Lecture 9 - Metallic Biomaterials

Question 1

Why Metals?

Answer 1

Mechanical Profile - failure modes, modulus, strength, modes of deformation (plastic)

Question 2

Why Metal Alloys?

Answer 2

• Fine-tune mechanical profile
• Reduce Corrosion
• Exceptions: Au, Pt, CP Ti

Question 3

Applications of Metallic Biomaterials

Answer 3

• Fracture plates
• Tibia rods and intramedullary nails (center cavity of bone shafts - storage of bone marrow)
• Bone screws
• Joint replacements

Question 4

Types of Metallic Biomaterials

Answer 4

Main:
- Stainless steels
- Co-based alloys
- Ti-based alloys
Others:
- Copper
- Gold
- Platinum
Degradable:
- Mg alloys (quick degradation profile)

Question 5

Goal of Metallic Biomaterials

Answer 5

Replace or repair diseased or damaged organs

Question 6

Use of Stainless Steels

Answer 6

Removable or low commodity devices:

• plates
• screws
• pins
• replacement joints

Question 7

Types of Stainless Steels

Answer 7

• ASTM F138
• ASTM F139
• Austenitic (FCC)
• 316L

Question 8

316L Stainless Steel

Answer 8

• Low C content (< 0.03%)
• Fe (60-65 wt.%)
• Cr (17-19 wt.%)
• Ni (12-14 wt.%)
• Minor additions: N, Mn, Mo, P, Si, S
• do not want cementite (brittle), maintain austenite
• 0% Ni (Ferrite), 5% Ni (Duplex), >8% Ni (Austenite)

Question 9

Why Cr Alloying Addition in 316L?

Answer 9

• Forms Cr2O3 surface oxide (passivating layer - preventing interaction between aqueous fluid and base metal)
• Corrosion resistant
• Strongly adhered to base metal
• Con: stabilizes weaker ferritic phase (BCC) than austenite

Question 10

Why Mo Alloying Addition in 316L?

Answer 10

• Resists pitting corrosion

- Con: stabilizes weaker ferritic phase (BCC) than austenite

Question 11

Why Ni Alloying Addition in 316L?

Answer 11

• Corrosion resistant
• Work hardening
• Austenitic stabilizer to counter Cr & Mo effects

Question 12

Why low C Alloying Addition in 316L?

Answer 12

• Don’t want to form carbides
• If > 0.03% C, Cr23C3
• Precipitate at grain boundary (forms at gb because free space between unaligned grains)
• Depletes grain boundary of Cr, decreases Cr2O3 layer

Question 13

Alloying Additions of 316L

Answer 13

Balance between corrosion (Cr, Mo) and mechanical (Ni, C) properties

Question 14

Carbides

Answer 14

• Not great mechanically
• Deplete grain boundary zones of Cr2O3 passivating layer
• Loss of individual grains
• Release abrasive particles

Question 15

Corrosion

Answer 15

More preferential at grain boundaries because higher energy

Question 16

Stainless Steel: ASTM Standards

Answer 16

• Single phase austenite (FCC)
• No carbides (act as sites of stress concentrators)
• ASTM grain size 6 or finer (100um or less)
• Hall-Petch relationship
• Additional processing (cold working)

Question 17

Hall-Petch Relationship

Answer 17

• Yield strength increases with decreasing grain size

- Grain boundaries impede dislocation motion (more gb with smaller grains, and therefore higher yield strength)

Question 18

Cold Working

Answer 18

• Adding dislocations, which move preferentially along slip plane/slip direction (move easiest on atoms close together), and can add more dislocations along the way, which slows dislocation motion
• Increases yield strength

Question 19

Co-Based Alloys

Answer 19

• Superior corrosion resistance and strength to stainless steels
• More expensive and more dense than stainless steels

Question 20

Use of Co-Based Alloys

Answer 20

• Bone plates
• Wires
• Screw nails
• Joint replacement parts (areas where we don’t want a lot of ossteointegration)
• Heart valves and rings

Question 21

Types of Co-Based Alloys

Answer 21

• F75
• F799
• F90
• F562
• Select based on mechanical properties and corrosion resistance

