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HeatTreatment

Dr.SantoshS.Hosmani

III. Grain-size / Grain-boundary hardening, fgb:

12

= n

N

at 100X magnification

nASTM grain size number

645 dGrain diameter inmeters101 10)2(

n nASTM grain size number

49

V. Solid solution hardening, fss:

Mixture of two or more metals

Solute atoms: a zero dimensional defect or a point defect

Two types:

1. Interstitial solid solution

.

50

V. Solid solution hardening, fss:

Interstitial Solid Solution

Perfect Cr stal Distortion caused by a

large interstitial atom

51

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V. Solid solution hardening, fss:

Substitutional Solid Solution

Small solute atom Large solute atom

Solute atom: a zero-dimensional point defect52

V. Solid solution hardening, fss:

Strains in theSolute

Obstacle to dislocationStrong

Alloys stronger than pure metals53

V. Solid solution hardening, fss:

annealed ferrite

Here, factors affecting hardness are:

Size difference between solute

and solvent atoms,

Elastic modulus of solute

54

V. Solid solution hardening, fss:

This conce t is relevant to allo desi n

55

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IV. Precipitation Hardening, fppt:

This conce t is directl relevant to heat-

treatment of alloys, e.g. ???

56

IV. Precipitation Hardening, fppt:

Hardness increases as a function of time.

Al-Cu alloys:

In 1906 Alfred Wilm a metallurgist working

at Dren in Germany quenched an

experimental AlCu-alloy after annealing it and

left the specimen on the bench over the

weekend. Testing it a few days later heeekend. Testing it a few days later he

found that both hardness and strength had

increased simply by having been left at room

t t W l th D lemperature. Wilm gave the name Duralumin

to his alloys after the place where they were

first made.

Alfred WilmBorn: June 25, 1869

Died: August 6, 1937

IV. Precipitation Hardening, fppt:

Hardness increases as a function of time.

Al-Cu alloys:

As-quenched

As-quenched

hardness

Ref.: Book by D.A. Porter, & K.E. Easterling 58

IV. Precipitation Hardening, fppt:

59

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IV. Precipitation Hardening, fppt:

b =

64 65Ref.: Book by D.A. Porter, & K.E. Easterling

Lattice misfit =>

66Volume misfit =>

Ref.: Book by D.A. Porter, & K.E. Easterling

Misfit strain energy:

67Ref.: Book by D.A. Porter, & K.E. Easterling

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68Ref.: Book by D.A. Porter, & K.E. Easterling 69Ref.: Book by D.A. Porter, & K.E. Easterling

70

Ref.: Book by D.A. Porter, & K.E. Easterling

Thermodynamics & Kinetics aspects for aging process

G

0Q

Greactants

Gdriving force =

G1+G2+G3+G4 4+

Gproducts

Reaction state

The activation ener barrier to the

formation of each transition phase (i.e.

& ) is very small in comparison to the

barrier against the direct precipitation of

71

e equ r um p ase .e. u2

Ref.: Book by D.A. Porter, & K.E. Easterling

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Thermodynamics & Kinetics aspects for aging process

Figure: Schematic diagram showing the total free energy of the alloy versus time

72

Ref.: Book by D.A. Porter, & K.E. Easterling

Thermodynamics & Kinetics aspects for aging process

73

Ref.: Book by D.A. Porter, & K.E. Easterling

Figure: Hardness as

time for an Al-4Cu

alloy.

Transmission electron micrographs:

74

Thermodynamics & Kinetics aspects for aging process

Why / How precipitates coarsen by consuming other precipitates?

75Figure:Schematic illustration of formation ofprecipitates in the matrix (a

and b) and their coarsening (c to f)

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Thermodynamics & Kinetics aspects for aging process

Why / How precipitates coarsen by consuming other precipitates?

76

Ref.: Book by D.A. Porter, & K.E. Easterling

Although cold-rolled and annealed sheet steels have

Strain Aging

,

close to the A1 temperature, some carbon and nitrogen

are always taken into solution (unless the steels areultralow-carbon or interstitialfree steels). Figure shows

are carbon-rich side of the Fe-C diagram. Carbon has

its maximum solubility at the A1 temperature, and itssolubility decreases with temperature to a negligible

.

relationship. Thus, if a steel is cooled from around A1

at a rate that prevents gradual relief of supersaturationby cementite formation during cooling, the ferrite at

room temperature may e g y supersaturate w t

respect to carbon and nitrogen. These interstitial

elements then may segregate to dislocations in

strained structures a rocess referred to as strain

aging, or they may precipitate out as fine carbide ornitride particles, a process referred to as quenchaging. The aging processes may occur at room

77

empera ure or urng ea ng a empera ures us

above room temperature because of the high diffusivity

of carbon and nitrogen in the bcc ferrite structure.

IV. Precipitation Hardening, fppt:

Movement of one-dimensional defects

called dislocations causes plastic

Obstacles to the movement of

dislocations cause strengthening

78

Howtointroducetheobstaclestothemotionofdislocations?

II. other dislocations giving what is calledWork Hardening (fwh),

III. grain boundaries introducingGrainsize Hardening (fgb),

IV.precipitates or dispersed particles giving Precipitation

Hardening (fppt),

V. by adding alloying elements to give Solid Solution Hardening

ss .

These techniques for manipulating strength are central

to alloy design.79

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80