1. Lecture 4 Chem 442/Mase 542
Metals as Biomaterials cont.

2. Metals Used as Biomaterials
• Stainless Steels
• Cobalt- based alloys
– Always mixed with chromium
• Titanium
• Titanium- based alloys
– Very light with relatively high strength
– Oxygen content affects strength
– Relatively pure
• Noble metals- Au, Au, Pt, Pd, Ir
– Expensive and poor material properties
– Used in electrodes- highly corrosion resistant
Fracture Fixation Plates
Hip Prostheses
Heart Pacemaker Housings
Hip Prostheses
Alloying is usually to modify corrosion and mechanical properties

3. Metals as Biomaterials
Alloying is usually to modify corrosion and mechanical properties

4. Dislocation Motion & plastic deformation
Metals - plastic deformation occurs by slip – an edge dislocation (extra half-plane of atoms) slides over adjacent plane half-planes of atoms.
So we saw that above the yield stress plastic deformation occurs. But how? In a perfect single crystal for this to occur every bond connecting tow planes would have to break at once! Large energy requirement
Now rather than entire plane of bonds needing to be broken at once, only the bonds along dislocation line are broken at once.
If dislocations can't move, plastic deformation doesn't occur!
Adapted from Fig. 7.1, Callister & Rethwisch 8e.

5. Deformation Mechanisms: Slip plane
Slip plane - plane on which easiest slippage occurs
Highest planar densities (and large interplanar spacings)
FCC Slip occurs on {111} planes (close-packed) in <110> directions (close-packed)
total of 12 slip systems in FCC
For BCC & HCP there are other slip systems.
Strength is increased by making dislocation motion difficult!

6. Slip Motion in Polycrystals
300 mm
• Polycrystals stronger than
single crystals – grain
boundaries are barriers
to dislocation motion.
• Slip planes & directions
change from one grain to another.
• The grain with the
largest shear stress yields first.
• Other (less favorably
oriented) grains
yield later.
Adapted from Fig. 7.10, Callister & Rethwisch 8e.
(Fig is courtesy of C. Brady, National Bureau of Standards [now the National Institute of Standards and Technology, Gaithersburg, MD].)
WHEN RESOLVED SHEAR STRESS CREATED UNDER TENSİLE STRESS IS LARGER THAN THE CRITICAL SHEAR STRESS AT WHICH MATERIAL YİELDS, THEN SLIP OCCURS

7. Reducing grain size increases the strength…
• Grain boundaries are
barriers to slip.
• Barrier "strength"
increases with
Increasing angle of
misorientation.
• Smaller grain size:
more barriers to slip. •
Adapted from Fig. 7.14, Callister & Rethwisch 8e. (Fig is from A Textbook of Materials Technology, by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.)

8. Precipitation Increases Strength
• Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
Dislocation
“advances” but
precipitates act as
“pinning” sites .

9. Impact of Cold Work As cold work is increased
• Yield strength (sy) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR)
decreases.
low carbon steel
Adapted from Fig. 7.20, Callister & Rethwisch 8e.

10. Strain Hardening
Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point (cold working).
The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work).
The average distance between dislocations then decreases and dislocations start blocking the motion of each one.

11. Cold Work (Strain Hardening)
• Deformation at room temperature (for most metals).
• Common forming operations reduce the cross-sectional area:
Adapted from Fig. 11.8, Callister & Rethwisch 8e.
-Forging A o d
force
die
blank
-Rolling
roll A o d
-Drawing
tensile
force A o d
die
-Extrusion
ram
billet
container
force
die holder
die A o d
extrusion

12. Dislocation Structures Change During Cold Working
Dislocation structure in Ti after cold working.
• Dislocations entangle with one another during cold work.
• Dislocation motion becomes more difficult.
Fig. 4.6, Callister & Rethwisch 8e.
(Fig. 4.6 is courtesy of M.R. Plichta, Michigan Technological University.)

13. Grain Size Influences Properties
Metals having small grains – relatively strong and tough at low temperatures
Metals having large grains – good creep resistance at relatively high temperatures

14. Fracture mechanisms Ductile fracture
Accompanied by significant plastic deformation
Brittle fracture
Little or no plastic deformation
Catastrophic

15. Ductile vs Brittle Failure
Very
Ductile
Moderately
Brittle
Fracture
behavior:
Large
Moderate
%AR or %EL
Small
• Classification:
Adapted from Fig. 8.1, Callister & Rethwisch 8e.
• Ductile fracture is
usually more desirable
than brittle fracture!
Ductile:
Warning before fracture
Brittle: No warning

16. 1. Stainless Steel Main advantage: corrosion resistance
Most common: 316L
Mixture of Fe (60-65%), Cr (17-20%), and Ni(12-14%)
Chromium oxidizes to limit corrosion (Cr2O3 formation on surface)
But, high chromium content reduces strength (increases BCC over FCC)

17. Stainless Steels
Low carbon content (L) : less than 0.03% to reduce corrosion
If High C content:
Carbides (ex. Cr23C6 ) ppt at grain boundries
Prevents protective Cr2O3 formation
Corrosion assisted fracture at grain boundries

18. Stainless Steels Ni: added to increase strength
Stabilizes austenite (FCC)
Mo: increases resistance to pitting corrosion
N: increases mechanical strength

19. Microstructure and Properties
316L : single phase FCC
grain size 100 micron or less
Smaller grain means more strength
Grain size: Manufacturing history
Plastic deformation within grains
Cold worked metal versus annealed
Higher tensile and fatigue strength
Decreased ductility (not a problem in implants)
Texture
Preferred orientation of grains
Typical microstructure of cold-worked 316L stainless steel, ASTM F138,

20. Microstructure and Properties
Texture
Preferred orientation of grains
Evidence of textured grain structure in 316L stainless steel ASTM F138, as seen in a longitudinal section through a cold-worked bone screw. The long axis of the screw is indicated by the arrow.

