THE FORMATION OF VACANCIES IN ORDERED Fe-Al ALLOYS WITH B2 STRUCTURE

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Submitted on 1 Jan 1977

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THE FORMATION OF VACANCIES IN ORDERED

Fe-Al ALLOYS WITH B2 STRUCTURE

D. Paris, P. Lesbats

To cite this version:

JOLKSAL. DE PHYSIQUE Colloque C7. suppliment cru no 12. Tonie 38, dicembre 1977. pnge C7-364

THE FORMATION

OF

VACANCIES

IN ORDERED Fe-AI

ALLOYS

WITH

B2 STRUCTURE

D.

PARIS and P. LESBATS

Departement d e Mktallurgie. Ecole des Mines de Saint-Etienne, France

Abstract.

-

The vacancies retained by quenching or at thermal equilibrium were studied experi- mentally in ordered Fe-A1 alloys with B2 structure over a range of compositions from 46 to 51 at

%

Al. The study, which was initially mainly dilatometric, shows that :

-

The formation of vacancies is a property of the B2 structure in iron-aluminium.

-

Quenching is very efficient for retaining vacancies at least up to 1 000 O C .

-

At constant temperatures, the concentration of vacancies increases monotically with the aluminium content and can reach very high values (> 2

X ) .

- At constant aluminium content, the vacancy concentration as a function of temperature exhibits an anomaly characterized by a negative deviation from the classical exponential law. The temperature at which this anomalous behaviour is observed is independent of aluminium content and is equal to 1 000 OC. A systematic study of the possible causes allows one to conclude that this anomaly must be related to a decrease in the degree of order.

This study was completed by observations using field ion microscopy which shows that :

-

The vacancy interpretation of the effects previously observed by different methods is correct. - It is possible to evaluate the distribution of vacancies among both sub-lattices, and that the quenched vacancies preferentially occupy iron sites.

A thermodynamical model was developed using the approximation of pair potentials and the Bragg-Williams theory. This model introduces two energies of atom-vacancy bonding, the numerical determination of which is performed by fitting the theoretical vacancy concentration curves to the above-mentioned anomaly, for the stoichiometric alloy. This model allows one to find the main experimental results : the anomaly temperature is independent of aluminium content, and vacancy concentration, at constant temperature, increases monotically with the aluminium content in all the B2 phase field.

Moreover it transpires that the vacancy with smallest energy is that situated on an iron site and with 8 aluminium atoms neighbours, and that at low temperature vacancies occur mainly on the iron sub-lattice; as temperature increases, other vacancy types appear and, in particular at high temperature, vacancies are created on the aluminium sub-lattice.