Metallurgy PDF

Online Sessions: Materials Engineering & Metallurgy (UME517) Dr. Tarun Nanda Professor, MED Thapar Institute of Engineering and Technology, Patiala Module: SCOPE OF METALLURGY  SCOPE OF METALLURGY...

Online Sessions: Materials Engineering & Metallurgy (UME517) Dr. Tarun Nanda Professor, MED Thapar Institute of Engineering and Technology, Patiala Module: SCOPE OF METALLURGY  SCOPE OF METALLURGY Presentation 1 Contents Covered 1. Metallurgy defined 2. Main Branches of Metallurgy 3. Scope of Extractive Metallurgy 4. Scope of Industrial Metallurgy  METALLURGY Metallurgy defined: Metallurgy is defined as the science and technology of procuring the metals from their ores and adapting them to satisfy human needs. In simple words, metallurgy is the science and technology of METALS. This engineering field has two main branches as follows: 1. EXTRACTIVE METALLURGY 2. PHYSICAL METALLURGY Figure 1.1 Fields under metallurgy 3  Scope of EXTRACTIVE METALLURGY Why extractive metallurgy is required? Plenty of metal atoms present on the earth’s surface but majority of them not present in usable form. Combined with other non-metal atoms such as oxygen etc. in substances similar to stones, clay etc. So, metals not present in their native state but exist in combined state. First step in making the metals available for human use is to free the metal atoms from this combined state. It is where extractive metallurgy comes into picture. Extractive metallurgy defined: Extractive Metallurgy is that branch of metallurgy which deals with the liberation of metals from their ores by means of various processes/chemical reactions. Extractive metallurgy makes use of the following processes: 1. Mining 2. Crushing 3. Pulverization 4. Concentration 5. Extraction 6. Refining 3 Figure 1.2 Various steps of extractive metallurgy 6  7 Figure 1.3 Images showing scope of extractive metallurgy  8 Figure 1.4 Images showing scope of extractive metallurgy PHYSICAL METALLURGY (INDUSTRIAL METALLURGY) Why industrial metallurgy is required? Metals are obtained in their pure form/free state/native state after following all the steps of extractive metallurgy. Pure metals (obtained after so much effort) have some desirable physical properties (thermal conductivity, electrical conductivity, luster, magnetic characteristics etc.) but they lack in mechanical properties (strength, hardness, toughness etc.) required for engineering applications. So, pure metals have to adapted/transformed to bring about desirable mechanical properties in them. Industrial metallurgy defined: Physical Metallurgy is defined as that branch of metallurgy which deals with the adaption of metals to satisfy human needs. Physical Metallurgy studies the mechanical properties of metals/alloy systems and investigates how these properties are affected by: i. Changing the chemical composition of metal/alloy system ii. Providing mechanical treatment to the metal/alloy system iii. Providing heat treatment to the metal/alloy system 9 SCOPE OF INDUSTRIAL METALLURGY Physical Metallurgy studies the mechanical properties of metals/alloy systems and investigates how these properties are affected by three main types of processes shown in Figure 1.4. Figure 1.4 Main control variables in industrial metallurgy Control factors in physical metallurgy i. Alloying: Alloying means changing the chemical composition of the metal/alloy system. Physical metallurgy investigates the affect of change in chemical composition of a materials system on its mechanical properties. ii. Mechanical treatment: Any process which brings about a change in the shape, or size, or both of a materials system (generally by application of force/load) is called a mechanical treatment viz. forging, rolling, drawing, machining etc. Physical metallurgy investigates the affect of providing these treatments on the mechanical properties of a materials system. iii. Heat treatment: Any process which involves heating and cooling of a materials system timed and applied when material is in the solid state to bring changes in the microstructure is a heat treatment process viz. annealing, normalizing, hardening etc. Physical metallurgy investigates as to how different heat treatment processes affect the mechanical properties of 10 metal/alloy systems.  SCOPE OF PHYSICAL METALLURGY (INDUSTRIAL METALLURGY) PHYSICAL METALLURGY investigates as to why metals/alloys behave in the manner as they show. In other words, physical metallurgy finds out as to why a metal/alloy system of a particular composition and processed under some given conditions of temperature, pressure etc. shows a particular set of mechanical properties and how/why do its properties change with change in conditions of composition, temperature, pressure etc. Physical metallurgy deals with production of new or improved metals/alloys through mainly heat treatment and alloying. 11 11 Online Sessions: Materials Engineering & Metallurgy (UME517) Dr. Tarun Nanda Professor, MED Thapar Institute of Engineering and Technology, Patiala Module: BASIC TERMS IN INDUSTRIAL METALLURGY Dr. Tarun Nanda, MED, TIET, Patiala BASIC TERMS IN INDUSTRIAL METALURGY Presentation 2 Contents Covered 1. Phases (Micro-constituents) defined 2. Phases – Macroscopic view 3. Phase Vs State 4. Phases – Microscopic View Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - PHASE Phase defined: A phase/micro-constituent of a materials system is a region present in the materials system which is chemically homogenous within itself, is physically distinct from other regions, and is mechanically separable from other regions (these other regions being other phases present in that materials system). In the above definition, - ‘Chemically homogenous’ means that when we are within a phase, the chemical composition remains same throughout the region/phase. - ‘Physically distinct’ means that one phase may be different from another phase either in terms of state (i.e. liquid, gas, solid), or colour (white, black etc.), or appearance (physical morphology/shape). - ‘Mechanically separable’ means that each phase has some unique mechanical properties which are different from other phases. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - PHASE At the macroscopic level, we can observe three states of matter (viz. liquid, solid, gas). Each state represents a different unique phase. Consider the H2O materials system. In Figure 2.1 (a), the materials system is in a complete liquid state; there is only one phase present in the system. The phase can be called liquid water or water, or simply liquid phase. In Figure 2.1 (b), the materials system is in a complete solid state; there is again only one phase but this phase type is different. It can be called solid water, or ice, or simply solid phase. In Figure 2.1 (c), though the composition is same throughout the system, but physical appearance (in terms of state) is changing; there are two different states, and hence two different phases present in the system viz. water and ice phases. Figure 2.1 Phase changes with change in state  BASIC TERMS - PHASE Further, let us see what are the number of phases that can be observed when a given materials system exists in a particular state.  GASEOUS SYSTEM: A materials system which is in the gaseous state is always a single phase system (in other words, if a particular materials system is in complete gaseous state, the materials system has only one phase present in it). This is because gases are completely miscible in each other irrespective of their type and amount present in a gaseous system. So, when you look into a gaseous materials system, there is no variation in chemical composition, physical state, properties; hence a single phase system always gets formed.  LIQUID SYSTEM: For a materials system in the liquid state, if the constituents are such that they form a liquid solution, then the materials system has only one phase in it; otherwise, if the constituents form a mixture, the liquid system will have to be investigated to see as to how many different regions and hence phases are formed in it. But, yes, for such a liquid system forming a mixture, the number of phases will be more than one.  SOLID SYSTEM: For a materials system in the solid state (metals/alloy system), generally for a pure metal (at ambient conditions) and for an alloy system with a single solid solution, the number of phases is one. In other cases, the number of phases will be more than one. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - PHASE At the microscopic level, one phase differs from another phase in terms of one or more of the following: (i) Chemical composition (ii) Physical appearance (iii) Crystal structure (iv) Crystal lattice dimensions (lattice parameters) Example: Different phases in pure iron depending on external conditions of temperature at constant pressure.  Pure iron (Fe) at room temperature and atmospheric pressure conditions (i.e. 1 atm) has 100% pure Fe chemical composition, is in the solid state, has B.C.C. crystal structure and has some definite value of lattice dimensions. These four parameters are same throughout pure iron. This means under these ambient conditions, there exists only one homogeneous region and hence one phase in pure iron. For temperatures between 0 ℃ to 910 ℃, iron shows no changes in any of these four parameters. So, for these conditions (0-910 ℃; 1 atm), iron has only one phase in it. This unique phase is called alpha phase (-phase) and the iron containing it is called -iron. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - PHASE Example (continued……)  For temperatures between 910 ℃ and 1410 ℃, pure iron is still 100% Fe, is in the solid state, but crystal structure changes to FCC (and so, also the lattice parameters). Thus, iron is now in a different phase and this phase is called gamma phase (-phase). So, for these conditions (910-1410 ℃; 1 atm), iron again has only one phase in it but the phase is different called - phase and the iron is called -iron. For temperatures between 1410 ℃ and 1539 ℃ (i.e. melting point of pure iron), iron is still in the solid state but crystal structure again changes to B.C.C. Iron again has one phase in it called delta phase (-phase) and the iron containing it is called -iron. Both -iron & -iron are in the same state (solid state), have same composition (pure iron), and have the same crystal structure (B.C.C.) but still represent different phases because lattice dimensions in -iron are greater than those of -iron. Hence, these two represent different phases. Dr. Tarun Nanda, MED, TIET, Patiala Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Materials Engineering & Metallurgy (UME517) Dr. Tarun Nanda Professor, MED Thapar Institute of Engineering and Technology, Patiala Module: BASIC TERMS IN INDUSTRIAL METALLURGY Dr. Tarun Nanda, MED, TIET, Patiala BASIC TERMS IN INDUSTRIAL METALURGY Presentation 3 Contents Covered 1. Components defined 2. Components of a metallic system 3. Exception to the general rule 4. Components of plain carbon steels Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - COMPONENTS Components defined: Components of a materials system are the number of stable individual substances which describe completely the chemical composition of the system at a given temperature and pressure which is of interest. In other words, components are the independent chemical species comprising the materials system. Components of a materials system may be in the form of elements, ions, or compounds (there is no fourth form). All pure substances are one component systems. For example, a materials system containing pure nickel only or pure copper only or any other pure substance is a one component system. Ice-water-steam system (means H2O at its triple point) is again a one component system; the component present in this pure substance is H2O. Please note, in this system with one component, the number of phases is three. