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MSE 3611 > Lecture 9 - Metallic Biomaterials > Flashcards

Lecture 9 - Metallic Biomaterials Flashcards

1
Q

Why Metals?

A

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

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2
Q

Why Metal Alloys?

A
  • Fine-tune mechanical profile
  • Reduce Corrosion
  • Exceptions: Au, Pt, CP Ti
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3
Q

Applications of Metallic Biomaterials

A
  • Fracture plates
  • Tibia rods and intramedullary nails (center cavity of bone shafts - storage of bone marrow)
  • Bone screws
  • Joint replacements
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4
Q

Types of Metallic Biomaterials

A 
Main:
- Stainless steels
- Co-based alloys
- Ti-based alloys
Others:
- Copper
- Gold
- Platinum
Degradable:
- Mg alloys (quick degradation profile)
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5
Q

Goal of Metallic Biomaterials

A

Replace or repair diseased or damaged organs

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6
Q

Use of Stainless Steels

A

Removable or low commodity devices:

  • plates
  • screws
  • pins
  • replacement joints
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7
Q

Types of Stainless Steels

A
  • ASTM F138
  • ASTM F139
  • Austenitic (FCC)
  • 316L
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8
Q

316L Stainless Steel

A
  • 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)
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9
Q

Why Cr Alloying Addition in 316L?

A
  • 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
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10
Q

Why Mo Alloying Addition in 316L?

A
  • Resists pitting corrosion

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

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11
Q

Why Ni Alloying Addition in 316L?

A
  • Corrosion resistant
  • Work hardening
  • Austenitic stabilizer to counter Cr & Mo effects
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12
Q

Why low C Alloying Addition in 316L?

A
  • 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
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13
Q

Alloying Additions of 316L

A

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

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14
Q

Carbides

A
  • Not great mechanically
  • Deplete grain boundary zones of Cr2O3 passivating layer
  • Loss of individual grains
  • Release abrasive particles
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15
Q

Corrosion

A

More preferential at grain boundaries because higher energy

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16
Q

Stainless Steel: ASTM Standards

A
  • 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)
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17
Q

Hall-Petch Relationship

A
  • Yield strength increases with decreasing grain size

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

18
Q

Cold Working

A
  • 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
19
Q

Co-Based Alloys

A
  • Superior corrosion resistance and strength to stainless steels
  • More expensive and more dense than stainless steels
20
Q

Use of Co-Based Alloys

A
  • Bone plates
  • Wires
  • Screw nails
  • Joint replacement parts (areas where we don’t want a lot of ossteointegration)
  • Heart valves and rings
21
Q

Types of Co-Based Alloys

A
  • F75
  • F799
  • F90
  • F562
  • Select based on mechanical properties and corrosion resistance
22
Q

Co-Based Alloying Additions

A
  • 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)
23
Q

Co-Based Alloying Additions: More HCP at Room Temp

A

Add Cr, Mo, W to increase transformation temp

24
Q

Co-Based Alloying Additions: More FCC at Room Temp

A

Add Fe, Ni to decrease transformation temp

25
Q

Co-Based Alloy Processing Technique

A
  • 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
26
Q

Co-Cr Multiphasic Alloy

A
  • Wide range of microstructures

- Use microstructure to control different properties

27
Q

Solid Solution Hardening

A
  • 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
28
Q

F75 Fabrication

A
  • 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)
29
Q

F799 Fabrication

A
  • Like F75 but forged after casting
  • Additional cold work (more energy driving transformation)
  • Results in fine two-phase HCP-FCC microstructure
30
Q

F90 Fabrication

A
  • W and Ni added to improve machinability
31
Q

F562 Fabrication

A
  • Co-Ni-Cr-Mo alloy with 50% cold work
  • Very fine HCP-FCC microstructure
  • Resistance to dislocation motion
32
Q

Titanium/Ti-Based Alloys

A
  • “New”
  • Very good biocompatibility
  • Good strength
  • Good corrosion resistance
  • Lighter weight
33
Q

Use of Titanium/Ti-Based Alloys

A
  • Screws
  • Nails
  • Pacemaker cases
  • Hip replacement stems
34
Q

Titanium/Ti-Based Alloying Additions

A
  • Multiphasic alloys

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

35
Q

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

A

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

36
Q

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

A

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

37
Q

Types of Titanium/Ti-Based Alloys

A
  • Ti-6Al-4V: two phase resulting in inhibiting dislocation motion
  • ASTM F67-CP Ti: single phase alpha (HCP)
  • F136 (Ti-6Al-4V)
38
Q

Ti-6Al-4V Fabrication

A
  • 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)
39
Q

ASTM F67-CP Ti Fabrication

A
  • 30% cold work (because no additional phases) which increases yield strength
  • Interstitial impurities (O, C, N) which increases yield strength
40
Q

F136 (Ti-6Al-4V) Fabrication

A
  • 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
41
Q

Why Ti?

A
  • 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)
42
Q

Summary

A
  • 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)

MSE 3611 (16 decks)

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