prof. a.k.m.b. rashid - bangladesh university of...

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Prof. A.K.M.B. RashidDepartment of MMEBUET, Dhaka

Concept of alloyingClassification of alloysClassification of alloysIntroduction to the phase diagramUses and limitations of phase diagramsClassification of phase diagramsConstruction of phase diagrams

© Rashid, DMME, BUET . 2009 MME 291, Lec 03: Introduction to phase diagrams P 02

Reference:1. Avner. Introduction to Physical Metallurgy, Ch. 5 and 6.

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An alloy is a substance that has metallic properties and is composed of two or more chemical elements, of which at least one is a metal.

Alloying elements are deliberately introduced into a metal to enhance properties (especially, mechanical properties)

Cu Ni

60% NiMonel

Wt % NickelNote the difference

Alloy and alloy system Cu NiWt % NickelAlloy and alloy systemAlloying element and impurity element

Alloy systemsBinary system (Fe-C system, Cu-Zn system)Ternary system (Fe-C-Mn system, Al-Si-Mg system)


Property DirectionTensile strength UpYield Strength Up% Elongation DownElectrical Conductivity Down

Classifications based on:Structure (e.g., austenitic stainless steel)Type of phase diagram (e.g., eutectic alloys)yp p g ( g , y )

In the solid state, there are three possible phases exist:Pure metalIntermediate alloy phase or compound

Alloy can be homogeneous or mixture.

y p pSolid solution

If the alloy is a mixture, then any combination of the above three phases is possible in the solid state.


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Intermediate Alloy Phases, or CompoundsIntermediate chemical composition; different crystal structureExpressed by chemical formula

Intermetallic compounds (Mg2Pb, Mg2Sn, ....)formed between chemically dissimilar metalshave strong ionic/covalent bondnonmetallic properties (poor ductility, poor conductivity)complex strucutre

Interstitial compounds (Fe3C, Fe4N, TiC, .....)

Congruently melting, exhibiting definite melting point (like metals)

Interstitial compounds (Fe3C, Fe4N, TiC, .....)formed between transition metals and H/O/C/N/Bsmall atoms of nonmetals go into the interstitices of metalsmetallic properties, narrow composition range, extremely hard, high melting point

Electron compounds (AgZn, FeAl, Cu3Si, ....)form structure similar to constituting elementshave a definite ratio of valence electronsproperties similar to solid solutions (wide composition range, high ductility, low hardness

Solid Solutions

Solution in the solid state where two kinds of atoms bi d i l l iare combined in one crystal lattice.

Distortion resulted during formation of solution causes an increase in strength of the alloy. This is the primarybasis for strengthening of a metal by alloying.

Solubility of solute in solution depends on temperature and pressure of the system, and method of forming.

Unsaturated, saturated, and supersaturated solutions


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Substitutional and interstitial solid solutions

Solid Solutions

InterstitialSubstitutional

Hume-Rothery Rule for formingHume Rothery Rule for forming substitutional solid solution

Crystal structure factorRelative size factorChemical affinity factorRelative valence factor

Example:

Cu – Ni alloy ⇒ Completely solubleCu – Zn alloy ⇒ Partially solubleCu – Pb alloy ⇒ Completely insoluble


Summary of Possible Alloy Structures


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We have seen how the structure of materials control their properties.

So it is important that we must knowabout the structure of a material that has been developed during its manufacture, and the method of controlling (and/or modifying) the structure to enhance its properties.

Phase diagram is an important tool for materials scientists that tells which phases are stable in a system under specified conditions (e.g. of temperature, overall composition, pressure)


solidliquid

liquid-gas

Phase diagram is basically a map that presents the domains of stability of phases and the limits of stability of

Temperature

gasliquid-gas equilibria

Typical phase diagram for one component system

phases in a graphical form.

Reading the map will tell you, at thestate when it comes to equilibrium,1. what phases are present,2. the state of those phases, and3 the relative quantities of each phase

Reading a phase diagram will also tell what phase transformations we can expect when we change one of the parameters of the system (T, P, X).

3. the relative quantities of each phase.


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Chemically recognisable species that are mixed to f h ll

Components

form the alloy.In Brass: Cu, Zn (element)In steels: Fe, C (element)In ceramics: SiO2, Al2O3 (compound)

Binary alloy contains 2 components, ternary 3, etc.

