Transformation diffusive

We saw in Chapter 6 that diffusive transformations (like the growth of metal crystals from the liquid during solidification, or the growth of one solid phase at the expense of another during a polymorphic change) involve a mechanism in which atoms are attached to the surfaces of the growing crystals. This means that diffusive transformations can only take place if crystals of the new phase are already present. But how do these crystals - or nuclei - form in the first place ... 

In this chapter we have shown that diffusive transformations can only take place if nuclei of the new phase can form to begin with. Nuclei form because random atomic vibrations are continually making tiny crystals of the new phase and if the temperature is low enough these tiny crystals are thermodynamically stable and will grow. In homogeneous nucleation the nuclei form as spheres within the bulk of the material. In... 

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 two samples have such divergent mechanical properties because they have radically different structures the structure of the as-received steel is shaped by a diffusive transformation, but the structure of the quenched steel is shaped by a displacive change. But what are displacive changes And why do they take place ... 

We saw in Chapter 6 that the speed of a diffusive transformation depends strongly on temperature (see Fig. 6.6). The diffusive f.c.c. b.c.c. transformation in iron shows the same dependence, with a maximum speed at perhaps 700°C (see Fig. 8.1). Now we must be careful not to jump to conclusions about Fig. 8.1. This plots the speed of an individual b.c.c.-f.c.c. interface, measured in metres per second. If we want to know the overall rate of the transformation (the volume transformed per second) then we need to know the area of the b.c.c.-f.c.c. interface as well. 

Fig 8 2 In a diffusive transformation the volume transforming per second increases linearly with the number of nuclei. 

In order to get the iron to transform displaeively we proceed as follows. We start with f.c.c. iron at 914°C which we then cool to room temperature at a rate of about 10 °C s . As Fig. 8.6 shows, we will miss the nose of the 1% curve, and we would expect to end up with f.c.c. iron at room temperature. F.c.c. iron at room temperature would be undercooled by nearly 900°C, and there would be a huge driving force for the f.c.c. b.c.c. transformation. Even so, the TTT diagram tells us that we might expect f.c.c. iron to survive for years at room temperature before the diffusive transformation could get under way. 

Interactions between diffusion and chemical transformation determine the performance of a transformation process. Weisz (1973) described an approach to the mathematical description of the diffusion-transformation interaction for catalytic reactions, and a similar approach can be applied to sediments. The Weisz dimensionless factor compares the time scales of diffusion and chemical reaction ... 

Diffusive transformations (including polymorphic transitions) abound in solid state chemistry. Component diffusion is always involved. However, the kinetics of a... 

Ultimately the Schottky maximum becomes severly deformed and when a critical value for the aggregation, Aw rj, is exceeded, the maximum becomes discontinuous on its high-temperature side. The diffuse transformation is then changed into a discontinuous transformation (Fig. 2.8). The reason for this change to a first order transition is the loss of stability of the system if... 

Au = 2.55 kJ/moI q = 6 and L = 11. As the tendency to aggregation increases, the Schottky anomaly assumes more and more the shape of a comparatively clearly defined, but diffuse transformation... 

Unstable phases cannot be thermodynamically and kinetically maintained. Any small change in X or T leads to a decrease in G(x, T). The condition from a metastable phase to an unstable phase is known as the stability limit of phases. A nucleus of new phases is formed with (j = 0 and (Tg line) = 0 (t, is the induction time at the stability limit). Because r Tp, the transformation must be a non-diffusion transformation (only... 

The obtained simplified systems of equations are solved analytically, and then balance equations for fluxes at the moving boundary of diffusion transformation are written down using Boltzmann s substitution. 

In the previous chapter, we considered the problem of selection of the path for the reactive diffusion when the choice is made among a finite number of phase formation modes. Let us now treat the problems allowing for an infinite set of solutions, all of which are compatible with the matter balance equations. These are mainly the problems of morphology choice when different modes are possible, and the task is to find the optimal one to be realized in practice. In the first two sections of the chapter, we consider the issue of low-temperature phase diffusion transformations, namely discontinuous precipitation and DIGM. At that, the evolution equations based on matter conservation laws allow an infinite number of solutions corresponding to different thicknesses of the phase formed and different velocities of phase transformation front movement. 

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