SOLIDIFICATION PHASE DIAGRAM & STEELS · 2017. 8. 24. · Solidification in solid solution alloys...

Pruthvi Loy, Chiranth B. P. 1 SJEC, Mangaluru

4. SOLIDIFICATION

Introduction

Mechanism of solidification

- crystallization and development of cast structure

- nucleation and grain growth

- dendritic growth

- coring & homogenization

- solidification in pure metals and alloys

Cast metal structures

- significance and practical control of cast structure

- grain shape, grain size and orientation

- refinement and modification of cast structure

4.1 INTRODUCTION

The pouring of molten metal into a relatively cool mould initiates the process of solidification

wherein the phase transformation from liquid to solid occurs. The mode of freezing has a twofold

influence upon the final properties of the casting; the metallographic structure acquired by the

casting is determined during solidification, besides structure the soundness of the casting also

depends upon the solidification mechanism.

4.2 MECHANISM OF SOLIDIFICATION

4.2.1 Crystallization of the melt

Crystal lattice represents a more closely packed state of matter than the liquid, thus freezing is

associated with volume contraction. Moreover, freezing results in reduced molecular motion

leading to liberation of energy in the form of latent heat of crystallization which affects the rate

and mode of crystal growth.

Crystallization from the melt involves successive stages of nucleation and grain growth. The

location and relative rates of these two phenomena determines the final structure.

MODULE TWO

SOLIDIFICATION, PHASE DIAGRAM

& STEELS

Module2 4. Solidification


Figure 4.1: Nucleation and grain growth

At the melting point the thermal fluctuations result in the formation of tiny particles of the

product phase which grow further by the transfer of atoms across its interface. The process of

formation of the first stable tiny particle is called nucleation and the process of increase in the

size of these particles is called grain growth.

4.2.2 Nucleation

Nucleation is the beginning of phase transformation. There can be two types of nucleation:

Homogeneous nucleation and

Heterogeneous nucleation

i) Homogeneous nucleation:

This type of nucleation is observed when solids are formed within its own melt without the aid of

any foreign particles. It occurs in perfectly homogenoeus materials such as pure metals.

Homogeneous nucleation requires some amount of under cooling and the nucleation of super

cooled grains depends on two factores, viz, volume free energy and surface free energy.

a) Volume free energy (fV): the free energy available from the solidifction process and it

depends upon the volume of particle formed.

Where, FV – free energy change per unit volume

(the –ve sign indicates that the free energy decreases)

fV = – 4

3𝜋𝑟3 FV



b) Surface free energy (fS): the energy required to form a solid-liquid interface.

Where, – interfacial free energy per unit area

Figure 4.2: Free energy changes in Homogeneous nucleation

Therefore, the total energy change is,

ii) Heterogeneous Nucleation:

It takes place due to the influence of foreign particles (container or insoulble impurities); the

presence of impurites lower the liquid-solid interface energy and help in nucleation there by

reducing the amount of super cooling needed to actuate nucleation.

The basic requirements for hetergeneous nucleations are

a) Nucleating agents: the presence of foreign particles so that nucleation takes place easily

b) Low contact angle: a good wetting between the liquid metal and the foreign particles.

fS = 4𝜋𝑟2

f = – 4

3𝜋𝑟3 FV + 4𝜋𝑟2



Figure 4.3: Contact angle for Heterogeneous nucleation

(with small contact angles the interface has low surface energy and nucleation can occur

at small amount of under cooling)

Once the intial nuclei are established two possiblities exist for further crystallization. i.e., more

solid may be deposited upon the first nuclei or fresh nucleation may occur, but growth might be

expected to predominat over further nucleation. However there could be barriers to growth

resulting from the evolution of latent heat of crystallization and in case of alloys from the change

in compostion of the adjacent liquid through differential freezing.