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - COMPONENTS Sodium chloride (NaCl) is a one component system. Though this materials system has two elements in it (Na and Cl) but neither Na nor Cl have any independent identity when present in NaCl. The stable substance with independent chemical identity is only one i.e. NaCl as a whole. COMPONENTS OF A METALLIC SYSTEM In metallic systems, generally the number of components is equal to the number of elements present in the metallic system. For example, a copper-nickel metallic system (Cu-Ni alloy) has two components in it (i. copper, and ii. nickel). Metallic systems containing two components in them are called ‘binary systems’ or ‘binary alloys’. Similarly, Cu-Ni-Fe is a three component system (ternary systems/ternary alloys). Similarly, there are quaternary, quinary systems/alloys and so on. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - COMPONENTS COMPONENTS OF A METALLIC SYSTEM In metallic systems, generally the number of components is equal to the number of elements present in the metallic system. However, there are a few exceptions to this general rule. In a metallic system, if there are constituents which form some truly stable compounds, then the above general rule is NOT followed. For example, in the metallic alloy system ‘Cu-Ni-Mn-S’, the number of constituent elements is four but the number of components is NOT four. The number of components in this alloy system is three. This is because Mn and S combine to form a truly stable compound which has its independent chemical identity. Thus, the components of this alloy system are i. Cu, ii. Ni, and iii. MnS. Plain carbon steels under ambient conditions have two components in them i. iron (Fe) and ii. Iron carbide (Fe3C). Dr. Tarun Nanda, MED, TIET, Patiala Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Materials Engineering & Metallurgy (UME517) Dr. Tarun Nanda Professor, MED Thapar Institute of Engineering and Technology, Patiala Module: BASIC TERMS IN INDUSTRIAL METALLURGY Dr. Tarun Nanda, MED, TIET, Patiala BASIC TERMS IN INDUSTRIAL METALURGY Presentation 4 Contents Covered 1. Equilibrium defined 2. Significance of the term, ‘equilibrium’ 3. Equilibrium in a physical sense 4. Classification of phases based on equilibrium 5. Meaning of equilibrium heating/cooling. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - EQUILIBRIUM Equilibrium defined: Equilibrium is defined as the condition under which the net forces (whether mechanical, thermal etc.) acting on the system are zero, and thus, there is no unbalanced force acting on the system. Equilibrium is also defined as the state of minimum free energy of the system under any given set of conditions of composition, pressure, and temperature (Gibb’s free energy is defined as the energy available with a system which can be converted into work/or can bring about a transformation/change in the system at a constant temperature and pressure. Gibb’s free energy is a useful measure of the tendency for a transformation to occur). Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - EQUILIBRIUM Significance of Equilibrium: When a system is in equilibrium, there occurs no change in the system with time, however long this time period may be. If a system is not in equilibrium, this means that there are certain unbalanced resultant forces still acting on the system. The system under the effect of these unbalanced forces keeps on changing until it has attained the state of minimum free energy i.e. net forces acting on it have become zero. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - EQUILIBRIUM In a physical sense: Among the various configurations which a body can have, the configuration in which the body has least potential energy is called its stable state or equilibrium state. The configuration in which the body has maximum potential energy is called its unstable state or non-equilibrium state. All configurations in-between these two configurations are called metastable states. Figure 4.1 Different configurations representing equilibrium, non-equilibrium, and metastable states.  BASIC TERMS - EQUILIBRIUM Significance of the term ‘Equilibrium’ in metallurgy: Equilibrium term is used in two contexts in the field of metallurgy. A. Categorization of phases existing in a materials system B. Equilibrium heating/cooling A. Classification of phases based on equilibrium i. Equilibrium phase ii. Non-equilibrium phase iii. Metastable phase i. Equilibrium phase: If a phase existing in a materials system under some given conditions of pressure and temperature does not change with time (pressure, temperature conditions remaining the same) nor has any urge/tendency to change, then that phase is an equilibrium phase for the given materials system under the given conditions. For example, for a plain carbon steel, ferrite is an equilibrium phase under room temperature conditions. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS - EQUILIBRIUM ii. Non-equilibrium phase: If a phase existing in a materials system under some given conditions of pressure and temperature immediately changes with time to some other phase(s) (pressure, temperature conditions remaining the same), then that phase is a non- equilibrium phase for the given materials system under the given conditions. For example, for a plain carbon steel, ferrite is a non-equilibrium phase under high temperature conditions (generally above 910 ℃). Similarly, for a plain carbon steel, austenite is a non-equilibrium phase under room temperature conditions. iii. Metastable phase: If a phase existing in a materials system under some given conditions of pressure and temperature immediately changes with time to some other phase(s) (pressure, temperature conditions remaining the same), then that phase is a non- equilibrium phase for the given materials system under the given conditions. For example, for a plain carbon steel, ferrite is a non-equilibrium phase under high temperature conditions (generally above 910 ℃). Similarly, for a plain carbon steel, austenite is a non-equilibrium phase under room temperature conditions.  