Displacive transformation

So far we have only looked at transformations which take place by diffusion the so-called diffusive transformations. But there is one very important class of transformation - the displacive transformation - which can occur without any diffusion at all. 

The most important displacive transformation is the one that happens in carbon steels. If you take a piece of 0.8% carbon steel "off the shelf" and measure its mechanical properties you will find, roughly, the values of hardness, tensile strength and ductility given in Table 8.1. But if you test a piece that has been heated to red heat and then quenched into cold water, you will find a dramatic increase in hardness (4 times or more), and a big decrease in ductility (it is practically zero) (Table 8.1). 

In order to answer these questions as directly as possible we begin by looking at diffusive and displacive transformations in pure iron (once we understand how pure iron transforms we will have no problem in generalising to iron-carbon alloys). Now, as we saw in Chapter 2, iron has different crystal structures at different temperatures. Below 914°C the stable structure is b.c.c., but above 914°C it is f.c.c. If f.c.c. iron is cooled below 914°C the structure becomes thermodynamically unstable, and it tries to change back to b.c.c. This f.c.c. b.c.c. transformation usually takes place by a diffusive mechanism. But in exceptional conditions it can occur by a displacive mechanism instead. To understand how iron can transform displacively we must first look at the details of how it transforms by diffusion. 

Fig. 8.6. If we quench f.c.c. iron from 914°C to room temperature at a rate of about 10 °C s we expect to prevent the diffusive f.c.c. —> b.c.c. transformation from taking place. In reality, below 550°C the iron will transform to b.c.c. by a displacive transformation instead.

Fig. 8.7. The displacive f.c.c. —> b.c.c. transformation in iron. B.c.c. lenses nucleate at f.c.c. groin boundaries and grow almost instantaneously. The lenses stop growing when they hit the next grain boundary. Note that, when a new phase in any material is produced by o displacive transformation it is always referred to os "martensite". Displacive transformations ore often called "martensitic" transformations os o result.

Stereoelectronic control also plays a role in mechanistic stereoselectivity. One such case is the very fundamental 8 2 process which proceeds rigorously with inversion of configuration at carbon. Because of that intrinsic and predictable stereoselectivity, the C-C disconnective Sn2 displacement transform is very important even though it does not directly reduce the number of stereocenters, e.g. 153 => 154. 

J.W. Christian, G.B. Olson and M. Cohen, Classification of displacive transformations what is a... 

K.M. Ho and B.N. Harmon, First-principles total energy calculations applied to displacive transformations. 

The behavior of cristobalite PON has been studied as a function of pressure. No in situ evidence for pressure-induced amorphization was noticed. Whereas cristobalite Si02 displays four crystalline phases up to 50 GPa (195), PON remains in a cristobalite phase (193, 196). By using Raman spectroscopy and synchrotron X-ray diffraction, Kingma et al. (193, 197) observe a displacive transformation below 20 GPa to a high-pressure cristobalite-related structure, which then remains stable to at least 70 GPa. The high value of the calculated bulk modulus (71 GPa) (196) is indicative of the remarkable stiffness of the phase. 

I.I The prediction of transformation diagrams after Kirkaldy et al. (1978). A model for the calculation of ferrite and pearlite was first presented by Kirkaldy et al. (1978) based on Zener-Hillert type expressions (Zener 1946, Hillert 1957). In this first effort, no attempt was made to differentiate between the diffusive and displacive transformations and a overall C curve was produced of the type shown schematically in Fig. 11.14. Kirkaldy ettd. (1978) used the formalism below where the general formula for the time (r) to transform x fraction of austenite at a temperature T is given by... 

Heterophase Interfaces. In certain cases, sharp heterophase interfaces are able to move in military fashion by the glissile motion of line defects possessing dislocation character. Interfaces of this type occur in martensitic displacive transformations, which are described in Chapter 24. The interface between the parent phase and the newly formed martensitic phase is a semicoherent interface that has no long-range stress field. The array of interfacial dislocations can move in glissile fashion and shuffle atoms across the interface. This advancing interface will transform... 

Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Vdgard s Law is followed. When the end members are not is structural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO4 compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 

With reference to the candidacy of generically-related crystalline materials to host and immobilize nuclear waste ions, these results prompt several comments regarding stability. Displacive transformations, as observed here, represent... 

The particle size and crystallite size of anatase increase markedly in the region of the crystal structure transformation. The unit cell of anatase seems to expand prior to the transformation to rutile. This expansion has been attributed to a displacive transformation of the type defined by Buerger. Smaller particle size and larger surface area seem to favour the transformation. 

A reversible minor displacive transformation of tetrahedral atoms can reduce the symmetry of the ZSM-5 framework to monoclinic, and thus make the appearance of the additional lines in the powder pattern possible. The observation of this type of line splitting in XRD patterns of ZSM-5 thus provides insights into subtle framework motions. 

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