heat treatments, phase diagram,
microstructure and mechanical
properties for the precipitation
hardening of Al-Cu alloys. Your answer
should identify the important
requirements to achieve good
precipitation hardening.
Von Mises condition
For a crystal to be able to undergo any arbitary
change of shape it must have at least 5 independent
slip systems. – Von Mises Criterion.
For a polycrystalline material to deform plastically
without cracking or the formation of voids requires
that all of the grains can deform in a compatible
manner with the surrounding grains. They must
therefore be capable of undergoing an arbitrary
shape change.
Requirements for high strength materials
Maximise all possible strength materials
•
Work hardening,
w
•
Solid solution hardening,
s
•
Precipitation hardening,
p
•
Dispersion hardening,
D
•
Grain size refinement,
G
•
Intrinsic lattice friction,
0
How the combined strengthening mechanisms influence
-
behaviour is complicated and depends alos on the testing
conditions (eg, strain rate).
a) A high solid solution composition for the
matrix. Ordered solid solutions are strongest,
but the concentration of solute remaining
after any precipitation is usually too small for
long range order.
b) A high volume fraction of precipitates. To
achieve this requires a large decrease in the
solubility limit with decreasing temperature.
c) A fine grain size. This can be achieved by recrystallisation of
highly worked materials. At high temperatures a large grain
size may improve creep resistance.
d) A fine dispersion of precipitates with spacing << 1
micrometer. Homogeneous precipitation through the materials.
e) For high temperature applications the precipitate particles
should have a low surface energy with the matrix. The
surface/volume ratio for fine precipitates is large and there
would be a large drawing force for particle coarsening.
Effect of strain rate and temperature upon mechanical properties
= average dislocation velocity, and is a function of the applied
stress. The higher
is the faster the rate at which dislocations overcome
obstacles. And the faster they move.
The thermal vibrations of the atoms
around the dislocations assist them in
overcoming the obstacles. So that the
higher the temperature for the same
applied stress, the faster the dislocations
can move.
Stress dependence of dislocation velocity
Consider yield drop behaviour
Both m and
u
determine the magnitude of the yield drop.
If m is large the stress drop is small for a given increase in
.
If m is small the stress drop is very large.
If the increase in
is slow, the stress drop is also slow.
Properties of bcc metals
•
Bcc metals have very high stacking energy (
>0.2J/m
2
) and the partial
dislocations
are hardly separated.
