What are defects that block dislocation movement?
Grain boundaries
Point defects
Dislocations
Precipitations
What are the treatment phases of precipitation hardening?
Solution heat treatment
Aging
What is the temperature distribution of a dispersion hardening process and the corresponding microstructure at the different points?
Why does precipitation hardening introduce a strain into the mother lattice?
Coherent precipitates with a different lattice parameter strain the bonds
What is GPI and GPII?
Guinier Preston zones:
GPI: twodimensional coherent precipitation
GPII: Three dim. coherent precipitation
Which precipitations occur over which ageing time and how does that relate to the hardness?
What is the GPI zone?
A two dimensional cluster of a preciptiation that is fully coherent with the matrix.
What is the GPII zone?
A three dimensional cluster of a preciptiation that is fully coherent with the matrix, showing a crystallographic order.
Why do transition phases form in precipitation hardening and what is the effect for the overall phenomenom?
Lower energy barrier and therefore faster nucleation than the equilibrium phase.
Faster transition of through faster decrease in free energyover transition phases.
What are the effects of increasing the sollute concentration of another material?
the attainable hardness increases
the precipitation kinetics increase (higher driving force of the reaction)
Which type of precipitation gives a larger stress field?
“Coherent” precipitate gives larger stress field.
“Incoherent” precipitate reduces stress field.
What is the prequisite for the alloy to go throught the cycle of GP zones and transition phases?
Ageging below GP solvus temperature
What is described by the term overaging?
Formation of misfit locations, which reduce the hardness by reducing the tension field.
The precipitation of which of theses phases GPI, GPII, ϴ’, ϴ’’ leads to the highest strength?
ϴ’’
Why is there an maximum ageing temperature?
at high temperature the ageing has not enough driving force due to the low supersaturation
at low temperature the diffusion is too slow
What is the effect of combining strain and precipitation hardening?
The precipitations nucleate at the dislocations.
What are the two interaction methods of a dislocation with a precipitation during plastic deformation?
The dislocation can shear through the material or bypass them.
In which ways increases a coherent precipitation the strength?
the increase in surface of the precipitation consumes energy
strain hardening through lattice mismatch
different stacking fault energy (energy to create dislocation)
The difference in modulus (which?) increases hardness
particle cut with two dislocations is energetically favourable and creates a super dislocation
How is the critical radius in coherent precipitates obtaines and how is it defined? What are typical values?
Minimum of the bowing and cutting function
typical values:5-30nm
What is a diffuse obstacle?
An obstacle, which interacts with dislocations over a distance, via a strain field.
What is a strong obstacle?
An obstacle which only interacts with dislocations in close contact.
What is described by the Orowan mechanism?
What is the critical angle and what are the effects of a weak, intermediate and strong obstacle?
strong obstacle: 0<φ<60°:large curvature of dislocation line
intermediate obstacle:φ=90°:less curvature fewer loops
weak obstacle: φ=180°:no loops no bending
What is the driving force of overaging?
The minimation of the surface energy. (Proportional to the inverse of r)
Why is a precipitation hardened material unstable in its hardest form?
The small evenly distributed precipitations tend to coarsen to minimize the surface area.
What is the difference between artificial and natural ageing?
artificial: T>RT
natural: T=RT, requires multiple days to achieve maximum hardening, peak hardness higher than in artificial, no overaging
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