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ORDERING IN CuNiZn ALLOYS
P. Bronsveld, W. Alsem, E. van Royen, S. Radelaar
To cite this version:
ORDERING
IN
CuNiZn
ALLOYS
P. M. BRONSVELD, W. H. M. ALSEM, E. W. VAN ROYEN and S. RADELAAR (*) Metal Physics Laboratory, Materials Science Centre, University of Groningen, Groningen, The Netherlands
Resume.
-
Par microscopie electronique et en mesurant la variation de rbistivitk les auteurs ont ttudie quatre alliages de structure c.f.c., de composition Cu,NiZn, Cu3Zn, Ni,Zn et CuNi,Zn.La cinetique de grossissement isotherme de domaines ordonnb dans Cu,NiZn suit une loi = Ktn avec n voisin de 0,3. 11 n'existe pas d'ordre a grande distance dans les alliages Cu3Zn et Ni3Zn m&me aprb de longs recuits de mise en ordre aux temptratures de 61 et 110 oC respectivement. Les micro- graphies en champ sombre fondamental de I'alliage CuNi,Zn prksentent un fort cc clustering )).
Abstract. - Electron microscopy and electrical resistivity have been used in a comparative study of Cu,NiZn, Cu,Zn, Ni3Zn and CuNi,Zn.
Quantitative measurements on the kinetics of domain growth in ordered .Cu,NiZn have been made and a t1j3 law is observed for the growth process. Alloys near the composition Cu3Zn and Ni,Zn do not show long range order even after long anneals at 61 and 1 10 OC respectively. Dark field micrographs of CuNi,Zn using a fundamental reflection indicate strong clustering.
1. Introduction. - In a number of f.c.c. CuNiZn alloys ordering and clustering have been investigated. As is well known from existing data on the three corresponding binary alloys clustering is present in CuNi, while both CuZn and NiZn exhibit ordering.
A neutron diffraction experiment by Vrijen et al. [l] revealed that below 425 OC a L1, superstructure exists in the alloy Cu2NiZn, such that Zn and Ni each occupy one of the four simple cubic sublattices while Cu occupies the remaining two. In the temperature range 425-475 a L12 structure is stable in which Zn still occupies one sublattice exclusively while Cu and Ni occupy the remaining three sublattices. An electron microscopic investigation [l] has confirmed the long- range ordered structure : superlattice reflections and ordered domains separated from each other by so called antiphase boundaries or APB's have been observed. The APB's are characterized by an APB displacement vector and by an APB plane. For thermal APB's these quantities are 112 a, ( 11 0 ) and { 100 ) respectively. If the APB plane contains the displacement vector one calls the APB of type I and otherwise of type 11. It can easily be checked that in the case of type 1 boundaries the nearest neighbors d o not change and the boundary energy is zero if only nearest neighbor interactions are taken into account. In a previous study on Au,Cu [2] it was observed that type I boundaries prevail after long anneals.
However the ternary ordered Cu2NiZn can not be described by pair interactions consistent with the behaviour of the corresponding binary alloys. This may be seen by considering a nearest-neighbor CuNi
(*) Physical Laboratory, University of Utrecht.
pair in the completely ordered L1, situation. From experiments on the binary CuNi (Vrijen et al., this conference) one knows that it has a clustering interac- tion in the first shell, W,.c,,i 3 0 and a smaller
ordering behaviour in the second shell, W2,,,i < 0 with
I
W,I
>I
W2I.
Interchanging a
Cu
and a Ni atom in Cu2NiZn gives an energy changeTherefore by destroying the CuNi ordering in the Ll, superstructure one decreases the energy, and of course also increases the entropy, resulting in a decrease in the free energy. To stabilize the Cu2NiZn structure more body interactions have to be included. In this respect it is of interest to determine experimentally how far the ordering region extends around the compo- sition Cu2NiZn in the ternary CuNiZn phase diagram. We report here on the results of such a study using electron microscopy and electrical resistivity measure- ments. A parallel study of the Zener-relaxation effect in cc-CuNiZn alloys is also underway (van der Veen
et a[., to be published).
2 . Experimental technique. - Starting material for the electron microscopic study as well as for the elec- trical resistance measurements were strips rolled to a thickness of 40 pm, approximately 4 mm wide and 12 cm long. These strips were encapsulated under vacuum in a narrow tube of fused quartz and annealed .at 930 OC for three days. Subsequently the samples were kept in a N 2
+
3%
H2 atmosphere at 800 OC for less than a minute and quenched in water with aORDERING IN CuNiZn ALLOYS C7-029
quenching rate of 7 x lo4 degrees per s. This pro- cedure should keep Zn evaporation as low as possible while on the other hand a large number of excess vacancies are frozen in. For a vacancy formation energie of Q = 1 eV/at about lo-' vac/at may be assumed to be present. The strips for the electron microscopic measurements were encapsulated for a second time under vacuum in fused quartz and annealed during 70 min, 10 h and I00 h at temperatures of 350,400 and 450 OC in a salt bath. The composition of the strips after these thermal treatments was
Cu
: Ni : Zn = 52 : 25 : 23 (at%).
