15 Dislocations

530.352 Materials Selection

Lecture #16: Dislocations Tuesday October 18th, 2005

Theoretical strength : .

U 2σTh = Eε= E ∆L/Lo 2σTh = E 0.25ro/ro = E/4

~ 2σTh

F

E

σTh

ro 1.25ro

σTh = E/8

r

Atomic description of plasticity:

Slip easier with defects

Hard to pull all at once ∆x

Much easier

Defects in crystals 0-D • vacancies

1-D • dislocation lines

2-D • fault planes • grain boundaries

Dislocation is the boundary between slipped and unslipped regions.

Edge dislocations

Burger’s Circuit Burger’s vector (b) F

S

RHSF convention :

Start with atom “S”. Count 4 to the right. Count 4 down. Count 4 to the left. Count 4 up. Finish with atom “F” b connects S with F

Geometry of Edges

t

Note: b is perpendicular to t

b

t x b -> extra half plane

Motion of Edges τ τ t b

τ

|b|

Motion of Edges τ

t |b| b

τ

Screw dislocations

Burger’s Vector of Screws

Note: F S

b is parallel to t

Motion of Screws

τ

Motion of Screws

τ