability because of a very long incubation time for the nucleation of the complex p-phase structure. excess vacancies

01.X) Al 0l.W Al ol.XIAl

2.25 103/T

2.30

2.35

,2.40

O/K)

Fig. 6. Arrhenius plot of characteristic ordering times of Ag-7.5 at.% Al (e, 0). Ag-11.5 at.% AI (A, A) and Ag-lf.Sat.%Al (D, IJ) showing the increasing atomic

mobility with increasing Al-content in spite of equal activation energy.

perature interval of about 20°C between the minimum and the straight equilibrium curve, where the resistivity increment continually becomes larger. By contrast, the linear increase for the present a-AgAl samples exactly begins at the resistivity minimum with a sharp bend. Such a behaviour is expected for a single exponential time-dependence, when the atomic mobility at increasing annealing temperatures for the first time is high enough to achieve an eq~lib~um state of micros~cture within the isochronal time interval. On the other hand, for two different processes simultaneously determining SROkinetics the isochronal curve after its minimum at first must show an increase with a growing rate until the slower of the two processes also finishes within the isochronal time interval, thus reaching the constank rate of the equilibrium curve [13]. An Arrhenius plot of the characteristic times of Table 1 (Fig. 6) gives the activation energy for the present SRO-kinetics (Table 2). Independent of the direction of temperature change (ordering or disordering) a value of 1.37 + 0.1 eV is common to all concentrations investigated, it lies quite below the value for self-diffusion of about 1.80 eV 114). Because

Table 2. Activation energy of SRO adjustment from Fig. 6, accuracy: +O.l eV cAl(aW

of very long annealing times above the temperature, where free vacancies become mobile and of only

Disordering Q (eV)

7.5 11.5

I .40 I .36

t 5.5

1.36

Ordering

Q &V) I .40 I.31 1.41

CONCLUSIONS (i) Isochronal annealing of a-AgAI shows that the establishment of SRO is accompanied by a reduction of electrical resistivity. An almost linear increase of resistivity indicates stable states of mi~rost~cture. (ii) The equilibrium values of resistivity observed by isothermal annealing at various neighbouring temperatures lie on the extrapolation of the increasing part of the isochronal curve to lower values. (iii) The equilibrium values of SRO are adjusted by one smgle exponential process with an activation energy of 1.37 -I_0.1 eV. A further hint at singleprocess kinetics arises from the sharp transition from decreasing resistivity to an almost linear increase during isochronal annealing. The microstructure of SRO is therefore assumed to be homogeneous. Ack~o~fed~e~enis-The continued interest of Professor Dr K. Lintne; is gratefully acknowledged. The authors are indebted to Dr K. Siebinaer and Dr M. Zehetbauer for valuable discussions and to Doz. Dr H.-P. Kamthaler for a critical reading of the manuscript. The work was financially supported by the Austrian “Fonds zur FGrderung der wissenschaftlichen Forschung, Projektnummer 4134”.

REFERENCES 1. R. I. Bagdasaryan et CI!..IZW.Akad. Nauk. Arm. SSR. Fiz. IO, 372 (1975).

MEISTERLE

and PFEILER:

SHORT

2. P. S. Rudman, SC. D. thesis, Massachusetts Institute of Technology. MA (1955). 3. W. Hume-Rothery, G. W. Mabbott and K. M. Channel-Evans, Phil. Tram. R. Sot. (A) 233, I (1934). 4. W. Hume-Rothery and G. V. Raynor, J. Sri. Insrrwz. 18, 76 (1941). 5. F. Foote and E. R. Jctte, Trans. Am. Inst. Min. Engrs 143, 151 (1941). 6. E. A. Owen and D. P. Morris, J. Insr. 14Je1ul.s 76, I45 ( I949/50). 7. L. Trieb and G. Veith. ACIN rwdall. 26, I85 (1978).

RANGE

ORDER

IN a-AgAl

1547

8. F. Adunka, M. Zehetbaucr and L. Trieb, Phy.ka status .didi (a) 62. 213 09801. 9. D. Tr&erind G. Pfkiler, J. Phys. F, Metal phys. 13, 739 (1983). IO. 1. I. Kidin and M. A. Shtremel, Phys. Met. MeraN. 11, I (1961). I I. H. E. Cook, J. Ph_w. Cltem. So&& 30, 2427 ( 1969). 12. S. Radelaar, J. Phys. Cbn. So&& 31, 219 (1970). 13. D. Trattner and W. Pfeiler, Scrip& meraIl. To be published. 14. R. E. Hoffman, D. Tumbull and E. W. Hart, Am mefall. 3, 417 (1955).