Question

Please use the characteristics of chemical bonding, crystal structures and dislocations to explain why metallic materials are strong and ductile and why ionic and covalent materials are strong but brittle?

Answer 1

Dislocations are a type of defect in a crystal. If you imagine a crystal as a lattice, a large array of evenly stacked atoms, a dislocation is a missing half plane of atoms. When you look at dislocations under an electron microscope, they look like spaghetti, lots of little lines.

When you apply a force to a crystal, it takes a lot of force to break the bonds between atoms, but not a lot of force to slide the bonds around a dislocation, letting it move. Moving dislocations let you change the shape of the material. Dislocations can glide along, but they can also tangle and even multiply.

Dislocations exist in large quantity in almost all crystalline materials, but it some materials they just can't move very easily, and when they do move they build up fast. Dislocations not being able to move means that it becomes easier for the material to change shape by cracking instead of moving dislocations. Brittle materials are ones that crack before they ever bend very much.

For example, Coal breaks because it has strong, directional bonds that mean that sliding a dislocation takes a lot of energy, so it's easier to crack. Rocks have complicated crystal structures, meaning you can't just slide one atom in a plane to move a dislocation, you have to slide a group of atoms before you get to a new stable point. Gold has a close-packed crystal structure with very little directional bonding, means it's really easy for the dislocations to slide. Iron has a different type of crystal structure called body-centered cubic, which has a little more directional bonding, meaning it's less ductile in general.

The field that deals with bending things without breaking is called plasticity, and it's pretty neat! Fracture and fatigue are two fields of study that involve how things break.