JOURNAL DR PHYSIQUE Colloque C7, supplement au n° 12, Tome 38, decembre 1977, page C7-55
ORDERING AND DOMAIN COARSENING KINETICS IN SUBSTITUTED PERMALLOYS
H. FERJANI, F. BLE Y and Y. CALVAYRAC Laboratoire de Métallurgie Structurale des Alliages Ordonnés, E.N.S.C.P. 11, rue Pierre-et-Marie-Curie 75231 Paris Cedex 05, France
Résumé. — Nous avons étudié la mise en ordre dans des alliages ternaires à base de Ni3Fe avec de faibles additions de Cr, Mo, Mn ou Al. L'influence de l'addition sur la température critique et sur les cinétiques isothermes de mise en ordre et de croissance des domaines ordonnés a été étudiée par diffraction des rayons X.
Abstract. — We have studied ordering in ternary alloys based on Ni3Fe with small amounts of Cr, Mo, Mn or Al. The influence of the addition on the critical temperature and on the isothermal kinetics of ordering and domain growth was investigated, by X-ray diffraction.
X-ray diffraction techniques have been used to study ordering in ternary alloys based on Ni3Fe with small additions of Cr, Mo, Mn or Al.
Experimental. — The specimens consist of unidi- rectionnally cold rolled strips, annealed 30 minutes in vacuum at 650 °C to give a small grain size and a high { 100 } < 001 > texture. Then the samples are ordered by isothermal annealing in sealed pyrex tubes. The (100) X-ray superstructure line is recorded by point counting, using CoKa radiation.
Results. — Effect of the addition on the order- disorder transition :
As for Ni3Fe [1] a hysteresis region exists in the neighbourhood of the critical temperature for ternary alloys with Cr, Mo or Mn. So, two transition tempe- ratures must be defined : we call 7cj the disorder to order transition temperature and Tc2 the order to disorder one. Their values are given in table I. The existence of a hysteresis zone is likely due to the impossibility of nucleation of the ordered phase in this temperature range.
For (Ni3Fe + Al) alloys, the phase diagram is different [2] : There is no suppression of nucleation.
TABLE I
Tcj : is the disorder -> order transition temperature Tc2 '• is the order -* disorder transition temperature
Alloy r e , Tc2
Ni3Fe, 75 %, 25 % 503 °C 512 °C Ni3Fe, 0,5 % C r 495 °C 511 °C .Ni3Fe, l , 5 % C r 483 °C 499 °C Ni-Fe-Mo, 75 %, 23 %, 2 % 481 °C 498 °C Ni-Fe-Mo, 73 %, 25 %, 2 % 484 °C 502 °C
Ni3Fe, 1,7 % Mn 520 °C 534 °C
Between the ordered and the disordered phases a two-phase region exists the temperature limits of which we call TcK and TcB (table II).
TABLE II
7cA = is the order ^±
^± order + disorder transition temperature 7cB = is the order + disorder ^±
^ disorder transition temperature
Alloy TcK 7cB
Ni-Fe-Al, 73,5 % 24,5 %, 2 % 540 °C 570 °C Ni-Fe-Al, 75 %, 21 %, 4 % 560 °C 654 °C Ni-Fe-Al, 75 %, 19 %, 6 % 600 °C 746 °C The results of tables I and II show the influence of alloying Ni3Fe with a third element on the critical temperatures : Al or Mn increases them and Cr or Mo lowers them.
— Evolution of the order parameter versus time of isothermal annealing.
We have plotted the ratio of the intensities of the 100 and 200 reflections as a function of annealing time (figure 1). Absolute values of the long range order parameter S cannot be deduced from this ratio, because the texture is not homogeneous. Only the evolution of order on annealing can be followed.
The addition of Al increases the rate of ordering.
That of Cr reduces it. This reduction of ordering rate due to Cr addition is more important when the anneal- ing temperature is closer Tc. This fact seems to be due to a greater difficulty of nucleation. Some preliminary experiments show that the equilibrium degree of order decreases as the annealing temperature increases towards the critical temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1977710
C7-56 H. FERJANI, F. BLEY AND Y. CALVAYRAC
FIG. I . . -- Evolution of the ratio x
%
versus annealingI200 J-PI00
time. For the points 0 , ,the last anneal was made at the other temperature : 470°C for the 478 OC kinetics, and 478 OC for the
470 OC kinetics.
- Evolution of domain size during isothermal annealing.
We determine the domain size by Fourier analysis of the superstructure (100) profile [3]. The variation in structure factor with scattering angle about the Bragg position was taken into account [4]. This correction is a very important one ; a striking effect on the measu- rement of small domain sizes may be seen in Ni3Fe kinetics : In the present work, no two-stage domain growth behaviour is observed, in disagreement with previous works [5, 61. For Ni3Fe and for ternary ailoy Ni3Fe with A1 the coarsening of the ordered domains follows a law of the form ( D )" - D," = kt (figure 2). The exponent n is always greater than 2.
It increases with the concentration of the addition.
The kinetics of isothermal coarsening of the ordered domains, in (Ni3Fe
+
Al) alloys show that the exponent n decreases with temperature[ I .
For (Ni3Fe
+
Cr) alloys a two stage domain growth behaviour is observed (figures 3 and 4). Higher slope at short ageing times cannot be explained by the influence of the Do term. It seems likely that the first stage corresponds to a nucleation and growth stage, and the second stage to a coalescence stage. The value of n obtained for the first stage is 2.0, and for the second stage is 4.6.FIG. 2. - Mean domain size
<
D ) versus annealing time at 480 OC for Ni,Fe and Ni,Fe,,,,Al,,,, alloys.FIG. 3. - Mean domain size ( D ) versus annealing time at 470 OC for Ni,Fe and (Ni,Fe
+
1,5 % Cr) alloys.FIG. 4. - Mean domain size ( D ) versus annealing time at 478 OC for Ni,Fe and (Ni,Fe + 1,5 0/, Cr) alloys.
The addition of A1 or Cr to Ni3Fe slows the kinetics of domain coarsening. We think this is due to a varia- tion of the composition in the vicinity of the anti- phase boundaries [8]. During the first stage, perhaps the ordering kinetics are slowed by inhibited nuclea- tion due to the Cr addition; as a consequence, the stage of nucleation and growth is longer and the time when the ordered domains come into contact may be observed.
References
[I] CALVAYRAC, Y. and FAYARD, M., Mat. Res. BUN. 7 (1972) 891. [6] MORRIS, D. G., BROWN, G. T., PILLER, R. C. and SMALLMAN, [2] BLEY, F. and FAYARD, M., 18e Colloque de Metallurgie, Saclay R. E., Acfa Met. 24 (1976) 21.
1975, p. 405, Ed. C.E.A. [7] BLEY, F., FAYARD, M., Acta Met. 24 (1976) 575.
(31 CHEARY, R. W. and GRIMES, N. W., Acta Crysfallogr. (A) 28 (8) PANIN, V. E. et al., Ordered alloys, Proceedings of the third
(1972) 454. Bolton Landing Conference, Sept 1969 (Claitor's Publish-
[4] BLEY, F., FAYARD, M., J. AppL Cryst. 9 (1976) 126. ing Division, Baton Rouge).
[5] CALVAYRAC, Y. and FAYARD, M., Phys. Status Solidi (a) 17 (1973) 407.
