Metals I: Structure

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Metals IStructure Properties

• Lecture 5
• February 3, 2009

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Metals as Biomaterials

• Metals were the first materials used in medical
  implants
• Bone repair, tooth replacement
• Very versatile in function
• Electrical applications
• Structural, load-bearing applications
• Wear surfaces
• Fatigue applications
• What gives metals their properties?
• What properties are desirable?

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General Properties

• Dense atoms are tightly packed
• High melting and boiling points strong forces
  of attraction exist between particles.
• Good heat conduction - kernels transmit the
  energy of vibrations to its neighbors.
• Good electrical conduction mobile delocalized
  electrons within the lattice.
• Malleable and ductile the distortion does not
  disrupt the metallic bonding.
• Metals are lustrous free electrons causes most
  metals to reflect light (non-metals are
  transparent).

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Applications of Metals

• Dental
• Fixed dental prostheses (endosseous screws,
  subperiostial implants)
• Maxillofacial reconstruction
• Fillings
• Electrodes
• Pacemakers
• Neural Stimulators
• Urologic Stimulators
• Orthopedic devices
• Plates, screws, pins and wires rods (temporary)
• Total joints (permanent)
• Clips and staples

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Implantation Requirements

• Corrosion resistant
• Appropriate mechanical properties for the desired
  application
• Stiffness
• Density
• Wear resistance
• Good fatigue properties if under cyclic loading
• Insulated or protected electrical applications

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Metals for Implantation

• Orthopedic implants
• Stainless steels
• Cobalt-chromium alloys
• Titanium alloys
• Dental implants
• Amalgam
• Gold

SS
amalgam
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Structure of Metals

• The metallic bond has no directionality
• Metals atoms pack in simple, high-density
  structures (like miniature ball bearings shaken
  down in a box
• Face-centered cubic (FCC)
• Hexagonal close-packed (HCP)
• Body-centered cubic (BCC)
• Some metals have more than one crystal structure
• Phenomenon referred to as polymorphism
• Polymorphism can be brought about by temperature
  or by alloying

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Polymorphism Examples

• Iron (Fe) and Titanium (Ti)

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Alloys

• Very few metals are used in their pure state
• Adding other elements creates alloys
• Alloying commonly gives better properties
• Alloying elements dissolve in the host metal to
  create solid solutions
• Substitutional solid solutions
• Dissolved atoms replace host atoms
• Interstitial solid solutions
• Small atoms fit between larger host atoms

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Solid Solubility

• Solubility of alloying element in the basic metal
  can very between lt0.01 and 100 depending on the
  combinations of elements
• Fe can dissolve only 0.007 C at room temperature
• Cu can dissolve more than 30 Zn (to form brass)
• Excess alloying element will precipitate
  (separate) out if its concentration is above the
  solubility limit
• Precipitates are usually chemical compounds
• Examples Fe3C in carbon steels CuAl2 in Al-Cu
  alloys

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Phases

• Phase Region of material that has uniform
  physical and chemical properties
• Water (single phase) Ice (single phase)
• Water ice (two-phase mixture)
• Metal structures
• Single phase - pure metal or solid solution
• Two-phase mixture
• Alloy containing more of alloying element than
  the host metal can dissolve
• Solid solution with 2 distinct crystal structures
  throughout
• Phase diagrams give information on phases and
  their composition under conditions of temperature
  and composition

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Metal Structure

• Structure of metal is defined by
• Composition
• Elements a metal contains and relative
  concentrations
• Number of phases and their concentrations
• Composition of each phase
• Geometric parameters, microstructure
• Size and spacing of each phase
• Shape of each phase
• Effects of phases on properties

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Steel

• Steel is the most widely-used metal
• Alloy of iron (Fe) and carbon (C)
• Simplest type is carbon steel with lt 2 wt C
• Good mechanical properties
• Manufacturing is easy and cheap
• But not biocompatible corrodes in saline
  environment
• Instead stainless steels are used
• 1913, Harry Brearly accidentally discovered
  that adding chromium to low carbon steel made it
  stain resistant - rifle barrels
• Modern stainless steal also contains nickel,
  niobium, and molybdenum to enhance corrosion
  resistance

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Iron Steel Crystal Phases

• Pure iron can be stable as three important
  crystal phases.
• Austenite
• Ferrite
• d Fe
• Stability Iron (FeC) depends on
  temperature and composition

Phase diagram of pure iron
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Steel Phase Diagram

• A few things to note
• Carbon content is very low, only up to 2 shown
  here
• Some single phase solids are possible
• Multi-phase solids are more common with higher C
  content
• Forms shown here include
• Liquid
• Austenite
• Ferrite
• Fe3C
• Stable phases are shown

Carbon content
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Steel Crystal Structure

• FCC
• Larger interstitial sites, higher C solubility
• Stable at high temps

• BCC
• Low solubility for C
• Stable at room temp

• Distorted BCC body centered tetragonal
• Higher C solubility than Ferrite
• Metastable at room temp
• results from quenching Austenite

• More complex crystal
• Precipitate that forms C solubility is exceeded
• Stable at room temp

x
x
x
x
x
x
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Phase Transformation in Steels
Austenite
Martensite
Ferrite

• The austenite-martensite phase transformation
  occurs by non-diffusional, distortion
  rearrangement of atoms.

