Final Project On Course:

SOLIDIFICATION OF CASTING TEHCHNOLOGY

Based on article: Lauren A., Calabrese, T., and Silverman, B.S. (2003). Metallurgical and Materials Transactions, 21, 267-294.

Written by: Dov BarkayStudent no: 22385991

Course No: 316424

OUTLINE

The Liquid Phase

Nucleation

Crystal Growth, Solid-Liquid Interface

Dendritic Growth

Freezing in Alloys

Freezing of Ingots

Eutectic Freezing

Metallic Glass

SOLIDIFICATION

Why do we need to know about solidification?

Many metal components are formed as castings

The primary initial form for wrought alloys is the cast ingot

Welding processes involve solidification phenomena

Alloy powders are often atomized, and rapidly solidified

Metastable microstructures, phases and glasses are becoming

more widely used.

What is the liquid phase?

We know a great deal about ordering and atomic arrangements

in the solid phase because of information from x-ray, electron,

neutron diffraction and TEM observations.

The gas phase is at the opposite extreme, with little or no

interaction between atoms, complete randomness, and

disorder.

*The average separation of atoms is close to that found insolid. (The change in density on melting of metals isabout 2-6%)

*The latent heat of fusion is 1/25 to 1/40 of the latent heatof vaporization.

*X-ray diffraction shows about the same coordination ofatoms as in the solid. There is short range order, but nolong range order.

*Diffusion rates are several orders of magnitude greaterthan in the solid state, meaning that there are smallerenergy barriers for atom movement.

*Most liquid metals have similar properties, while their

solids do not.

Temperature

TemperatureTemperatureTemperature

SL

G

S S S

LLL GGG

Single componentphase diagram withisobars, and relatedfree energy plots

NUCLEATION

The solidification of metals occurs by nucleation and growth.

The same is true of melting (maybe) but the barriers are much less.

Thus, it is possible to achieve s ignificant supercooling of pure metal

liquids.

Undercooling or supercooling is achieved by suppressingheterogeneous nucleation. “Prenucleants” are assumed to exist in

the liquid metal. In many processes, homogeneous nucleation isassumed to occur, but experimental evidence suggests otherwise.

W hat is the evidence?

Before stirring After stirring

Temperature Temperature

The Experiments of Perepezko

A n emulsion is made with the metal dispersed in a molten salt. W hen the

mulsion makes much finer droplets, more undercooling can be achieved

efore the evidence of freezing is noted.

e

b

Metal Max undercooling, K

g 88d 110b 153l 160n 187g 227u 230u 236

HCPASAACFe 286Mn 308Ni 365PNt 370b 525

igure 14.4 in text shows undercooling for a complete range of Cu-Ni alloysF

Temperature

Nucleation RateGrowth Rate

Nucleation is fastest at lowertemperature

Crystal Growth from the Liquid Phase:

he movement of a boundary separating liquid from solid, under theT

influence of a temperature gradient normal to the boundary, is the result of

two different atomic movements.

A

A

toms leave the liquid and join the solid = rate of attachment

toms leave the solid and join the liquid = rate of detachment

Nature of the Solid-Liquid Interface:

Usually we consider only the two extreme cases, as illustrated below:

P lanes of looser atomic packing can better accommodate an atom that

eaves the liquid.l

A

T

growing crystal will assume faces that represent s low growing planes.

he rate of movement of a solid-liquid interface varies linearly with the

amount of undercooling at the interface.

T he velocity, V, is related to the net frequency of atoms joining the solid,

ince atoms are continuously leaving or joining the interface.s

A simplified relation is:

V = BDT

S

R

table Interface Freezing:

emoval of the heat of fusion from the interface–if there is a thermalradient perpendicular to the interface, increasing into the liquid, a stablelanar interface can move forward as a unit. This also depends onndercooling.

gpu

LiquidSolid

Velocity

Temp.Tm

Heat FlowA sketch of the moving solid-liquid interface where the temperature gradien til

s high, the velocity is also high, and the heat flow is sufficient to remove theatent heat of solidification.

DENDRITIC GROWTH

ure Metals:PTm

SolidLiquid

This represents a temperature inversion during freezing. The temperature

ecreases into the liquid, ahead of the solid-liquid interface.d

Liquid

GrowthDirection

Solid

Dendrite arm

The first stage of dendritic growth

Dendritic growth can be quite complicated!

Ta

TaTb Secondary

arms

How do such complicated structures arise?

Secondary arms arise because there is afalling temperature gradient between armssuch that Ta > Tb

Dendritic Growth Directions in Various Crystal Structures

CC <100>

CC <100>

F

B

H

B

CP <10-10>

CT (tin) <110>

n cubic crystals the <100> dendrite arm growth d irection leads to theI

s

a

econdary arms being perpendicular to the primary arms. Also, the tertiary

rms are perpendicular to the secondary arms.

a

c

bd

e

Compositio n, Percen t B0

Freezing In Alloys

Part of an isomorphous binary phase diagram

dxHeat Flow Direction

Solid of co mp osition a Liquid of composition b

eba

eba

simple case of one-dimensional freezing, showing how compositionA

changes as solidification proceeds from left to right.

METE 327 Physical Metallurgy Copyright 2008 Loren A. Jacobson 5/16/08

Freezing of Ingots

Very important practical implications

Much effort devoted to controlling this

Plate, beams and sheet are worked from ingotsto their final shape

Several zones:

-

-

-

Chill zone—nucleation and growth

Columnar zone—dendritic growth

Central zone-- constitutional supercooling

Eutectic Freezing

Usually a coupled microstructure

-

-

Plates

Rods

Growth is controlled by diffusion in the liquidahead of the two phase solid

Spacing can be based on the growth rate

Very rapid solidification

-

-

Glass

Uniform distribution of particles

A

To

B

Tosolid

so lu tion

Metallic Glass (1)

Intersecting orcontinuous To curvesgive metastable solidsolution

A

formation. Tg is theglass transitiontemperature.

B

TgTo To

glass

Metallic Glass (2)

Non-Intersecting Tocurves lead to thepossibility of glass

Rapid cooling to a temperature below Tg, for compositions between the twoTo curves can result in the formation of a metallic glass.

Viscosity of the liquid increases rapidly as the temperature is lowered, andfinally atomic movement is too slow to permit crystallization.

Metallic glass applications include transformer cores and golf clubs.