modulation and failure Flashcards
modulation
Alloying elements
Melting and cooling rates
Strain hardening (Cold working)
Heat treatment
Alloying example
Adding Cu to Au to improve Au mechanical properties
Slow cooling
Large grain size
Soft/ductile material
Corrosion resistant
Quick Cooling
Small grain size
hard/brittle material
Extremely corrosive
Strain Hardening (Cold Working)
- Strengthening a metal by plastic deformation
- plastic deformation increases amount of dislocations making dislocations interact and movement difficult
- increases strength but decreases ductility
Heat treatment
Annealing
Tempering
Annealing
Heat metal to > Tmelt/3
Hold for short time
Cool very slowly
soft, ductile, and corrosion resistant material
heals dislocations
Tempering
Heat metal to critical temperature
age for a fixed time
Cool quickly
maintain strength
reduce brittleness
releases atoms from unfavorable positions such as grain boundaries
Failure of metallic implants
Fracture
Fatigue
Metal-to-metal abrasion
Corrosion
Stress shielding
Fractures of metallic implants
Failure due to sudden impact
ex. accidental fall
Fatigue of metallic implants
Age of the implant
ex. titanium hip implants need to be replaced every 15 years
Metal-to-metal abrasion
Abrasion releases metal ions into the blood stream resulting in bone/tissue damage
ex. All-metal hip implants
many lawsuits have been filed
Corrosion of metallic implants
Blood and other body electrolytes, such as proteins and enzymes are highly corrosive
Stress shielding of metallic implants
Occurs due to differences in mechanical properties of metallic implant and native bone
implant bears the majority of the load resulting in the removal of normal stress from native bone
bone begins to resorb and loosen the implant
How to prevent fatigue of metallic implants
Add oxygen to Ti
Oxygen acts as an interstitial impurity and forms strong covalent bonds improving fatigue life of Ti implants
