MAE 3324 Ch10 Flashcards
The development of microstructure in both single and two-phase alloys typically involves some type of ____.
Phase transformation.
Ref: Pg 303
What is the general definition of a phase transformation?
An alteration in the number and/or character of the different microstructures that compose an element.
Phase transformation occurs when phase boundaries on PD’s are crossed as temperature and/or pressure is changed.
Ref: Pg 303
What is the “transformation rate?”
The rate at which a given transformation takes place with respect to time.
Ref: Pg 303
What are the three classifications of transformations?
- Diffusion dependant transformations in which there is no change in either the number or composition of the phases present.
- Diffusion dependant transformations in which there is some alteration in the phase compositions, often in the number of phases present.
- Diffusionless transformations.
Ref: Pg 304-405
Give examples of diffusion dependant transformation in which there is no change in either the number or composition of the phases present.
Solidification of a pure metal, allotropic transformations, recrystallization, and grain growth.
Ref: Pg 305
Give an example of a diffusion dependant transformation in which there is a change in either the number or composition of the phases present.
The eutectoid reaction described by Equation 9.19.
Extra notes: See extra information in section 10.5.
Ref: Pg 305
True or False
With phase transformations, normally at least one new phase is formed that has different physical/chemical characteristics and/or a different structure than the parent phase.
True
Ref: Pg 305
True or False
Most phase transformations occur instantly.
False
Ref: Pg 305
The process of a phase transformation may be broken down into two distinct stages. What are they?
- Nucleation
- Growth
Ref: Pg 305
Describe the nucleation stage.
Nucleation involves the appearance of very small particles, or nuclei of the new phase (often consisting of only a few hundred atoms), which are capable of growing. The nuclei act as templates on which crystals grow.
Ref: Pg 305
Describe the growth stage.
The nuclei increase in size, which results in the disappearance of some or all of the parent phase.
Ref: Pg 305
When is a phase transformation said to be complete?
When the growth of the new particles is allowed to proceed until the equilibrium fraction is attained.
Ref: Pg 305
What are the two types of nucleation and what are their differences.
- Homogeneous and heterogeneous
- The distinction between the two is made according to the site at which nucleating events occur.
Ref: Pg 305
Where does homogeneous nucleation occur?
For the homogeneous type, nuclei of the new phase form uniformly throughout the parent phase. Nuclei form in the bulk of liquid metal and require considerable supercooling (80 to 300C).
Ref: Pg 305 and Lecture Slides
Where does heterogeneous nucleation form?
Nuclei from preferentially at structural inhomogeneities, such as container surfaces, insoluble impurities, grain boundaries, and dislocations.
Ref: Pg 305
What is the general definition of Gibbs free energy?
Gibbs free energy is a thermodynamic parameter that is a function of the internal energy of the system( enthalpy, H) and a measurement of the randomness of the atoms in the system (entropy, S).
Ref: Pg 306
A transformation occurs spontaneously only when the change in Gibbs free energy has a ____.
Negative value.
Ref: Pg 306
What are the two contributors to the total free energy change that accompany a solidification transformation?
- The free energy difference between the solid and liquid phases, or the volume free energy.
- The second is the energy contribution that results from the formation of the solid/liquid phase boundary during the solidification transformation.
Ref: Pg 306
What is the activation free energy?
The free energy required for the formation of a stable nucleus.
For nuclei to form what condition must be present?
The rate of addition of atoms to the nucleus must be faster than the rate of loss.
Ref: Lecture slides
What is the driving force to nucleate as we supercool a material?
Eutectic, eutectoid reactions
Ref: Lecture Slides
What is the driving force to nucleate as we superheat a material?
Peritectic
Ref: Lecture Slides
How does large supercooling effect nucleation?
Rapid nucleation rate and small crystals.
Ref: lecture slides
How does small supercooling effect nucleation?
Slow nucleation rate with large crystals.
Ref: Lecture Slides
Is it easier or harder for heterogeneous nucleation to take place? Why?
It is much easier since a “nucleating surface” is already present.
Ref: Lecture slides
To determine reaction rate, what does one need to do?
Measure the degree of transformation as a function of time while holding temperature constant.
Ref: Lecture Slides
What are some experimental techniques to measure the degree of transformation?
x-ray diffraction (many specimens)
Electrical conductivity measurements (single specimens)
Propagation of sound waves (single specimens)
Ref: Lecture Slides
True or False
The rates of transformation are often so slow that the attainment of equilibrium states is not possible.
True
Ref: Lecture slides
True or False
The degree of supercooling for homogeneous nucleation may be significant (on the order of several hundred degrees Kelvin) for some systems.
True
Ref: Pg 309
During the cooling of a liquid, an appreciable nucleation rate will be fine only after the temperature has _____. This phenomenon is known as ____.
been lowered to below the melting temp
supercooling (or undercooling)
Ref: Pg 309
True or False
It is easier for nucleation to occur at surfaces and interfaces than at other sites. This type of nucleation is termed heterogeneous.
True
Ref: Pg 310
When does the growth step begin?
Once an embryo has exceeded the critical size r* and becomes a stable nucleus.
Ref: Pg 313
Particle growth occurs via ____.
Long-range atomic diffusion.
Ref: Pg 313
Kinetic deals with ____.
The time dependence of a transformation rate.
Ref: Pg 314
True or False
During kinetic investigations, the fraction of reaction that has occurred is measured as a function of time while the temperature is maintained constant.
True
Ref: Pg 314
The Avrami equation shows ____.
The dependence of the fraction of transformation on time.
Ref: Pg 315
What is a limitation of phase diagrams with respect to phase transformations?
They do not tell you how long a phase transformation will take.
Ref: Pg 317
What is metastable with respect to phase transformations?
An intermediate material state in which the solid is somewhere in between initial and equilibrium states.
Ref: Pg 317
True or False
Particle Growth Occurs by long-range diffusion.
True
Ref: Pg 313
With respect to the Iron Carbon system, most steels have less than ___% of carbon.
6.7%
Ref: Pg 288
Gamma iron is analogous to ___.
Austenite
Ref: Pg 287
With respect to the Iron Carbon system, beyond 6.7% carbon, what compound forms.
Cementite (Fe3C)
Ref: Pg 288
When does cementite form?
When the solubility limit of carbon and alpha-ferrite is exceeded below 727 degrees C.
Perlite forms when ___.
Austenite is colled blow approximately 720 degrees C to the alpha + Fe3C region.
Ref: Pg 291
If Austenite is allowed to slowly cool what is the result?
Pearlite (alpha + Fe3C layers + a proeutectiod phase)
Ref: Lecture Slides
If Austenite is allowed to moderately cool what is the result?
Bainite
Ref: lecture Slides
If Austenite is rapidly quenched what is the result?
Martensite
Ref: Lecture Slides
If martensite is reheated, what forms?
Tempered Martensite
Ref: Lecture Slides
