Phase ttthlkkl uiuiu ioioioii Diagram.pptx

phase diagram

1. Phase Diagrams Dr. Umesh Vishwakarma

2. Introduction Metals in actual commercial use are almost exclusively alloys, and not pure metals. A homogeneous mixture of two or more metals or a metal and a non-metal when fused together at a certain temperature forms a new metal after solidification, termed as an alloy. Alloys are normally harder than their components, less ductile and may have a much lower conductivity, whereas the highly purified single crystal of a metal is very soft and malleable, with high electrical conductivity.

3. Introduction Alloys are classified as binary alloys, composed of two components; as ternary alloys, composed of three components; or as multicomponent alloys. Most commercial alloys are multicomponent. The composition of an alloy is described by giving the percentage (either by weight or by atoms) of each element in it . Metal alloys by virtue of composition, are often grouped into two classes: ferrous and non-ferrous. Ferrous alloys are those in which iron is the principal constituent, include steels and cast irons. The nonferrous alloys are all alloys that are not iron based.

4. Introduction Metallic objects are invariably manufactured from alloys instead of pure metals. The alloying elements are added to improve one or more of the following properties: ( a) tensile strength , hardness and toughness (b ) corrosive and oxidation resistance, ( c) machinability, ( d) elasticity ( e) hardenability ( f) creep strength and ( g) fatigue résistance, etc . The characteristic of any series of alloys can be easily studied by using alloy phase diagrams. A phase diagram is a graphical description of the kinds and amounts of the phases that can be expected in an alloy as a function of its composition, temperature, and pressure, when it has reached thermodynamic equilibrium.

5. Introduction The following terms are frequently used in the study of solid phases and phase diagrams: System : A system may be composed of solids, liquids, gases or their combinations and may have metals and non-metals separately or in any combination . Components : These are the substances, either chemical elements or chemical compounds, whose presence is essential and sufficient to make a system . Phase : It is a homogeneous portion of a system that has uniform physical and chemical characteristics. Phase diagram: A graphical representation of the relationships between environmental constraints ( e.g. temperature and sometimes pressure), composition, and regions of phase stability, ordinarily under conditions of equilibrium . Phase Equilibrium: The state of a system where the phase characteristics remain constant over indefinite time periods. Phase Transformation: A change in the number and/or character of the phases that constitute the microstructure of an alloy.

6. Hume- Rothery’s Rules While developing an alloy, it is frequently desirable to increase the strength of the alloy by adding a metal that will form a solid solution. Hume- Rothery has framed empirical rules that govern the choice of alloying elements in the formation of substitutional solutions. We may note that if an alloying element is chosen at random , it is likely to form an objectionable intermediate phase instead of a solid solution. Extensive solid solubility by substitution occurs, when The solute and solvent atoms do not differ by more than 15% in size, i.e. diameter. Within this limit of size factor, each of the metals will be able to dissolve appreciably (to the order of 10%) in the other metal. However, if the atomic size factor is greater than 15%, solid solution formation tends to be severely limited and is usually only a fraction of one percent.

7. Hume- Rothery’s Rules T he electronegativity difference between the elements is small. If the chemical affinity of two metals is greater, then the solid solubility will be more restricted. When the chemical affinity of two metals is great, they tend to form an intermediate phase rather than a solid solution . The valency and the crystal structures of the elements are the same. If the alloying element has a different valence from that of the base metal, the number of valence electrons per atom (called the electron ratio), will be changed by alloying.

8. Hume- Rothery’s Rules Ag-Au, Cu-Ni and Ge -Si are the systems which satisfy Hume Rothery conditions very well (Table 3.1 ). Obviously, these systems form complete solid solutions, i.e. the elements mix in each other in all proportions . 3.1

9. P hase Diagram The study of phase relationships plays an important and vital role in the better understanding of the properties of materials. Much of the information about the control of microstructure or phase structure of a particular alloy system is properly displayed in what is called a phase diagram, also called as an equilibrium or constitutional diagram. Phase diagrams are clear maps that give the relationships between phases in thermodynamic equilibrium in a system as a function of temperature, pressure and composition . Phase diagrams are of following three types : Unitary or single-component phase diagram Binary or two-component phase diagram Ternary or three-component phase diagram Binary phase diagrams are extensively used.

