Metallic alloys Flashcards
What is steel made of?
Alloy of Fe and C
Forms of steel based on temperature
Austenite
Ferrite
Martensite
Austenite
FCC
Desirable
Not stable at room temperature
Ferrite
BCC
Stable at room temperature
Martensite
Body-centered tetragonal
Very brittle
Stable at RT
How can you make steel stainless
Add large amounts of Cr as a substitutions impurity (20%)
A passive layer of Cr2O3 forms protecting steel from corrosion
How can you make austenite stable at room temp?
Add large amounts of Ni
How can you improve the mechanical properties of stainless steel with an interstitial impurity?
Add Nitrogen
forms covalent bonds with Fe
Further improves corrosion resistance
What is the atomic geometry of titanium alloys according to temperature?
Alpha phase: HCP structure at room temp
Beta phase: allotropic transformation to BCC T>885 celcius
What is the transition temperature
Temperature at which allotropic transformation takes place
How do beta stabilizers affect titanium alloys?
Reduces transition temperature
results in the retention of the beta phase at room temperature
essential to increase strength and attain high strength-to-modulus ratio of Ti alloys
limitations of titanium alloys
- Toxicity from alloying elements
- short fatigue life due to wear and surface corrosion
- Lower young’s modulus compared to stainless steel
- stress shielding still a concern for the material and alternatives
NiTi
An equiatomic alloy of Ti and Ni
Shape memory properties
Temperature dependant crystal structures of NiTi
High temp: Austenite, FCC phase
Low Temp: Martensite, Body-centered tetragonal
Shape memory
Caused by temperature phases of the alloy
superelasticity
Ability to return to its original shape after unloading after substantial deformation
