Definition of solidification, Cooling Curves of metal and alloy, Nucleation and Crystal Growth. Reference: Material Science and Engineering, William Callister

1. CHAPTER 2
SOLIDIFICATION MECHANISMS AND PHASE DIAGRAMS
 Homogeneous and heterogeneous nucleation
 Nucleation and Crystal Growth
 Solid solutions and types
 Phase diagrams
 Lever rule and its applications
 Iron-Carbide system
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2. CONTENTS
 Introduction
 Definition of solidification
 Melting of Metals
 Cooling curves
 Nucleation and Crystal Growth
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3. INTRODUCTION
 It is the process where liquid metal transforms into solid upon cooling.
 The properties of the solidified metal depends on its microstructure and the microstructure in turn depends
on the Solidification Mechanism.
 The rate of cooling of liquid metals, the type of mould used, the temperature at which the molten metal is
poured into the mould are all a part of solidification process.
 Transformation from liquid metal to solid metal is accompanied by shrinkage of volume.
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4. DEFINITION
 The transformation of metals and alloys from liquid state to solid state
(crystalline state) is called solidification or crystallization.
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5. MELTING OF METALS
 Consider a pure metal being melted from room temperature.
 When the melting temperature is reached, the solid metal starts
melting, but at constant temperature.
 This can be understood by considering the graph of temperature
versus time as shown in figure.
 The process of melting takes place at constant temperature as the
heat added is utilized in breaking the bonds of the solid crystal
structure.
 The heat that is added to the metal to convert all the solid metal
into liquid state is called the “Latent heat of fusion”. AB represents
the constant temperature line.
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6. FREEZING OR SOLIDIFICATION OR CRYSTALLIZATION OF METALS
 Solidification is the reverse of melting.
 The pure metal in its molten state first gives up its superheat upon
cooling.
 When the temperature drops to the freezing (solidification)
temperature, the solidification process starts.
 The metal starts to give out its latent heat at constant freezing
temperature.
 This continues until all the metal in the liquid state is converted into the
solid state.
 Further the metal in the solid state is cooled to room temperature.
 This type of curve (graph) is called a cooling curve as shown in figure .
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7. COOLING CURVE FOR PURE METAL AND ALLOY
 The solidification of a pure metal takes place at a
constant temperature TS as shown in figure .
 The solidification line DE is horizontal on the cooling
curve for a pure metal which means that solidification
begins and ends at the same temperatureTS .
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8.  Whereas, the solidification of an alloy takes place over
a range of temperatureT1 to T2 as shown in figure.
 The solidification line DE is inclined on the cooling
curve for an alloy which means that solidification
begins at temperatureT1 and ends at temperatureT2.
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9.  The solidification of metal can be divided into:
Nucleation of minute crystals – The formation of stable nuclei in the liquid melt.
Growth of nuclei into crystals and formation of grain structure.
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10. Nucleation
 This is the starting point of the solidification process of any metal or alloy.
 In this stage, a number of minute crystals nucleate through out the melt.
 Metal in molten state possesses high energy and atoms are in random state.
 As the metal cools, atoms gradually loose their energy and start colliding with each other.
 During these collisions, energy is released out and attractive forces are set up between them.
 These attractive forces result in formation of small cluster of atoms (10-15 atoms) at several places. These cluster
of atoms is called “nuclei.”
 Nucleation in metals and alloys occur by two mechanisms:
i. Homogeneous Nucleation
ii.Heterogeneous Nucleation
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11. Crystal Growth
 In this stage, the molten metal continues to solidify around the nuclei formed.
 The nuclei and the metal surrounding them continuously release heat as the process further cools down.
 As the temperature decreases due to cooling the nuclei grow rapidly with atoms attaching themselves in identical
layers around the nuclei and a dendritic formation(in case of alloys)/columnar formation(in case of metals) takes place.
 The dendrites or columns are skeleton in the microstructure of metal or alloy.
 When one big dendrite/column meets another, a grain boundary is formed along their border while dendrites
themselves become the crystals.
 The liquid metal along the grain boundaries is the last to solidify and this marks the end of solidification.
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