section%208.1%20-%20Nucleation%20and%20Growth

Section 8.1 - C hapter 8 M icrost ructu re Kinetics 8.1 Nucleation and Growth Objectives Describe the steps of phase t ransformations Describe the

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Chapter 8: Microstructure – Kinetics 8.1 Nucleation and Growth Objectives Describe the steps of phase transformations. Describe the factors affecting the rate of phase transformations. Chapter 7 was all based on thermodynamic equilibrium; in other words, phase diagrams represent the lowest free energy state of a mixture. This chapter is about kinetics , how fast different microstructures form and the ways in which those microstructures evolve. One important thing to keep in mind is that there may be kinetic barriers to achieving an equilibrium state. Even if the phase diagram says that certain phases should exist, we can use a processing method that doesn’t give those phases enough time to form, and thus freeze in a non-equilibrium structure. Later in this chapter we will talk a lot about steel as an example of how we can use processing methods to change the microstructure and the properties. But before we get to that we need to understand some general aspects of phase transformations . Phase transformations occur by a two step process. In order to understand this, think of a liquid that is cooled to below its melting temperature, and then solidifies. The first step in this process is called nucleation . During nucleation small particles of the solid, called nuclei , form in the liquid. The second step is growth , during which the particles get bigger. Even thought the examples in this section are all for solidification from a liquid, you should keep in mind that the same concepts also apply to solid-solid transformations, for example the formation of a β phase from an α phase. The process of phase transformations is governed by changes in the free energy of the system. We need to consider two aspects of the free energy: the bulk free energy and the surface free energy. Phase transformations occur when the total free energy of the system (bulk plus surface) is reduced. GI 8.1.1 What is the first step in the solidification process? What happens during that process? GI 8.1.2 At a temperature below the melting temperature, which form of the material is more stable, the liquid or the solid?
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GI 8.1.3 At a temperature below the melting temperature, which form of the material has the lower free energy, the liquid or the solid? GI 8.1.4 If a nucleus is formed, there is a surface formed between the nucleus and the
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This note was uploaded on 03/30/2010 for the course EMA 3010 taught by Professor Unknown during the Spring '08 term at University of Florida.

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Section 8.1 - C hapter 8 M icrost ructu re Kinetics 8.1 Nucleation and Growth Objectives Describe the steps of phase t ransformations Describe the

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