Metallurgy Flashcards
What is wrought metal
Cast metal or alloy that is permanently deformed
What is malleabilty
Ability to form thin shset by hammering or roling
What is dictility
Ability to stretch into wire
What is effect of strain/work hardening/cold working
Increase hardness, ultimate tensile strength (ie harder to elongate), increase brittleness
Decrease ductility and corrosion resistance
How does cold working make metal harder
Dislocation tends to happen at grain boundaries. Atomic slip occurs on intersecting slip planes, point defects increase and grains are distorted. More stress required for firther slip
What happens if there is further increase in cold work
Fracture as atoms get more detached from one another, weakened
Are orthodontic wires prone to work hardening
No. Bending force is generally within working range
How does proportional limit relate to elastic deflection
Increased proportional limit, higher elastic deflection
Relationship between elastic deflection and force applied
For given elastic modulus, elastic deflection is proportional to force applied
Is nickel titanium easy to shape
Hard to bend, does not undergo plastic deformaiton easily. Can be shaped with increase in temperature
What does it mean that nickel titanium ahs no shape memory
When force is applied, atoms will find and hold the new position
What is shape memory
Ability to return to original shape thermally induced eg deform at rtp (20-25ºC), return to original shape at 500ºC or 37ºC over a long time
What features enable shape memory
No dissociation of grains (change shape eg sqaure torhombus but atom arrangement and position same)
Twinning
What is super elasticity
Transition from austenite (fcc) to martensite (highly strained tetragonal form) induced by stress
At the same temperature, when stress is released, go back to orginial shape
Grains do not dissociate. Twinning responsible
Stress memory vs superelasticity
Stress memory thermally induced
Superelasticity stress induced
What is twinning
Structures on either side of boundary between two different orientations are crystallographically identical
Boundary is called a twinning plane
Properties of NiTi
Excellent corrosion resistance
Non magnetic
High fatigue strength
Moderate impact and heat resistance
Biocompatible
BC FIN
What is unloading and loading curve (NiTi)
Loading: force applied to activate austenite to martensite
Unloading: force it delivers when martensite deactivated to austenite
Hysteresis loop (history dependent ie memory)
What is load deflection rate
Force generated by unit length deformation
What happens if orthodontic wires have very high load deflection rate
Excessive force on teeth
Strength decreases quickly with tooth movement
Compare load deflection rates of NiTi and stainless steel
NiTi LDR lower, more constant and continuous force than stainless steel
Limitations of NiTi
Not joinable
Poor formability ie cannot undergo plastic deformation without behind damaged
Expensive