Table 4.1: Comparison of Homogeneous and Heterogeneous nucleation

Homogeneous nucleation Heterogeneous nucleation

Occurs in pure metals Occurs in alloys

Nucleation is by deposition of atoms from

its own melt

Nucleation is by the influence of foreign

particles

Requires some amount of undercooling

for nucleation.

Undercooling is less as compared to

homogeneous nucleation.

Starts below the equilibrium freezing

temperature

Starts comparatively at much higher

temperature

4.2.3 Grain growth

Growth follows nucleation and it determines the final crystallographic structure of the solid. The

mode of growth depends upon the thermal conditions in the solidification zone and the

constitution of the alloy.



Duing growth, the material is transferred by diffusion and the rate of transfer by diffusion can be

analyzed by an equation of Arrhenius type, as shown below;

Where,

Q – Activation energy

R – Gas constant

T – absolute temperature

The growth is controlled by the rate of heat transfer from the casting. Since there is a temperature

gradient towards the casting surface, the growth occurs in a direction opposite to heat flow. For

growth to occur more atoms must join the solid than leave it and for this to happen the

temperature of the interface must be slightely below the equillibrium freezing temperature. This

means some amount of undercooling must exist if the interface is to advance.

Figure 4.4: Material transfer during growth

In case of pure metals, the undercooling can be produced only by thermal means (thermal

undercooling) but in alloys the undercooling may be produced by the changes in temperature as

well as composition which is termed as constitutional undercooling.

Pure metals – thermal undercooling

Alloys – constitutional undercooling

The presence of undercooling alters the growth morphology (i.e., the advance of solidification

front); a faster advancing interface experiences a transition of the solidification front from planar

to cellular and to dendritic from at even higher rates.

Rate of transfer = constant x e - Q/RT



Figure 4.5: Transition of growth morphologies due to undercooling

i) Growth in pure metals:

The heat transfer from the casting to the cooler mould produces a positive temperature gradient,

as a result the solid-liquid interface advances progressively as a flat plane under conditions of

slow cooling and steep temperature gradients. Here the latent heat of crystallization is insuficient

to reverse the direction of heat flow.

If the evolution of latent heat is sufficient enough to reverse the temperature gradient at the

interface i.e., a negetive temperature gradient, then minimum temperature in the liquid will no

longer be adjacent to the interface; hence growth by general advance of a smooth solidification

front gives way to other modes of growth.



Figure 4.6: Temperature gradient

ii) Growth in alloys:

The growth structure in alloys depends on constitutional undercooling.

Let k0 – be the distribution coefficient.

For k0 < 1, i.e., when the concentration of the solute in the solid is less than that of the liquid,

there must be a rejection of the solute into the liquid at solid-liquid interface and hence the liquid

is enriched with the solute concentration and if sufficient time is not allowed for the solute to

distribute itself throughout the liquid, a concentration gradient is developed which promotes

constitutional undercooling.

Also for k0 > 1, there is a depletion of solute concentration in the liquid adjacent to the interface

which inturn gives rise to constitutional undercooling if sufficient time is not allowed for solute

redistribution.

The variation in concentration of solute in the liquid and solid gives rise to concentration

gradient which in turn gives rise to differential freezing. Differential freezing promotes growth in

a manner other than by the advance of a smooth interface.

𝑘0 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑜𝑙𝑖𝑑

𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 =

𝐶𝑆

𝐶𝐿



Figure 4.7a: Variation of solute concentration (when k0 < 1)

Figure 4.7b: Variation of solute concentration (when k0 > 1)



4.2.4 Dendritic growth

Pure metals solidifying with a negative temperature gradient may result in uneven projection of

interface due to the thermal undercooling. The tip of the projection is at a region of greater

undercooling than the remainder of the interface and will have a tendency to grow further into

the liquid. The solids grow in a stem perpendicular to the surface. The latent heat evolved tends

to lower the amount of undercooling at the main interface. The protrusion grows into a spike

while the growth of main interface is somewhat retarded. The spike grows and branches develop

on it, this branched structure is known as a dendrite. The rate of dendritic growth depends upon

the amount of undercooling in the liquid ahead of the advancing dendrite.

