Chapter 10 Flashcards
Homoegeneous Nucleation
Occurs anywhere in the old phase and results from the formation of a spherical seed when the drop in volume energy is more than the increase in surface energy of the seed
Requires undercooling to occur
Heterogeneous Nucleation
Happens with the assistance of surface inhomogeneities or third phase particles, and does not require much undercooling
Growth
Growth of the new solid phase requires diffusive rearrangement of atoms
Growth occurs quickly at higher temperature and slowly or not at all at lower temperature
Temperature slightly below equilibrium transformation temperature
Homogenous nucleation very difficult, growth moderate
Total rate of transformation is slow
Temperature moderately below equilibrium transformation temperature
Homogeneous nucleation moderate, growth moderate
Maximum rate of total transformation (minimum time for transformation on TTT)
Temperature far below equilibrium transformation temperature
Homogeneous nucleation very easy, growth very difficult
Total rate of transformation slow
Temperature extremely far below equilibrium transformation temperature
Nucleation impossible despite extreme driving force for nucleation because there is insufficient time/thermal energy for new phase to precipitate
Growth impossible because of very low temperature
Leads to supersaturated solid solutions and/or meta-stable phases
Age Hardening steps for age-hardenable alloys
- solution treatment
- Quenching
- Aging
Age Hardening
Solution Treatment
Alloy raised to a high temperature where particle-forming element goes entirely into solid solution
Age Hardening
Quenching
Very rapid cooling locks particle forming element into matrix of alloy by not giving it time to precipitate out.
Slow cooling often results in brittle phase at old grain boundaries instead of fine dispersion of spherical particles everywhere.
Age Hardening
Aging
Slight increase in temperature now causes the particle forming element to precipitate out in the form of extremely small spherical particles throughout the alloy
Homogeneous nucleation dominates and the particles form everywhere rather than on grain boundaries
Higher aging temperature
Fewer and larger particle
Lower alloy strength
more critical timing
faster and cheaper heat treatment
Lower aging temperature
Greater number of smaller particles
higher alloy strength
timing not critical, easy to achieve optimum age and uniform properties
Longer, more expensive heat rteatment
TTT and CCT Diagram use
Predict the final micro-structure of a steel if the heat treatment is given to you
to construct a heat treatment to obtain a desired final micro-structure
TTT Diagram
Always start with an austenitizing treatment to wipe out previous microstructure
Remember: every time you change temperatures, the time goes back to zero
At each hold temperature, the only part of the material that is transforming is the remaining unstable austenite. Austenite that has changed to ferrite, cementite, pearlite, or bainite will no longer change
