VACANCY THERMODYNAMICS AND THE KINETICS OF SUPERSTRUCTURE

							Paper 4.3

         VACANCY THERMODYNAMICS AND THE KINETICS OF
         SUPERSTRUCTURE FORMATION IN INTERMETALLIC
                        COMPOUNDS.
              KOZUBSKI R. 1, KOZŁOWSKI M. 1, PARTYKA E.1, PIERRON-BOHNES V.2, PFEILER W.3
                  1
                    M. Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4,
                                              30-059 Krakow, Poland,
             2
               Institut de Physique et Chimie des Matériaux de Strasbourg, 23, rue du Loess, BP43,
                                       67034 Strasbourg CEDEX 2, France
              3
                Institut für Materialphysik, Universität Wien, Strudlhofgasse 4, 1090 Wien, Austria

A number of technologically attractive intermetallic compounds show high stability of crystalline superstructure
and, therefore, “order-disorder” transitions in those materials occur at very high temperatures. In many cases the
state of chemical long-range order is maintained even up to the melting point. Nevertheless, fine temperature
dependence of the degree of long-range order is experimentally observable in so-called “order-order” processes;
the phenomena controlled by atomic migration via vacancy mechanism. Atoms migrate, however, in non-steady-
state conditions and hence, the corresponding investigation is complementary with respect to standard diffusion
studies.

Extensive experimental research of “order-order” kinetics (carried out mainly by means of various resistometric
techniques) covered intermetallics with different types of superstructures: Ni 3Al (L12), NiAl (B2), FeAl (B2),
and FePd (L10). It has been found out that specific features of the observed “order-order” relaxations are due to
superstructure geometry. An interest has been focused on the observed correlation between the superlattice
stability and the activation energy for ordering. Complete understanding of the observed effects requires,
however, a study of vacancy thermodynamics in systems with varying superlattice stability.

In parallel, specific atomistic mechanisms of “order-order” processes are investigated by means of computer
simulation.