PhaseA phase is a homogenous, physically distinct and mechanically separable portion of the material with a given chemical composition and structure.


What and how many phases materials possess?

Solid, liquid, or gas, (and plasma)?

Is it possible to have more than one solid phases?

Iron, being an allotropic material, has more than one solid phases:

When iron first freezes from its liquid state, it is BCC (δ-iron)As it cools it changes to FCC (γ-iron)Upon further cooling it changes to BCC (α-iron)


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The properties of an alloy

Microstructureβ phase(lighter)

Microstructure of Al-Cu Alloy

The properties of an alloy depend not only on proportions of the phases but also on how they are arranged structurally at the microscopic level.

Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space

pearlite(finger print)

( g )

α phase(darker)

Microstructure of Cast Iron

their arrangement in space.

Phase diagrams will help us to understand and predict the microstructures like the one shown in this page

print)

graphite(grey)


A system is at equilibrium if at constant T d iti th t i

Equilibrium state and Metastable state

T, pressure and composition the system is stable, not changing with time. The equilibrium state always has the minimum free energy.

Equilibrium state requires sufficient time to achieve. When this time is too long (due to g (slow kinetics), another state along the path to the equilibrium may appear to be stable. This is called a metastable state.A system at a metastable state is trapped in a local minimum of free energy, which is not the global one.© Rashid, DMME, BUET . 2009 MME 291, Lec 03: Introduction to phase diagrams P 14

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Selection of alloys showing enhanced characteristics in:B i d ld iBrazing and solderingDiffusion problemsCorrosionElectrical resistivity

Manipulation of phase transformations of materials to control their properties


Phase diagrams are also known as the equilibrium diagrams.Rate of phase transformations is missing.TTT (Temperature-Time-Transformation) diagrams are a complement to phase diagrams.

One component (unary) phase diagramsAlso known asP T diP-T diagrams.The simple case is Water.

How many single-phase regions?How many two-phase regions?

Unary phase diagram of water

Is there any three-, or more-phase regions?

Gibb’s Phase Rule:F = C–P+2

F = # variablesC = # componentsP = # phases


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Binary (two-component) phase diagramsHow does mixing of A into B effect the bond energies

d th lti t t f th lt t ll ?and the melting temperature of the resultant alloy?

Interaction of A and B resulted three bonds: A-A, B-B and A-B bonds.

In ideal cases:(A-B) = x (A-A) + (1-x) (B-B) Must followTAlloy = TA + x (TB – TA)where x is mole fraction of A in B

Example: Copper - Nickel, Silicon - GermaniumCompletely miscible/soluble phase diagrams

Must follow Hume Rothery Rule


Binary (two-component) phase diagrams

Ni-Cu phase diagram (completely miscible)

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Ge-Si phase diagram (completely miscible)


When A-B < 0.5 (A-A + B-B)

thenTAlloy < TA , TB

Example: Lead Tin Gold Silicon Copper SilverExample: Lead - Tin, Gold - Silicon, Copper - SilverEutectic phase diagrams

Completely immiscible/insoluble phase diagramsPartially miscible/soluble phase diagrams


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Au-Si phase diagram (Completely insoluble)


Cu-Ag phase diagram (Partially soluble)

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Pb-Sn phase diagram (Partially soluble)


Au-Ge phase diagram (Partially soluble)

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When A-B > 0.5 (A-A + B-B)

thenTAlloy > TA , TB

E l G lli A i Al i i A tiExample: Gallium - Arsenic, Aluminium - AntimonyIntermetallic compound formation



Al-Sb phase diagram

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Theoretical ConstructionBy applying thermodynamic principlesUse of software like Thermocalc

Experimental MethodsThermal analysis

Generation of cooling curves (temperature vs. time) for a number of alloys of the alloy system to obtain arrest points (temperatures where a change in slope is observed)Solid-state phase changes are difficult to obtained in this method

M ll hi h dMetallographic methodHeating samples of an alloy to different temperatures, and quench them after equilibrium to retain the high-temperature structureObserve the structure microscopicallyRapidly cooled samples do not always retain high-temperature structures; considerable skill is required to interpret the microstructure correctly


prof. a.k.m.b. rashid - bangladesh university of...

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