Solidification Heat Treatment

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Sp’ 05 W. Li

Solidification and HeatTreatment

Solidification

Crystal structures

Structure-property relationships

Heat treatment



Pure Metal Solidification

• Temperature remains

constant while grains

grow.

• Some metals undergo

allotropic transformation

in solid state.

• For example on cooling

bcc -iron changes tofcc -iron at 1400 C,

which again to bcc -

iron at 906 C.



Nucleation and Grain Growth

Nucleation;

Homogeneous nucleation: very pure metal, substantialundercooling (0.2Tm)

Heterogeneous nucleation: nucleation agents (5ºC

undercooling) Grain growth

Planar: pure metal

Dendritic: solid solution

Grain size

depends on number of nuclei and cooling rate.



Crystal Nucleation and Growth

“ Manufacturing Processes for Engineering Materials,” by Serope Kalpakjian



Crystal Structure of Metals

Atoms arrange themselves into various orderlyconfiguration, called crystals.

The arrangement of the atoms in the crystal is

called crystalline structure.

The smallest group of atoms showing the

characteristic lattice structure of a particular

metal is known as a unit cell.



Crystal Structure of Metals



Slip Systems

Deformation (dislocation) occurs on preferentialcrystallographic planes and directions, called slip

systems.

The slip plane/direction is the plane/direction with the

most closely packed atoms.

6x2=12 4x3=12 1x3=3



Slip Systems

BCC has 6 slip planes and 2 slip directions per plane (12slip systems), but distance between slip planes is small,

therefore the required stress is high. Good Strength and

moderate ductility, e.g. Steel, Titanium, Molybdenum,

Tungsten. FCC has 4 slip planes and 3 slip directions per plane (12

Slip Systems), but distance between slip planes is larger

than BCC. Therefore, probability of slip is moderate,

shear stress to cause slip is low. Moderate Strength andGood Ductility, e.g., Aluminum, Copper, Gold, Silver

HCP has 1 slip plane and 3 slip directions on that plane

(3 systems). Low probability of slip. Generally brittle

materials, e.g., Beryllium, Magnesium, and Zinc



Plastic Deformation of Single

Crystals



Theoretical Shear Strength and

Tensile Strength

Theoretical shear stress is the shear stress tocause permanent deformation in a perfectcrystal.

Theoretical or ideal tensile strength of material is

the tensile stress required to break the atomicbonds between two neighboring atomic planes.

30/~10/2

max GGbetween

a

bG

10/max

E



Solid Solutions

Most metals are not pure but contain a number of

other metallic or non-metallic elements, eitheralloying elements or contaminants. Alloying elements

are uniformly distributed in the base metal, forming a

solid solution.

Substitutional solid solution

Interstitial solid solution



Effect of Imperfections

Pure metal: dislocation

Solid solutions

Solute atoms of slightly different size distort the lattice and makesdislocation propagation more difficult, thus strength increaseswithout necessarily reducing ductility.

Interstitial elements play a similar role in impeding dislocationmobility although they can have an embrittling effect.

Interfaces, inclusions, gases



Grain Size Effect

Grain boundaries presentobstacles to dislocation

propagation. Therefore, it is

generally found that the yield

strength of a material

increase with decreasinggrain size according to the

Hall-Petch equation.

However at low strain rate

and close to Tm, dislocation

is resolved by diffusion.Material deforms by sliding

of grains or reshaping of

grains. Both processes are

easier if grain size is small.



Phase Diagrams

• A phase diagram, also called equilibrium diagram or a constitutionaldiagram, graphically illustrates the relationships among temperature,

composition, and the phases present in a particular alloy system.



Lever Rule

The composition of various phases in a phase

diagram can be determined by a procedure

called the lever rule.

Example: Calculate the relative proportions of

the phases in a Cu-Ag alloy of eutectic

composition just below the eutectic temperature.

L s

s

L s

L

C C

C C

LS

Lor C C

C C

LS

S

00

%2.232.919.7

2.919.71

C C

C C E



The Structure of a Cu-Ag Solid

Solution with 20% Ag



Iron/Iron Carbide Phase Diagram



Nonequilibrium Solidification

Microsegregation or coring



Heat Treatment

Most parts will require heat treatment either after orduring the processing for proper in-service properties

Annealing

Heat to elevated temp, hold, cool

Softens the material and removes stress

Precipitation Hardening

Diffusion of alloys to produce two phase structure that

promote good strength and ductility

(Aging – Aluminum for example)

Heat Treatment of Steel



Heat Treatment of Steel (TTT

Diagram)



Summary

Solidification process affects crystal structures

which in turn affect material properties.

Single crystal materials behave very differently

than metal alloys.

The effect of imperfections and grain size in

solid solutions.

Heat treatment can modify material properties by

changing the crystal structure.

Solidification Heat Treatment

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