Chapter 7 Dislocation and Strengthening Mechanism

Question 1

What are the different strengthening mechanisms?

Answer 1

• Grain size-reduction
• Solid solution alloying
• Precipitation hardening
• Cold working

Question 2

Describe Slip System

Answer 2

• combination of the slip direction and slip plane of material
• dislocation is most probable to occur due to the high planar and linear density of atoms which promotes more movement

Question 3

Why are HCP brittle? and FCC ductile?

Answer 3

Even both of the structure is close packed, FCC has more slip system than HCP this rendering easier slip motion thus imparting the ductile property in contrast to HCP that has significantly fewer slip system

Question 4

Briefly define deformation

Answer 4

Plastic shear or slip where one plane of atoms slides over adjacent plane by defect motion (dislocations).

Question 5

What is Slip Direction?

Answer 5

Slip direction - direction of movement - Highest linear densities

Question 6

What is Slip Plane

Answer 6

Slip plane - plane allowing easiest slippage

Wide interplanar spacings - highest planar densities

Question 7

Give the Slip Plane and Direction for FCC

Answer 7

Slip Plane, Slip Direction Number of Slip System

{111}, <1 1̅ 0>, 12

Question 8

Give the Slip Plane and Direction for BCC

Answer 8

Slip Plane, Slipe Direction Number of Slip System

{110}, , 12
{211}, , 12
{321}, , 24

Question 9

Give the Slip Plane and Direction for HCP

Answer 9

Slip Plane, Slipe Direction Number of Slip System

{0001}, <11 2̅ 0>, 3
{10 1̅ 0}, <11 2̅ 0>, 3
{10 1̅ 1}, <11 2̅ 0>, 6

Question 10

How does grain size-reduction strengthen the material?

Answer 10

• grain boundaries are barriers/obstacles to dislocation/slip
• smaller grain = more grain boundary = more obstacle
• more obstacle impede dislocation/slip movement = strengthen the material.

Relationship is presented by Hall-Petch Everything

Question 11

How does solid solution alloying strengthen the material?

Answer 11

• impurity atoms distort the lattice generate stress.
• stress can produce a barrier to dislocation motion.
• small impurities = compressive stress
• large impurities = tensile stress

Question 12

How does cold working strengthen the material

Answer 12

• introduces dislocations

* dislocation impedes dislocation movements.

Question 13

How does precipitation hardening strengthen the material?

Answer 13

Hard precipitates are difficult to shear