BASIC TERMS - EQUILIBRIUM B. Equilibrium heating/cooling When a materials system is heated/cooled, it is sometimes referred that it is being heated/cooled under equilibrium conditions. What is meant by this? Heating/cooling under equilibrium conditions means that the rate of heating/cooling is infinitesimally small. As such, there is ample (sufficient) of time available with the materials system at each step of heating/cooling so that whatsoever changes/processes (diffusion, recrystallization, etc.) want to occur in the system, can get completed.  BASIC TERMS - EQUILIBRIUM When a materials system is cooled under equilibrium conditions of cooling, the system obtained at room temperature will be stable with no tendency to change with time. The chemical composition/structure will be completely homogeneous. No residual stresses are present. On the contrary, if a materials system is heated/cooled under non-equilibrium conditions (rapid heating/cooling), the system obtained under final conditions will NOT be stable. Due to non-equilibrium heating/cooling, certain unbalanced forces (locked-in/residual stresses) are still present in the system which tend to bring about a change in it. Dr. Tarun Nanda, MED, TIET, Patiala  a nd Na Online Sessions: Industrial Metallurgy (UME733) un ar.T Dr Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala  a nd Na Module: BASIC TERMS IN INDUSTRIAL un METALLURGY ar.T Dr Dr. Tarun Nanda, MED, TIET, Patiala BASIC TERMS IN INDUSTRIAL METALURGY Session 5 a nd Contents Covered Na 1.Degrees of freedom (DOF) 2.DOF in one component system having one phase. un 3.DOF in one component system having two ar phases..T 4.DOF in one component system having three phases. Dr Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS – DEGREES OF FREEDOM Degrees of freedom defined: Degrees of freedom of a materials system under some given conditions is the number of variables (parameters viz. temperature, pressure, composition, magnetic force etc.) which can be changed independently for that system a without bringing out the disappearance of an already existing phase or formation of a new phase (i.e. without any change in the existing phase(s)) nd Na Variables/parameters can be an internal variable like concentration of the components of the materials system (e.g. weight percentage of constituents) or external variables un like temperature, pressure etc. ar.T Dr Dr. Tarun Nanda, MED, TIET, Patiala  FREEDOM Example to explain meaning of degrees of freedom - Figure 5.1 is equilibrium diagram of a pure a substance. nd - Three single-phase regions, three two-phase regions, and one three-phase region (triple Na point). Case 1. When materials system has temperature and pressure values such that it lies in any of the single phase regions. un How many DOF? (P = 1; DOF = ?)  Let this materials system be defined by point ‘a’ having pressure Pa and temperature Ta. ar  Under these conditions (Pa, Ta), the material has one phase and type is gas phase.  Now for these conditions, it is to be checked if pressure and.T temperature are degrees of freedom or not.  It can be seen that as the temperature of the material is changed from Ta to Ta’, the value of temperature could be changed independently of pressure value and still the number Dr and type of phase (i.e. gas phase) remains same in the materials system. Thus, temperature can be varied independently and is a degree of freedom for this material under the given condition.  Similarly, pressure could have be varied independently Figure 5.1 Equilibrium diagram of a typical pure substance without change in phase.  Thus, for the given condition, both temperature and pressure  FREEDOM Case 2. When materials system has temperature and pressure values such that it lies in any of the two phase regions. a How many DOF? (P = 2; DOF = ?)  Let this materials system be defined by point ‘b’ having nd pressure Pb and temperature Tb.  Under these conditions (Pa ,Ta), the material has two phases, these are solid phase and gas phase. Na  Now for these conditions, it is to be checked if pressure and temperature are degrees of freedom or not.  It can be seen that temperature of the material can be changed from Tb to Tb’, however as temperature value is un changed (), the pressure value will have to be changed by so that the materials system still remains on the two phase boundary of (solid + gas) region.  Here, the value of temperature could be changed ar independently of pressure value but change in pressure value was dependent on change in temperature value..T Thus, temperature can be varied independently and is a degree of freedom for this material under the given condition.  Similarly, pressure could have be varied independently Dr but then temperature change would have become dependent on pressure change.  Thus, for the given condition, either temperature or pressure is a degree of freedom.  Thus, for this one component system, when P = 2, DOF = Figure 5.2 Equilibrium diagram of a typical pure 1. substance  FREEDOM Case 2. When materials system has temperature and pressure values such that it lies at the triple point (three phase region). a How many DOF? (P = 3; DOF = ?)  Let this materials system be defined by point ‘O’ having nd pressure PO and temperature TO.  Under these conditions (PO ,TO), the material has three phases, these are solid phase, liquid phase, and gas Na phase.  Now for these conditions, it is to be checked if pressure and temperature are degrees of freedom or not.  