2 Metals Bio Material

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Biomaterials, Artificial Organs and

Tissue Engineering

Chapter 2

Metals

Dr E. Jane Minay and Dr Aldo R. Boccaccini

Imperial College London


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Acti-

nide

seriesRaFr

RnAtPoBiPbTlHgAuPtIrOsReWTaHfRare

earth

series

BaCs

XeITeSbSnInCdAgPdRhRuTcMoNbZrYSrRb

KrBrSeAsGeGaZnCuNiCoFeMnCrVTiScCaK

ArClSPSiAlMgNa

NeFONCBBeLi

HeH

LwNoMdFmEsCfBkCmAmPuNpUPaThAc

LuYbTmErHoDyTbGdEuSmPmNdPrCeLa

The Periodic Table

Fig. 2.1 The periodic table showing the metals (yellow), non-metals (red) and

intermediate (blue) elements. Metals are good conductors of heat and electricity, opaqueto visible light, and have a familiar metallic lustre.


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+

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e-e

-

e-

e-

e-

e-

e-e-

e-e-

e-

e-

e-e-

e-

e-e-

e-

e- e-

e-

e-

e-

e- e-

e-

e-

e-

e-e-

e-

e-

e-

e-

e- e-e-

e- e-e-

Fig. 2.2 One model of a metal, positive ions are surrounded by a sea or cloud of

electrons. These free electrons are not bound to any particular metal ion but are free

to move around within the structure. It is these free electrons that result in some ofthe typical properties of a metal including the good conductance of heat and

electricity, and the metallic lustre of a polished surface.

Model 1


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A A

A AA A

AAA

AA

AA

A

B B B B

B B B

C CC

C C C C

B B B B

B B B

C

C

C C

C CC

A

A

A AA A

AAA

AA

AA

A

Model 2: f.c.c.

Fig. 2.3 (animated) The face centred cubic (f.c.c.) structure can be considered as arepeating stack of closely packed layers of hard spheres.


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Fig. 2.3 shows that the face centred cubic (f.c.c.) structure can be considered as a

repeating stack of closely packed layers of hard spheres. Each layer has the

highest possible density of spheres, each sphere touching six neighbouringspheres. The first layer can be considered as being in the A position. The next

layer sits on top of this layer in the B position. The third layer can sit on the

second layer either with each sphere directly above the spheres in the A layer or

in the third possible position, C. In the f.c.c. structure the third layer sits in the Cposition. The overall structure is a sequence of layers stacked in the order

ABCABCABC. Each sphere touches 12 others.

Aluminium, nickel and iron (at high temperatures) all have this type of structure.


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Fig. 2.4 (animated) The face centred cube structure


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Fig. 2.4 shows that the face centred cube in this structure can be easily seen if youtake two closely packed layers, each containing 6 spheres arranged in a triangle,

pointing down in the lower layer and up in the other layer. Place one sphere in the

recess of the spheres below the lower layer and another sphere in the recess above

the upper layer. Now stack the two structures so the atoms sit in a ABCAsequence. The cube has a sphere centred on each corner and a sphere in the centre

of each face.


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A A

A AA A

AAA

AA

AA

AB B B B

B B B

C CC

C C C C

B B B B

B B B

A A

A AA A

AAA

AA

AA

A

Fig. 2.5 The hexagonal close packed structure also consists of a stack of closely

stacked planes, however this time the stacking sequence is ABABAB. Like the

f.c.c. structure each sphere is touching 12 others.Zinc, magnesium and titanium (at room temperature) all have this type of structure.

Model 2: h.c.p.


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a

z

h.c.p.

Fig. 2.6 A hexagonal prism can be seen in this structure by taking atoms fromthree of the closely packed planes, ABA.


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< B >

< B >

< B > < B >

Fig. 2.7 The bcc structure does not contain closely packed planes. The structure can be

contained in a cube with a sphere centred on each corner and a sphere in the centre of

the cube. Each sphere is only touching 8 other spheres.

Chromium, Iron (at room temperature) and Ti (at high temperature) all have this typeof structure.

Model 2: b.c.c.


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50m

Fig. 2.8 Microstructure of an Fe-C alloy observed using an optical microscope. The

material has been polished to give a smooth surface and etched using nital to revealgrain and phase boundaries. Light coloured grains are outlined and dark areas

consist of a finer structure which is not resolved at this magnification.

