Processing; Solidification, especially of metals

Question 1

What is Solidification?

Answer 1

It is the transformation of a liquid into a solid.

Question 2

How does Solidification work when producing steel elements?

Answer 2

Typically start from a liquid solution of carbon in iron at high temperature (2000 degrees celsius) and we cool it down to convert it into solid steel, via transformations to austenite first and to ferrite and cementite at lower temperatures.

Question 3

What is casting?

Answer 3

The process of pouring liquid steel into moulds where it is left to solidify.

Question 4

How does Solidification work when producing concrete?

Answer 4

A cement powder is dissolved in water and, when the concentration of calcium and silicon irons released in solution becomes sufficiently high, they start to react chemically with the water and assemble various new solid phases called hydration product, or gel.

Question 5

What does the first step in solidification consist of?

Answer 5

A few atoms in the liquid aggregating to form a first so-called nucleus. This process is called “nucleation”.

Question 6

What theory can we refer to when describing how atoms in a liquid assemble to form solid nuclei?

Answer 6

The Classical Nucleation Theory (CNT).

Question 7

What is the second law of Thermodynamics?

Answer 7

A always tries to maximise its entropy (thermal energy).

Question 8

What the consequence of the second law of thermodynamics?

Answer 8

When a system tries to maximise its entropy by consuming free energy then it will minimise its Gibbs free energy G.

Question 9

What is Gibbs free energy?

Answer 9

The energy that can be used to generate work;

1) Energy stored as pressure
2) Energy stored as temperature
3) Energy released from chemical reactions

Question 10

What is Gibbs energy G a function of?

Answer 10

Temperature, it changes with the temperature of the system.

Question 11

When will a liquid solidify?

Answer 11

At low temperatures, for which the free energy of the molecules in a solid-state is less than the free energy of the molecules in the liquid state. (Gsolid

Question 12

What is the co-existence region on the phase diagram?

Answer 12

The region where the free energy of the solid is the same as the free energy of the liquid.

Question 13

What is undercooling?

Answer 13

undercooling is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid.

Question 14

What is supersaturation?

Answer 14

Supersaturation occurs with a chemical solution when the concentration of a solute exceeds the concentration specified by the value equilibrium solubility.

Question 15

When working under the CNT what assumption do we work with?

Answer 15

Spherical nucleus approximation, assuming that the molecules will form perfect spherical nuclei.

Question 16

When is a solid stable?

Answer 16

When the free energy is lower than the liquids.

Question 17

When will a nucleus form?

Answer 17

When DG<0.

Question 18

What does DG equal?

Answer 18

DG = Gsolid - Gliquid

Question 19

What are the 2 contributions for a nucleus to form?

Answer 19

1) DG stored in the volume (due to DG of individual molecules going from liquid to solid). If undercooked or supersaturated, this DG<0.
2) DG stored on the solid-liquid interface, >0 because interfaces are “unhappy”.

Question 20

What does DGsurf equal?

Answer 20

DGsurf = 4πr²γ

Question 21

What does DGvol equal?

Answer 21

DGvol = 4/3πr³DGv

Question 22

What is homogenous nucleation?

Answer 22

In this process, vapour atoms or molecules are transformed into solid particles in a supersaturated vapour without the presence of a foreign phase.

Question 23

What is heterogeneous nucleation?

Answer 23

When a nucleus forms on a pre-existing surface.

Question 24

What is Planar Growth?

Answer 24

Crystals grow one layer at a time.

Question 25

When does Planar Growth occur?

Answer 25

Occurs when the heat is removed slowly from a liquid or when reactants are added slowly to a solution.

Question 26

When is planar growth more likely to occur?

Answer 26

When different facets of the crystals have similar interfacial energies associated with them.

Question 27

When does dendritic growth occur?

Answer 27

When heat is removed rapidly from a liquid or reactants are added rapidly in a solution, or when crystals are very anisotropic.

Question 28

What does dendritic growth create?

Answer 28

The crystals grow abruptly and this creates inhomogeneities in the distribution of temperature or iron concentrations near the growing front of the crystal.

Question 29

What is the main growth mechanics in steel production?

Answer 29

Dendritic growth because it is when liquid metal is cooled rapidly.

Question 30

Where can micro-segregation happen?

Answer 30

In an alloy. Think for example of an alloy made of two elements, let’s call them generically A and B, where element A is significantly more abundant than element B. Starting from a liquid and cooling it down, when the liquid-solid coexistence curve is reached element A will start to crystallise forming dendrites (assuming there are the conditions to form dendrites - see the previous lecture on growth). Depending on the solubility of B in the solid form of A, and depending on the cooling rate, it can happen that element B is pushed out of the crystal dendrites being formed by element A. As a result, atoms or molecules of B get concentrated in-between dendrite arms of A, where they precipitate forming small lamellar inclusions. These lamellae can provide a starting point for fractures, reducing the strength and ductility of the material.

Question 31

When can Shrinkage-induced cavities appear?

Answer 31

Appear due to the thermal expansion properties of the metal. As a consequence of the material shrinking, it is possible that in some points you are left with crack or cavities that will reduce the strength and overall quality of the metal.

Question 32

When can Entrapped gas cavities appear?

Answer 32

Form when a gas, dissolved in the liquid metal, is unable to bubble out and evaporate and remains entrapped during solidification. The gas can just be air, but also other types of gases produced by the reaction of impurities within the metal or within the casts into which the hot liquid metal is poured during casting.

Question 33

What is Slag inclusion?

Answer 33

Pieces of ceramic material that remain incorporated into the metal. The origin of this ceramics has to do with the fact that metals are typically obtained starting from minerals, via the smelting process.

Question 34

What is smelting?

Answer 34

Separating a metal from its other elements.