Phase transformations diffusional

The diffusion-dependent transformations of austenite (to ferrite, pearlite, and bainite) compete with the martensitic transformation such that the volume fraction available for the latter will decrease as the volume transformed by the former increases. This transformation kinetics of the diffusional phase transformations is strongly dependent on alloy composition. 

Hirth J P (1991), Interface dislocations and ledges in oxidation and diffusional phase transformations . Met Trans A, 22A(6), 1331-1339. 

Simultaneously, the ArBs layer thickness increases at interface 3 as a result of the phase transformation of A/Bn into ArBs by reaction (4.18). The diffusional constant klB2 characterises partial chemical reaction (4.1) in which 5 diffusing B atoms take part. Again, according to equation (4.18) the loss of r AtBn molecules (rn-ls) B atoms results in the formation of l ArBs molecules. Therefore, the growth rate of the ArBs layer at interface 3 is... 

In this book we are concerned only with mass transport, or diffusion, in solids. Self-diffusion refers to atoms diffusing among others of the same type (e.g., in pure metals). Interdiffusion is the diffusion of two dissimilar substances (a diffusion couple) into one another. Impurity diffusion refers to the transport of dilute solute atoms in a host solvent. In solids, diffusion is several orders of magnitude slower than in liquids or gases. Nonetheless, diffusional processes are important to study because they are basic to our understanding of how solid-liquid, solid-vapor, and solid-solid reactions proceed, as well as [solid-solid] phase transformations in single-phase materials. 

Our focus in this text will be on diffusional transformations. Diffusional transformations can be further subdivided into two main types continuous and discontinuous. Gibbs (of the Gibbs phase rule and the Gibbs free energy) articulated the difference between continuous and discontinuous phase transformations as follows ... 

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

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. 

The curvature method also has some inherent limitations. Since the method entails determination of plastic response through the imposition of a temperature change or phase transformation, which is known to alter the plastic properties of metals, care should be exercised in the interpretation of strain relaxation phenomena from curvature measurements. There are also no clear means of isolating the individual contributions to overall curvature evolution seen experimentally from such factors as plastic yielding, strain hardening, diffusional creep, or microstructural changes, without recourse to other independent experimental and observational tools. 

Of course, the borderline between the two pictures (the one-phase and the two-phase ones) becomes diffused when the second transforms into the first with the decrease of the typical pore radius. In fact, as will become clear in Chapter 6, 6.4, distinction between the phases becomes meaningless as soon as the typical pore radius becomes shorter than some typical electro-diffusional length scale—the Debye length—to be defined below. 

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. 

In spite of their seeming variety, theoretical approaches of different authors to the consideration of solid-state heterogeneous kinetics can be divided into two distinct groups. The first group takes account of both the step of diffusional transport of reacting particles (atoms, ions or, in exceptional cases if at all, radicals) across the bulk of a growing layer to the reaction site (a phase interface) and the step of subsequent chemical transformations with the participation of these diffusing particles and the surface atoms (ions) of the other component (or molecules of the other chemical compound of a binary multiphase system). This is the physicochemical approach, the main concepts and consequences of which were presented in the most consistent form in the works by V.I. Arkharov.1,46,47... 

Ti-6A1-4V is an alpha-beta alloy that can be modified extensively by both thermal and thermomechanical processing to produce a large variety of microstructures and hence a wide spectrum of mechanical properties. The beta-transus temperature is approximately 1000 °C (1830 °F) and is a function of interstitial content (Ref 1). Samples of Ti-6A1-4V cooled at relatively slow rates from elevated temperatures contain mainly the alpha and beta phases as a result of diffusional transformations, while those cooled rapidly may also contain martensitic phases such as the cc (hep structure) or the a" (orthorhombic structure) phases. 

Bor] Borgenstam, A., Engstroem, A., Hoeglund, L., Agren, J., DICTRA, a Tool for Simulation of Diffusional Transformations in Alloys , J. Phase Equilib., 21(3), 269-280 (2000) (Thermodyn., Calculation, Kinetics, 59)... 

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