Chapter 5- Looking inside materials Flashcards
define crystaline
a solid material whoes atoms/molecules/ions are arranged in a ordered lattice structure that can extend in an direction
define amorphous
a material whoes atoms are not ordered in a defined lattice shape
Why are ceramics brittle?
they are brittle because dislocations are very uncommon and therefore slip and plastic deformation cant happen, also the impurity s make it easy for cracks to propagate through the material. The covelant bonds are directions and cannot slip. Cracks act as stress concentrators.
ceramics are strong becuase
they have lots of strong covelant bonds holding it together in a rigid lattice, which means the whole material is strong.
glass is a
amorphous solid , made from strong directional covelant bonds.
general properties of a ceramic
brittle, hard, high compressive strength, prone to thermal shock, resistant to heat, insulators (exempt some are superconductors when very cold)
what is toughened glass?
glass that has been annealed so that the edge is under compressive stress and the inside is under tensile stress. when broken it turns into a fine powder.
what makes rubbers very elastic
cross links in the polymer chains that allow the chains the stretch from their tangled state and be returned.
What increases tensile strength in polymers?
longer polymer chains, and crosslinks
explain what happens when you stretch a polymer past its elastic limit.
Once its past its elastic limit the cross links cant pull it back together.
the chain starts off as a amorphous jumbled chain, and stress is applied these chains unravel and straighten to become more crystalline, which allows the plastic to extend drastically, once they have been straightened fully the C-C bonds will start to break and form a neck in the plastic, which then breaks.
What are plasticizes?
small molecules added to plastics to improve their flexibility.
what is a matrix?
the material that binds the reinforcing fibers together in a composite eg plastic in glass fibre.
what is a reinforcement?
a composite building material eg glass in fibre glass.
what is a fibre-reinforced composite?
the fibre is the primary load bearing component(eg glass)
What is a dispersion strengthened composite?
The matrix is the major load bearing component.
What is a particle reinforced composite?
the load is shared by the matrix and the particles (eq MDF)
What is a hole?
A hole is the absence of an electron in a particular place in an atom, its a positive electrical charge carrier.
What is doping?
the addition of foreign atoms to a pure semi-conductor crystal in order to improve its conductivity.
what is a semi conductor
A material which is an insulator at 0K but conducts better with increasing temperature.
intrinsic semiconductors havent been doped.
Covelantly bonded.
N-type semiconductor?
have impurities added with add extra ELECTRONS which improves conductivity.
P-type semiconductor?
have impurities added with add extra HOLES which improves conductivity.
define polycrystline
have structures composed of many smaller crystallites. These grains(crystallites) have a crystalline structure. the grains form in different sizes and directions to create a overall polycrystaline structure.
Grain boundarys are…
where two or more crystallites(grains) meet in a polycrystaline structure.
2/3 types of metalic lattice structures
Body centered cubic packing- each particle touches 8 others.
Hexagonal (and cubic closest packing) closest packing- each particle touches 12 others. made from 3 layers that are stacked so the particles sit in the depressions of the lower layer. (more ductile due to small congregations in layers that allow slip to happen more easily)
The two processes that happen when a metal cools?
whole process is called Crystallization, which is split into;
nucleation-formation of stable nuclei
crystal growth-branching tendrils that happen as the liquid becomes a solid.
What makes metals very ductile and malleable?
the non-directional metallic bonds allow slip to occur very easily, also lattice defects make it more ductile.
What are the kinds of lattice defects in a metal?
Dislocations - plane defect
Grain Boundary’s -lattice defects
Stacking defects
vacancy - point defect
What is a dislocation?
An extra hale a plane of atoms. The dislocation means that the rows of atoms can slips one at a time which reduces the stress required to deform it.
What is a stacking defect?
a defect in how a plane rests on the lower plane eq being directly on top and not sitting in the depressions.
What is a vacancy?
where a atom is missing from the ideal crystal structure, fixed with a solute atom being substituted in.
Why are metals much weaker then in theory.
Metals have stone non-directional metallic bonds and are polycrystlline in structure. However they contain dislocations which can move throughout the structure requiring only a few bonds to break at once , which can reduces stress required by 1000x .
Why are alloys usually more resilient to stress?
the bigger atoms in the lattice pin the dislocation and stop them from moving which reduces slip .
What is annealing?
Heat metal then allow to cool slowly
The structure disperses- which increases toughness but lowers yield strength.
What is Quenching
Heat metal then cool rapidly
Freezes the structure in the high temp form
which makes it very brittle but very hard and strong
What is tempering
Quench (heat and cool rapidly) then reheat slowly
This controls the dispersion between the structures
so it regains toughness
and regains some yield strength
Three ways to changing structure to a metal (in order to change properties)
Healing +cooling to get the right combination of strength and toughness.
Alloying = harder +stronger but reduce toughness
Work hardening- increases the number of dislocations and reduces grain size which means they entangle this improves yield strength and reduces toughness.
Things to include in a questions relating structure to properties.
Amorphous, crystalline or polycrystalline structure
type of bonding and theire strenght etc
then other relevant stuff eq imperfections in the lattice,
MAKE SURE TO USE SCIENCY WORDS
then relate them to the properties.