• The austenite-ferrite phase transformation occurs
  by diffusional rearrangement of atoms.

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Alloying in Steels
Temp.

• Different alloying elements can
• increase austenite stability to lower
  temperatures.
• encourage martensite formation by slowing down
  the ferrite transformation.
• Enough Chromium in steel leads to corrosion
  resistance

Austenite
Ferrite
Unstable Austenite
Martensite
Time
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Why is Stainless Steel Stainless?

• Stainless steels are stainless because of
  passivation
• A protective layer of oxides on the surface of a
  metal which resists corrosion, Cr2O3

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Passivity of Stainless Steel

• Passivity is due to a self-repairing oxide film
• A compact, continuous film requires 11wt
  chromium
• Passivity increases with chromium content up to
  17wt chromium
• Most stainless steels contain 17-18wt chromium
• Corrosion resistance depends on maintenance of
  the passive film
• This is optimised for different environments by
  alloying with other elements
• e.g. Ni, Mo, N, Cu....

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Stainless Steel Families

• Stainless steels can be divided into five
  families.
• Austenitic
• Ferritic
• Martensitic
• Martensitic-Austenitic
• Ferritic-Austenitic
• These families are based on the polymorphic
  crystal structures of iron

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Alloying in Stainless Steels

• Stainless steels are alloyed to control both
  microstructure and corrosion resistance
• Alloying elements can be austenite or ferrite
  stabilizers
• Austenite, ferrite and martensite have different
  properties due to different crystal structures
• Cementite precipitates are very hard and brittle
• The stable phase or phases depends on the balance
  of alloying elements

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Effects of Alloying Elements

• Chromium (Cr)
• Increases resistance to corrosion/oxidation
• Increase hardenability
• Increases high temperature strength
• Stainless steel at least 10.5 Cr
• Molybdenum (Mo)
• Promotes a fine grain structure
• Increases resistance to corrosion in saline
• Increases hardenability
• Nickel (Ni)
• Promotes an austenitic structure (counter acts Cr
  and Mo, which stabilize Ferrite)
• Increases hardenability
• Increases toughness

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Summary of Properties
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Stainless SteelsStrength and Ductility
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Stainless Steel as a Biomaterial

• Most common for implants 316L
• Austentitic family, with lt 0.030 carbon in order
  to reduce the possibility of in vivo corrosion
• Composition typically (wt)
• Fe 60-65 Cr 17-19 Ni 12-14 Mo 2-3
• Low resistance to stress corrosion cracking,
  pitting and crevice corrosion, better for
  temporary use
• Corrosion accelerates fatigue crack growth rate
  in saline and in vivo
• Intergranular corrosion at chromium poor grain
  boundaries - leads to cracking and failure
• Wear fragments - found in adjacent giant cells

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Corrosion in Stainless Steels

• If the carbon content of the steel exceeds
  0.030, carbides may form such as Cr23C6
• Carbides tend to precipitate at grain boundaries
  and start to grow
• Carbide growth depletes the region of chromium
  which forms the protective oxide

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Cobalt-Based Alloys

• General Information
• Cobalt and chromium are dominant elements forming
  a solid up to 65 wt Co
• High Cr content leads to formation of passivating
  Cr2O3 surface layer
• Higher Youngs modulus than either SS or Ti
  alloys
• Can produce implants with highest available
  strengths and endurance limits
• Molybdenum, when added, produces finer grains
• What is the result of this on mechanical
  properties?

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Co Alloys as Biomaterials

• CoCrMo alloy
• Typically cast into desired form
• Used for many years in dental implants more
  recently used in artificial joints
• Good corrosion resistance
• CoNiCrMo Alloys
• Typically used for stems of highly loaded
  implants, such as hip and knee arthroplasty
• High degree of corrosion resistance in salt water
  when under stress
• Poor frictional properties with itself or any
  other material
• CoCrWMo
• Better machinability

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Co Cr Mo

• Cast CoCrMo alloys, ASTM F75 (59-69 Co, 27-30
  Cr, 5-7 Mo)
• Worst mechanical properties of all CoCr alloys
  (similar to some SS and Ti)
• Widely used due to lower cost and ability to
  easily produce intricate shapes investment
  casting
• Used in dentistry and some joint replacements
• Wrought CoCrMo Alloys, ASTM 799 (58-59 Co,
  26-30 Cr, 5-7 Mo)
• Hot forging after casting
• Yield strength, fatigue strength, and UTS are
  about 2x that of F75
• Not as commonly used

