Lecture 9 - Metallic Biomaterials Flashcards
Why Metals?
Mechanical Profile - failure modes, modulus, strength, modes of deformation (plastic)
Why Metal Alloys?
- Fine-tune mechanical profile
- Reduce Corrosion
- Exceptions: Au, Pt, CP Ti
Applications of Metallic Biomaterials
- Fracture plates
- Tibia rods and intramedullary nails (center cavity of bone shafts - storage of bone marrow)
- Bone screws
- Joint replacements
Types of Metallic Biomaterials
Main: - Stainless steels - Co-based alloys - Ti-based alloys Others: - Copper - Gold - Platinum Degradable: - Mg alloys (quick degradation profile)
Goal of Metallic Biomaterials
Replace or repair diseased or damaged organs
Use of Stainless Steels
Removable or low commodity devices:
- plates
- screws
- pins
- replacement joints
Types of Stainless Steels
- ASTM F138
- ASTM F139
- Austenitic (FCC)
- 316L
316L Stainless Steel
- 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)
Why Cr Alloying Addition in 316L?
- 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
Why Mo Alloying Addition in 316L?
- Resists pitting corrosion
- Con: stabilizes weaker ferritic phase (BCC) than austenite
Why Ni Alloying Addition in 316L?
- Corrosion resistant
- Work hardening
- Austenitic stabilizer to counter Cr & Mo effects
Why low C Alloying Addition in 316L?
- 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
Alloying Additions of 316L
Balance between corrosion (Cr, Mo) and mechanical (Ni, C) properties
Carbides
- Not great mechanically
- Deplete grain boundary zones of Cr2O3 passivating layer
- Loss of individual grains
- Release abrasive particles
Corrosion
More preferential at grain boundaries because higher energy
Stainless Steel: ASTM Standards
- 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)
Hall-Petch Relationship
- Yield strength increases with decreasing grain size
- Grain boundaries impede dislocation motion (more gb with smaller grains, and therefore higher yield strength)
Cold Working
- 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
Co-Based Alloys
- Superior corrosion resistance and strength to stainless steels
- More expensive and more dense than stainless steels
Use of Co-Based Alloys
- Bone plates
- Wires
- Screw nails
- Joint replacement parts (areas where we don’t want a lot of ossteointegration)
- Heart valves and rings
Types of Co-Based Alloys
- F75
- F799
- F90
- F562
- Select based on mechanical properties and corrosion resistance
Co-Based Alloying Additions
- 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)
Co-Based Alloying Additions: More HCP at Room Temp
Add Cr, Mo, W to increase transformation temp
Co-Based Alloying Additions: More FCC at Room Temp
Add Fe, Ni to decrease transformation temp
Co-Based Alloy Processing Technique
- 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
Co-Cr Multiphasic Alloy
- Wide range of microstructures
- Use microstructure to control different properties
Solid Solution Hardening
- 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
F75 Fabrication
- 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)
F799 Fabrication
- Like F75 but forged after casting
- Additional cold work (more energy driving transformation)
- Results in fine two-phase HCP-FCC microstructure
F90 Fabrication
- W and Ni added to improve machinability
F562 Fabrication
- Co-Ni-Cr-Mo alloy with 50% cold work
- Very fine HCP-FCC microstructure
- Resistance to dislocation motion
Titanium/Ti-Based Alloys
- “New”
- Very good biocompatibility
- Good strength
- Good corrosion resistance
- Lighter weight
Use of Titanium/Ti-Based Alloys
- Screws
- Nails
- Pacemaker cases
- Hip replacement stems
Titanium/Ti-Based Alloying Additions
- Multiphasic alloys
- Pure Ti: HCP at room temp (alpha phases). BCC at high temps (beta phase). Transformation is slow.
Titanium/Ti-Based Alloying Additions: More HCP at Room Temperature
Add H, C, N, Al to increases transformation temp
Titanium/Ti-Based Alloying Additions: Less HCP at Room Temperature
Add Cr, V, Fe, Ni to decrease transformation temp
Types of Titanium/Ti-Based Alloys
- Ti-6Al-4V: two phase resulting in inhibiting dislocation motion
- ASTM F67-CP Ti: single phase alpha (HCP)
- F136 (Ti-6Al-4V)
Ti-6Al-4V Fabrication
- 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)
ASTM F67-CP Ti Fabrication
- 30% cold work (because no additional phases) which increases yield strength
- Interstitial impurities (O, C, N) which increases yield strength
F136 (Ti-6Al-4V) Fabrication
- 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
Why Ti?
- 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)
Summary
- 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)
