Department of Mechanical Technology

Theory of Technological Processes

Theory of phase conversions

Author: Pau Miralles Ferrs

Teacher: doc. Ing. Jitka Podjuklov

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METALS

Even the fact that now a days we are discovering new ceramic

and plastic materials that in some industrial applications are replacing

the metals; they are far away replacing them fully.

The main inconvenient using of the metals are the exhaustion of

the metal mines, new industry uses and the oxidation by corrosion of

them by some chemic and atmospheric substances. From the point of

view of them use we can sort them in alloy and pure metals.

Pure metals

The use of pure metals is focused in a few uses, despite the fact

that they are hard to get them they are very resistance to the corrosion

and they also have a very well electric conductivity, which make them

appropriate for concrete applications.

Cristal structure of pure metals

One of the main characteristic of these metals is that they

solidificate into a determinate crystal structure formed from the core.

According to the velocity of coldness, in a pure metal piece can form

more or less cores giving as a result grains which size will set some

mechanic properties.

ALLOY

We can define it as every product result of the bond of two or

more chemical elements, one of them must be metallic. For describing

it as alloy two conditions must be obeyed:

The elements of the joint must be totally miscible in liquidstate.

The resultant product must have majority of metallicbond.

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Alloys improve significantly the mechanic properties of the pure

metals as the tenacity, hardness, oxidation resistance Nevertheless

some of them like electrical and thermic conductivity get worse.

Elements that form the alloys

In the alloys, for producing a stable solid solution, is necessary

that the constituent elements are part of the same crystal net.

For a two-element alloy that have the same crystal structure, the

element with higher proportion is called solvent, and the one with less

proportion is the solute.

As stated above, the pure metals solidify forming a specific

crystal structure, therefore, the introduced atoms must be part of the

crystalline structure, distinguishing two types of solutions:

Substitutional solid solution: in this case, the solute and solvent

have a similar crystal structure, so solute atom occupies the position

of other solvent atom in the final crystal structure.

Insert solid solution: it happens when the solute atoms are very

small and occupy the interstitial voids of the solvent. This causes an

increase of the strength of the alloy, because final product deformation

becomes harder.

PHASE DIAGRAMS

From the structural point of view, a material phase is a

homogeneous part which is different from the other in composition,

state or structure. The whole representations of the possible states is

called phase diagram.

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Gibbs phase rule

Gibbs equation allow us to calculate the number of phases that

can exist in equilibrium in any system.

f+N = C+2

f: Is the number of phases that are in the analysis point.

N: Freedom degrees, that means, number of variables (pressure,

temperature or composition in systems that have more than one

component) that you can modify without having changes in the system

phases.

C: Is the number of system components.

Equilibrium diagrams in alloys

If there is a two-metal alloy (A and B), we represent the

temperature in ordinate, and the composition in abscissa. In phase

diagrams, solid solutions are usually represented by the first letters of

the Greek alphabet.

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0% A 20 40 60 80 100% A

+ L

0% B

Liquidus

Solidus

Liquid (L)

C0100% B

(oC)

D

Tem

pera

ture

CL C

Liquidus line: is the top line in the diagram; it represents the

start of the solidification, and set the transition between liquid phase

and liquid+solid phase.

Solidus line: is the bottom line in the diagram, and represents

the transition between liquid+solid phase and the solid phase.

LEVER RULE:

In the last diagram, the D point is in a biphasic state when there

are a solid phase and another liquid L. The chemical composition of

the solid and the liquid can be determined by the horizontal rule,

making a horizontal line through the point D and cutting phase lines,

determining C and CL.

If we call WL to the parts per unit that we have in liquid mass in

the D point, and W to the parts per unit that we have in solid mass in

the same point, we can determine that masses using some equations,

applying what is known as the lever rule.

C0 = Concentration of the element A or B corresponding to the D point.

CL = Concentration of the liquid corresponding to A or B

C = Concentration of the solid corresponding to the element A or B.

If we are using concentrations of the A element, the equations

for W and WL are:

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IRON-CARBON DIAGRAM

In order to receive the alloy name, an iron-carbon solution cant

have more carbon concentration than 6,67%, since, if greater, it would

lose the metal qualities and would receive the name of chemical

compound.

In the next iron-carbon diagram, we can see the next

fundamental components:

Iron: it has a carbon content between 0.008% and 0.025%. The

pure iron is difficult to obtain because the carbon concentration at

ambient temperature must be less than 0.008%. Moreover their

applications are limited almost exclusively to cores inductances.

Steels: For considering steel an iron-carbon alloy, the carbon

concentration has to be between 0.025% and 1.76% at ambient

temperature. The field of application of the steels is very wide, covering

all fields of industry. Among its main features there are: high hardness,

good mechanical strength, malleability, ductility, etc.

Foundries: this iron-carbon alloys have a carbon concentration

between 1.76% and 6.67%. The main feature of the foundry is its

extraordinary hardness, which makes it ideal for cutting tools.

Particular constituents:

Ferrite: Also known as alpha iron (Fe). For temperatures below

900 C has a cubic body-centred structure. Depending on the

temperature at which it is found, the ferrite is ductile, magnetic,

but becomes nonmagnetic at temperatures above 768 C. Its

ability to form insertion solid solutions is very weak because its

available interatomic spaces are small.

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Austenite: Component also known as gamma iron (Fe) with

cubic face-centred structure. This allotropic variety of iron is stable at

temperatures between 910 C and 1400 C and is denser than the

nonmagnetic alpha form. The Fe has greater ability to form solid

solutions than alpha, because the interatomic space in the center of

the cubes can easily accommodate elements with small atomic

diameter.

Cementite: This constituent is iron carbide, with 6.67% carbon,

Fe3C formula. It is very fragile and hard, at low temperatures is

ferromagnetic and loses this property at 212 C. Probably melts above

1950 C and is unstable at temperatures below 1200 C.

Perlite: Is a mixture that happens in the eutectoid point (0.8%

C and 723 C) and is made up of ferrite and cementite. Its structure is

made up of alternate layers of ferrite and cementite. The mechanical

properties of perlite are intermediate between ferrite and cementite

and although it is harder and stronger than ferrite is softer and more

malleable than cementite.

Martensite: It is a supersaturated solid solution of carbon in Fe.

It is obtained by rapid cooling of austenite steels, after being heated to

achieve an austenitic constitution. The proportion of carbon is not

constant, and if we heat it, also increases the mechanical strength,

hardness and brittleness of steel.

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Theory of phase conversions