Metals and Alloys 2 Flashcards
What is an alloy
An alloy is a combination of two or more metals or a metal with a metalloid
What is the advantage of an alloy
improved properties
lower melting point than the individual metal
What are the improved properties of an alloy
→ Mechanical (EL, UTS, hardness)
Corrosion resistance
What is a phase
physically distinct homogenous structure (can have more than one component)
What is a solution
homogenous mixture at an atomic scale
Are metals usually soluble or insoluble when molten?
soluble
What can happen to the metals regarding solubility on crystallization
they may be insoluble
they may form an inter metallic compound with specific chemical formulation
they may be soluble
If the metals are soluble on crystallization what will they form
a solid solution
What happens if the metals are insoluble on crystallization
there is no common lattice network and there is two phases
What are the 2 types of solid solution
substitutional
interstitial
What is a substitutional solid solution
atoms of one metal replace the other metal in the crystal lattice
What are the 2 types of substitutional solid solutions
random
ordered
What is a random substitutional solid solution
metal atoms similar in size, valency and crystal structure and are randomly positioned in the lattice
What is a ordered substitutional solid solution
metal atoms in a regular lattice arrangement. The metals must be similar in size, valency and crystal structure
What is an interstitial solid solution
□ Atoms markedly different in size (DIFFERENCE FROM SUBSTITIONAL)
Smaller atoms located in spaces in lattice/grain structure of larger atom
What is a metal cooling curve like
In a metal cooling curve it shows that after all the metal has crystallised then temperature of the metal will drop resulting in a ‘plateau phase’
Why is the cooling curve for alloys different from metals
METALS CRYSTALLISE AT ONE TEMPERATURE BUT ALLOYS CRYSTALLISE OVER A TEMPERATURE RANGE
What is the liquidus
represents the temperature which different alloy compositions begin to crystallise
What is the solidus
represents the temperature which different alloy compositions have completely crystallised
What is the cooling slowly result in
metal atoms to diffuse through the lattice which ensures that grain composition is homogenous but results in large grains
What does cooling rapidly result in
Rapid cooling prevents atoms diffusing through lattice and causes coring as the composition varies throughout the grain
For coring to occur what needs to happen
→ Fast cooling of liquid state
Liquidus and solidus must be separated and determines the extent of coring
What is the effect of coring
may reduce the corrosion resistance of the solid form of an alloy
What can coring be resolved
through homogenising annealing
What happens in homogenizing annealing
Once a solid cored alloy is formed it should be reheated to allow atoms to diffuse and so cause grain composition to become homogenous
Why do alloys forming a solid solution and consisting of metals of different atomic size have improved mechanical properties
has a distorted grain structure which impedes dislocation movement
instead falls into the larger space existing between large and small atoms meaning more energy is needed for the defect to overcome the different sized atoms meaning greater stress it required to move dislocations and so alloys are inherently more fracture resistant and so stronger than metals
What are the properties of a eutectic alloy
→ Metals are soluble in liquid state
→ Metals are insoluble in solid state
Each metal forms physically distinct grains
What is the eutectic composition
When the liquidus and solidus coincide (where the crystallisation process occurs at a single temperature) and where grains of individual metals formed simultaneously
What is the lowest melting point at eutectic composition used for
solder
What are those alloys of eutectic composition like
hard but brittle
poor corrosion resistance
What is the non-eutectic composition
The excess metal crystallises first then the liquid reaches eutectic composition and then BOTH the metals crystallise (forming separate grains)
What are solubility limit lines
ndicates that a range of compositions are not possible
What are the two grain structures possible
alpha and beta solid solution
What do partially soluble alloys undergo on annealing
a supersaturated alloy will undergo precipitation hardening
What is a solid solution
A solid solution is a mixture of elements at the atomic level, and is analogous to a mixture of liquids which are soluble in one another
