Nanostructured Metallic Materials
Processing and Mechanical Properties
Sung Whang
Historical Use of Metals
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Gold
Copper
Bronze (Cu–Sn)
Brass (Cu-Zn)
Iron
Steel
~ 35,000 years ago
~ 4,000 BC
~ 1,200 BC
2,000 – 1,000 BC
~ 1,200 BC
~ 500 BC
Development of strong Metallic Materials
• i) Alloying with two or more elements by melting or
other atomic mixing technique.
• Ii) Strengthening metals by reducing grain size or
other second phase or precipitates, etc,.
• Iii) Permanent or plastic deformation increase
hardening and strength – called strain hardening or
work hardening. In the microscopic terms, it means
“Increase dislocation density” from 103 /mm2 up to
max 109 /mm2.
Polycrystalline Metals
Yield Stress vs. Ave grain diameter in Brass
Hall-Petch relation
Hall-Petch Relation
• σy = σo + k d-1/2
• Where σy is yield strength, σo yield strength
for a very large grain or reference point, k
material proportional constant, and d average
grain diameter.
Dislocations
Dislocations in Deformed Metals
Strain Hardening
Classification of Metals by Grain size
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Nanostructured metals:
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A nanostructured material is made of
grains or other microstructural entities that
have average size 100 nm or less in length
at least in one dimension. Other entities
include precipitates, second phase, a third
phase, etc.
• Ultrafine grained metals (UFG)
A materials that is made of grains of 100 nm –
1,000 nm diameter.
But, practically often grain refined metals
contain a mixture of both nano-grains and
ultrafine grains. An alloy of such a mixture
possess excellent strength and ductility.
• Coarse grained metals (CG) or Conventional
metals
• grain size larger than 1,000 nm (1 micron) –
few mm in diameter.
Processing
• ( a ) Severe Plastic Deformation: Starting
materials have a bulk form.
• ( b ) Mechanical Attrition: Starting materials
have powders or fine pieces
• ( C ) Electrodeposition: Anode – metal bar
• ( d ) Amorphous Metals Route: Devitrify
amorphous metal into fine crystalline precipitates
(1) Severe Plastic Deformation
High Pressure Torsion
Equal Channel Angular Pressing
Severe plastic deformation &
Grain refinement
• 1). SPD at/below o.4 Tm produces dislocation
density as high as 1012 / mm2 under
hydrostatic pressure.
• 2). A portion of high density dislocations
transform into grain boundaries under the
proper heat treatment. Thus, this creates very
fine grains of 100 nm – 500 nm in diameter.
Pure Ti after ECAP
Commercially pure Ti – 4 passes
Grain size 300 – 400 nm
Pure Ti + Thermomechanical
Treat grain size 100 nm
(2) Mechanical Attrition
or Mechanical Alloying
Pure metal or two metals or more
Layered particle in mechanical alloying
Mechanism of Mechanical alloying
• (a) Large impact on particles by milling balls
under protective atmosphere leads to large strain
and very high dislocation density in the matrix.
The grain size decreases as the milling progresses.
• (b) For two different metals, the attrition leads to
atomic mixing of two metals.
• (c) The resulted grain size becomes much smaller
for higher melting-temperature metals.
Consolidation of milled powders
• 1) Cold pressing  high temperature sintering
Issue: grain growth
• 2) Cold pressing  hot extrusion or hot isostatic pressing
• 3) In-situ consolidation of low melting metals
into spheres of a few mm diameter by
cryomilling in liquid- N (77 K) or liq-Ar (87 K) .
In-situ consolidation
room temperature, inert gas atmosphere
True stress-true strain for copper
grain size 23 nm
Mechanical Properties of
Nanostructured Metals
• Strength : The yield strength of nanostructured
metals increases with decreasing grain size from
100 nm down to 20 nm, obeying Hall-Petch rule;
But the yield stress decreases decreasing grain
size below 20 nm. Instead of hardening, the
material shows softening, i.e., the violation of
Hall-Petch rule.
• Ductility vs. grain size: The ductility is
decreasing with decreasing grain size.
Superplasticity
• Definition: The ability of a polycrystalline
metal to exhibit very high tensile elongation,
say 200 %.
• Superplasticity of metals facilitates sheet
metal forming for manufacturing.
• Superplasticity occurring temperature has a
strong dependence on grain size.
Summary on Mechanical Properties
• 1) With decreasing grain size in nanostructured
metals, the yield strength increases up to grain size
of ~ 20 nm. Below 20 nm grains, the yield strength
decreases with decreasing grain size, exhibiting a
softening behavior. Thus, the deformation
mechanism for nanostructured metals with above 20
nm grains is governed by dislocation pile-up
mechanism while for the nano-grained material with
grains less than 20 nm, the deformation is governed
by dominant grain boundary sliding.
Practical material can be produced from grain sizes
from 20 nm to 1,000 nm. These metals contain
nano-grains and ultrafine grains as well.
2) Nano- and ultrafine grained metals trade off
ductility with higher strength. But, the
increase in specific strength would be more
than off-set any reduction in ductility.
3) When the grain size becomes ultrafine size or
less, the alloy exhibits superplastic behavior at
lower temperatures where the grain growth is
minimal.
4) The fatigue testing shows that ultrafine
grained metals exhibit superior performance
in the high stress range compared to
counterpart coarse grained metals (of same
compositions). This is not the indication
that nanograined metals with grains less
than 20 nm would have the same fatigue
behavior.