NUCLEATION & CRYSTALLIZATION

Introduction

“The formation of first nano-sized crystallites from molten material”.

In a broader sense, the term nucleation refers to the “initial stage” of formation of one phase from another phase.

So nucleation is a phenomenon associated with the phase transformation.

As water begins to freeze, nano-sized ice crystals forms 1st.

Process Thermodynamics

At the thermodynamic melting or freezing temperature, the probability of forming stable, sustainable nuclei is extremely small.

Therefore, solidification does not begins at thermodynamic melting or freezing temperature.

If the temperature is decreased from the freezing point of the material, the liquid is considered as under-cooled.

Process Thermodynamics

A material is solidify when the liquid cools to just below its freezing temperature because the energy associated with the crystalline structure of the solid is less than the energy of the liquid.

This energy difference between the liquid and the solid is the free energy per unit volume and is the “driving force” for solidification.

Process Energetic

When the solid forms, a solid-liquid interface is created. A surface free energy associated with this interface. The larger the solid the greater the increase in the surface energy.

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An interface is created when a solid forms from the liquid

An interface is created when a solid forms from the liquid

Process Free Energy

The total energy change is given as:∆G=4/3πr3 ∆Gv + 4πr2σsl

4/3πr3=the volume of a spherical embryo of radius r

4πr2=the surface area of spherical embryoσsl=surface energy of solid liquid interface∆Gv = free energy change per unit volume.

The “Top Curve” shows the parabolic variations of total surface energy.

The “Bottom Curve” shows the total volume free energy change. The “Middle Curve” shows the net variation of ∆G.

Nucleation

The total free energy of solid-liquid system changes with the size of the solid.

The solid is an “Embryo”, if it’s radius is less than the critical radius and is a “Nucleus” if its radius is greater than the critical radius.

“The critical radius is the minimum size of the crystal that must be formed by the atoms clustering together in a liquid before the solid particle is stable and begins to grow.”

The total free energy of the solid-liquid system changes with the size of the solid. The solid is an embryo if its radius is less than the critical radius, and is a nucleus if its radius is greater than the critical radius

Initiation of Growth

The new solid is then stable and sustainable since nucleation has occurred, and growth of solid particle which are now called a “Nucleus” begins.

Growth

The growth process which follow nucleation determine the following crystallographic structure of the solid.

The mode of the growth, both of individual grains and of general mass of solid, depends upon thermal conditions in the solidification zone.

Segregation

Segregation means “to isolate something” according to specific laws.

Here, Segregation means “Change in the average composition of the metal as one moves from place to place in an ingot”.

The liquid which are frozen to form industrial alloys usually contain many impurity elements.

These impurity elements are frequently eliminated during smelting and refining operation.

The refractory lining of furnaces and the gases in the furnace atmospheres may be the sources of these impurities.

Segregation

Elements enter the liquid metal in the form of dissolved gases.

The various elements dissolved in the liquid metals can and often do react with each other to form compounds.

In many cases, these compounds are less dense than the liquid and rise to the surface and join the slag which float on the top of the liquid metals.

On the other hand, it is quiet possible for small impurity particles to exist in the liquid.

Segregation

When an alloy is frozen, a rule applies is that solute elements, whether present as alloying elements or as impurities, are more soluble in liquid state than in the solid state.

This fact usually leads to the segregation of solute elements in finished castings.

Segregation

There are two basic ways of looking at the resulting non-uniformity of the solute. As freezing point progresses, the solute

concentration in a casting tend to rise in those regions which solidify in the last (centre of ingot).

In general, segregation means “ to change in the average composition of the metal as one moves from place to place in an ingot.

Segregation

Gravitational effects are often a factor in producing the segregation.

The crystal which form freely in the liquid often have a different density from that of liquid. As a result, they may either “Rise” towards the surface of the casting or “Settled” towards the bottom.

Macro-Segregation

The basic factor is the accumulation of rejected solute by transport over long distances through the casting.

The other factor is the movement of growing crystals from their nucleation sites from gravity or turbulence and long range capillary flow of liquids during the final stages of freezing.

Micro-Segregation

It results from accumulation of rejected solute between the growing crystals and its failure due to either inadequate time to diffuse into the main body of residual liquid.

The micro-segregation of alloying elements and impurities can effect the strength and the ductility.

Gravity Segregation

Macro-segregation are frequently influenced by the mass movement of precipitated phases due to difference in density compared with their parent liquid.

Metals crystals growing independently in the melt tend to sink and produce a corresponding upward displacement of solute enriched liquid.

Gravity concentration is mainly encountered in heavy sections.