PHASE DIAGRAM OF THE ORDER-DISORDER TRANSITION IN Ni3Fe

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PHASE DIAGRAM OF THE ORDER-DISORDER TRANSITION IN Ni3Fe

J. van Deen, F. van der Woude

To cite this version:

J. van Deen, F. van der Woude. PHASE DIAGRAM OF THE ORDER-DISORDER TRANSITION IN Ni3Fe. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-367-C1-368. �10.1051/jphyscol:19801138�.

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JOURNAL DE PHYSIQUE Colloque

Cl ,

suppl&ment au

n

^O 1 , Tome

41,

janvier

1980,

page

Cl-367

PHASE DIAGRAM

OF

ME

ORDER-DISORDER TRANSITION IN Ni3Fe

J.K. Van Deen and F. Van Der Woude

Solid s t a t e Physics Laboratory, Materials Science Center, University of Groningen, GRONINGEN, The Netherlands.

1. Introduction. - The phase diagram of the Ni-Fe binary alloy system in the region around the

3:

1 composition has been investigated by means of

MES.

.The NigFe system is a well known example of an al- loy, ordering in the LIZ structure, a superstructure of the fcc-lattice. The transition temperature rea- ches a maximum at 789 K and 27 at.% Fe, slightly away from the stoichiometric

3 : l

composition.

790 780

770

_ordered

Fig. I . Tentative phase diagram of the Ni-Fe system near Ni3Fe.

Because of the first order character of the transition, a two phase region is expected between the ordered and the disordered phase. According to Gibbs' phase rule this two phase area should disap- pear at the

maxjnnnn

transition temperature. X-ray measurements, however, suggested coexistence of or- dered and disordered phases at all compositions bet- tween 22 and 32 at.% Fe /I/. A theoretical interpre- tation of this result was recently proposed by Bar-

tis

/2/.

Another interesting aspect of the order-disor- der transition in Ni3Fe is the occurrence of a broad hysteresis zone in the phase diagram. When a dis- ordered sample is cooled slowly, ordering is sup- pressed over 15

K.

2. Order to disorder reaction i n Ni7e27. - ^{TO set-}

tle the question of the two phase region in the top of the hysteresis zone, the temperature of an orde- red sample was increased in steps of 1 K and 1 week

Fig. 2. Average hyperfine f i e l d us. annealing time.

Details are explained i n the t e x t .

anneals, starting from 787 K (see fig. 2, circles).

The first change in the spectra was observed at 790 K; at the same temperature the reaction came to c m - pletion in 10 days. When the temperature of the corn- pletely disordered material was lowered

1

K no sign

.

of the reverse transformation was observed. In a se- cond experiment, the disordering reaction at

790 K

was interrupted when about 30% of the material had transformed (see fig. 2, triangles]. The temperature was lowered

1

K and now the reaction reversed. Due to the presence of a large amount of ordered mate- rial, nucleation of the ordered phase was not neces- sary, and growth could proceed on the boundaries of the not yet disordered material.

In this way we demonstrated directly that the disordering temperature in Ni73Fe27 is well-defined within 1 K.

This observation is, however, contrary to the findings of Calvayrac and Fayard

/ I / ,

who report a reversible transformation between order and disorder in a

5 K

wide two-phase region, determined by means of electron microscopy and X-ray diffraction.

A

theo- retical eglanation of this violation of Gibbs' phase rule was given by Bartis /2/, who suggests internal stresses to be responsible for a smeared-out transi- tion temperature. Some COlnnentS on Bartis' explana- tion are published elsewhere /3/. In the second place we demonstrated by these experiments that the disordering temperature is also the thermodynamical equilibrium temperature. Any hysteresis between or- dering and disordering processes is caused specifi-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801138

C1-368 ^{JOURNAL DE PHYSIQUE}

cally by suppression of the ordered phase. cia1 support from the "Nederlandse Organisatie voor 3. Order t o disorder reaction i n other compositions. Zuiver Wetenschappelijk Onderzoek" (ZWO).

The ,existence of a two phase region of about 2 K

References wide was confirmed a t the compositions Ni71Fe29 and

Ni75Fe25 (see ref' 14/) ' The order to disorder /1/ Y. Calvayrac and M. Fayard, Mat. Res. Bull.

reaction in Ni77Fe23, where the two phase region i s 11972) 891.

.

^,

a t l e a s t 4 K wide, was studied in more d e t a i l . Six- /2/ F. Bartis, Acts Metall.

26

(1978) 879.

-line f i t s during the disordering run give excessive /31 J.K. van Deen & F. vm der Woude, Phys, Rev.

linewidths of about 1 -10 4 s . This value is much

July 1979.

larger than from the difference in /4/ J.W. k i j v e r , F. van der Woude & S. Radelaar, fine fields between ordered and disordered material.

Phys. Rev.

B16

(1977) 993.

The same effect was found in Ni75Fe25

14/

_{/5/ R.J. Wakelin &} E.L. Yates, Proc. Phys. Soc.

B66

and was attributed t o phase separation which leads (1953) 221.

t o Fe-rich ordered material and Fe-poor disordered material, enhancing the difference between the hy- perfine fields. This interpretation is confirmed by the absence of excessive l i n e broadening in Ni73Fe27, where no phase separation occurs, and i n Ni71Fe29, where the effect of ordering and phase separation partly cancel each other.

&en the outer l a e s of t h e spectra of Ni77Fe23 during the disordering process are f i t t e d with two components the relative deviation from the nominal composition can be estimated for both phases from the r e l a t i v e intensities (Fig. 2).

4. Disorder t o order reaction.

-

^As mentioned before, the ordering reaction takes place a t To, about 15 K below the equilibrium temperature. I t has been sug- gested before t h a t t h i s large hysteresis i s of mag- netic origin. Nucleation of the ordered phase is hindered by the difference i n magnetization of both phases. As shown in ref. /4/, t h i s yields the cor- rect order of magnitude for the width of the hyste- r e s i s zone. In Ni77Fe23 the width of the hysteresis zone should be l e s s , a s the difference i n magnetiza- tion between the two phases is m c h l e s s there /5/.

Our measurements are not yet conclusive i f t h i s i s indeed the case.

From the time dependence of the ordering reac- tion it appears t h a t the reaction is a nucleation and growth process with low nucleation rate. The or- dering process of samples coming from s l i g h t l y above, compared t o samples coming from 20 K above the orde- ring temperature is markedly different. An explana- tion can be found i n the presence of ordered nuclei immediately above To, which are hoGever too small t o become viable /3/.

Acknowledgement.

-

This work was performed as a part of the research program of the "Stichting voor Fun- damenteel Onderzoek der Materie" (FOM), with finan-