Displacive phase transformation

Many stainless steels, however, are austenitic (f.c.c.) at room temperature. The most common austenitic stainless, "18/8", has a composition Fe-0.1% C, 1% Mn, 18% Cr, 8% Ni. The chromium is added, as before, to give corrosion resistance. But nickel is added as well because it stabilises austenite. The Fe-Ni phase diagram (Fig. 12.8) shows why. Adding nickel lowers the temperature of the f.c.c.-b.c.c. transformation from 914°C for pure iron to 720°C for Fe-8% Ni. In addition, the Mn, Cr and Ni slow the diffusive f.c.c.-b.c.c. transformation down by orders of magnitude. 18/8 stainless steel can therefore be cooled in air from 800°C to room temperature without transforming to b.c.c. The austenite is, of course, unstable at room temperature. Flowever, diffusion is far too slow for the metastable austenite to transform to ferrite by a diffusive mechanism. It is, of course, possible for the austenite to transform displacively to give... 

EXPERIMENTAL STUDIES ON PRECURSOR PHENOMENA IN DISPLACIVE PHASE TRANSFORMATIONS... 

A. Kmmhansl and Y. Yamada, Some new aspects of first-order displacive phase transformations ... 

In addition to the individual and uncorrelated particle motions, we also have collective ones. In a strict sense, the hopping of an individual vacancy is already coupled to the correlated phonon motions. Harmonic lattice vibrations are the obvious example for a collective particle motion. Fixed phase relations exist between the vibrating particles. The harmonic case can be transformed to become a one-particle problem [A. Weiss, H. Witte (1983)]. The anharmonic collective motion is much more difficult to treat theoretically. Correlated many-particle displacements, such as those which occur during phase transformations, are further non-trivial examples of collective motions. 

Naturally, the fixed composition phase transformations treated in this section can be accompanied by local fluctuations in the composition field. Because of the similarity of Fig. 17.3 to a binary eutectic phase diagram, it is apparent that composition plays a similar role to other order parameters, such as molar volume. Before treating the composition order parameter explicitly for a binary alloy, a preliminary distinction between types of order parameters can be obtained. Order parameters such as composition and molar volume are derived from extensive variables any kinetic equations that apply for them must account for any conservation principles that apply to the extensive variable. Order parameters such as the atomic displacement 77 in a piezoelectric transition, or spin in a magnetic transition, are not subject to any conservation principles. Fundamental differences between conserved and nonconserved order parameters are treated in Sections 17.2 and 18.3. 

Diffuse crystal/crystal interfaces often appear in systems subject to incipient chemical or structural instabilities associated with phase separation, long-range ordering, or displacive phase transformations [2], Examples of interfaces associated with the first two types are shown in Fig. 18.7. 

An important class of phase transformations occurring in solids, particularly metallic alloys, are termed martensitic transformations. These solid-solid transformations involve no change of composition or atomic diffusion, but merely a local displacive change which can be usefully studied by NMR since a local atomic-scale rearrangement is involved. A series of papers from Rubini et al. has shown that the NMR spectra of metals can be used to observe and quantify the degree of martensitic transformations, since there... 

Giddy AP, Dove MT, Pawley GS, Heine V (1993) The determination of rigid rmit modes as potential soft modes for displacive phase transitions in framework crystal structures. Acta Crystallogr A49 697-703 Grimm H, Domer B (1975) On the mechanism of the a-p phase transformation of qrrartz. Phys Chem Solids 36 407-413... 

As single-phase substances are heated or cooled, they can undergo a number of polymorphic transformations. Polymorphs are different crystalline modifications of the same chemical substance. These transformations are quite common and include crystallization of glasses, melting, and many solid-solid phase transformations, some of which are described below. In general, there are two types of polymorphic transformations, displacive and reconstructive. 

In contrast to reconstructive transformations, displacive transformations do not involve the breaking of bonds, but rather occur by the displacement of atomic planes relative to one another, as illustrated in Fig. 8.1/>. These reactions occur quite rapidly, and the resulting microstructures are usually heavily twinned. In these transformations, the role of thermal entropy is important since the enthalpies of the phases on either side of the transformation temperature are quite comparable. It follows that the transformation usually results in the formation of more open (less dense) structures at higher temperatures, for reasons that were touched upon in Chap. 5, namely that the more open structures have higher thermal entropies. " ... 

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