10. The Phase Rule or Gibb’s Phase rule or Condensed Rule This expresses mathematically the general relationships for the existence of stable phases corresponding to the equilibrium conditions. It enables us to predict and check the processes that occur in alloys during heating or cooling. Using this rule, it is possible to determine whether the solidification process takes place at a constant temperature or within a certain temperature interval; it can also indicate the number of phases that can exist simultaneously in a system. The phase rule enunciated by J.W. Gibbs relating number of phases P, number of components C, and number of degrees of freedom F has a simple form: P + F = C + n P + F = C + 2

11. The Phase Rule or Gibb’s Phase rule or Condensed Rule n = number of external factors = 2 (temperature and pressure) In applying the phase rule to metal systems the effect of pressure is neglected, leaving only one variable factor ,—temperature. Equation (1) reduces to F = C + 1 – P The number of degrees of freedom is essentially the number of independent variables, both internal (composition and phases) and external ones (temperature, pressure, concentration etc.), which can be changed without changing the number of phases in equilibrium.

12. Cooling Curves (Time-Temperature Curve) Cooling curves shows the temperature changes with time as the liquid metal solidifies . Figure 3.2 shows a cooling curve which is distinctly divided into two portions while exhibiting the fall of temperature of time, the cooling curve exhibit that the temperature remains practically constant over a period of time. This constant temperature is called as the point of arrest. The solidification occurs during temperature arrest. During this period, heat is still lost from the mass of metal but release of kinetic energy compensates the heat loss whereby temperature remains constant. The released heat at constant temperature is called the latent heat.

13. Cooling Curves (Time-Temperature Curve) Curve (a): Applying Eq. 1 ( P + F = C + n ) under constant pressure, for region AB when P = 1, C = 1, F = 1, i.e. system has single degree of freedom (called univariant ). Obviously, only variant that changes is temperature. Between B and C both liquid and solid phases are present, P = 2, C = 1 so that F = 0, i.e. system has no degree of freedom ( called as non-variant). Obviously, temperature remains constant (pressure is already constant) and the mass between B and C is marshy (partly liquid and partly solid). On further cooling from C to D the system reaches room temperature.

14. Cooling Curves (Time-Temperature Curve) When two or more metals are mixed in liquid state to form an alloy and allowed to cool, the solidification occurs over a range of temperature. Figure 3.2(b ) shows a cooling curve for an alloy of metals A and B. We can see that curve AB is the same as for pure metals. The freezing line BC drops until the whole mass is solid at point C. Applying Eq. (1) to the system between B and C (Fig. 3.2b ) with P = 2, C = 2, F = 1, i.e. there is one degree of freedom . The temperature will change (the pressure is constant).

15. Cooling Curves (Time-Temperature Curve) Figure 3.2(c ) is the freezing curve for another binary system whose two components are completely soluble in liquid state but not at all soluble in solid state. They are liquid along AB upto point B of the cooling curve. At point B the component with larger content starts solidifying and temperature falls along BC . At point C the components solidify simultaneously at constant temperature, the lowest for a given alloy system , and are termed as eutectic alloys. At D the only phase that is present is solid and cools along DE as usual. Point D on the curve is called as eutectic point.

16. Construction of a Phase Diagram Depending upon the number of components and solubility characteristic, the phase diagrams are usually categorised as follows: Solid Solution Type: In this case two metals are completely soluble in solid as well as in liquid state. They have the same type of lattice and similar atomic size. Eutectic Type: When two metals are completely soluble in the liquid state but partly or completely insoluble in the solid state, is termed as eutectic type. Fe-C, Al- Mn , Pb-Sn form an eutectic system . Peritectic Type: In this case liquid and solid combine to form a new solid. The melting points of two metals differ considerably. Ag and Pt form such a system.

17. Construction of a Phase Diagram Monotectic Type: In this case the two liquid solutions are not soluble in each other over a certain composition range, i.e., there is a miscibility gap in liquid state between the two metals. In this type one liquid decomposes into another liquid solid. Cu and Pb form monotectic system . Eutectoid Type: In this one solid decomposes into two different solids. Obviously, solid to solid transformation takes place. Fe-C, Cu-Zn, Al-Cu, Cu- Sn , etc form eutectoid system.

18. Lever Rule This rule helps to calculate the relative proportions of solid and liquid material present in the mixture at any given temperature. The number and composition of phases can be obtained from the phase diagram. If the composition and temperature position is located within a two-phase region, things are more complex. In a two-phase region, one can determine the relative amount of each phase that is present from the phase diagram , using a relationship known as lever rule (or the inverse lever rule), which is applied as follows:

19. Lever Rule Construct the tie line across the two-phase region at the temperature of the alloy. The overall alloy composition is located on the tie line. The fraction of one phase is computed by taking the length of the line from the overall alloy composition to the phase boundary for the other phase, and dividing the total tie line-length. One can determine the fraction of the other phase in the same manner. In case if phase percentages are desired, each phase fraction is multiplied by 100. When the composition axis is scaled in weight percent; the phase fractions computed using the lever rule are mass fractions – the mass (or weight) of a specific phase divided by the total alloy mass (or weight).