It can be seen that triple point occurs at a definite value un of temperature and pressure.  Neither temperature value can be disturbed/varied nor pressure value can be disturbed, else the triple point (three phase region) will be lost. ar  Thus, for the given condition, neither temperature nor pressure is a degree of freedom..T  Thus, for this one component system, when P = 3, DOF = 0. So, for a one component system, Dr  There are 02 degrees of freedom when system has 01 phase in it  There is 01 degree of freedom when system has 02 Figure 5.3 Equilibrium diagram of a typical pure phases substance in it  There are 0 degrees of freedom when system has 03 phases in it  a nd Na THANKS FOR WATCHING un ar.T Dr Dr. Tarun Nanda, MED, TIET, Patiala  a nd Na Online Sessions: Industrial Metallurgy (UME733) un ar.T Dr Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala  a nd Na un Module: BASIC TERMS IN INDUSTRIAL METALLURGY ar.T Dr Dr. Tarun Nanda, MED, TIET, Patiala BASIC TERMS IN INDUSTRIAL METALURGY Session 6 a nd Contents Covered Na 1. Gibb’s phase rule 2. General Gibb’s phase rule un 3. Condensed Gibb’s phase rule ar 4. Significance/applications of the rule.T Dr Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS – GIBB’S PHASE RULE Gibb’s Phase rule: Gibb’s phase rule is a mathematical expression which provides the number of phases present in a materials system under equilibrium conditions. It gives a a nd quantitative relationship between the number of components (C) present in a materials Na system, the number of phases (P), the number of degrees of freedom (F or DOF), and the number of external variables (E) affecting the phase relationships. un Gibb’s phase rule is given as Equation 6.1. ar P+F=C+E.T …………….. (Eq. 6.1) Dr - Gibb’s phase rule is valid for a materials system only if the system has been treated under strict conditions of equilibrium (i.e. Dr. Tarun slow Nanda, MED,heating/slow TIET, Patiala cooling).  BASIC TERMS – GIBB’S PHASE RULE Generally, only two external variables (pressure and temperature) are a considered to be affecting the phase relationships in a materials system, and nd as such the most general form of Gibb’s phase rule is given by Equation 6.2. Na General Gibb’s phase rule is given as Equation 6.2. un P+F=C+2 ar …………….. (Eq. 6.2).T Dr However, there can be several other variables which can affect the phase relationships of a materials system. These may include electrostatic forces, magnetic forces, gravitational forces, surface tension forces etc. Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS – GIBB’S PHASE RULE For any given materials system under a given condition, it has to be noted as a to how many external variables are affecting its phases. For example, if for a nd particular system, there are four external variables influencing the phase Na relationships, the Gibb’s phase rule of this system will be modified and given as Equation 6.3. un P+F=C+4 ar …………….. (Eq. 6.3).T Dr Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS – GIBB’S PHASE RULE For binary alloy systems (alloys containing two components), pressure is a generally fixed at 1 atm. The only parameter that is varied and affects the nd phase relationships is temperature. If for any materials system, there is only Na one variable (temperature) affecting the phases of the system, the phase rule is modified and is called Condensed Gibb’s phase rule. un Condensed Gibb’s phase rule is given as Equation 6.4. ar P+F=C+1.T …………….. (Eq. 6.4) Dr Dr. Tarun Nanda, MED, TIET, Patiala  BASIC TERMS – GIBB’S PHASE RULE Significance (applications/use) of Gibb’s phase rule a 1. For a given materials system under some given conditions, if out of the four parameters of nd P, F, C, and E, any three are known, the fourth can be obtained using Gibb’s phase rule. Na 2. Equilibrium diagrams are generally constructed using experimental methods un viz. thermal analysis, X-ray diffraction, metallography etc. There is always a possibility that ar errors/mistakes are present in these diagrams. Gibb’s phase rule is used to.T check the accuracy of equilibrium diagrams. There are no exceptions to the Dr Gibb’s phase rule and in case any conclusion is drawn from any equilibrium diagram which violates this rule, means that there is definitely something wrong with that equilibrium diagram.  Numerical Problem – GIBB’S PHASE RULE 1. Enumerate the various degrees of freedom of a three component materials system with various number of possible phases. a nd Solution: If nothing is mentioned, it is assumed that this system was treated under strict conditions of equilibrium and there are two external variables (pressure and temperature) affecting the phase relationships of this materials Na system. As such, General Gibb’s phase rule will be applied. Three component system  C = 3 un P + F = C + 2 (general Gibb’s phase rule) Þ F=C-P+2 ar.T Number of phases (P) Corresponding degrees of freedom (F) 1 F = 3-1+2 =4 (minimum number of phases in a system can b1) Dr Final Answer: 2 F = 3-2+2 = 3 3 F = 3-3+2 = 2 Degrees of Freedom Corresponding Phases 4 1 4 F = 3-4+2 = 1 3 2 5 F = 3-5+2 = 0 2 3 6 F = 3-6+2 = -1 (* Not Possible) 1 4 0 5 * Not possible because degrees of freedom cannot be less than zero.  a nd Na un THANKS FOR WATCHING ar.T Dr Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Module: BASIC TERMS IN INDUSTRIAL METALLURGY Dr. Tarun Nanda, MED, TIET, Patiala BASIC TERMS IN INDUSTRIAL METALURGY Session 7 Contents Covered 1. Microstructure and mechanical properties 2. Meaning and definition of the term ‘microstructure’ 3. Constituents of microstructure 4. Schematics showing different microstructures 5. Actual micrographs showing microstructures Dr. Tarun Nanda, MED, TIET, Patiala  SIGNIFICANCE OF MICROSTRUCTURE The mechanical properties which any metal/alloy system possesses under any given conditions (of