Chapter 3 - Thermodynamics Flashcards
What is thermodynamics?
general laws, relationships, and procedures for understanding macroscopic temperature-dependent phenomena
What are intensive variables? Examples
properties independent of the mass of the system
these includes Pressure and Temperature
What are extensive variables? Example
properties that change with the mass of the system
these includes Volume
What is equilibrrium?
a state of a system in which its variables have definite values that remain constant as long as the external conditions are unchanged depends on the nature of the boundaries separating this system from its surroundings
Name the four types of boundaries?
adiabatic, diathermic, closed, and open
What is adiabatic
no transfer of heat
what is diathermic
heat can be transfered
What is a closed boundary
no mass transfer
what is an open boundary
mass can be transfered
What is work?
system undergoes displacement under the action of a force
What is an hydrostatic system?
an isotropic system of constant mass and composition that exerts uniform pressure on its surroundings
What is the PVT system
a hydrostatic system that can be desecribed by 3 variables: P (pressure exerted by the system on its surroundings), V, and T
What is heat?
a form of energy which is transferred between a system and its surroundings due to a temperature difference
Does a system contain heat?
No, system does not “contain heat”, but rather heat is a process of energy transfer between a system and its surroundings (or two systems)
What is the 1st law of thermodynamics?
Conservation of energy: energy can be transformed from one form to another, but it cannot be created or destroyed
The change in internal energy is equal to the heat transferred to/from the system plus the work performed by/on the system. differential form:
𝑑𝑈=𝑑𝑄+𝑑𝑊
What is the 2nd law of thermodynamics?
Entropy never decreases
Work CAN however be obtained from a temperature difference between two systems (Thigh and Tlow), but some loss of energy via heat will occur in this case as the work is done and the entropy increases (this is why it is impossible to convert heat into work with perfect efficiency with any kind of process)
address the observation that natural processes have a preferred direction of progress
What is the third law of thermodynamics?
The entropy of a homogeneous material is zero at absolute zero temperature (T=0K)
What is the gibbs energy
Now we can see that G is a function of P and T as we desired, and it can be minimized if P and T are held constant (so that dP and dT = 0).
A system in equilibrium with its surroundings will have dG=0. A system which is undergoing a spontaneous (irreversible) process will have a change in Gibbs energy < 0. The Gibbs energy decreases during the process, and then reaches a minimum at equilibrium.
What is enthalpy?
Enthalpy is useful for expressing thermal energy changes under constant pressure, and for problems involving heat capacity
What are the three types of equilibria in p hase diagrams?
stable, metastable and unstable
What is the stable equilibrium in phase diagram?
exists when the object is in its lowest energy condition
What is metastable equilibrium in phase diagram?
exists when additional energy must be introduced before the object can reach true stability.
What is unstable eequilibrium in phase diagrams?
exists when no additional energy is needed before reaching metastability or stability. Under some conditions, metastable crystal structures can form instead of stable structure. Rapid freezing is a common method of producing metastable structures, but some (such as Fe3C, or cementite) are produced at moderately slow cooling rates. With extremely rapid freezing, even thermodynamically unstable structures (such as amorphous metal glasses) can be produced.
What are phase diagrams?
In order to record and visualize the result of studying the effect of stage variable on a system, diagrams were introduced to show the relationships between the various phases that appear within the system under equilibrium conditions
What could be a single component phase diagram?
can be simply a one- or two-dimensional plot showing the phase change in the substance as temperature and/or pressure change.
What information can be derived from a phase diagram for a given composition and temperature?
(1) What phases are present (2) The states of those phases (3) The relative quantities of those phases
What is a binary phase diagram?
2 components, and diagram is typically expressed as a two-dimensional plot of temperature versus composition
For a binary system with two components that are completely soluble in each other, what are the two
single-phase fields?
- Liquid
2. Solid
For a binary system with two components that are completely soluble in each other, what are the two phase fields boundaries?
- Liquidus
2. solidus
What is the liquidus?
the boundary between the liquid field and the two-phase field - represents the temperatures at which the alloy of various compositions of the system begin to freeze during cooling, or finish melting during heating
What is the solidus?
boundary between the solid field and the two-phase field – represents the temperatures at which the alloy finishes freezing during cooling or begins melting during heating
What is the eutectic alloy?
where the liquid phase freezes into a mixture of two solid phases.
How can you determine the phase fraction at a particular composition using a binary diagram?
With the lever rule
How is the shape of liquidus, solidus, and solvus curves (or surfaces) in a phase diagram determined
by the gibbs energies of the relevant phases
What is the driving force for solidifcation explain.
Remember that under constant temperature, T, and pressure, P, the equilibrium state is the one for which the Gibbs energy is at a minimum. In order for a phase transformation to occur, the destination phase (state) must have a lower Gibbs energy, such that there is a decrease in the Gibbs energy as a result of the phase change. The difference in Gibbs free energy between twFo phases, ΔG, is what provides a driving force for solidification to occur
For an alloy to be steel the carbon content must be…
<2.1 wt%
If an alloy has a carbon content above 2.1 wt%, is it steel?
No, it is cast iron
What crystaline structure is the alpha-ferrite?
BCC iron
What is the stable form of iron at room temperature?
ferrite
At what temperature does ferrite convertes into austenite?
912 C
What cyrstalline structure is austenite?
FCC iron
Below what temperature is ausenite not stable?
727 C
If austenite is not stable below 727C hwo do we obtain it at lowe rtemperatures?
By rapid cooling
Is there another phase above austenite? If so, what temperature?
Yes, there is S-ferrite starting at 1394 C
What is the max. solubility of C in alpha-ferrite?
0.022wt%
Why is the austenite carbon solubility much higher than the alpha-ferrite
Because the internstitial sites in the FCC cyrstal are much larger.
Whta are the general properties of cementite?
hard and brittle.
What is the effect of rapid cooling in critical radius? What does this mean?
The faster we cool the lower the critical radius. This means many more grains are being initiated.
What is the critical radius?
it is the lowest radius formed by atoms or molecules clustering together (in a gas, liquid or solid matrix) before a new phase inclusion (a bubble, a droplet or a solid particle) is viable and begins to grow.
