Materials- Metals: Phase Transformation for Materials Processing Flashcards
What is diffusion-depend transformation 1?
Where there is no change in number or composition of phases present (e.g solidification of pure metal)
What is diffusion-depend transformation 2?
Where there is some alteration in phase compositions and possibly the number of phases present. The final microstructure normally has 2 phases present.
What is produced in a diffusionless transformation?
A metastable phase
What are the two stages in the progress of a phase transformation?
Nucleation (only few hundred atoms) and growth
Difference between homogeneous and heterogeneous nucleation
Homogeneous: nuclei of the new phase form uniformly throughout the parent phase.
Heterogeneous: nuclei form preferentially at structural inhomogeneities such as container surfaces, soluble impurities, grain boundaries, dislocations.
When will a spontaneous phase transformation occur?
Only when the Gibbs free energy has a negative value
What are the two energy contributions to free energy change that accompanies a solidification transformation?
Free energy difference between solid and liquid phases (volume free energy, ΔGv) which will be negative if the temperature is below the equilibrium solidification temperature.
Formation of the solid-liquid phase boundary during solidification transformation (surface free energy, γ) which is positive.
Describe the change in free energy during solidification
Solid particle begins to form as atoms in liquid cluster together and it’s free energy first increases. If this cluster reaches a size corresponding to the critical radius r, then growth will continue with the accompaniment of a decrease in free energy to form a nucleus. But if the cluster has a radius less than r then it will shrink and re-dissolve (this was an embryo).
What is activation free energy?
ΔG*. The free energy required for the formation of a stable nucleus (or energy barrier to nucleation)
In the homogeneous nucleation equations, what are ΔHf, Tm, T?
ΔHf is latent heat of fusion
Tm in melting temperature in K
T is temperature of system
What does lowering the temperature at temperatures below equilibrium solidification temperature mean for nucleation?
Nucleation occurs more readily because critical radius and ΔG* decrease
Formula for number of stable nuclei having radius greater than r, n
n=K1e^(-ΔG/kT)
k is Boltzmann’s constant
Graph of T against n* like decay curve
Rate of diffusion equation
νd=K2e^(-Qd/kT)
νd is frequency at which atoms from the liquid attach themselves to the solid nucleus
Qd is activation energy for diffusion
Graph of T against νd forms cave
Formula for nucleation rate (nuclei per unit volume per second), N•
N•=K3n*νd
Describe and explain how lowering temperature below Tm affects nucleation rate
The nucleation rate first increases, achieves a maximum, then diminishes. Nucleation rate suppressed at high temperatures because of small activation driving force. At lower temperatures, low atomic mobility suppresses nucleation rate.
For heterogeneous nucleation, why is the activation energy lowered when nuclei form on pre-existing surfaces or interfaces?
Because the surface free energy is reduced
What are the three steps of particle growth by long-range atomic diffusion?
Diffusion through the parent phase, across a phase boundary, into the nucleus
Equation for growth rate
G•=Ce^(-Q/kT)
G• is growth rate
Graph of T against rate forms cave
What does the temperature at which something is solidified do to the grain size?
High temperature means coarse grains. Low temperature means fine grains
Equilibrium cooling
Nucleation, growth and transformation rates are a function of temperature
Non-equilibrium cooling
Nucleation, growth and transformation rates are a function of time
Super cooling
For other than equilibrium cooling, transformations are shifted to lower temperatures than indicated by the phase diagram
Super heating
For other than equilibrium heating, transformations are shifted to higher temperatures than indicated by the phase diagram
Limitation of phase diagrams
Inability to indicate the time period required for attainment of equilibrium or what phases will form under non-equilibrium cooling conditions.
Typical graph shape for fraction of transformation against logarithm of heating time
S shape. Cuts x axis at end of nucleation stage and then enters growth stage.
Avrami equation for fraction of transformation
y=1-e^(-kt^n)
y is fraction of transformation
t is time
Formula for transformation rate
Rate=1/(t0.5)
t0.5 is when half has transformed
Describe a time-temperature-transformation plot for top part of one for plain carbon steel
y axis temperature. x axis logarithmic time. Eutectoid temperature isotherm near top. Left and top curve is begin curve (0% transformation). Right and bottom curve is completion curve (100% transformation). Middle dotted curve is 50% completion curve. All of shape y=rt(x). Curves never reach eutectoid temperature. Based on S shaped curves of y against time.
What and where are the regions of the full time-temperature-transformation plot for plain carbon steel?
Above eutectoid temperature is stable austenite. To left of begin curve is unstable austenite. To right of completion curve and high is coarse pearlite. To right of completion curve and low is fine pearlite (both above where begin curve turns back round). To right of completion curve below turning point is bainite. Where curves stop there are isotherms where different compositions of martensite form.
What does martensitic transformation depend on?
Not time. Only temperature to which the alloy is quenched or rapidly cooled.
If you continually cooled the alloy to the same temperature as if you had rapidly cooled it and held it at that temperature, would the microstructure of the products be the same?
Yes
Differences between fine and coarse pearlite
Fine pearlite has thinner layers. Harder and stronger but not as ductile as coarse.
Can bainite, martensite or pearlite transform into each other?
No. Must go from austenite.
How is martensite formed?
When austenitised Fe-C alloys are rapidly cooled (quenched) to relatively low temperatures. This means that carbon cannot diffuse to form ferrite and cementite phases.
Unit cell structure of martensite and how it forms
Body centred tetragonal. Basically BCC stretched vertically. Formed from middle of two FCC unit cells of austenite with interstitial carbon atoms. Atoms cannot diffuse to BCC position because C gets in the way. Forms a supersaturated solid solution of C in Fe.
What is supersaturation?
A state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances
Properties of pure martensite
Extremely hard and brittle
How does tempering martensite work?
Heat martensitic steel to a temperature below eutectoid temperature for a specified time period (normally 250-650°C). Allows C atoms to diffuse and form Fe3C and the BCT structure transforms to equilibrium BCC.
Structure and properties of tempered martensite
Consists of extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix. Can be nearly as strong as martensite but with substantially enhanced ductility and toughness from the α properties.
What is the structure of bainite?
Forms in grains within martensite and consists of elongated and needle-shaped particles of Fe3C within a ferrite matrix.
Formation of bainite
Austenite cooled past a critical temperature (e.g 727°C). Starts with nucleation of ferrite plates at the austenite grain boundaries. Growth of these plates is similar to martensite (displacive). But temperature high enough for C to diffuse out of lattice so FCC can fully transform to BCC. As ferrite plates form, austenite becomes richer in C. Eventually cementite precipitates in remaining austenite layers between ferrite platelets.
How does spheroidite form?
Steel alloy having pearlitic or bainitic microstructures is heated to and left at temperature below eutectoid for a sufficiently long period of time. E.g 700°C for 18-24hrs. Forms α iron grains with relatively small spherical Fe3C in their matrix. It’s diffusion dependent.
Properties of spheroidite
Soft and ductile
Properties of bainite
Harder and stronger than fine pearlite, less hard than martensite but more ductile.