21. Corrosion
Even the 316L stainless steels may corrode inside the body under certain circumstances in a highly stressed and oxygen depleted region, such as contact under screws or fracture plates.
Thus, stainless steels are suitable to use only in temporary implant devices, such as fractures plates, screws and hip nails.

22. Corrosion Corrosion Leaching into body Crack formation and fatigue
Mechanism
– Lowest energy state is oxidized state
– Metal atoms ionize, go into solution, and combine with oxygen
– Similar to rusting of iron: metal just flakes off
Degradation : oxides, hydroxides, and other compounds
Biological fluids contain water, dissolved oxygen, ions, etc
– Very aggressive mixture
Tendency to corrosion based on electrochemical series

23. Corrosion
– Different regions of the body affect corrosion levels differently – Wounds and infections can also change pH dramatically • Corrosion and fatigue can be synergistic effects • Rubbing on surfaces can disrupt passivation • Pitting is corrosion in local area • General problem is assuring metals in screws and plates are identical • Surgeons must be careful not to scratch metals or leave metals in tissues

24. Cobalt-Based Alloys Four main alloys ASTM F 75 – Co-28Cr-6Mo
ASTM F 90 – Co-20Cr-15W-10Ni
ASTM F 799 – Co-28Cr-6Mo
ASTM F 562 – Co-35Ni-20Cr-10Mo

25. Co-alloys Strength > Stainless steel
Lost wax method (femoral stems, oral implant, dental bridge, etc)
Grain size can be controlled

26. Cobalt-Based Alloys Large grain size ( 4mm) Casting defects
Microstructure of as-cast Co–Cr–Mo ASTM F75 alloy
Large grain size ( 4mm)
Casting defects
HIP (Hot isotatic pressing):
8 micron grain size
Higher yield strength and better ultimate and fatigue properties
Lost-wax process °C
Pressurized into ceramic molds

27. Cobalt-Based Alloys Most widely used Co-Cr-Mo: F75
Corrosion resistant in chloride environment (Cr2O3)
Aerospace and biomedical use
Uses
Femoral stems, oral implants, dental bridges
Microstructure
Co-rich matrix
grain boundry
Dendritic structure
Lost-wax process °C
Pressurized into ceramic molds

28. Scanning electron micrographs of the
fracture surface from a cast F75 subperiosteal dental implant
Macrophoto of the fracture surface of the same Co–Cr–Mo ASTM F75 hip stem

30. Ti & Ti Alloys High strength/weight ratio
Can be strengthened by alloying and deformation processing
Excellent biocompatibility – little to no reaction with surrounding tissue
Corrosion resistance derives from stable TiO2 surface oxide layer

31. For biomedical applications
CP-Ti
Ti-6Al-4V
Ti-6Al-4V ELI

32. CP-Ti: Commercially Pure
ASTM F 67
Essentially all α (hcp)
Relatively low strength, high ductility
Excellent corrosion resistance
Yield strength varies widely (25 ~ 70 ksi) depending upon interstitial & substitutional levels
Used for :
pacemaker cases, ventricular-assist devices,
implantable infusion drug pumps, dental implants,
craniofacial implants,
screws and staples for spinal surgery

33. Alpha-beta allays used for Ti-6Al-4V is an alpha-beta alloy
(V – beta stabilizer)
Contains both alpha and beta phases
Also contains alpha stabilizer and beta stabilizer
Solution heat treated and quenched, followed by aging to precipitate alpha phase
Ti-6Al-4V and Ti-6Al-4V ELI (ASTM F 136)
extra-low interstitial: ELI
used for
total joint replacement arthroplasty
hips and knees
Arthroplastjoint surgery

34. Alpha-beta allays
Ti-6Al-7Nb (ASTM F 1295) : Aluminum Niobium allow used for femoral hip stems, fracture fixation plates, spinal components, fasteners, nails, rods, screws and wire
Ti-3Al-2.5V (ASTM F 2146) used for tubing
Arthroplastjoint surgery

35. Callister: page 413

37. Metal Implant Reliability
depends largely on the:
corrosion,
Structural integrity
Host reaction
wear,
Reaction of body to wear debris
Longevity of implant
fatigue resistance of the materials

38. Cobalt-chromium-molybdenum
Ti-6Al-4V ELI Titanium AlloyInterface

39. Fatigue Surface condition, e.g., smooth vs rough
Loading type & characteristics (place of the implant)
Microstructure (grain size, allignment, phases)
Site of use

40. Friction & Wear
Passive & tenacious TiO2 film remains under low loads and slow sliding in articulating conditions
Wearing away of TiO2 film can lead to high wear rates of mating surfaces
Wear debris leads to adverse tissue reactions
Wear of the mating surfaces leads to loosening of implant

41. Shape memory Allos NITINOL nickel±titanium
alloy of near-equiatomic composition
high elastic deformation
self-expanding stents
Exceptionally flexible
short and long-term biocompatibility
chemical (degradation,corrosion, and dissolution) stability