mme 323 materials science week 7-8 - dislocations & strenghtening mechanisms

MME 323: MATERIALS SCIENCE WEEK 7-8 :

Dislocations and Strengthening Mechanisms*

Adhi Primartomo, PhDEmail: [email protected]: Room 191 – JIC Academic Building* Source: Materials Science and Engineering; 9th Edition; W.D.Callister;

Wiley; 2011

https://sites.google.com/site/primartomo/file-cabinet

~ LECTURE OUTLINE ~

Chapter 9: Dislocations and Strengthening Mechanisms (p. 253 - 279)*

• Why Study Dislocations and Strengthening Mechanisms?

• Introduction,

• Dislocations and Plastic Deformation,

• Characteristics of Dislocations,

• Slip Systems,

• Mechanism of Strengthening in Metals,

• Strengthening by Grain Size Reduction,

• Solid Solution Strengthening,

• Strain Hardening.

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WHY STUDY DISLOCATIONS AND STRENGTHENING

MECHANISMS?(page 254)

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With knowledge of the nature of dislocations and therole they play in the plastic deformation process, weare able to understand the underlying mechanism ofthe techniques that are used to strengthen and hardenmetals and their alloys.

INTRODUCTION(page 254)

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• On a microscopic scale, plastic deformationcorresponds to the movement of a large number ofatoms in response to an applied stress.

• During this process, interatomic bond must be brokenand then re-formed.

• In crystalline solids, plastic deformation involves themotion of dislocations (linear crystal defects – Ch.6)

• Characteristics of dislocations and their involvementin plastic deformation will be discussed.

DISLOCATIONS AND PLASTIC DEFORMATION(page 254-259)

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Basic Concepts:

Edge Dislocation

Screw Dislocation


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Basic Concepts:

• Plastic deformation corresponds to the motion of large numbers of dislocations.

• An edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line.


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Basic Concepts:


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Basic Concepts:

• The process by which plastic deformation is produced by dislocation motion Slip

• The crystallographic plane along which the dislocation line moves Slip Plane

• Macroscopic plastic deformation simply corresponds to permanent deformation that results from the movement of dislocations or slip in respond to applied shear stress.


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Basic Concepts:

Edge Dislocation

Screw Dislocation


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Basic Concepts:

• All metals and alloys contain some dislocations that were introduced during: solidification, during plastic deformation and as consequence of thermal stress from rapid cooling.

• The number of dislocations dislocation density: total

dislocation length per unit volume or per square millimeter, i.e.: 109 – 1010 mm-2

heavily deformed

metals.


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Characteristics of Dislocations:

Strain energy:

• When metals plastically deformed, some fraction of the deformation energy (5%) retained internally.

Lattice strain:

• Some atomic latticedistortion exist arounddislocation line because ofthe presence of the extrahalf plane of atoms.


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Slip Systems:

• Dislocations don’t move with the same degree of ease on all planes.

• There is a preferred plane Slip Plane

• In the plane, there is preferred direction Slip Direction

• Slip plane usually has the densest atomic packing greatest planar density

MECHANISMS OF STRENGTHENING(page 266-271)

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Strengthening by Grain Size Reduction:

• The size of the grains influences mechanical properties.

• During plastic deformation, dislocation motion must take place across this boundary.

• The grain boundary acts as a barrier to dislocation motion for two reasons:

1. The two grains are of different orientations, a dislocation passing into next grain must change its direction of motion becomes more difficult as the misorientation increases.

2. The atomic disorder within a grain boundary region results in a discontinuity of slip planes from one grain into the other.


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• Hall-Petch Equation – dependence of yield strength on grain size.

“A fine-grained material (small grain) is harder and stronger than coarse-grained (big grain) because fine-grained has a greater total grain boundary to restrict dislocation motion”


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Solid-Solution Strengthening:

• High-purity metals are almost softer and weaker than alloys composed of the same base metal.

• Increasing concentration of the impurity results in increase in yield strength.

• Alloys stronger than pure metals because impurity atoms that go into solid solution impose lattice strains on the surrounding host atoms.

• Lattice strain field interaction between dislocation and these impurities atoms result: dislocation movement is restricted.


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Solid-Solution Strengthening:


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Strain Hardening / Work Hardening:

• A ductile material becomes harder and stronger as it is plastically deformed.

• The dislocation density increases with cold work because of dislocation multiplication of formation of new dislocation.

• Consequently the average distance of separation between dislocation decreases – dislocation positioned closer together.

• This result that the motion of a dislocation is restricted by the presence of other dislocation.


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Strain Hardening / Work Hardening:

MECHANISMS OF STRENGTHENING(page 272)

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Example Problem 9.2:

mme 323 materials science week 7-8 - dislocations & strenghtening mechanisms

Education

plastic deformationpage

plastic deformation

movement of dislocations

nature of dislocations

study dislocations

dislocations dont

deformation energy

dislocation line