Dislocation and Strengthening Mechanisms

Materials Science-Part II
• Instructor: Dr. Lou X. W. (David) • Textbook: Materials Science and Engineering: An Introduction, by William D. Callister, 7th Edition • Office: N1.2-B1-09 • Email: [e-mail] • www.ntu.edu.sg/home/xwlou/ • Lecture: LT12, Thursdays 10:30 am – 12:30 pm • Tutorials: B12, TR18, Mondays 9:30 – 10:30 am
• • B11, TR18, Mondays B10, TR18, Fridays 10:30 – 11:30 am 2:30 – 3:30 pm

• • • •

Tutorials weeks will be announced. CA: 2 Homeworks; 30% (50x30%=15 marks) Office hours: by appointment or drop by Final exam: Close book.
Chapter 7 - 1

Chapter 7:
Dislocations & Strengthening Mechanisms
ISSUES TO ADDRESS...
• How are strength and dislocation motion related? • How do we increase strength? • How can heating change strength and other properties?

Chapter 7 - 2

Dislocations & Materials Classes
• Metals: Disl. motion easier.
-non-directional bonding -close-packed directions for slip. electron cloud
+ + + + + + + + + + + + + + + + + + + + + + + +

ion cores

• Covalent Ceramics
(Si, diamond): Motion hard. -directional (angular) bonding

• Ionic Ceramics (NaCl):
Motion hard. -need to avoid ++ and - neighbors.

+ +

+ -

+ +

+ -

+ +

+ -

+ +

Chapter 7 - 3

Dislocation Motion
Dislocations & plastic deformation • Cubic & hexagonal metals - plastic deformation by plastic shear or slip where one plane of atoms slides over adjacent plane by defect motion (dislocations).

• If dislocations don't move, deformation doesn't occur!

Chapter 7 - 4

Dislocation Motion
• For an edge dislocation, dislocation moves along slip plane in slip direction perpendicular to dislocation line • Slip direction same direction as Burgers vector

Edge dislocation

Screw dislocation
Chapter 7 - 5

Deformation Mechanisms
Slip System
– Slip plane - plane allowing easiest slippage
• Wide interplanar spacings - highest planar densities

– Slip direction - direction of movement - Highest linear
densities