For each of the following observations, identify what is the likely strengthenin

Question

For each of the following observations, identify what is the likely strengthening mechanism and describe briefly (half page maximum per observation) how the mechanism increases strength, making reference to dislocation movement.

a) Plastic deformation of an aluminium alloy at room temperature increases its hardness.

b) An alloy of 80wt% copper with 20wt% zinc has higher tensile strength than pure copper.

c) The tensile strength of nickel is increased by the addition of fine particles of thorium oxide.

d) A sample of commercially pure titanium with an average grain size of 20?m shows higher tensile

strength than a sample of the same alloy with an average grain size of 500?m.

Explanation / Answer

a)

Plastic deformed Al develops a 3d dislocation cell structure , the scale of which depends upon the amount of deformation and temperature. With increasing strain the homogenous rotation of the individual cells within cell blocks becomes gradually more difficult and thus the size of cell blocks decreases. As a consequence , a high rate of work hardening is retained at high stresses leading to the improved combination of hardness and strength.

b)

. Small amounts of an alloying element added to molten copper will completely dissolve and form a homogeneous microstructure (a single phase). At some point, additional amounts of the alloying element will not dissolve; the exact amount is dependent on the solid solubility of the particular element in copper. When that solid solubility limit is exceeded, two distinct microstructures form with different compositions and hardnesses. because of the formation of these microstructures the strength increases.

c)

In oxide dispersed materials the dispersed particles influence the room temperature yield strength of the material by preventing grain growth during consolidation. The oxide particles impede dislocation motion, as they will not shear. A fine and even distribution of the oxide particles has a greater effect on tensile strength than just large volume of particles.
d)

A decrease in the grain size increases the number of grains in a polycrystalline specimen.the role of strain compatability in individual grains increases leading to a increase in the density of geometrically necessary dislocations.So the tensile strenght increases with decrease in grain size.

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