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BULACAN STATE UNIVERSITY

CITY OF MALOLOS, BULACAN

COLLEGE OF ENGINEERING

ENGINEERING MATERIALS

CHAPTER III: ALLOYING OF METALS

GROUP 2:

ADRIANO, Marlon

ALCANTARA, Carla Mae

LIMPAHAN, Rey

SALVO, Nico Bryan

BSCoE3A

Engr. Sarah M. Faustino


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CHAPTER III ALLOYING OF METALS

3.1 ALLOYS

Pure metal objects are used for good electrical and thermal conductivity and corrosion

resistance. However, pure metals usually lack the strength required for structural materials.

Therefore alloys are mainly used for structural materials since they can be formulated to give

superior mechanical properties such as tensile strength, yield strength and hardness. However,

ductility is reduced.

An alloy is an intimate association of two or more component materials which form a

single metallic liquid or solid that may contain metal elements or combination of metal and non-

metal elements.

It is important to distinguish alloying elements from impurities. Alloying elements are

added in controlled quantities to modify the properties of a material while impurities are

undesirable elements that are usually carried over from some previous processes.

Alloys should be completelymiscible that is, components which are soluble in each other

in the molten state.

In any alloy, the metal with larger proportion is referred to as theparent metalorsolvent,

while the metal (or non-metal) present with the smaller proportion is known as the alloying

component orsolute.

Alloys are formed in three ways:

If the alloying components in the molten solution have similar chemical properties andtheir atoms are of similar size, they will form asolid solution upon cooling.

If the alloying components in the molten solution have different chemical properties,they may attract each other and form chemical compounds. If the alloying components

are both metals, these compounds are referred to as intermetallic compounds

When atoms with different chemical properties attract each other less than those withsimilar chemical properties, then both intermetallic compounds and solid solutions willbe present at the same time. Upon cooling, they will tend to separate out to form a

heterogeneous mixture.


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3.2 ALLOYING ELEMENTS

Aluminum

The presence of up to 1 percent aluminum in alloy steels enables them to be given a hard,

wear-resistance skin by nitriding.

Chromium

The presence of small amounts of chromium improves the ability of steels to respond to

hardening by heat treatment. The presence of large amounts of chromium improves the corrosion

resistance and heat resistance of steels. Unfortunately, the presence of chromium also promotes

grain growth. It is more often associated with nickel in alloy steels because nickel tends to refine

the grain structure.


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Cobalt

The presence of cobalt induces sluggishness into the heat treatment

transformations and improves the ability of tool steels to operate at high temperatures without

softening.

Copper

The presence of 0.5 percent copper helps to improve the corrosion resistance of

alloy steels.


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Lead

The presence of up to 0.2 percent lead improves the machinability of steels.

Unfortunately, it also reduces the strength of the steel to which it is added.

Manganese

This element is always present in steels. It improves the wear resistance of steels

by spontaneously forming a hard skin when subjected to abrasion. Manganese alloy steels can

also have high strength and toughness.


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Molybdenum

The presence of molybdenum in alloy steels raises their high-temperature creep

strength, stabilizes their carbides and reduces brittleness.

Nickel

The presence of nickel in alloy steels results in increased strength by grain

refinement. It also improves the corrosion resistance of steels. Chromium offsets the

disadvantages of nickel as an alloying element. Chromium improves the ability of the alloy to

respond to heat treatment while nickel increases the strength of the alloy by grain refinement.


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Phosphorus

This is a residual element from the smelting (extraction) process that causes

weakness in the steel. A trace of phosphorus can improve machinability and in larger quantities,

can improve the fluidity of casting steels.

Silicon

The presence of silicon also improves the fluidity of casting steels without the

reduction of strength. It also improves the heat resistance of steels and its magnetic properties by

increasing the permittivity of the alloy. Unfortunately, silicon is also a powerful graphetizer.


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Sulphur

This is a residual element from the smelting (extraction) process that reduces the

strength and toughness of steel. It is sometimes added to low-carbon steels to improve their

machinability.

Tungsten

The presence of tungsten in alloy steels promotes the formation of very hard

carbides and induces sluggishness into the heat-treatment transformations during hardening. It

enables steels to retain their hardness at high temperatures.


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Vanadium

This element enhances the performance of the other alloying elements. Its effects

in alloy steels are various:

Promotes formation of carbides Stabilizes the martensite and improves hardenability Reduces grain growth Enhances the hot hardness of tool steels and die steels Improves the fatigue resistance of steels Improves the life of valve steels used in internal combustion engines


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3.3 SOLUBILITY

Example:

The water is thesolvent. The salt is thesolute. The resulting liquid is thesolution.

Solvent is a substance, usually a liquid, capable of dissolving another substance and

eventually forms a solution.

Solute is a substance dissolved in another substance, usually the component of a solution

present in the lesser amount. It is the one being dissolved in a solution.

