Metals- Industrial Materials Processing Flashcards
When is strain proportional to stress?
During elastic deformation
This is Hooke’s law
What does Young’s modulus depend on?
The strength of the atomic bonds and crystal structure so is almost unchanged by composition or microstructure (heat treatment)
What happens during elastic deformation?
When a small stress is applied to the metal the bonds between the atoms are slightly stretched and the metal deforms a little. When this stress is removed the bonds regain their original length and the metal regains its original shape. The strain is conserved.
What happens during plastic deformation?
As the stress is increased beyond the yield stress, the planes of atoms in the crystal can slip over one another. This occurs at and beyond the elastic limit. Because atoms have slipped over each other when the stress is removed the elastic strain is recovered but the plastic strain is permanent
What type of deformation needs to occur during processing of metals?
Plastic deformation
Examples of point defects
Vacancy, substitutional impurity, vacancy, impurity interstitial, self interstitial
What physical mechanism is responsible for plastic deformation and ductility in metals?
The movement of atomic defects
How can vacancies be produced?
By solidification or a result of atomic vibrations. Vacant atomic sites cause distortions of planes
For creep, what is the strain a function of?
Stress, temperature and time
What type of defects are the basis of alloying?
Substitutional atoms (like nickel in copper) or interstitial atoms (like carbon in iron
What type of defect is a dislocation?
Linear defect
Where is a slip plane relative to a dislocation?
Perpendicular to and at the bottom of the half plane of atoms
What is the active slip system?
The one for which shear stress is greatest
What is a slip system made from?
A slip plane and slip direction along which slip occurs most easily
Where is the slip plane and direction in a slip system?
Slip plane is usually the plane of greatest packing density
Slip direction is usually the closest-packed direction within a plane
Number of possible slip systems for FCC, BCC and HCP metals
FCC: 12
BCC: 48
HCP: 12
Not all are always operational
What is DBTT?
Ductile to brittle transition temperature. Temperature below which a metal is brittle. Generally better if this is lower
What is toughness related to?
Area under stress-strain curve, crystal structure, temperature and ductility of the metal. Most important property regarding DBTT
Graph of energy absorbed on impact vs temperature
Starts low, curves up to steep straight line, levels off. DBTT in middle of straight section. This is for charpy test
Von Mises criterion
For plastic deformation at least 5 independent slip systems must be operational
Temperature dependence of slip systems in FCC, BCC and HCP
FCC: 5 of 12 slip systems are independent and operate at all temperatures so a FCC metal is ductile at all temperatures.
BCC: the 48 slip systems are highly temperature dependent
HCP: the 12 slip systems are temperature dependent
Which unit cells have a DBTT?
FCC don’t have DBTT
BCC and HCP do
How to determine DBTT?
It is not an exact temperature and there are many ways of estimating it. Can also look at fracture surface of charpy specimen to estimate DBTT of FATT (fracture appearance transition temperature)
What is the critical resolved shear stress CRSS?
Plastic deformation occurs when the resolved shear stress on the most favourably oriented slip systems reaches a critical value (the CRSS).
τR=σCosφCosλ
φ is angle from applied force to the normal of the slip plane
λ is angle from applied force to the slip direction
What does the amount of obstacles to dislocation movement mean?
Few obstacles=ductility (or toughness)
Many obstacles=strength
Examples of obstacles to dislocation motion
Other dislocations, grain boundaries, precipitates, cracks
What assists motion of dislocations?
Elevated temperatures, availability of slip systems
