Describe the difference between the martensite and austenite

Describe the difference between the martensite and austenite structures. Also, briefly discuss how memory wire (nickel titanium) is applied in everyday life along with advantages and disadvantages of its use.

Solution

Martensite

Martensite is formed in steels when the cooling rate from austenite is sufficiently fast.

It is a very hard constituent, due to the carbon which is trapped in solid solution.

The transformation to martensite does not involve atom diffusion, but rather occurs by a sudden diffusionless shear process.

The term is not limited to steels, but can be applied to any constituent formed by a shear process which does not involve atom diffusion or composition change.

Austenite

Austenite was originally used to describe an iron-carbon alloy, in which the iron was in the face-centred-cubic (gamma-iron) form.

It is now a term used for all iron alloys with a basis of gamma-iron.

Austenite in iron-carbon alloys is generally only evident above 723°C, and below 1500°C, depending on carbon content. However, it can be retained to room temperature by alloy additions such as nickel or manganese. Similarly, ferrite was a term originally used for iron-carbon alloys, in which the iron was in the body-centred cubic (alpha- or delta-iron) morphology, but is now used for the constituent in iron alloys, which contains iron in the alpha- or delta-iron form.

Delta ferrite is the high temperature form of iron, formed on cooling low carbon concentrations in iron-carbon alloys from the liquid state before transforming to austenite.

In highly alloyed steels, delta ferrite can be retained to room temperature.

Nickel-titanium smart alloys are the most successful
but the most expensive.

The material is called Nitinol.

Advantages
Nitinol is is used to make dental braces, spectacle frames and stents.

Dental braces are wires which are used to correct the growth of teeth.
Nitinol can provide a constant force on the teeth
as it tries to return them to the correct shape.

Spectacles are glasses which people use to see better.
The frames often get bent and
Nitinol frames can return to their original shape on warming.

A stent is a tube inserted into a damaged blood vessel.
The stent strengthens the blood vessel to prevent it
collapsing or bursting. Small nitinol stents are inserted cold
and expand inside the blood vessel to the correct diameter
as the stent warms up to the patient\'s body temperature.

Disadvantages

High Cost
and their increased metal fatigue.
Metal fatigue happens when bending or twisting
of a metal causes it to break.