Diffusionless phase transformations

G.M. Woken, Diffusionless Phase Transformations in Zirconia and Hafnia, Journal of the American Ceramics Society, 46 9, pp.418-422 (1963). 

E. Dow Whitney, Electrical resistivity and diffusionless phase transformation of zirconia at high temperatures and ultrahigh pressures, J. Electrochem, Soc. 112(1), 91-94 (1965). 

FIGURE 6.1 (a) Illustration of a diffusionless phase transformation involving a displacive... 

Diffusionless phase transformations do not require the net transport of atoms across a phase boundary. For example, phase transformations involving a change in spin or magnetic moment or certain changes in crystal structure or symmetry do not require diffusional fluxes. Examples of such processes include the martensitic transformation in steel or certain cubic-to-tetragonal phase transformations. 

Many metals and metallic alloys show martensitic transformations at temperatures below the melting point. Martensitic transformations are structural phase changes of first order which belong to the broader class of diffusion js solid-state phase transformations. These are structural transformations of the crystal lattice, which do not involve long-range atomic movements. A recent review of the properties and the classification of diffusionless transformations has been given by Delayed... 

L. Delaey, Diffusionless transformations, in Phase Transformations in Materials, Materials... 

Martensitic phase transformations are discussed for the last hundred years without loss of actuality. A concise definition of these structural phase transformations has been given by G.B. Olson stating that martensite is a diffusionless, lattice distortive, shear dominant transformation by nucleation and growth . In this work we present ab initio zero temperature calculations for two model systems, FeaNi and CuZn close in concentration to the martensitic region. Iron-nickel is a typical representative of the ferrous alloys with fee bet transition whereas the copper-zink alloy undergoes a transformation from the open to close packed structure. ... 

The symmetry of the high temperature phase A is higher than the symmetry of the low temperature phase B. The A, B phase transformation occurs by temperature variation inside the same crystal. The new structure appears in epitaxial growth on the first one and the displacement of the atoms is very small, so that the transformation is a cooperative phase transformation (diffusionless... 

The structures and phase transformations observed in steels have been dealt with in some detail not only because of the great practical importance of steels, but also because reactions similar to those occurring in steels are also observed in many other alloy systems. In particular, diffusionless transformations (austenite - martensite), continuous precipitation (austenite - pearlite) and discontinuous precipitation (austenite - bainite and tempering of martensite) are fairly common in other alloy systems. 

Condensed-matter phase transformations can be broadly divided into two main categories diffusional transformations and diffusionless (or fluxless ) transformations. 

The titanium-nickel alloys show unusual properties, that is, after it is deformed the material can snap back to its previous shape following heating of the material. This phenomenon is called shape memory effect (SME). The SME of TiNi alloy was first observed by Buehler and Wiley at the U.S. Naval Ordnance Laboratory [Buehler et al, 1963]. The equiatomic TiNi or NiTi alloy (Nitinol) exhibits an exceptional SME near room temperature if it is plasticaUy deformed below the transformation temperature, it reverts back to its original shape as the temperature is raised. The SME can be generally related to a diffusionless martensitic phase transformation which is also thermoelastic in nature, the thermoelasticity being attributed to the ordering in the parent and martensitic phases [Wayman and Shimizu, 1972]. Another unusual... 

Phase transformations in steels Volume 2 Diffusionless transformations, high strength steels, modelling and advanced analytical techniques... 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Let us regard a binary A-B system that has been quenched sufficiently fast from the / -phase field into the two phase region (a + / ) (see, for example, Fig. 6-2). If the cooling did not change the state of order by activated atomic jumps, the crystal is now supersaturated with respect to component B. When further diffusional jumping is frozen, some crystals then undergo a diffusionless first-order phase transition, / ->/ , into a different crystal structure. This is called a martensitic transformation and the product of the transformation is martensite. 

From the EPMA data in Table 3.7 (see also Fig. 3.14a), it follows that the Co-bordering layer consists of the y and yi phases, with the last phase being dominant. Another important point is a smooth concentration distribution within the bulk of this layer, without any discontinuity due to the existence of the two-phase y + Yi field of 85.4-87.4 at.% Zn on the phase diagram, indicative of a diffusionless transformation. Note that the restrictions on the number of simultaneously occurring layers, following from physicochemical considerations, are clearly inapplicable to compounds which are formed by a diffusionless (shear) mechanism. 

---