Microstructure of a steel


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5m10m

Fig. 2.9 Scanning electron microscope image of a metallic fracture surfaces. The top

image is of a metal which has fractured along the grain boundaries in a brittle

manner. The lower image shows a typical fracture surface of a metal that has failed

in a more ductile manner. The large depth of field can be seen in these images.

Metallic Fracture


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200nm

Fig. 2.10 Transmission electron microscope image taken through a thin film of metalshowing small bright cubic precipitates in a darker matrix.

Precipitation


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Fig. 2.11 Graph showing the effect of grain size on the yield stress of a commercially

pure titanium alloy. YS=231+10.54 d-1/2

where the yield stress is in MPa and the grainsize is in mm.

Effect of grain size


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0

Plastic DeformationNon-reversible

Fracture

TensileStrength

Yield

Stress

RecoverableElastic Strain

Fig. 2.12 Typical stress strain behaviour of a metallic material tested in tension. The

metal behaves elastically for the blue portion of the curve, permanent (plastic)

deformation (red section) begins at the yield stress of the metal, a maximum stress isreached, the stress then decreases until ultimately the metal fractures.

Tensile properties of metals


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Youngs

Modulus

(GPa)

Yield Stress

(MPa)

Tensile

Strength

(MPa)

%

elongation

Fracture

Toughness

(MPam1/2)

Steels

(General)

210 210-1600 400-1800 10-40 50-90

StainlessSteels 190-210 200-1650 400-1800 5-40 80

Titanium

Alloys

100-115 170-1100 400-1200 10-30 40-70

Platinum 170 10 120-240 1-40 -

Fig. 2.13 Table of typical properties of some metals and alloys

Mechanical properties of metals


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Bend

ingStres

s,

S,

MP

a

Number of Cycles to Failure, N105 106 107 108 109

100

400

300

200

0

Mild Steel

Aluminium

Fatigue Limit

Fatigue properties of metals

Fig. 2.14 The typical way of presenting fatigue data is a graph of the alternating stress

against the log of number of cycles to failure. The two types of fatigue behaviour are

shown here, the first in green is the behaviour of many non-ferrous alloys such as

aluminium. The number of cycles to failure increases as the alternating stress decreases.

The second type of behaviour, in red is typical of ferrous alloys and titanium alloys, a

fatigue limit is observed. Below this stress the metal has an effectively infinite fatigue life.


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Heat

Cool

Correspondence

Variant A

Correspondence

Variant B

Correspondence

Variant A

CorrespondenceVariant A

Correspondence

Variant B

CorrespondenceVariant A

Fig. 2.15

The shape memory effect


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Figure 2.15 shows a schematic mechanism of the shape memory effect. As

the metal is heated and cooled the atomic arrangement changes between one

of high symmetry at high temperature (in this schematic a square array) to alower symmetry arrangement at low temperature (in this case a diamond

array). Several variants of the lower symmetry phase will form. The

arrangement will change backwards and forwards as the metal is heated and

cooled with no overall shape shape being observed. If the metal is deformed

whilst it is in its low temperature structure the atoms move in such a way that

one variant grows at the expense of the others. If the metal is then heated to

return to the higher symmetry structure (the square array) the metal will

return to is original shape. This is the shape memory effect.


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M+

M+ M+

M+

M+

OH-

OH-OH-OH-

O2

O2O2O2

e e e

e eO2

O2 Cl-Cl-

Cl-

Na+

Na+

Na+

M+

M+

OH-

OH-

OH-OH-

O2

O2O2O2

e e

eeO2

O2

Cl-

Cl-Na+

H+

M+

M+

M+ Cl-H+

H+Na+

Cl-

Cl-

Cl-

eOH-

O2

O2

Fig. 2.16 The mechanism of crevice corrosion.

Crevice Corrosion

Magnification of metal joint and crevice


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Figure 2.16 shows the mechanism of crevice corrosion. A stagnant zone exists in the

crevice. Initially corrosion occurs at a uniform rate over the entire surface of the

metal. After a short time the oxygen in the crevice is used up. No reduction ofoxygen to hydroxyl ions occurs in this area, although the dissolution of metal

continues. An excess of positive charge is produced in the solution by the metal

ions. This is balanced by the migration of chloride ions into the crevice. The

concentration of metal chloride increases within the crevice. The metal ions andchloride ions react with water to form an insoluble metal hydroxide, MOH and a

free acid H+Cl-.Both chloride and hydrogen ions accelerate the dissolution rates of

most metals and alloys and the process becomes auto catalytic.

2 Metals Bio Material

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