Figure 4.8: Dendritic growth structure

In case of alloys as it solidifies, it tends to reject solute at the solid-liquid interface. The rejected

solute elements lower the melting point of the liquid adjacent to the freezing front and tend to

inhibit further solidification. Freezing then continues by dendrites reaching out into the residual

liquid, the dendrite is the result of the preferred growth at an edge or corner of an existing

crystallite. Dendritic growth is most common in alloys (commercial casting alloys forming solid

solution).

Dendritic growth in a pure metal can only be detected by interrupted freezing and decantation,

but is evident in alloys through the persistence of compositional differences, which can be

revealed on etching.



4.2.5 Solidification curves

A solidification curve (or cooling curve) is a record of material temperature vs time, as it cools

down from its molten state to room temperature. It can be used to determine the phase transition

temperatures of materials (pure metals or alloys).

i) Solidification in pure metals

Pure metals have a clearly defined melting or freezing point. i.e., it solidifies at a constant

temperature. The evolution of latent heat is associated with solidification and as a result

temperature remains constant. An equilibrium cooling of pure metal maybe assumed as shown in

figure 4.9. It is also seen that if a pure metal is cooled rapidly or otherwise when it is very pure it

may cool with some amount of undercooling.

Figure 4.9: Cooling curve for pure metals

Skin formation in pure metals:

When pure metals are allowed to solidify in a mould, the portion of the molten metal next to the

mould wall begins to solidify, this metal solidifies in the form of solid skin and then liquid metal

tends to freeze on it. The solid skin progresses towards the center of the mould from mould walls

and due to this progressive solidification, successive layers of molten metal buildup in the form

of solid skin. As the solid metal wall thickness increases the liquid level in the mould falls

because of solidification shrinkage. This causes pipe defects and hence use of risers becomes

necessary.



Figure 4.10: Skin formation in pure metals

Also pure metals have high melting temperature as compared to alloys due to which they exhibit

difficulties in casting especially while pouring due to severe metal-mould reactions. Moreover it

is prone to cracking and other defects due to their mode of solidification.

ii) Solidification in alloys

Solidification in solid solution alloys occurs over a range of temperature as shown in figure 4.11

below. In alloys the amount of undercooling required is less as compared to pure metals and

hence the nucleation occurs with ease. A rapidly solidifying alloy may show considerable

amount of undercooling owing to differential freezing.

Figure 4.11: Cooling curve for alloys



Alloy solidifying with a mushy zone:

Figure 4.12: Mushy type solidification in alloys

When an alloy solidifies by rejecting solute at the solid liquid interface, the rejected solute

elements lower the melting point of the liquid adjacent to the interface. A mushy zone is thus

formed due to several dendrites reaching out into the residual liquid. Pure metals will have a

narrow mushy zone (steep temperature gradient). Chilling effect brings about steep temperature

gradients thus a narrow mushy zone can be achieved.

4.3 STRUCTURE OF CASTINGS

4.3.1 Significance and control of cast structure

The metallographic structure of a casting consists of

Grain size, shape and orientation

Distribution of alloying elements

Underlying crystal structure and its imperfections

The above stated attributes of a metallographic structure are acquired during solidification and

they define the properties of a casting. These attributes are influenced by various factors such as

the thermal conditions, alloy constitution, conditions for nucleation and growth, etc.



The principal factors governing the final metallographic structure are:

1. Casting alloy properties (alloy constitution and thermal properties)

2. Mould properties (design and thermal properties)

3. Solidification process parameters (pouring and casting temperature, conditions for

heterogeneous nucleation, mode of solidification, etc.)

