Transformation diffusionless

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 transformation is beHeved to occur by a diffusionless shear process (83). It is often referred to as martensitic transformation, having a thermal hysteresis between the cooling and heating cycles. The transformation is dependent on particle size finer particles transforming at a lower temperature than... 

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. ... 

Displacive lattice transformations, whieh are characterised by a diffusionless shear proeess have been extensively studied by metallurgists and physieists. In the language of the former these are referred to as martensitie, for the latter soft-mode . 

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... 

A between translationally related molecules to give a dimer of mirror symmetry (m-dimer) and (c) the y-type crystal, which is photochemically stable because no double bonds of neighboring molecules are within 4 A. On the basis of mechanistic and crystallographic results it has been established that in a typical topochemical photodimerization, transformation into the product crystal is performed under a thermally diffusionless process giving the space group quite similar to that of the starting crystal (5,6). 

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. 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

We have mentioned above the tendency of atoms to preserve their coordination in solid state processes. This suggests that the diffusionless transformation tries to preserve close-packed planes and close-packed directions in both the parent and the martensite structure. For the example of the Bain-transformation this then means that 111) -> 011). (J = martensite) and <111> -. Obviously, the main question in this context is how to conduct the transformation (= advancement of the p/P boundary) and ensure that on a macroscopic scale the growth (habit) plane is undistorted (invariant). In addition, once nucleation has occurred, the observed high transformation velocity (nearly sound velocity) has to be explained. Isothermal martensitic transformations may well need a long time before significant volume fractions of P are transformed into / . This does not contradict the high interface velocity, but merely stresses the sluggish nucleation kinetics. The interface velocity is essentially temperature-independent since no thermal activation is necessary. 

Diffusionless transformations have been sometimes called military , in contrast to the more civilian diffusion controlled transformations. Considering their technical relevance, the crystallographic theory of martensite transformation has been worked out in much detail, and particularly for the habit plane selection of the given 0-0 lattice structural change. The reader is referred to the corresponding metallurgical literature (D.A. Porter, K.E. Easterling (1990) D.S. Liebermann (1970) C.M. Wayman (1983)]. 

But, aside from these unique properties, Nitinol has a number of commonalities with other known martensitic transition systems (1) it is an athermal transformation, (2) it is diffusionless, (3) it involves displacive or shear-like movement of atoms, (4) the activation energy for the growth of martensite (continuous atomic shear in Nitinol) is effectively zero, i.e., the propagation rate of transformation (transition in Nitinol) is fast and independent of temperature. 

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. 

These observations can be represented as a special case of the general rate equation derived by the application of order-disorder theory to diffusionless transitions in solids.3 According to this equation, the shape of the rate curve is determined by the relative numerical values of zkp/kn and of c. The larger the factor is relative to c, the more sigmoidal the curves become. This is understandable since the propagation effect which is responsible for the autocatalytic character of the transformation becomes more noticeable when kPlkn is large and c small. Under these conditions some time elapses before a sufficient number of nucleation sites are formed then the... 

The fcc-bct conversion, known as the Bain transformation, is a diffusionless process. That is, unlike the previous high-temperature conversions we saw earlier e.g., austenite to ferrite), martensite can form at temperatures significantly below room... 

In contrast to this behaviour a topochemical reaction can be described as a diffusionless transformation of the parent crystal into the daughter crystal. All reactivity comes about by very specific rotations of the monomers on their lattice sites. Both crystallographic position and symmetry of the monomer units are retained in this process which is schematically shown in Fig. 1. 

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