EngEvans

Question 7

Dislocations are the underlying cause of plasticity in most metals and alloys. As such, they play an important role in determining the mechanical behaviour of many materials.

a) Draw a left-hand screw (LHS) dislocation and give the shear stress that will move it to the left on the page. Do this graphically.

Marks 4

Use your knowledge of dislocations to explain each of the following observations about material properties.

b) Ceramics are generally very strong when loaded in compression but fail in a brittle manner at relatively low stresses when loaded in tension.

Marks 5

c) Many materials, like low C steel, exhibit a fatigue limit. What is a fatigue limit and how is it related to dislocations?

Marks 5

d) Why is the fracture toughness of aircraft grade titanium alloys greater than that of armour grade silicon carbide?

Marks 5

e) Explain why BCC metals exhibit a Ductile-Brittle Transition Temperature while FCC metals do not.

Marks 4

The strengthening mechanisms that are available for improving the strength of commercial alloys are also related to dislocations. Here, the name of the game is slowing down or impeding the dislocations. In each of the following cases identify the obstacles that are used to slow down the dislocations.

f) Almost all FCC metals are stronger after cold rolling or forging – why?

Marks 4

g) An Al alloy with 3% Cu that is solutionized, quenched, and heat-treated or aged at a moderate temperature has a strength that at first increases with heat-treat time and then decreases if the alloy is “over-aged”. Why?

Marks 4

h) The room temperature yield strength of a pure metal that is heat-treated such that the grain size of the material increases from 10 microns to 40 micron will decrease by a

factor of 2 – why?