4. Subsequent heat treatments such as annealing, normalizing, etc.

Table 4.2: Influences of casting variables upon structure

Variable Effect Structural tendency

Increasing pouring

temperature

Decreases freezing rate and inhibits nucleation Coarse columnar

Increases temperature gradient Columnar

Decreasing mould

Temperature

Increases freezing rate Fine equiaxed


Decreasing pouring

rate


Increases mechanical disturbance Fine equiaxed

4.3.2 Grain structure

The grain structure is defined by the grain size, grain shape and its orientation. Grain size is a

very important factor in relation to strength, usefulness and other physical properties. A fine

grained structure offers better strength over a coarse grained structure but suffers low ductility.

The other advantages of a finer grained structure includes

increased impact toughness

improved machining finishes

mitigate cracks and quenching distortions

The grain size in the final product phase depends on the relative rates of nucleation and growth.

As each nucleating particle becomes a grain in the final product; a high nucleation rate means a

large number of grains, moreover with low growth rate more time is available for further

nucleation to take place in the parent phase. Thus with a high nucleation rate and low growth rate

a fine grained structure can be obtained.



Typical grain structure

A solidifying melt can assume any of the grain structures as shown in figure 4.13 below.

In case of pure metals a columnar grain structure with the grains growing from the mould wall

towards the casting is observed with a thin layer of equiaxed grains near the mould due to

chilling effect. On the other hand alloys experience better undercooling as compared pure metals

and hence they usually solidify with a completely equiaxed structure.

The metals with minor alloying constituents may solidify with a grain structure intermediate to

pure metals and alloys. The solidification begins similar to pure metals with an equiaxed chill

zone near mould walls and then followed by a columnar zone; as the concentration of the solute

in the remaining melt becomes richer the mode of solidification switches to that of an alloy type

due to increased undercooling and the remaining melt solidifies as equiaxed crystals.

Figure 4.13: Typical grain structures in (a) pure metals (b) metals with

minor alloying constituents, and (c) alloys



4.3.3 Grain refinement and modification

Grain refinement is a process of refining the grain structure to impart better properties. It is

simply a technique to control the grain size of the cast structure. Grain refinement aims at

reduction or elimination of columnar zone and increase in number of equiaxed grains

Mechanism of grain refinement and modification

Various practical measures employed for grain refinement and modification are

i) variation of cooling rate

ii) chemical treatment of melt

iii) agitation during freezing

iv) subsequent heat treatments

i) Variation of cooling rate

The association of rapid cooling with fine grain size arises from the influence of undercooling on

the comparative rates of nucleation and growth. Higher nucleation rate is observed due to better

undercooling and less time is available for the grain growth.

ii) Chemical treatment of the melt

Effective grain refinement can be achieved by inoculation which involves the addition to the

melt of small amount of substances designed to promote nucleation. The effectiveness of

inoculants or refiners are determined by their structural affinity (i.e., similarity of symmetry and

lattice parameter)

Example: Silica, Aluminium, titanium are used as refiners during casting of steel.

iii) Agitation during freezing

Nucleation can be brought about by physical disturbance of undercooled liquid, for example by

stirring or gas evolution.

This may be attributed to

a) the widespread distribution of nuclei originally produced at the surface

b) the fragmentation of already growing crystals.

Vibration too is a well-known method of structural refinement. Other practical possibilities

include electromagnetic stirring with the aid of induction coils.

iv) Subsequent heat treatment

With the above techniques the grain size control can be achieved during the casting stage but if

they are found ineffective then the cast specimens may be subjected to subsequent heat treatment

operations such as annealing, normalizing, etc.



References:

1. A text book of Foundry Technology – O.P. Khanna

2. Castings - ASM Handbook, Volume 15

3. Foundry Technology – Peter Beeley

4. Castings – John Campbell

5. Material Science & Metallurgy – K.R.Phaneesh

6. Materials Science for Engineers – James F. Shackleford

SOLIDIFICATION PHASE DIAGRAM & STEELS · 2017. 8. 24. · Solidification in solid solution alloys...

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Transcript of SOLIDIFICATION PHASE DIAGRAM & STEELS · 2017. 8. 24. · Solidification in solid solution alloys...