Solidification of Single-Phase Alloys_2007

8/14/2019 Solidification of Single-Phase Alloys_2007

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Guided by

Prof. B.J. Chauhan Sir

Prepared by

Purvesh K. NanavatyME-I (Materials Technology)

Solidification of Single-Phase Alloys


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Introduction

The solidification process by which a liquid metal freezes ina mold plays a critical role in determining the properties of

the as-cast alloy

The initial uniform composition in liquid becomes non

uniform as the liquid transforms to solid

Different solidification conditions give rise to different

microstructures of the solid

Many casting defects, such as porosity and shrinkage,

depend on the manner in which the alloy is solidified in a

mold


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1. Alloy Composition

A pure metal has a specific melting point Tm,

while an alloy freezes over a range of

temperatures

This freezing range is generally represented by a phase

diagram, as shown in Fig. 1. The liquids

line represents the temperature at which the liquid alloybegins to freeze, and the freezing process is complete when

the solidus temperature is reached,

Two important factors that control solidification

microstructures


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The ratio of the solid to liquid

composition at a given temperature

is called the solute distribution

coefficient k.The first solid that forms at

temperature TL will have a

composition kCo, which is lower

than the liquid composition Co.

Thus, the excess solute rejected

by the solid will give rise to a

solute-rich liquid layer at the

interface.

This increase in liquid

composition, along with thelowering of

temperature, gives rise to solute

segregation patterns in the solid

A single-phase region of a phase diagram showing the liquidus and the solidus lines


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The buildup of solute in

liquid requires diffusion of

solute in liquid for further

growth. For efficient

distribution of the

solute in liquid.

the interface may

change its shape. In

addition to the solute

transfer, the interface

shape is governed by theeffective removal of the

latent heat

of fusion.


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Heat Flow Conditions

Two distinctly heat flow conditions may exist in a

mold. different In the first case, the temperature gradients in theliquid and the solid are positive such that the latent heatgenerated at the interface is dissipated through thesolid. Such a temperature field gives rise to directional

solidification and results in the columnarzone in acasting.

In the second case, an equiaxed zone exists if the liquidsurrounding the solid is under cooled so that a negativetemperature gradient is present in the liquid at thesolid/liquid interface. In this case, the latent heat offusion is dissipated through the liquid. Such a thermalcondition is generally present at the center of the mold.


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Fig. 2 Effect of increasing growth rate on the shape of the solid/liquid

interface in a transparent organic

system, pivalic acid-0.076 wt% ethanol, solidified directionally

atG = 2.98 K/mm (75.7 K/in.). (a) v = 0.2 m/s(8 in./s). (b) v = 1.0m/s (40in./s). (c) v = 3.0m/s (120

in./s). (d) v = 7.0m/s (280in./s)

DA B C


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Planar interface growth occurs only under directional

solidification conditions and, for alloys, only under low

growth rate or high-temperature gradient conditions.

consider an interface that is moving at a

constant velocity v, with heat flowing from the

liquidto the solid under temperature gradients

GL and GS in liquid and solid, respectively

Constitutional supercooling diagram. The

solute concentration profile in the liquid

gives rise to the variation in the equilibrium

freezing temperature Tf of liquid near the

interface. The actual temperature in liquidis given by line 1, and the slope ofTf at the

interface is given by line 2. A supercooled

liquid exists in the shaded region.


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Cellular and Cellular Dendritic Structures. Under

directional solidification conditions.

Cellular /cellular dendritic interface is observed

Which have two important characteristics. First, the length of the

cell is small, and it is of the same order of magnitude as the cellspacing (Fig. b).

Second, the tip region of the cell is broader and the cell has alarger tip radius. At higher velocities, a cellular dendritic structureforms (Fig. c) in which the length of

the cell is much larger than the cell spacing. Also, the cell tipassumes a sharper, nearly parabolic shape, which is similar

to the dendrite tip shape so that the term cellular dendritic is usedto characterize this structure


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Dendritic Structures.

. Dendritic structures are characterized by the formation of side branches

(Fig. 2d). These side branches, as

well as the primary dendrite, grow in a preferred crystallographic direction,

Directionally solidified peritectic cobalt-

samarium-copper alloy showing primarycobalt dendrites when

the Co17Sm2 matrix is etched away. The

cut surfaces in the foreground indicate the

structure that would be

observed on the plane of polish if the

matrix were not etched away.

Formation of cells with intercellular

eutectic in the directionallysolidified Sn-20Pb alloy. G = 31

K/mm

(79 K/in.) and v = 1.2m/s (48

in./s). The nearly flat eutectic

interface is at the eutectic

temperature


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Dendritic structure in a directionally solidified

transparent organic system, succinonitrile-4.0

wt%

acetone. G = 6.7 K/mm (170 K/in.) and v = 6.4m/s (260 in./s). The secondary dendrite arm

spacing

increases with the distance behind the tip.

Formation of equiaxed crystals at the center of the

mold during the solidification of transparentammonium chloride-water mixture


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Schematic of microstructure zoneformation in castings. Directional

solidification conditions give rise to

a columnar zone, while an equiaxed

zone is formed at the center where

the liquid is under cooled

References:-

ASM Metals Handbook Vol.15 Casting (R. Trivedi, Iowa State

University; W. Kurz, Professor, Swiss Federal Institute of Technology,

Switzerland)

ASM Metals Handbook Vol.09 Metallography & Microstructures

Solidification of Single-Phase Alloys_2007

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