Question 22

Co-Based Alloying Additions

Answer 22

• Multiphasic alloys (grains that are different crystal structures)
• Pure Co: In equilibrium is HCP at room temp (difficult). Slowly transforms from FCC at high temps to HCP at low temps. Retained FCC if cooled quickly (quenched-cooled faster than could transform because diffusion requires time and temp). Increases yield strength (dislocation motion from FCC to HCP hard, requiring higher energy, and results in high hardness)

Question 23

Co-Based Alloying Additions: More HCP at Room Temp

Answer 23

Add Cr, Mo, W to increase transformation temp

Question 24

Co-Based Alloying Additions: More FCC at Room Temp

Answer 24

Add Fe, Ni to decrease transformation temp

Question 25

Co-Based Alloy Processing Technique

Answer 25

- Quench to get FCC - Cold work (increases drive for transformation, adding energy and pushing closer to equilibrium) - HCP forms fine platelets within FCC grains - Resists dislocation motion (to greater extent) and increases yield strength

Question 26

Co-Cr Multiphasic Alloy

Answer 26

- Wide range of microstructures | - Use microstructure to control different properties

Question 27

Solid Solution Hardening

Answer 27

- Lattice distortion by interstitial and substitutional atoms in solid solution - Add Mo, W, Cr, Mn, and Si to harden alloys - Distortion caused by large interstitial atom - Substitutional solid solutions: small or large solute atom - Systems under tension/compression

Question 28

F75 Fabrication

Answer 28

- Cast into ceramic mold at 1350-1450C - Co-rich (alpha) matrix plus interdendritic and grain boundary carbides - Molds can add impurities (could lead to failure in vivo)

Question 29

F799 Fabrication

Answer 29

- Like F75 but forged after casting - Additional cold work (more energy driving transformation) - Results in fine two-phase HCP-FCC microstructure

Question 30

F90 Fabrication

Answer 30

- W and Ni added to improve machinability

Question 31

F562 Fabrication

Answer 31

- Co-Ni-Cr-Mo alloy with 50% cold work - Very fine HCP-FCC microstructure - Resistance to dislocation motion

Question 32

Titanium/Ti-Based Alloys

Answer 32

- "New" - Very good biocompatibility - Good strength - Good corrosion resistance - Lighter weight

Question 33

Use of Titanium/Ti-Based Alloys

Answer 33

- Screws - Nails - Pacemaker cases - Hip replacement stems

Question 34

Titanium/Ti-Based Alloying Additions

Answer 34

- Multiphasic alloys | - Pure Ti: HCP at room temp (alpha phases). BCC at high temps (beta phase). Transformation is slow.

Question 35

Titanium/Ti-Based Alloying Additions: More HCP at Room Temperature

Answer 35

Add H, C, N, Al to increases transformation temp

Question 36

Titanium/Ti-Based Alloying Additions: Less HCP at Room Temperature

Answer 36

Add Cr, V, Fe, Ni to decrease transformation temp

Question 37

Types of Titanium/Ti-Based Alloys

Answer 37

- Ti-6Al-4V: two phase resulting in inhibiting dislocation motion - ASTM F67-CP Ti: single phase alpha (HCP) - F136 (Ti-6Al-4V)

Question 38

Ti-6Al-4V Fabrication

Answer 38

- Difficult - 1668C and extremely reactive (H, O, N) - Ti very efficient getter of O2 - Investment cast (must control casting enviroment) - PM gaining popularity (customize part to patient)

Question 39

ASTM F67-CP Ti Fabrication

Answer 39

- 30% cold work (because no additional phases) which increases yield strength - Interstitial impurities (O, C, N) which increases yield strength

Question 40

F136 (Ti-6Al-4V) Fabrication

Answer 40

- Heat treat to alter microstructure (control) - Forging (introduce dislocations) and annealing (bring microstructure back) used to produce fine alpha with isolated beta at grain boundaries - Add energy to allow transformation of phases

Question 41

Why Ti?

Answer 41

- Case study: Ti rod fused to bone (normally lose) - Osseointegration: naturally bonds directly to bone - Outer surface of Ti has gamma TiO/TiO2. Bone cells attach to that surface (form anchorage points, deposit minerals, firm attachment)

Question 42

Summary

Answer 42

- Each metal has disadvantages/advantages - High strength/stiff - Biomedical metals have superior corrosion resistances to other metals - Ti-enhanced integration - Balance of properties - Implants can fail no matter what (surgical error)