pressure, temperature, composition etc.) depend on the microstructure which the metal/alloy system possesses under those conditions. This means that a given metal/alloy system shows some specific mechanical properties because there is some peculiar microstructure present in it. This also means that if the mechanical properties of a metal/alloy system have to be changed, its microstructure will have to be changed. Dr. Tarun Nanda, MED, TIET, Patiala  SIGNIFICANCE OF MICROSTRUCTURE To do that, we must know: 1. which specific microstructure provides a particular set of mechanical properties 2. what processing route is required to achieve a particular microstructure and hence properties in a metal/alloy system. If we know the answers to the above two key points, we can learn to have total control over the mechanical properties of a metal or an alloy system. This is where INDUSTRIAL METALLURGY comes to our rescue. Dr. Tarun Nanda, MED, TIET, Patiala  Meaning of the term, ‘Microstructure’ The mechanical properties which any metal/alloy system possesses under any given conditions (of pressure, temperature, composition etc.) depend on the microstructure which the metal/alloy system possesses under those conditions.  Meaning of the term, ‘Microstructure’ Microstructure is the micro-scale (very fine scale) structure of a metal/alloy (or any material) generally revealed for its prepared surface through optical/electron microscope (generally above 25X magnification). Microstructure of a metal/alloy system is a micrograph which shows the shape, size, and distribution of various phases present in a particular grain size in a metallic system when its prepared surface is viewed under an optical/electron (SEM, TEM etc.) microscope. Meaning of the term, ‘Microstructure’ Figure 7.1 Constituents of the microstructure  Meaning of the term, ‘Microstructure’ Microstructure of a metal/alloy system is a micrograph which shows the shape, size, and distribution of various phases present in a particular grain size in a metallic system when its prepared surface is viewed under an optical/electron (SEM, TEM etc.) microscope. Figure 7.1 Schematic microstructure of a typical metallic system  Schematics showing various microstructures Figure 7.1 Schematic micrographs representing microstructures of typical metallic systems  Schematics showing various microstructures Figure 7.1 Schematic micrographs representing microstructures of typical metallic systems  Micrographs showing actual microstructure Figure 7.1 Actual SEM micrographs of a dual phase steel showing microstructure at (a) 4000X, and (b) 60,000X  SIGNIFICANCE OF PHASES The main constituents of the microstructure are the phases present and their details. So, mechanical properties which any metal/alloy system possesses under any given conditions (of pressure, temperature, composition etc.) mainly depend on the phases present in the microstructure of that metal/alloy system under those conditions. Phases affect the properties of a metallic system by two main ways: 1. Volume fraction of the continuous phase 2. Morphology of the secondary phase Dr. Tarun Nanda, MED, TIET, Patiala Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Module: BASIC TERMS IN INDUSTRIAL METALLURGY Dr. Tarun Nanda, MED, TIET, Patiala BASIC TERMS IN INDUSTRIAL METALURGY Session 8 Contents Covered 1. Significance of phases 2. How do the phases affect the properties? 3. Primary phase (continuous/background phase) 4. Secondary phase (discontinuous phase) 5. How do primary and secondary phases affect the properties? Dr. Tarun Nanda, MED, TIET, Patiala  SIGNIFICANCE OF PHASES The main constituents of the microstructure are the phases present and their details. So, mechanical properties which any metal/alloy system possesses under any given conditions (of pressure, temperature, composition etc.) mainly depend on the phases present in the microstructure of that metal/alloy system under those conditions. Phases affect the properties of a metallic system by two main ways: 1. Volume fraction of the continuous phase 2. Morphology of the secondary phase(s) Dr. Tarun Nanda, MED, TIET, Patiala  SIGNIFICANCE OF PHASES Volume fraction of the continuous phase ⁃ Phase which occupies maximum volume (area) fraction in the microstructure (volume fraction means the %volume or %area occupied by a phase in the microstructure). ⁃ Also referred to as continuous phase or background phase. ⁃ Affects properties like strength, hardness, ductility etc. of metals/alloys. ⁃ Affects properties by its volume fraction. Dr. Tarun Nanda, MED, TIET, Patiala  SIGNIFICANCE OF PHASES Morphology of the secondary phase(s) ⁃ Phase(s) other than the primary phase (means all the phases whose volume fraction is not maximum in the microstructure) is/are referred to as secondary phase(s). ⁃ Also referred to as discontinuous phase. ⁃ Affects properties like machinability, toughness etc. ⁃ Affects properties by its morphology (i.e. shape, size, and distribution). Dr. Tarun Nanda, MED, TIET, Patiala  SIGNIFICANCE OF PHASES Phases affect the properties of a metallic system by two main ways: 1. Volume fraction of the continuous phase 2. Morphology of the secondary phase(s)  Ferrite phase has - Extreme softness; least hardness (30 HRB) - Moderate tensile strength (235 MPa) - Extreme ductility (50% on 5 cm gauge length)  Cementite phase has - Highest hardness (65 HRC) - Extreme brittleness (~ 0% ductility) Microstructure of this steel is such that it will have: - Very low hardness, moderate tensile strength, very high ductility - Poor toughness, good machinability Figure 8.1 Schematic microstructure of a typical steel Dr. Tarun Nanda, MED, TIET, Patiala  SIGNIFICANCE OF PHASES Phases affect the properties of a metallic system by two main ways: 1. Volume fraction of the continuous phase 2. Morphology of the secondary phase(s) Microstructure of this steel is such that