20. Allotropic forms of Iron Pure substances may exist in more than one crystalline form and each such crystalline form is stable over more or less well defined limits of temperature and pressure. This is termed as allotropy or polymorphism. Pure iron is relatively soft and ductile and its melting point is 1539°C. The pure iron exists in three important allotropic forms, i.e. alpha ( α ), gamma ( γ ) and delta ( δ ) iron. The existence of phases depends upon the temperature to which the iron is heated. An ideal curve for pure iron, showing the temperature ranges over which each of these crystallographic forms are stable at atmospheric pressure is shown in Fig. 3.4.

21. Allotropic forms of Iron From Fig. 3.4, it is evident that from room temperature to 910°C pure iron has a BCC structure and is called alpha ( α ) iron ( Fe). It is highly ferromagnetic and remains so upto 768°C (Curie point). On heating it becomes non-magnetic, i.e., ferromagnetism disappears. However, the crystal structure still remains BCC. Non-magnetic α -iron is stable upto 910°C. The non-magnetic α -iron was earlier known as β -iron . However, the X-ray crystallography revealed no change of crystal structure at 768°C. In order to avoid confusion, the original naming of the sequence retained with the β -phase deleted. Hence it is known as α -iron ( Fe). Above 910°C, it is transformed into FCC and allotropic change takes place. It is transformed from to γ -iron structure. Upon heating to 1404°C, again allotropic change takes place and δ -iron is transformed back into the BCC structure called δ -iron . It is stable upto the melting point, 1539°C of pure iron. The BCC structure δ -iron has a longer cube edge than BCC structure of α -iron.

22. Iron-carbon System It is the most important binary system in engineering alloys. The alloys of iron-carbon system containing from 0 to 2.0% carbon are called steels and those containing from 2.0% to 6.7 % are called cast irons. However , in practice, the steels are manufactured with carbon content upto 1.4%. It is due to the fact that steels with carbon content more than 1.4% are brittle and hence are not useful. Similarly, the cast irons that are manufactured in practice contain carbon from 2.0% to 4.5% only. It will be interesting to know that iron-carbon alloys exist in different phases in steels and cast irons. In steels, the iron and carbon exists as two separate phases, ferrite and cementite. The ferrite is a solid solution of carbon in α -iron , with negligible amount of carbon and cementite is an intermetallic compound called iron-carbide (Fe 3 C). Cementite is a stable phase in steels only. But it is not stable in cast irons under all conditions and hence cementite is called a metastable phase. Under certain conditions, cementite decomposes into the more stable phases of graphite and iron.

23. Iron-carbon System However, once cementite is formed, it is very stable for all practical purposes and therefore can be treated as an ‘equilibrium phase’. For this reason , we have the following two phase diagrams of iron-carbon system : Iron-Iron carbide (Fe-Fe3C) phase diagram. Iron-Carbon (Fe-C) phase diagram.

24. Iron-carbon System Fig. 3.5 shows the iron-iron carbide (Fe-Fe 3 C) phase diagram. In this diagram, the carbon composition (weight percent ) is plotted along the horizontal axis and temperature along the vertical axis. The diagram shows the phases present at various temperatures for very slowly cooled iron-carbon alloys with carbon content up to 6.7%. This diagram gives us information about the following important points : Solid phases in the phase diagram Invariant reactions in the phase diagram Critical temperatures Eutectoid, hypo-eutectoid and hypereutectoid steels.

25. curve ABCD is called the liquidus All alloys represented by compositions and temperatures in the region above ABCD are completely liquids. Point A in the diagram represent the melting point of pure iron (1539°C). Point D represent the melting point (1539°C) of iron carbide or cementite . With the fall in the temperature of the liquid along the line ABC, crystals of austenite separate from the liquid. Similarly, in the same way, crystals of iron carbide ( FeC ) separate from the liquid along the line CD .

26. High temperature transformation take place at upper left hand portion of phase diagram The peritectic reaction (L+S=S)HJB represents the formation of austenite, i.e. solid solution of carbon in gamma iron ( γ -Fe or Fe γ ). crystals of δ -iron ( Fe δ ) begin to separate from the liquid along the line AB.

27. The complete solidification of iron-carbon alloys proceeds along the line HJECF called the solidus.

28. The alloys containing 0.18 to 1.7% carbon represent the solidus HJE those with carbon contents ranging from 1.7 to 6.67% become solid along the line ECF at temperature 1130°C. At point C ( 4.3% carbon), austenite and cementite are simultaneously precipitated from the alloy to form the eutectic, also called as Lede-burite . The iron-carbon phase diagram indicates a peritectic point J, a eutectic point C, and a eutectoid point S.

