PHASE DIAGRAM of the ORDER-DISORDER TRANSITION in Ni3fe J

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. jpa-00219622

HAL Id: jpa-00219622 https://hal.archives-ouvertes.fr/jpa-00219622 Submitted on 1 Jan 1980

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PHASE DIAGRAM OF ME ORDER-DISORDER TRANSITION IN Ni3Fe

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

Solid state 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 alloy, 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.

Fig. 2. Average hyperfine field us. annealing time. Details are explained in the text. 790 anneals, starting from 787 K (see fig. 2, circles). The first change in the spectra was observed at 790 780 K; at the same temperature the reaction came to cm- pletion in 10 days. When the temperature of the corn- pletely disordered material was lowered 1 K no sign . 770 of the reverse transformation was observed. In a se- ordered 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 Fig. I. Tentative phase diagram of the Ni-Fe system to the presence of a large amount of ordered mate- near Ni3Fe. rial,nucleation of the ordered phase was not neces- sary, and growth could proceed on the boundaries of Because of the first order character of the the not yet disordered material. transition, a two phase region is expected between In this way we demonstrated directly that the the ordered and the disordered phase. According to disordering temperature in Ni73Fe27 is well-defined Gibbs' phase rule this two phase area should disap- within 1 K. pear at the maxjnnnn transition temperature. X-ray This observation is, however, contrary to the measurements, however, suggested coexistence of or- findings of Calvayrac and Fayard who report a dered and disordered phases at all compositions bet- /I/, reversible transformation between order and disorder tween 22 and 32 at.% Fe /I/. A theoretical interpre- in a 5 K wide two-phase region, determined by means tation of this result was recently proposed by Bar- of electron microscopy and X-ray diffraction. A theo- tis /2/. retical eglanation of this violation of Gibbs' phase Another interesting aspect of the order-disor- rule was given by Bartis /2/, who suggests internal der transition in Ni3Fe is the occurrence of a broad stresses to be responsible for a smeared-out transi- hysteresis zone in the phase diagram. When a dis- tion temperature. Some COlnnentS on Bartis' explana- ordered sample is cooled slowly, ordering is sup- tion are published elsewhere /3/. In the second pressed over 15 K. place we demonstrated by these experiments that the 2. Order to disorder reaction in Ni7e27. - TO set- disordering temperature is also the thermodynamical tle the question of the two phase region in the top equilibrium temperature. Any hysteresis between or- of the hysteresis zone, the temperature of an orde- dering and disordering processes is caused specifi- red sample was increased in steps of 1 K and 1 week

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 to disorder reaction in other compositions. Zuiver Wetenschappelijk Onderzoek" (ZWO). The ,existence of a two phase region of about 2 K wide was confirmed at the compositions Ni71Fe29 and References 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 is 11972). , 891. at least 4 K wide, was studied in more detail. Six- /2/ F. Bartis, Acts Metall. 26 (1978) 879. -line fits during the disordering run give excessive /31 J.K. van Deen & F. vm der Woude, Phys, Rev. linewidths of about 1 -10 4s. This value is much July 1979. larger than from the difference in /4/ J.W. kijver, 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 to phase separation which leads (1953) 221. to Fe-rich ordered material and Fe-poor disordered material, enhancing the difference between the hyperfine fields. This interpretation is confirmed by the absence of excessive line broadening in Ni73Fe27, where no phase separation occurs, and in Ni71Fe29, where the effect of ordering and phase separation partly cancel each other. &en the outer laes of the spectra of Ni77Fe23 during the disordering process are fitted with two components the relative deviation from the nominal composition can be estimated for both phases from the relative intensities (Fig. 2). 4. Disorder to order reaction. - As mentioned before, the ordering reaction takes place at To, about 15 K below the equilibrium temperature. It has been suggested before that this large hysteresis is of mag- netic origin. Nucleation of the ordered phase is hindered by the difference in magnetization of both phases. As shown in ref. /4/, this yields the cor- rect order of magnitude for the width of the hysteresis zone. In Ni77Fe23 the width of the hysteresis zone should be less, as the difference in magnetization between the two phases is mch less there /5/. Our measurements are not yet conclusive if this is indeed the case. From the time dependence of the ordering reaction it appears that the reaction is a nucleation and growth process with low nucleation rate. The ordering process of samples coming from slightly above, compared to samples coming from 20 K above the ordering temperature is markedly different. An explana- tion can be found in the presence of ordered nuclei immediately above To, which are hoGever too small to 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-