A jet polishing method was applied for the electro- chemical thinning [I]. The electron microscope was a Philips EM 300 used at I00 kV.
The resistance measurements were made in a horizontal furnace under a He
+
3%
H, atmosphere. The furnace was first heated to the desired temperature then the sample, mounted on a resistance measuring tube, was brought from a water cooled entrance section into the hot middle section. It took 0.15 h before the furnace temperature was in equilibrium again (AT < 1 K). The resistance was measured with a Thomson bridge and normalized with respect to the resistance at 0.15 h.3. Results and discussion. - For the electron microscopic observations use was made of a 30 pm selected area diaphragm corresponding to 1 pm diaphragm at the sample. In a 1 pm grain there are as many as lo4 domains of on the average 100 A. A diffraction pattern giving the orientation of the grain still is an average of about 1 O4 domains. Each domain has occupation probabilities for Cu, Ni and Zn. We introduce two Bragg-Williams long-range order para- meters to describe the ordered state. One describes the ordering of Zn and is given by :
with rzn : fraction of Zn sublattice actually occupied by Zn atoms ;
xzn : concentration Zn in the alloy.
To describe the ordered structure of the CuNi we make the assumption that the Zn-ordering dominates the CuNi ordering, as is borne out by the fact that the L1, structure remains when the L1, structure disappears. The CuNi ordering takes place on the remaining three sublattices except the positions occupied by wrong Zn atoms (while extra available lattice sites for Cu and Ni on the Zn sublattice should be ignored for the description of the CuNi ordering). Furthermore we make the assumption that the wrong Zn-atoms are randomly distributed over the Cu and Ni sublattices. This leads to the following parameter, describing the CuNi ordering in the system
with r , : fraction of
Cu
on the available Cu sub- lattice sites ; i.e. the sites not occupied by Zn-atoms.xLU : concentration on the available CuNi sub- lattice sites.
These definitions are not exactly the same as in [l] but they are physically identical with them.
The occupation probabilities of all sublattice sites can now be expressed in S' and S". If we start with a total of N atoms located according to a L1, structure we find on the 114 N Zn-sublattice sites :
1 /4 N(1/2 - 112 S") Cu-atoms ,
114 N(1/4
-
114 S") Ni-atoms and1 /4 N(3/4 S "
+
114) Zn-atoms.
Similarly on the 114 N Ni-sublattice sites one expects :
114 N(2/3
-
213 S') (314+
114 S") Cu-atoms ,114 N(2/3 S f
+
113) (314+
114 S") Ni-atoms and114 N(1/4
-
114 S") Zn-atoms.
Finally on the 112 N Cu-sublattice sites one finds : 112 N(1/3 S f
+
213) (314+
114 S") Cu-atoms ,112 N(1/3
-
113 S') (314+
114 S") Niiatoms and112 N(1/4 - 114 S") Zn-atoms
.
The intensity of the diffraction spots is proportional to the structure factor of the unit cell squared :
with g : diffraction vector ;
rj : vector indicating the position of atom j in the unit cell ;
&(g) : atomic scattering factor of atom j for a certain g.
Assuming the L1, structure as described pre- viously [I] and introducing g = hb,
+
kb,+
l b 3 , bi being the reciprocal lattice vectors, we write~ ( g )
=
F(hk1) =An
+
fii exp {-
in(h+
k)f
+
+
f&[exp { - in(k+
1) )+
exp { - in(h+
1)11
where we have used average scattering factors cal- culated by multiplying the atomic scattering factors and the occupation probabilities expressed in terms of S" and St.Using the data for the scattering factors tabulated by Hirsch [3] we calculated for { F(g)
l2
the values as given in table I.TABLE I
Values for { F(g)
l2
as a function of the order parameters S' and S"{ F(g)
l2
x lo4S1=1/2, S f = 1 / 4 , St = 0,
sH
= 1str
=0.9su
112Beam g S f = SU.= 1 (350 OC) (400 OC) (450 OC)
- - - -
ooi
loo 225 169 100 25 010 225 169 100 25 110 1 49 16 9 010 100 225 169 100 25 001 1 36 49 25 101 121 8 1 49 9 100 001 1 010 225 011 121 110-
001 1 110 1 101 - 010 225 101 121 011 100 225oio
121350, 400 and 450 OC, namely 0.29, 0.16 and 0.36 respectively. This is the result assuming one domain within the grain and the tetragonal axis being aligned parallel. to the beam- direction. This confirms the observations in 111 where no 110 sumrlattice reflection
.