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Co Ni Cr

• Wrought CoNiCrMo Alloys, ASTM F562 (29-38 Co,
  19-21 Cr, 9-10.5 Mo, 33-37 Ni)
• Very high strengths due cold working and aging
• Highest endurance limit of all available metal
  alloys for implant applications (endurance limit
  700-800 MPa)
• Widely used in high-load joint replacements knee
  and hip
• More expensive than cast F75
• Wrought CoCrWNi Alloys, ASTM F90 (45-56 Co,
  19-21 Cr, 14-16 W, 9-11 Ni)
• W and Ni added to improve machinability and
  fabrication properties
• Annealed state has similar mechanical properties
  as F75 cold working to 44 more than doubles
  properties
• Very high yield and tensile strengths when cold
  worked
• Not commonly used

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Titanium-Based Alloys

• Ti alloys have some advantage over SS and Co
  alloys
• High strength to weight ratio
• Density of 4.5 g/cm3
• Density of 7.9 g/cm3 for 316 SS
• Density of 8.3 g/cm3 for cast CoCrMo
• Density of 2.0 g/cm3 for solid bone
• Modulus of elasticity for alloys is about 110 GPa
• Half the stiffness of the other metals E
• Still does not match bone - will cause stress
  shielding
• Cortical bone, E 15 GPa
• However, it has some disadvantages too
• Ti has poor shear strength
• Less desirable for bone plates, screws, etc
• Poor in sliding contact with itself or other
  metals, seizes

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Titanium as a Biomaterial

• Best biocompatibility
• Metal of choice where tissue or direct bone
  contact required
• endosseous dental implants
• porous uncemented orthopedic implants
• Corrosion resistance due to formation of a solid
  oxide layer on surface (TiO2) - leads to
  passivation of the material

zimmer
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Titanium Alloys

• Ti-6Al-4V (6 wt Al 4 wt V)
• Poor shear strength which makes it undesirable
  for bone screws or plates
• Tends to seize when in sliding contact with
  itself or other metals
• What application would this not be good for?
• Metal on metal motion
• Joint surfaces
• Screws to be inserted into metal plates

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Dental Metals - Amalgam

• Alloy containing mercury (45-55 mercury, 35-45
  silver, 15 tin)
• Forms a plastic mass at room temperature, when
  mixed with silver and tin, which hardens with
  time
• Strength increases with time to an asymptotic
  level
• 25 of final strength within 1 hour
• Almost 100 strength within 1 day

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Dental Metals - Gold

• Durable, stable, corrosion resistant
• This is especially important in the mouth (why?)
• Can use gold alloys as well
• Copper and platinum improve strength
• Higher gold content (better corrosion
  resistance/lower strength) used in areas not
  subject to high stresses
• Lower gold content used in crowns or other areas
  exposed to high stresses

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Gold and Platinum

• The only metallic biomaterials that do not form a
  self protecting oxide layer on the surface
• Generally used as electrodes and in dentistry due
  to their corrosion resistance, durability, and
  stability
• Gold never reacts with oxygen, which means it
  will not rust or tarnish
• Gold is among the most electrically conductive of
  all metals.
• EXPENSIVE!!

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NITINOL (Nickel Titanium Naval Ordinance
Laboratory)

• A family of materials which contain a nearly
  equal mixture of nickel (55 wt. ) and titanium.
  Other elements can be added to adjust or tune the
  material properties.
• Exhibits unique Shape Memory Effect and
  Pseudo-elasticity
• Biocompatible and corrosion resistant
• Ductile and strong

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Shape Memory Effect

• The process of restoring the original shape of a
  plastically deformed sample by heating
• A crystalline phase change known as
  "thermoelastic martensitic transformation".
• Below the transformation temperature, Nitinol has
  a soft martensitic microstructure and is easily
  deformed.
• Heating the material converts the material to its
  high strength, austenitic condition

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Shape Memory Effect
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Pseudo-elasticity

• Martensite in Nitinol can be stress induced if
  stress is applied in the temperature range above
  austenite finish temperature(100 austenite)
• Then the martensite is deformed using less energy
  then it would take to deform austenite
• Since austenite is the stable phase at this
  temperature under no load conditions, the
  material springs back to its original shape when
  the stress is removed.
• Nitinol alloys are at their optimum superelastic
  behavior at body temperature

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Nitinol Behavior
Pseudo-elasticity
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Summary

• Many alloys currently in use for various
  structural applications
• No single material works best for everything
• In general there are tradeoffs between
• Strength and ductility
• Strength and corrosion resistance
• Ideal properties and cost
• Alloys can be fine tuned to achieve good
  combinations of properties based on the intended
  application
• Next class we will look at how to process metal
  and control their properties