Mechanisms of strengthening in metals and alloys

Question 1

Describe what the yield stress associated with the obstacles in a dislocation line shows.

Answer 1

Stress needed to be applied in order for the dislocation to by-pass the obstacles

Question 2

Describe the Frank-Read mechanism.

Answer 2

A dislocation segment is pinned at its ends will blow out under the action of an applied stress (this is an example of a dislocation source)

Question 3

Describe solid solution strengthening.

Answer 3

-Solute atoms induce distortions, which inside a stress state
-The distortions can result in tensile or compressive stresses
-Interstitial atoms ‘pull’ neighbouring host atoms towards them, this can either induce a tensile stretch or a compressive stress
-The elastic field associated with solute atoms can interact with the dislocations, these can pin the dislocation line and restrict motion

Question 4

How does yield stress change with the concentration of solute atoms during solid solution strengthening?

Answer 4

As the yield stress of alloys increases, the concentration of solute atoms also increases

Question 5

What influence do elastic interactions have on dislocations?

Answer 5

They can pin them

Question 6

What effect does plastic deformation have on the number of dislocations?

Answer 6

Number of dislocations increase, as shown by the Orowan-Bailey relation

Question 7

How are yield stress and dislocation density related?

Answer 7

Yield stress increases parabolically with the dislocation density

Question 8

What is told about the yield stress of a material if is is said to work harden?

Answer 8

Yield stress increases with a decreasing positive gradient

Question 9

Describe the Hall-Petch effect.

Answer 9

Smaller grains have higher yield stress compared to larger grains