How Does Internal Structure Determine Material Properties?

• Atoms in a regular layout
• Neighbouring grains
• Needle-like phases
• Fibres in a matrix

What you will learn

• List the levels of internal structure in a material, from atoms to fibres.
• Give examples of structure changing properties, such as wood grain and the hardness of steel.
• Understand why the same substance can still show different properties.

Before you start

No prior knowledge is needed. Just know that everything is made of atoms.

What does a material's internal structure consist of?

Structure lies at several levels, from how atoms are laid out to how larger components are distributed. At the smallest level, a crystalline material has atoms in a regular arrangement. At a larger level is the microstructure: in a polycrystalline material many crystal grains sit side by side, and each grain's deformation must be compatible with its neighbours.

Some materials also have clearly visible components inside. The University of Cambridge Classes of Materials package notes that the internal material and microstructure may differ from that at the edge, and for composites, sectioning a sample may reveal fibres or particles.

How does structure change properties?

Structure decides how a material deforms. The Mechanical Properties package says plastic deformation of metals most commonly results from the glide of dislocations, driven by shear stresses. So how easily a metal bends depends on how these dislocations move in the crystal structure.

The direction of the structure matters too. The Mechanical Properties package describes wood as having a higher Young's modulus when tested along the grain and a lower one when tested across it.

• Load along the grain
• Load across the grain

Same substance, different structure?

A different structure gives different properties even when the composition stays the same. For steel, the Jominy end quench package says high hardness occurs where high volume fractions of martensite develop, while lower hardness indicates transformation to bainite or ferrite and pearlite. How the steel is cooled changes the microstructure, and the microstructure changes the hardness.

In a composite, the structure is a deliberate combination. The Cambridge package on fibre composites describes a composite as a combination of two materials that show the desired properties of both, such as the compressive strength of the matrix and the tensile strength of the fibres.

• Fast cooling: much martensite
• Slow cooling: ferrite and pearlite

Real-world example

Look at a wooden board. The grain runs along its length. According to the package, the board is stiffer when loaded along the grain than across it, so people consider grain direction when using it.

Common misunderstandings

The same substance means the same properties. Steel cooled at different rates gets different structures and hardness.

Properties come only from chemical composition. Structure, such as grain direction or microstructure, plays a role too.

The surface tells everything. The package notes that the inside may differ from the edge.

Frequently asked questions

What is microstructure?

It is how the grains, phases or small components inside a material are arranged.

What do dislocations have to do with bending metal?

According to the package, plastic deformation of metals most commonly results from the glide of dislocations.

Why does wood grain direction matter?

Because wood has a higher Young's modulus when tested along the grains and a lower one when tested perpendicular to them.

Summary

• Internal structure has several levels, from atoms to grains, phases and fibres.
• Plastic deformation of metals most commonly results from dislocation glide, and wood grain direction changes Young's modulus.
• The same substance with a different structure, like steel cooled fast or slow, gives a different hardness.