Metals- Strengthening Mechanisms Flashcards
What is the Hall-Petch equation?
σy=70+17.4d^-1/2
Where σy is yield strength (MPa)
d is grain size (mm)
Describes effect of ferrite grain size on yield strength for low carbon steel
What does reducing the grain size do?
It increases the strength and also the ductility of a material. Due to there being more grain boundaries to hinder dislocation motion
Effect of phase boundaries
E.g boundary between ferrite and austenite. Also hinders dislocation motion but not as effectively as a grain boundary
What is hot working?
Working where the deformation temperature divided by the melting temperature of the material is greater than 0.6
What is cold working?
Working where the deformation temperature divided by the melting temperature of the material is less than 0.3 (roughly)
What is warm working?
Working where the deformation temperature is between that for hot and cold working
How does cold working work?
When we deform a crystal, dislocations are generated in large numbers. These move (slip) in certain directions. They are quite mobile so interact with and instruct each other. Aka work hardening
How does flow stress vary for work hardening?
Flow stress increases with strain but not strain rate. As strain increases dislocation density increases so flow stress increases. Initially a sharp increase, then sharp transition to shallow curve
What is cold working better for than hot working?
Better surface quality because there is no oxidation. Better control over dimensional tolerances
Potential problem with cold working
The metal is not softened and gets stronger so needs a greater capacity mill to deform it
What affect does cold working have on yield strength, tensile strength and ductility?
Increases yield and tensile strength. Can cause large decrease in ductility so the material becomes more brittle
How to overcome problem of work hardening
When yield strength reaches a certain point, apply a heat treatment. The stored strain energy in the metal may revert back to the pre-cold worked state after the heat treatment
What is the heat treatment called for Al and steel?
For Al it is called annealing. For steel it is called tempering
3 mechanisms that cause restoration of mechanical properties
Recovery, recrystallisation, grain growth
How are the mechanisms of recovery activated?
Thermally activated and occur at high homologous temperatures (T/Tm>0.3) and lead to microstructural changes in the metal
Usual sequence of restoration mechanisms
Recovery, recrystallisation, grain growth. Although some metals more prone to recovery or recrystallisation than others.
How does tensile strength and ductility vary through the restoration mechanisms?
Tensile strength starts high but starts to decrease in the recovery zone. It curves down more in recrystallisation zone. Then levels off entering grain growth zone. Ductility shows opposite shape where it starts low and ends high
Recovery
Some stored internal strain energy can be relieved due to dislocation motion (in absence of externally applied stress) if temperatures are high enough (to give dislocations enough energy). When dislocations are more mobile they can cancel each other out by annihilation
What is the recrystallisation mechanism?
The mechanism that leads to a much greater reduction of dislocation density and the nucleation and growth of new strain free, dislocation free grains. Driving force is the difference in internal energy between strained and untrained material. Metal becomes less strong and more ductile
When does grain growth occur?
When a metal is held at elevated temperature, the recrystallised strain-free grains continue to grow