it Microstructure of this steel is such that it will have: will have: - Very low hardness, moderate tensile - Very low hardness, moderate tensile strength, very high ductility strength, very high ductility - Poor toughness, poor machinability - Moderate toughness, good machinability Figure 8.2 Schematic microstructures of typical steels Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) d a an N Module: ALLOY SYSTEMS n ru Ta r. D Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Dr. Tarun Nanda, MED, TIET, Patiala 1  ALLOY SYSTEMS a Session 9 d an Contents Covered 1. Meaning of phase diagrams N 2. Are phase diagrams valid for industrial conditions? n ru 3. Classification of phase diagrams Ta r. D Dr. Tarun Nanda, MED, TIET, Patiala 2  PHASE DIAGRAMS Phase diagrams defined: Phase diagram is a graphical representation a of what different phases are present in a materials system at various d an temperatures, pressures and compositions. N Also referred to as CONSTITUTIONAL DIAGRAMS or EQUILIBRIUM DIAGRAMS. n ru Ta For alloys, pressure is generally assumed to be constant at one atmospheric pressure, and so the phase diagrams show phase changes due to variations in r. temperature and composition. D 3 Dr. Tarun Nanda, MED, TIET, Patiala  PHASE DIAGRAMS For one component systems (composition is fixed; e.g. all pure substances like water, pure a metals etc.), phase diagram is drawn with temperature along the ordinate (Y-axis) and d pressure along the abscissa (X-axis) [composition = constant]. an N n For alloys (composition is NOT fixed but pressure is assumed fixed at 1 atm), phase ru diagram is drawn with temperature along the ordinate (Y-axis) and composition along the Ta abscissa (X-axis) [pressure = constant]. r. D 4 Dr. Tarun Nanda, MED, TIET, Patiala  PHASE DIAGRAMS Phase diagrams show the phase relationships only under equilibrium conditions. d a However, in actual industrial practice, materials are rarely heated and cooled under an equilibrium conditions. Materials are rapidly heated/cooled for productivity reasons. N n So actually, phase changes occur at slightly higher or lower temperatures than those shown in phase ru diagram for a given materials system. Also at room temperature etc., the system may contain Ta phases other than what are being shown in the phase diagram for the system. r. But still the study of phase diagrams is very important to have an insight as to how the D microstructure of a materials system can be controlled. 5 Dr. Tarun Nanda, MED, TIET, Patiala  CLASSIFICATION OF PHASE DIAGRAMS a Phase diagrams are classified on the basis of the number of d components present in a materials system. Phase diagrams are of an three main types: N n 1. Unary phase diagrams (equilibrium diagrams for one component systems) ru 2. Binary phase diagrams (equilibrium diagrams for two component systems) Ta 3. Ternary phase diagrams (equilibrium diagrams for three component systems) r. D 6 Dr. Tarun Nanda, MED, TIET, Patiala  d a an N n ru Ta r. D 7 Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) d a an N Module: ALLOY SYSTEMS n ru Ta r. D Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Dr. Tarun Nanda, MED, TIET, Patiala 1  ALLOY SYSTEMS a Session 10 d an Contents Covered 1. Unary phase diagrams N 2. Checking the accuracy of unary phase diagrams n ru Ta r. D Dr. Tarun Nanda, MED, TIET, Patiala 2  UNARY PHASE DIAGRAMS Unary Phase diagrams defined: Unary diagrams are phase diagrams for a materials systems having only one component (e.g. pure substances). d an N In single component systems, composition is NOT a variable and there are n only two variables (temperature and pressure) which generally affect the ru phases present in a system. Ta So, unary equilibrium diagram is a diagram between temperature and r. D pressure (it is a graphical representation of what different phases are present in a unary system under different conditions of temperature and pressure). 3 Dr. Tarun Nanda, MED, TIET, Patiala UNARY PHASE DIAGRAMS d a an N n ru Ta r. D Figure 10.1 Phase diagram of pure iron 4  CHECKING ACCURACY OF PHASE DIAGRAMS CASE 1. C = 1; P = 1 a Let materials system (pure iron) be existing at point (y) d defined by [Py , Ty] an Lies in single phase region (α phase) Pressure can be disturbed from the existing value Py to a newer value Py’ without any dependence on N temperature, without any phase change. So, pressure is a DOF or F. n Similarly, temperature could have be disturbed from ru the existing value Ty to a newer value without any dependence on pressure, without any phase change. Ta So, temperature is also a DOF. Thus, when a unary system has one phase in it, the system possesses two degrees of freedom. Both, r. pressure and temperature are degrees of freedom D (F = 2). Same would be the case for other single phase regions. Figure 10.2 Phase diagram of pure iron (case: one phase region) Dr. Tarun Nanda, MED, TIET, Patiala 5  CHECKING ACCURACY OF PHASE DIAGRAMS Now applying Gibb’s phase rule to this d a single phase region: an P+F=C+2 1+F=1+2 N F=2 n ru Ta Since the result for DOF obtained for the single phase regions from the diagram r. and from the Gibb’s rule match, the D diagram is absolutely correct with regard to single phase regions. 