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 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 20um shows higher tensile strength than a sample of the same alloy with an average grain size of 500um.

Explanation / Answer

Metals plastically deform through diclocation motion. Hence, any strengthening mechanism looks to impede dislocation motion.

a) Strengthening mechanism: Work hardening.

Plastic deformation leads to dislocation motion. Two dislocations may interact by generating stress fields which impede dislocation motion. Also, as a crystal has many slip directions, dislocations may cross slip and get entangled with one another (refer Frank Partials). This entaglement hinders dislocation motion, thus increasing hardness. Dislocations are effectively pinned.

b) Strengthening mechanism: Solid solution strengthening

As the two atoms of Cu and Zn have different size, they create compressive/ elastic stress fields in the crystal. These stress fields hinder dislocation motion, causing strengthening.

c) Strengthening mechanism: Precipitation hardening

Presence of fine particles cause lattice distortions in the original material. Depending on the size and crystal structure of these fine particles (precipitates), the lattice distortion can be tensile or compressive in nature. Both these stresses in the lattice hinder dislocation motion, causing strengthening.

d) Strengthening mechanism: Grain boundary strengthening

The smaller the grain size, the more grain boundaries in the material. Dislocations cannot jump accross the high potential barrier of grain boundaries. Thus dislocation motion is impeded, strengthening the material.

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