CHAPTER 7: DISLOCATIONS AND STRENGTHENING

EDGE DISLOCATIONS

SCREW DISLOCATIONS

CHARACTERISTICS OF DISLOCATIONS

SLIP SYSTEMS

SLIP IN SINGLE CRYSTALS

max(max) )cos(cos R

coscosR Resolved Shear Stress

Example ProblemConsider a single crystal of BCC iron oriented such that a tensile stress is applied along [010] direction.

Compute the resolved shear stress along a (110) plane and in a [ 11] direction when a tensile stress of 52 MPa is applied

1

STRENGTHENING BY GRAIN SIZE REDUCTION

yield o kyd 1/2

SOLID SOLUTION STRENGTHENING

SOLID SOLUTION STRENGTHENING

COLD WORKING

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tensile force

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COLD WORKING

• New crystals are formed that: --have a small disl. density --are small --consume cold-worked crystals.

33% coldworkedbrass

New crystalsnucleate after3 sec. at 580C.

Fig. 7.19 (a),(b), Callister 6e. (Fig. 7.19 (a),(b) are courtesy of J.E. Burke, General Electric Company.)

0.6 mm 0.6 mm

RECRYSTALLIZATION

• All cold-worked crystals are consumed.

After 4seconds

After 8seconds

Fig. 7.19 (c),(d), Callister 6e. (Fig. 7.19 (c),(d) are courtesy of J.E. Burke, General Electric Company.)

0.6 mm0.6 mm

FURTHER RECRYSTALLIZATION

GRAIN GROWTH

• Empirical Relation:

After 8 s,580C

After 15 min,580C

dn do

n Ktelapsed time

coefficient dependenton material and T.

grain diam.at time t.

exponent typ. ~ 2

0.6 mm 0.6 mm Fig. 7.19 (d),(e), Callister 6e. (Fig. 7.19 (d),(e) are courtesy of J.E. Burke, General Electric Company.)