Materials Properties of Metals and Alloys
Hypothetical situation:
Upon graduation, you go to work for Boeing.
Your job - select a high-strength A l alloy for jet airplanes.
Airplane: 500 tons
50 tons cargo
150 tons plane structure
300 tons fuel
}
If you can triple the alloy strength, you can increase
the cargo load to 150 tons.
Material
Tensile Yield Stress (psi)
pure (99.45%) annealed A l
pure (99.45%) cold drawn A l
4 x 10
3
24 x 10
3
3
A l alloy - precipitated, hardened
50 x 10
/
big improvement
But, "perfect" single crystal A l as
a yield stress of ca. 10 psi!
6
Defects in Metallic Crystals
Defects are responsible for important mechanical
properties of metals: malleability, yield stress, etc.
Non-directional bonding, high coord, number
=> metallic structures readily tolerate "mistakes"
o°£o°
o o oo oo
o oo oo o
o o o oo
O O Qt O O O 0R0O0R0
o o o/o o o
O O/O o o o ogogogo
vacancy
ogogogo
(missing
1
atom)
(point defect)
dislocation T
(extra plane I
of atoms)
(line defect)
grain boundary: planar defect
Moving a Dislocation Under Shear Stress
1
2 3
4
6
7
5
dislocation
Horizontal arrows
indicate applied
shearing stress
1
2
5
1
3
4
7
6
2
3
5 6
7
4
Dislocation
moves in
crystal by
breaking/making
a line of
metal-metal bonds
Work hardening - moves dislocations to
grain boundaries
planar defect
(stronger under stress)
"Cold working" or "drawing" of a metal
increases strength & brittleness
(e.g., iron beams, knives, horseshoes)
Annealing: crystal grains grow, dislocations move
(metal becomes more malleable)
Alloying - impurity atoms or phases "pin"
dislocations.
extra atom (e.g., Mo in Fe)
or crystal grain boundary
(e.g., FeaC in Fe)
Malleability of Metals and Alloys
Some metals are ductile (Au, Ag, Cu, etc.)
Some are hard (Mo, W, Cr, etc.)
Why?
Crystal structure is important:
close packed structures (hep, ccp) •=> ductile
non-close packed (bcc) => hard
small corrugations
in hep and ccp
=> planes slip
easily, metal
is ductile
o o o o
large corrugations
in bcc
=> less ductile
Some important alloys have bcc structure
E.g. Brass - a hard alloy of two soft metals (Cu and Zn)
pure Cu: ccp i
pure Zn: hep I
b o t h
s o f t
brass = CuZn
(hard)
Iron and Steel
Below 900 C, Fe has bcc structure ("hard as nails")
Between 900 - 1535 C, Fe is ccp (more ductile)
t
M.P.
can be worked
into various shapes
when hot
Steelmaking
Carbon steel contains ca. 1 % C by weight
t
dissolves well in ccp Fe,
but not in bcc
slow cooling
ccpFe/l%C
'tempering"
"austentite"
mixture of
bcc Fe
and F e C ("cementite")
3
"pearlite"
can be worked
into various shapes
(e.g., knives, swords)
hard, brittle
F e C grains stop dislocation motion in bcc Fe
3
=> very hard material
Amorphous (Glassy) Alloys
Most metals and simple alloys are polycrystalline.
Very rapid cooling rates – 106 K/s - are needed to
freeze them into a glassy (“liquid metal” state.
Polycrystalline
Liquid or Glass
More complex alloys are better glass formers and can
be made as glasses at slower cooling rates.
Year
1960
1969
1980s
1990s
Alloy
Au75Si24
Pd-Cu-Si
La-Al-Cu & others
Zr-Ti-Cu-Ni-Be
Cooling rate (K/s)
~ 106 - thin films & ribbons
100-1000
1-100
~ 1 (similar to oxide glasses)
No dislocation motion
=> strong, wear-resistant.
http://www.its.caltech.edu/
~vitreloy/development.htm