Phase Diagrams(equilibrium diagrams)

Prepared ByMr.P.Senthamaraikannan, M.E., (Ph.D).Assistant professor,Department of Mechanical engineering,Kamaraj college of engineering and technology,Virudhunagar.

Why we study Phase Diagrams?Many material systems and alloy systems exist in more than one phases depending on the conditions of temperature, pressure and compositions. Each phase will have different microstructure which is related to the mechanical properties. The development of microstructure is related to the characteristics of phase diagrams. Thus the knowledge and understanding of the phase diagrams is very important for engineers . Proper knowledge and understanding of phase diagrams will lead to design and control of heating procedures for developing the required microstructure and properties.

Phase (gas, liquid, solid):Gas (Atoms are not contact with each other)

Liquid( Limited mobility)( contact with each other but free to move)

Solid(should maintain contact with neighbour and “Fixed “in ordered form)

Physics definition for solid: In solid the atom /molecules are arranged in regular 3 dimensional patternEngineering Purpose : Rigid body is called solid

UNARY PHASE DIAGRAMS

What is Phase Diagram?A graphic representation showing the phase or phases present for a given composition, temperature and pressure. Also termed equilibrium diagrams.A phase diagrams show what phases exist at equilibrium and what phase transformations we can expect when we change one of the parameters of the system (T, P, composition)We will discuss phase diagrams for binary alloys only and will assume pressure to be constant at one atmosphere. Phase diagrams for materials with more than two components are complex and difficult to represent

Binary(2 components) Phase Diagrams:1.Components completely soluble in the liquid state anda, completelySoluable in solid state: Isomorphous Systems(Type I)

b.Insoluble in the solid state: the eutectic reaction (Type II)

c. Partly soluble in the solid state: the eutectic reaction (Type III)

e. Formation of a congruent-melting intermediate phase (Type IV)

f. The peritectic reaction (Type V)

2. Transformations in the solid statea. Allotropic changeb. Order-disorderc. The eutectoid reactiond. The peritectoid reaction

Interpolation of phase diagramIn a binary system(Cu-Ni) cooling experiments are conducted on some of the alloys and the results are given in table. Plot the phase diagram, label the phases draw the cooling curve for the alloy of 50%cu (50%Ni) and mark the micro structure change on cooling. At 1275°C find amount of phases and composition of phases.

S.No Pure Cu 80%Cu+20%Ni

60%Cu+40%Ni

40%Cu+60%Ni

20%Cu+80%Ni

Pure Ni

Ts 1085°C 1180°C 1280°C 1350°C 1400°C 1453°C

Te 1085°C 1140°C 1220°C 1290°C 1360°C 1453°C

Binary Isomorphous SystemsIso - sameMorphous - Structurecomplete solid solubility of the two components (both in the liquid and solid phases)

congruent-melting alloysIt begins and ends solidification at a constant temperature with no change in composition, and its cooling curve will show a horizontal line. Such alloys are known as congruent-melting alloys

The end product of a congruent melting alloy shall have only one phase

Variations of Type I

Two Metals Completely Soluble in the Liquid State and Completely Insoluble in the Solid State (Type II)

Technically, no two metals are completely insoluble in each other. However, in some cases the solubility is so restricted that for practical purpose they may be considered insoluble.

A-Cd(cadmium ) B-Bi(Bismuth)

A is added to B, the temperature for the beginning of solidification for those alloys is also lowered. Therefore, since each metal lowers the freezing point of the other, the line connecting the points showing the beginning of solidification, the liquidus line, must show a minimum.This is shown by the upper dotted line in the above figure, showing a minimum at point E, known as the eutectic point, for a composition of 40A-60It can be seen that over a wide range of compositions, a portion of the cooling curve that shows the end of solidification occurs at a fixed temperature

The lower horizontal line at TE, shown dotted in the above figure, is known as eutectic temperature. In one alloy, the eutectic composition 40A-60B, complete solidification occurs at a single temperature, the eutectic temperature. Although the freezing of the eutectic composition alloy thus resembles that of a pure metal, it is not congruent melting alloy since the resulting solid is composed of two phases

TE is called the eutectic temperature and 40A-60B the eutectic composition

The phase diagram consists of four areas. The area above the liquidus line is a single-phase homogeneous liquid solution, since the two metals are soluble in the liquid state (labeled as Liquid solution). The remaining three areas are two-phase areas.Since the two metals are assumed to be completely insoluble in the solid state, when freezing starts, the only solid that can form is a pure metal. Thus, every alloy when completely solidified must be a mixture of the two pure metals. Thus the area below FEG line in above diagram will be a mixture of two solid pure metals A and B (labeled as Solid A + Solid B).

It is common practice to consider alloys to the left of the eutectic composition as hypoeutectic alloys and those to the right as hypereutectic alloys.

Alloy 1 (Eutectic Composition)

Let us assume that a small amount of pure metal A solidified. This leaves the remaining liquid richer in B; the liquid composition has shifted slightly to the right. To restore the liquid composition to equilibrium value, B will solidify. If slightly too much B is solidified, the liquid composition will have shifted to the left, requiring A to solidify to restore equilibrium. Therefore, at constant temperature, the liquid solidifies alternately pure A and pure B, resulting in extremely fine mixture usually visible only under the microscope. This is known as eutectic mixture shown at (4) in the above figure.

Eutectic reaction The change of this liquid of composition E into two solids at constant temperature is known as the eutectic reaction and may be written as

Since solidification of eutectic alloy occurs at constant temperature, its cooling curve would be the same as that for a pure metal or any congruent melting alloy. The eutectic solidification however is not congruent as there is a difference in composition between the liquid and the individual solid phases.

Alloy 2 (Hypoeutectic Alloys)

Remains a uniform liquid solution as shown at (1) until the liquidus line temperature T1 is reached

At this point the liquid L1, is saturated in A, and as the temperature is dropped slightly, the excess A must solidify as shown at (2)

the amount of liquid is gradually decreasing. When the alloy reaches xE.

The conditions existing just a fraction of a degree above TE are:

When solidified, the alloy will consist of 67 percent of grains of primary or proeutectic A (which formed between T1 and TE or before the eutectic reaction) and 33 percent eutectic (A + B) mixture as shown at (5).Every alloy to the left of the eutectic point E, when solidified, will consist of grains of proeutectic A and the eutectic mixture. The closer the alloy composition is to the eutectic composition, the more eutectic mixture will be present in the solidified alloy.

Phase Liquid Solid AChemical

Composition 40A-60B 100ARelative amount

(TExE / TEE) x 100 = 33 percent

(xEE / TEE) x 100 = 67 pecent

Alloy 3 (Hypereutectic Alloys)

when the liquidus line is reached at temperature T3 the liquid deposits crystals of pure B instead of A as shown at (2).As the temperature is decreased, more and more of B will solidify, leaving the liquid richer in AThe remaining liquid now solidifies into the eutectic (A + B) mixture as shown at (5)After solidification the alloy will consist of 75 percent grains of primary B or proeutectic B and 25 percent eutectic (A + B) mixture.

In Si –Ag system melting point of Silicon is 1414°C and the melting point of Ag (Silver) is 962°C.An alloy of 20% Si is solidified in constant temperature of 363°C.Plot the phase diagram.(1) Label the phases(2) Draw the cooling curve an alloy of 50% Si and mark the micro structure change.

Two Metals Completely Soluble in the Liquid State but only Partly Soluble in the Solid State (Type III)

Alloy 1

Alloy 2

Alloy 3

Alloy 4