Mechanical Alloying and Characterisation

Question 1

Name the 3 main mechanisms in mechanical alloying

Answer 1

1) Particle size reduction
2) Grain refinement
3) Mechanochemical Reactions

Question 2

Explain cold-welding during mechanical alloying

Answer 2

Particles are welded together by 3-way collisions with other particles, and the ball in the mill
Temporarily larger particles form

Question 3

For metal powders, how does particle size reduction occur?

Answer 3

Plastic deformation for metals
Particle fracture for non-metals

Question 4

How does grain refinement occur in mechanical alloying?

Answer 4

Trans-grain fracture

Question 5

What does the smallest grain size correspond to?

Answer 5

The smallest grain size which prevents crack propagation and nucleation

Question 6

Outline the 3 step mechanism for grain refinement

Answer 6

1) Dislocation concentration in shear bands
2) Sub-grain formation by dislocation annihilation and recombination
3) Grain formation by sub-grain rotation and boundary sliding

Question 7

What provides the driving force for mechanochemical reactions?

Answer 7

Applied mechanical force

Question 8

Outline the basic mechanochemical reaction

Answer 8

M1X + M2 → M1 + M2X

Question 9

What can be used to document the size and morphology of grains during mechanical alloying?

Answer 9

Microscopy, SEM, TEM
Step-wise with time to document change

Question 10

Name a diffractive technique used to analyse structure during mechanical alloying

Answer 10

XRD
Peak location = Identity
Peak Width = Crystallite Size

Question 11

Name 2 techniques used to quantify phase changes

Answer 11

DSC - Differential Scanning Calorimetry
DTA - Differential Thermal Analysis

Graphs indicate points of transition during the process due to peaks which correspond to certain enthalpic changes