Dislocation and strengthening mechanisms

Question 1

Why is the actual yield strength much lower than the theoretical value

Answer 1

Due to dislocations and impurities

Question 2

Discuss dislocations in metals

Answer 2

-Easiest to dislocate
+non directional bonding
+close packed directions for slips

Question 3

Discuss dislocations in ceramics

Answer 3

• difficult to dislocate
  + (covalent) directional (angular) bonding
  + (ionic) need to avoid nearest neighbors of like sign (- and +)

Question 4

Discuss dislocation in polymers

Answer 4

Fail in brittle manner before slip
-dislocation is minor
- large Vol of amorphous materials -> no dislocation
-Perma deformations: stretching, rotation, disentanglement of monomers

Question 5

What is dislocation density

Answer 5

Number of dislocations/ unit volume

Question 6

Range of values of different materials regarding dislocation density (mm^-2)

Answer 6

-Carefully solidified metal crystals: 10^3
-Heavily deformed metals: 10^9 - 10^10
-Ceramic materials: 10^2 - 10^4
-Silicon single crystals: 0.1 - 1

Question 7

Define slip system

Answer 7

Combination of slip plane and slip direction

Question 8

Define slip plane

Answer 8

Plane with high PD slips easily

Question 9

Define slip direction:

Answer 9

Slip occurs most easily along direction with high LD

Question 10

There will be question about a diagram with theta, tao, slip plane and normal to slip plane

Answer 10

Ok

Question 11

How many potential slip systems in FCC, list them all

Answer 11

12 slip systems
(111) [1-10]
(-111) [01-1]
(1-11). [10-1]
(11-1)

Question 12

Write expression for resolved shear stress

Answer 12

Tr = o’ * cos(tao) * cos(theta)
Unit: Gpa

Question 13

What is the critical resolved shear stress and its range

Answer 13

When levels of RSS exceeds CRSS, slip occurs.

10^-4 - 10^-2 GPA

Question 14

At which condition and the associated minimum stress to initiate yielding

Answer 14

Tr = Tcrss -> slip/ yielding occurs

Min stress:
o’ = 2Tcrss

Question 15

Methods for strengthening materials:

Answer 15

-grain size reduction
-solid solution hardening
-Cold working

Question 16

How does grain size reduction increase material strength

Answer 16

-increase grain boundary area
-more barriers to dislocation motion
-increase yield strength, tensile strength and hardness

Question 17

How does solid solution strengthening improve strength

Answer 17

Impurity atoms distort lattice-> creates lattice strains

Lattice strains act as barriers to disc motion

Question 18

How does cold working (strain hardening) improve strength

Answer 18

Plastically deforming most metals at room temp makes them harder and stronger

-increases in disc density due to reduction in volume

Question 19

For edge, screw, mixed dislocations, cite the relation between app shear stress (apss) and the direction of disc line motion

Answer 19

Edge: disc line // apss
Screw: disc line perpendicular with apss