Materials- Metals: Phase Transformation for Materials Processing

Question 1

What is diffusion-depend transformation 1?

Answer 1

Where there is no change in number or composition of phases present (e.g solidification of pure metal)

Question 2

What is diffusion-depend transformation 2?

Answer 2

Where there is some alteration in phase compositions and possibly the number of phases present. The final microstructure normally has 2 phases present.

Question 3

What is produced in a diffusionless transformation?

Answer 3

A metastable phase

Question 4

What are the two stages in the progress of a phase transformation?

Answer 4

Nucleation (only few hundred atoms) and growth

Question 5

Difference between homogeneous and heterogeneous nucleation

Answer 5

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.

Question 6

When will a spontaneous phase transformation occur?

Answer 6

Only when the Gibbs free energy has a negative value

Question 7

What are the two energy contributions to free energy change that accompanies a solidification transformation?

Answer 7

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.

Question 8

Describe the change in free energy during solidification

Answer 8

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).

Question 9

What is activation free energy?

Answer 9

ΔG*. The free energy required for the formation of a stable nucleus (or energy barrier to nucleation)

Question 10

In the homogeneous nucleation equations, what are ΔHf, Tm, T?

Answer 10

ΔHf is latent heat of fusion
Tm in melting temperature in K
T is temperature of system

Question 11

What does lowering the temperature at temperatures below equilibrium solidification temperature mean for nucleation?

Answer 11

Nucleation occurs more readily because critical radius and ΔG* decrease

Question 12

Formula for number of stable nuclei having radius greater than r, n

Answer 12

n=K1e^(-ΔG/kT)
k is Boltzmann’s constant
Graph of T against n* like decay curve

Question 13

Rate of diffusion equation

Answer 13

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

Question 14

Formula for nucleation rate (nuclei per unit volume per second), N•

Answer 14

N•=K3n*νd

Question 15

Describe and explain how lowering temperature below Tm affects nucleation rate

Answer 15

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.

Question 16

For heterogeneous nucleation, why is the activation energy lowered when nuclei form on pre-existing surfaces or interfaces?

Answer 16

Because the surface free energy is reduced

Question 17

What are the three steps of particle growth by long-range atomic diffusion?

Answer 17

Diffusion through the parent phase, across a phase boundary, into the nucleus

Question 18

Equation for growth rate

Answer 18

G•=Ce^(-Q/kT)
G• is growth rate
Graph of T against rate forms cave

Question 19

What does the temperature at which something is solidified do to the grain size?

Answer 19

High temperature means coarse grains. Low temperature means fine grains

Question 20

Equilibrium cooling

Answer 20

Nucleation, growth and transformation rates are a function of temperature

Question 21

Non-equilibrium cooling

Answer 21

Nucleation, growth and transformation rates are a function of time

Question 22

Super cooling

Answer 22

For other than equilibrium cooling, transformations are shifted to lower temperatures than indicated by the phase diagram

Question 23

Super heating

Answer 23

For other than equilibrium heating, transformations are shifted to higher temperatures than indicated by the phase diagram

Question 24

Limitation of phase diagrams

Answer 24

Inability to indicate the time period required for attainment of equilibrium or what phases will form under non-equilibrium cooling conditions.

Question 25

Typical graph shape for fraction of transformation against logarithm of heating time

Answer 25

S shape. Cuts x axis at end of nucleation stage and then enters growth stage.

Question 26

Avrami equation for fraction of transformation

Answer 26

y=1-e^(-kt^n)
y is fraction of transformation
t is time

Question 27

Formula for transformation rate

Answer 27

Rate=1/(t0.5)

t0.5 is when half has transformed

Question 28

Describe a time-temperature-transformation plot for top part of one for plain carbon steel

Answer 28

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.

Question 29

What and where are the regions of the full time-temperature-transformation plot for plain carbon steel?

Answer 29

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.

Question 30

What does martensitic transformation depend on?

Answer 30

Not time. Only temperature to which the alloy is quenched or rapidly cooled.

Question 31

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?

Answer 31

Yes

Question 32

Differences between fine and coarse pearlite

Answer 32

Fine pearlite has thinner layers. Harder and stronger but not as ductile as coarse.

Question 33

Can bainite, martensite or pearlite transform into each other?

Answer 33

No. Must go from austenite.

Question 34

How is martensite formed?

Answer 34

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.

Question 35

Unit cell structure of martensite and how it forms

Answer 35

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.

Question 36

What is supersaturation?

Answer 36

A state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances

Question 37

Properties of pure martensite

Answer 37

Extremely hard and brittle

Question 38

How does tempering martensite work?

Answer 38

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.

Question 39

Structure and properties of tempered martensite

Answer 39

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.

Question 40

What is the structure of bainite?

Answer 40

Forms in grains within martensite and consists of elongated and needle-shaped particles of Fe3C within a ferrite matrix.

Question 41

Formation of bainite

Answer 41

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.

Question 42

How does spheroidite form?

Answer 42

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.

Question 43

Properties of spheroidite

Answer 43

Soft and ductile

Question 44

Properties of bainite

Answer 44

Harder and stronger than fine pearlite, less hard than martensite but more ductile.