.
in the (001) D.P. was found and where a preferential
" .
alignment of the tetragonal axes parallel to the foil A
normal was suggested. If we take the more probable case of the tetragonal axes being distributed equally over the three cubic directions, the following set of ratios 0.56, 0.46 and 0.36 are obtained. Thus the I ,
,,
is about half so intense as the I , , , for the beam direc- tion along a cube axis, and this is in agreement with what we found in the majority of our Cu2NiZn samples.In figure 1 domain patterns are shown for three
:
different temperatures after a 100 h anneal. The domains are made visible with a g = [ l i 0 ] diffractionvector and with the electron beam parallel to the [110]
.
-
direction. Domain sizes were determined by counting .-
the number of intersections of a circle, drawn on the micrographs, with the domain boundaries. The domain size then is given by 2/n l / N in which I is the circum- ference and N is the number of intersections. As one third of the domain boundaries are usually not made visible we have to multiply the resulting number by 312. In figure 2a the logarithm of the domain size versus the logarithm of annealing time is plotted. Through the points belonging to the same temperature the equation
5
= Kt"is fitted, where D i s the average diameter of a domain,
t is the isothermal annealing time, and K and n are
FIG. I . -Three electron micrographs of domain patterns in ordered Cu,NiZn taken after a 100 h anneal at 350,400 and 450 OC
respectively. The sample was previously quenched from 700 O C .
ORDERING IN CuNiZn ALLOYS C7-331
FIG. 2. - ( a ) Domain size and (b) electrical resistivity of Cu,NiZn at three different annealing temperatures as function of annealing
time.
is in contrast with our data on Au3Cu where a strong preference was observed for domain walk aligning parallel to cube planes.
Figure 2h shows the electrical resistance of Cu,NiZn.
Great care was taken to apply the same thermal treatment as was indicated for the domain size measu- rements of figure 2a. It is obvious that the resistance after 100 h at 350 OC is not yet in equilibrium, while 100 h at 400 OC is almost sufficient to reach equi- librium. For 450 OC the resistance started to rise after 30 h. This may very well be an indication that the effect of Zn evaporation on the resistance exceeded the effect due to ordering. The composition was checked after the resistance measurements and similar values were measured as on the samples used for the electron microscopic observations. The large decrease in resistance for all three runs is due to an increase in long-range order as well as due to a decrease in the total area of APB's. A comparison between figures 2a
and b shows that the main reason for the large decrease is the establishment of long-range order.
Data collected by Bartsch and Tehnzen [4] show that there exists a short range order peak at the
beginning of the annealing curves. However this peak will move towards t = 0 at higher temperatures and it may very well be that it occurred during the 0.1 5 h heating period. We also measured the electrical resis- tance of Cu3Zn and Ni3Zn. Annealing curves were recorded after a rapid quench from 775 and 950 OC
respectively and at the rather low annealing tem- peratures of 61 and 110 OC. Both resistance curves showed a continuously increasing resistance. No sign of long-range order could be found on the electron micrographs.
Moreover. two samples of CuNi,Zn were inves- tigated. One was quenched from 1 000 OC and annealed at 400 OC for 24 h and the other was quenched from 845 OC and annealed at 320 OC for 144 h.
A strong tweed structure was observed in both samples with characteristic distances of 100 and 200
A
respectively. Figure 3 gives an example of such a structure. Remarkable are also the dark and light patches. As we have only encountered this effect in CuNi,Zn we tentatively explain this by the assump- tion of Cu and Ni-rich regions being etched away preferentially.
FIG. 3. - Electron micrograph of CuNi,Zn quenched from
1 000 OC and subsequently annealed for 24 h at 400 OC. 4. Conclusion.
-
Electron microscopic observa- tions and electrical resistance measurements of CuNiZn alloys show long-range order in Cu,NiZn and clustering in CuNi,Zn. Cu3Zn and Ni3Zn do not show long-range order at temperatures as low as 61 and 110 OC respectively. These findings are in agreement with the sketched phase diagram of CuNiZn by Thomas [5], mainly based on electrical resistivity databy Koster and Storing [6] and on calorimetric measure-
ments by Kuszmann and Wollenberger [7]. References
[I] VRIJEN, J., BRONSVELD, P. M., VAN DW VEEN, J. and RADE- [4] BARTSCH. G . and TEHNZEN, B., Metallwissenschafi und Technik LAAR, S., Z. Metallkd. 67 (1976) 473. IS (1961) 127.
[2] BRONSVELD, P. M. and RADELAAR, S., J. Phys. Soc. Japan 38 (51 THOMAS, H., 2. Metallkd. 63 (1972) 106.
(1975) 1336. [6] KOSTER, W. and STORING, R., Z. Meta'llkd. 54 (1963) 182.
[3] Electron Microscopy of Thin Crystals (Buttenvorths, London) [7] KUSZMANN, A . and WOLLENBERGER, H., 2. Metallkd. 50 (1959)