Solution is a homogeneous mixture of two or more substances, which may be solids,

liquids, gases, or a combination of these.

A solution is said to be saturatedif it is combined with or containing all the solute that

can normally be dissolved at a given temperature. The excess solute that will not be dissolved

will remain as aresidue.


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3.4 SOLID SOLUTIONS

Most metals are completely and mutually soluble (they are miscible) in the liquid state

that is, when they are molten.

Two Sorts of Solid Solutions:

Substitutional Solid SolutionsThe copper-nickel alloy mentioned previously is an example of substitutional solid

solutions.

Insterstitial Solid SolutionsIt is a solid solution in which the solute atoms occupy positions between the atoms in the

structure of the solvent.

Important Factors Governing the formation of Substitutional Solid Solution:

Atomic Size

The atoms of the solute and the solvent must be approximately the same size. If the atom

diameters vary by more than 15% the formation of a substitutonal solid solutions is highly

unlikely.

Electrochemical Series

If there is only a small difference in charge between the alloying components then they

will probably form a solid solution. Conversely, if their charges are very dissimilar they are more

likely to form intermetallic compounds.

Valency

A metal lower than valency is more likely to dissolve one of higher valency then the

other way round, assuming the conditions stated above are also favourable.


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Fig. 3.3 Substitutional Solid Solution

Fig.3.4 Interstitial Solid Solution


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3.5 INTERMETALLIC COMPOUNDS

Intermetallic compounds tend to be hard and brittle and are thus less useful for

engineering alloys than the tough and ductile solid solutions. Intermetallic compounds are most

widely found in bearing alloys where they form hard, wear-resistant pads with a low coefficient

of friction, set matrix of tough, ductile solid solution.

Fig. 3.5 Intermetallic Compounds


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3.6 COOLING CURVES

A cooling curveis a line graph that represents the change of phase of matter, typically

from a gas to a solid or a liquid to a solid. The independent variable (X-axis) is time and

the dependent variable (Y-axis) is temperature.

Fig. 3.6 (a) Cooling curve for water

Fig. 3.6 (b) Cooling curve for a salt-water solution


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3.7 PHASE

A portion of a system which is uniform composition and texture throughout, and which is

separated from the other phases by clearly defined surfaces.

Water vapor (steam). Liquid salt solution ( sodium chloride). Crystals of water (ice). Crystals of salt (sodium chloride).

When a liquid solution of two metals solidifies:

Metals which are soluble in the liquid state may become totally insoluble in the solidstate and separate out as grains of two pure metals. Thus there will be two phases present,

with each phase consisting of many grains of the same composition.

Metals which are soluble in the liquid state may remain totally soluble in the solid stateresulting in a solid solution. Thus a single-phase solid solution will be present

consisting of many grains of the same composition.

The two metals may react together chemically to form an intermetallic compound.Again a single phase consisting of many grains of the same composition will be present.

Alloy may be built in different ways:

Two pure metals existing entirely separately in the structure. In practice this is extremelyrare since there is usually some solubility of one metal in another.

A single solid solution of one metal dissolved in another. A mixture of two solid solutions if the metals are only partially soluble in each other. An metallic compound and a solid solution mixed together. The individual grains found in any of these phases may vary considerably in size. Some

are large enough to see with the unaided eye, while others are so small that a high

powered microscope is required.


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3.8 ALLOY TYPES

Simple eutectic type

The two components are soluble in each other in the state, but are completely insoluble in

each other in the solid state.

Solid solution type

The two components are completely soluble in each other both in the liquid state and in

the solid state.

Combination Type

The two components are completely soluble in the liquid state, but are only partially

soluble in each other in the solid state. Thus this type of alloy combines some of the

characteristics of both the previous types, hence the name combination type phase equilibrium

diagram.


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3.9 PHASE EQUILIBRIUM DIAGRAMS

The eutectic reaction is defined as follows:

This type of reaction is

an invariant reaction, because it is

in thermal equilibrium; another way

to define this is the Gibbs free

energy equals zero. Tangibly, this

means the liquid and two solid

solutions all coexist at the same

time and are in chemical

equilibrium. There is also a thermal

arrest for the duration of the reaction.

At the liquidus there is a liquid solution of molten bismuth and molten cadmium. As the solution cools to the liquidus temperature, for the alloy under consideration,

crystals of pure cadmium precipitate out. This increases the concentration of bismuth and

reduces the concentration of cadmium present in the remaining solution. Thus the

solidification temperature is reduced to that appropriate for this new ratio of cadmium

and bismuth, and further crystals of pure cadmium precipitate out. This again reduces the

percentage of cadmium present in the remaining solution and solidification temperature is

further reduced with more pure cadmium crystals being precipitated.

At the eutectic composition, crystals of cadmium and bismuth precipitate outsimultaneously to form lamellar eutectic of the two metals.


