Metals II Flashcards
Name properties of metals:
Tough. Strong, large breaking stress. Ductile. Malleable. Hard. Stiff.
What is a metal lattice made up of? What sort of structure does it have?
A lattice consists of positive ions and delocalised, free electrons. Metals are crystalline with ions in regular repeating patterns.
What is polycrystalline?
A metal sample with grains which meet at grain boundaries.
What is the old model of ductility?
For a slip to occur, rows of ions must move as a unit, which assumes metals have 1000x greater yield strength for layer to move one atom spacing.
What is the new model of ductility?
Metals are malleable/ductile. Their ions slip one by one. Lattice defects such as vacancies, mobile dislocations, allow this to happen. Neighbouring layers are displaced to absorb strain, and less stress is needed as atoms move one by one, so the whole row eventually moves.
What are alloys and what do they do?
They are lattices with more than one element.
They pin dislocations due to sizing/stronger bonding and increase yield strength.
Alloy particles make metals less ductile/malleable and more brittle.
Describe forces in metals..
Metals are held by electrostatic attraction. Repulsion is short-range and rises rapidly with less distance, while attraction rises less rapidly. They are equal at equilibrium.
What happens in elastic and plastic deformation?
ELASTIC - metallic bonds are stretched slightly.
PLASTIC - metallic bonds overcome by external force.
Describe crack propagation and how it varies in metals..
In a brittle material, bonds are under massive stress and the bond breaks, so the stress moves to the bond below. A crack propagates.
In metals, which are ductile, they deform plastically under low stress due to mobile dislocations, and stress is spread over a grater surface area than materials like glass.