29. The following reactions takes place at these points :

30. Solid phases in iron-iron carbide phase diagram The iron-iron carbide phase diagram shown in Fig. 3.5 contains four solid phases, i.e. α -ferrite, austenite ( γ ), cementite(Fe 3 C ) and δ -ferrite. α -ferrite: the solid solution of carbon in α -iron is called α -ferrite or simply ferrite. This phase has a BCC structure and at 0% carbon, it corresponds to α -iron. The phase diagram indicates that the carbon is slightly soluble in ferrite. It is due to the fact that maximum solid solubility of the carbon in α -ferrite is 0.02% at 723 ° C. The solubility of carbon in α -ferrite decreases with decrease in temperature, until it is about 0.008% at 0 ° C as shown by the line GM in the phase diagram. The carbon atoms, because of their small size, are located in interstitial spaces or voids in the iron crystal lattice.

31. Solid phases in iron-iron carbide phase diagram Austenite: The solid solution of carbon in γ - iron is called austenite. It has face-centred cubic (FCC) structure and has a much greater solid solubility for carbon than α -ferrite . The solubility of carbon in austenite reaches a maximum of 2.11% at 1148°C and then decreases to 0.8% at 723°C as shown by the line CD in the phase diagram. The carbon atoms are dissolved interstitially (in the same way as in ferrite) but to a much greater extent in the FCC lattice. This difference in the solid solubility in austenite and α -ferrite is the basis for the hardening of most steels. The austenite is soft and ductile. It is not ferromagnetic at any temperature.

32. Solid phases in iron-iron carbide phase diagram Cementite: The intermetallic iron-carbon compound is called iron carbide or cementite. Its chemical formula is Fe 3 C. This means that in a cementite crystal lattice, the number of iron atoms are 3 times more that those of carbon atoms. Cementite has negligible solubility limits and contains 6.7% carbon and 93.3 % iron. Cementite has an orthorhombic crystal structure with 12 iron atoms and 4 carbon atoms per unit cell. As compared to austenite and ferrite, cementite is extremely hard and brittle. It is magnetic below 210°C . δ -ferrite: The solid solution of carbon in δ -iron is called δ -ferrite . It has a BCC crystal structure but with a different lattice parameter than α -ferrite . The maximum solid solubility of carbon in δ -ferrite is 0.09% at 1495°C .

33. Eutectoid, Hypoeutectoid and Hypereutectoid Steels We have already discussed that iron-carbon alloys containing carbon from 0 to 1.4 % are called steel. These steels are quite often referred as plain carbon steels, when they do not contain any alloying element. A plain carbon steel containing 0.8% carbon is known as eutectoid steel. If the carbon content of the steel is less than 0.8%, it is called hypo-eutectoid steel. Most of the steels produced, commercially, are hypo-eutectoid steels. The steels, which contain more than 0.8% of carbon are called hypereutectoid steels. Hypereutectoid steels with carbon content upto 1.4% When the carbon content of steel is more than are produced commercially. 1.4%, it becomes very brittle. Thus very few steels are made with carbon content more than 1.4%. In order to increase the strength of steels, other alloying elements are added. These elements increase the strength as well as maintain ductility and toughness.

34. Eutectoid, Hypoeutectoid and Hypereutectoid Steels The iron-carbon alloys containing carbon above 2% are called cast irons. The cast iron containing 4.3% carbon is called eutectic steel, If the carbon content of cast iron is less than 4.3% it is called hypoeutectic cast iron. Most of the cast irons, produced commercially are hypoeutectic cast irons. The cast irons containing more than 4.3% carbon are called hypereutectic cast irons. Hypereutectic cast irons with carbon content up to 4.5% are produced commercially.

35. Modified Iron-carbon System In phase diagram of Fig. 3.5, the upper left hand portion, the region ABJN, in which δ -iron may exist is not very important from practical point of view because temperature near 1401°C are neither used for heat treatment and not in mechanical working. This is why, it is essential to discuss all the practical aspects of primary solidification with the modified iron-carbon phase diagram shown in Fig. 3.6. The study of following two important transformations will clarify the interpretation of phase diagram, Fig . 3.6: ( i ) Primary solidification: i.e., transformation from liquid to solid state. ( ii)Secondary solidification or crystallization: i.e., solid state transformation.

36. Primary solidification To study these transformations, let us consider the sequence of events when liquid alloys of various carbon contents are cooled to a temperature just below the eutectic temperature, 1130°C, as shown by line ECF Fig . 3.6.

37. S econdary transformations