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3.10 PHASE EQUILIBRIUM DIAGRAMS

(SOLID SOLUTION TYPE)

Phase equilibrium diagram is a graphic relationship between temperature and

weight ratios of elements and alloys contribute to the built of the diagram.

Wherephase is a uniform part of an alloy, having a certain chemical composition

and structure, and which is separated from other alloy constituents by a phase boundary.

For example the salt . water solution have a four possible phases:

- Water vapor (steam)

- Liquid salt solution (sodium chloride in water)

- Crystals of water (ice)

- Crystals of salt (sodium chloride)

Alloying systems

Alloy is a metal composing of a mixture of elements. Most of alloys are composed of a

base metal with small amounts of additives or alloying elements. The typical examples of

alloys are steel/cast iron (iron base alloys), bronze/brass (copper base alloys), aluminum

alloys, nickel base alloys, magnesium base alloys, titanium alloys.

There are many types of alloying systems which they are:

Binary system. It means that alloying have two metals only.


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Phase Equilibrium Diagrams (Solid Solution Type):

Solid solution is a phase, where two or more elements are completely soluble in each

other. Depending on the ratio of the solvent (matrix) metal atom size and solute element atom

size, two types of solid solutions may be formed: substitution or interstitial.

Interstitial solid solution

If the atoms of the alloying elements are considerably smaller, than the atoms of the

matrix metal, interstitial solid solutionforms, where the matrix solute atoms are located in the

spaces between large solvent atoms. When the solubility of a solute element in interstitial

solution is exceeded, a phase of intermediate compound forms. These compounds (WC, Fe3C

etc.) play important role in strengthening steels and other alloys. Some substitution solid

solutions may form ordered phase where ratio between concentration of matrix atoms and

concentration of alloying atoms is close to simple numbers like AuCu3 and AuCu. Solid solution

formation usually causes increase of electrical resistance.


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3.11 PHASE EQUILIBRIUM DIAGRAM

(COMBINATION OF SOLID AND EUTECTIC TYPE)

Phase equilibrium diagrams (Eutectic type):

It is identical with this type of phase diagram is produced for a salt (sodium chloride) and

water solution, it is total solubility of the salt in water in the liquid state and total insolubility

(crystals of ice and separate crystals of salt) in the solid state. As an example of eutectic are

carbon steels .

Phase equilibrium diagrams (Combination of eutectic and solid type):

Many metals and non-metals are neither completely soluble other in the solid state nor

are they completely insoluble. Therefore form a phase equilibrium diagram of the type shown in

figure 5 system there are two solid solutions labeled and . The use of the letters , , , etc., in

phase equilibrium diagrams may be defined, general, as follows:

A solid solution of one component A in an excess of component B, such that A is thesolute and B is the solvent, referred to as solid solution .

A solid solution of the component B in an excess of the component A, so that B nowbecomes the solute and A becomes the solvent, referred to as solid solution .

In a more complex alloy, any further solid solutions or intermetallic compounds whichmay be formed would be referred to subsequent letters of the Greek alphabet. That is, ,

, etc.

If the entire process is slow enough so that equilibrium within the crystal is maintained

from the start, thendiffusionwill occur with copper atoms migrating into the core of the crystal

and nickel atoms migrating into the case of the crystal. By the time cooling is complete, the

composition should be uniform throughout with 70 % copper and 30 % nickel.

Diffusion

It describes the spread of particles through random motion from regions of higher

concentration to regions of lower concentration.


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Crystal Growth (a) dendritic nucleus at liquidus temperature [47% Cu, 53% Ni], (b)

diffusion of copper and nickel as crystal commences to grow [62% Cu, 38% Ni at 1200 C]

Example of Crystal growth:

Once diffusion is complete the rate of cooling is irrelevant. However, over-fast cooling creates

stresses in the metal. On the other hand, excessively long periods of heating excessively slow

cooling results in the grain growth which may improve ductility but reduce mechanical strength

and may cause machining problems as the metal will tend to tear and leave a poor surface finish

rather than cut cleanly.


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In phase equilibrium diagrams it is always assumed that cooling will be slow enough for

equilibrium to be maintained. Under production conditions in the foundry, where cooling is more

rapid than the ideal, there is insufficient time for diffusion to become complete and the nickel-

rich core will become apparent when an etched specimen is examined under a microscope.

The core of the crystal will have the outline appearance of the initial dendrite from which

the crystal has grown. The result of this more rapid cooling is calledcoring and, since coring

leads to lack of uniformity in the structure of the metal, this adversele affects its mechanical

properties.

Coring can largely be eliminated by heat treatment. The casting is heated to just below

the solidus for the alloy concerned until diffusion is complete.

3.12 CORING

It happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium

conditions. This causes the exterior of the material to harden faster than the interior.

Coring causes the exterior layers to retain more of the higher melting temperature

element. In this case, the dendrite arms formed from the exterior have a different composition

than the alloy in the inner regions, resulting in a local compositional difference.

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