Metals

Question 1

Advantages of Metals in Biomedical Applications

Answer 1

• they have the requisite mechanical properties necessary for many applications that no other class of bio- materials can replicate. (Strength, fatigue and corrosion resistance)
• No other biomaterial class can perform at the stress and degradation- inducing levels that metals can. ie. Cyclic loading results in fatigue failure modes, sustained wear mechanisms, the potential for mechanically assisted corrosion mechanisms, and other effects.

Question 2

Permanently Implantable metals

Answer 2

Metals intended for indefinite use within the body.

• 3 major
• Stainless steels (primarily 316L) [ surgical instrumentation and screws, rods, and plates for bone fixation and in spinal fusion devices. ]
• cobalt–chromium–molybdenum alloys [applications where high strength, fatigue resistance, and wear resistance are needed - one of the most wear resistant alloys - joint applications]
• titanium (ASTM-F76) and its alloys [ dental implants and total joint replacements ( high strength, low modulus, and is particularly good at interfacing with the biological system - particularly bone ingrowth via pourous surfaces)

Question 3

Degradable alloys

Answer 3

Designed to be temporary, ultimately degrading or biocorroding over time (biodegradable alloys).

• Mg,
• tin (Sn),
• iron (Fe),
• and zinc (Zn) alloys.
• the primary interest is to find alloys that will remain capable of carrying the applied loads and wear processes until the body has healed sufficiently that they are no longer required, and will then corrode away and be resorbed and eliminated by the body.

Question 4

Metal Processing

Answer 4

• Processing affects structure, structure affects properties, and properties affect performance.
• Once elemental metals are formed, they can be melted together to form alloys of specific weight or atomic concentrations.
• Followed by heating to melting and then casting.
• Usually immediately hot worked into stock.
• Deformation processes, thermochemical processing or thermal treatments may then be conducted.

Question 5

Thermomechanical Processing

Answer 5

• The combination of high temperature and plastic deformation so that the effects of each can work together to obtain the final form of the metal.
• Plastic deformation at low temperature, known as cold working or work hardening, imparts plastic deformation into the alloy.
• This is manifested structurally as the generation, movement, and entanglement of dislocations.
• These defect structures act to increase the yield stress and lower the ductility of the metal.
  ie. raise strength but increase risk of fracture in alloy.

Question 6

Thermal treatments

Answer 6

• aka. Annealing, are often performed on previously cold-worked alloys to provide thermal energy to the microstructure so that dislocations can be eliminated (recovery) or new deformation-free grains can be nucleated and grown (recrystallization) from the prior deformed (or cold-worked) grains.
• Tend to have lower yield strengths than their cold-worked counterparts, and higher ductility.

Question 7

Point defects

Answer 7

• Include vacancies and interstitial atoms.
• Vacancies can have an effect on the diffusion behaviour of substitutional - have a limited effect on properties in most biomed apps.
• Interstitial atoms can have profound effects on the strength properties of a metal.
• increase the yield strength and ultimate strength, and affect the work hardening of metals by creating dislocation atmospheres that can pin dislocations requiring higher stresses to induce plastic deformation

Question 8

Line Defects

Answer 8

• Plastic deformation of metals is associated with the sliding of atom planes over one another and dislo- cations are a localized elastic distortion of the crystal associated with this plane sliding.
• edge dislocations can be thought of as the distortion around the termination of an extra half-plane of atoms in the crystal lattice .

Question 9

Strengthening Mechanisms

Answer 9

The strength of metals is determined from a number of different tests that basically assess the stresses required to induce some amount of permanent (or plastic) deformation in the metal.

• Alloying: small additions of alloying elements, particularly the interstitial atoms, can have a large effect on the strength of the metal. eg. Carbon additions to iron to make steel (interstitial solid solution strengthening).
• Cold-working: the extent of plastic deformation that is imparted to the alloy during processing. These changes make it more difficult to induce additional dislocation generation and motion and therefore higher amounts of applied stress are needed to generate additional plastic deformation.
• Grain size: Hall–Petch relationship where the yield strength increases with decreasing gain diameter to the 1⁄2 power.
• Precipitation strengthening: generation of multiple phases in a single alloy.