6 Figure 10.2 Phase diagram of pure iron (case: one phase region) Dr. Tarun Nanda, MED, TIET, Patiala  CHECKING ACCURACY OF PHASE DIAGRAMS CASE 2. C = 1; P = 2 a Let materials system (pure iron) be existing at point (z) d defined by [Pz , Tz] Lies in two phase region (α + γ) an Pressure can be disturbed from the existing value Pz to a newer value Pz’ without any dependence on temperature, N without any phase change. So, pressure is a DOF or F. However, when pressure is changed independently, n temperature has to be changed so as to remain on (α + ru γ) line ‘ab’ and this change in temperature is dependent on change in pressure. Thus, if pressure is used as DOF, Ta temperature is a dependent variable. Similarly, temperature could have been used as a DOF, but then pressure would become dependent on it. r. Thus, when a unary system has two phases in it, the system possesses only one degree of freedom, either D pressure or temperature (F = 1). Same would be the case for other two phase regions. Figure 10.3 Phase diagram of pure iron (case: two phase regions) Dr. Tarun Nanda, MED, TIET, Patiala 7  CHECKING ACCURACY OF PHASE DIAGRAMS a CASE 1. C = 1; P = 3 d Two triple points in the diagram at points ‘b’ an and ‘g’. The system achieves this three phase N condition at unique combinations of pressure and temperature. Two such unique n combinations for pure iron at points ‘b’ and ru ‘g’. For each for these points, neither pressure nor temperature can be changed. So, neither Ta pressure nor temperature is a DOF. Thus, when a unary system has three phases coexisting in it, the system possesses zero r. degrees of freedom (F = 0). D Thus points ‘b’ and ‘g’ are invariant points. Figure 10.4 Phase diagram of pure iron (case: three phase region) 8 Dr. Tarun Nanda, MED, TIET, Patiala  d a an N n ru Ta r. D 9 Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) d a an N Module: ALLOY SYSTEMS n ru Ta r. D Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Dr. Tarun Nanda, MED, TIET, Patiala 1  ALLOY SYSTEMS a Session 11 d an Contents Covered 1. Binary phase diagrams N 2. Limitations of pure metals n ru 3. General procedure of alloying 4. Alloys defined Ta 5. Classification of phases in alloys based on properties r. D Dr. Tarun Nanda, MED, TIET, Patiala 2  BINARY PHASE DIAGRAMS Binary Phase diagrams defined: Binary equilibrium diagrams are phase a diagrams for materials systems having two components in them (i.e. binary alloys). d an N In two component systems, composition is also a variable and there are three variables (temperature, pressure, and composition) which can affect the phases n ru present in the system. However, pressure is generally fixed as constant at 1 atm. Ta r. So, binary equilibrium diagram is a diagram between temperature and composition D (it is a graphical representation of what different phases are present in a binary alloy under different conditions of temperature and composition). 3 Dr. Tarun Nanda, MED, TIET, Patiala  BINARY PHASE DIAGRAMS Need of Alloys: Pure metals are rarely used for engineering applications. a Only used when properties like high electrical conductivity, high corrosion d an resistance, high ductility are required. However, when other mechanical properties are required viz. strength, hardness, toughness etc. pure metals find limited use and N their properties are improved by alloying. n ru Alloys defined: An alloy is a materials system which has metallic properties Ta and is formed by the intimate combination of a metal with one or more than r. one metals and or non-metals (metal atoms must dominate in composition and D metallic bonding in crystal structure) 4 Dr. Tarun Nanda, MED, TIET, Patiala  BINARY PHASE DIAGRAMS Examples of alloys: Steels and cast iron are alloys of iron and carbon. Brass a is an alloy of copper and zinc. d an N General procedure for alloying n (most common method is melting, higher melting point constituent is ru melted and other element are then added. Other methods are sintering, sublimation, electrolysis) (a metallurgical useful alloy is formed only when the constituent elements dissolve in each other in the liquid state and form a completely homogeneous liquid solution. Ta Exceptions include undissolved lead in free cutting alloy etc.) r. Classification of phases observed in alloys on the basis of D properties/characteristics shown by phases. 5 Dr. Tarun Nanda, MED, TIET, Patiala CLASSIFICATION OF PHASES (based on properties) d a an N n ru Ta r. D 6 Figure 11.1 Various categories of phases in alloys based on properties of phases  CLASSIFICATION OF PHASES (based on properties) SOLID SOLUTION explained: a  Suppose forming a binary alloy with two elements X (solute; FCC) and Y (solvent; BCC). d an  Homogeneous liquid solution of these two elements obtained. N n  In liquid solution, no crystal structure exists. ru Ta  After solidification, if only one crystal structure exists (generally that of the solvent; here BCC) throughout the solid formed, the resultant phase is called a solid solution. r. D  Atoms of both the elements are present in this crystal lattice of the resultant solid at the lattice points in proportion to their original weight concentrations. 7  CLASSIFICATION OF PHASES (based on properties) SOLID SOLUTION defined: Solid solution is a solution in the solid state in which atoms of a the solute element do not destroy the crystal structure of the solvent element but become a d an part of it. Thus, solid solution is a solution in the solid state in which atoms of all the constituent elements share the same crystal structure. N n ru Ta r. D Figure 11.2 Atoms of different elements sharing the same crystal structure in a solid solution 8  CLASSIFICATION OF PHASES (based on properties) d a an N n ru Ta Figure 11.2 Atoms of different elements sharing the same crystal structure in a solid solution r.  The size and electronic structure of the solute and solvent atoms is always different. On formation of solid D solution, the crystal lattice of the solid solution (i.e. solvent element) is always distorted. This distortion interferes with the movement of dislocations on the slip planes and enhances the properties like strength, hardness etc. of the solid solution alloy. This is the primary reason which strengthens the metals on alloying.  d a an N n ru Ta r. D 10 Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) d a an N Module: ALLOY SYSTEMS n ru Ta r. D Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Dr. Tarun Nanda, MED, TIET, Patiala 1  ALLOY SYSTEMS a Session 12 d an Contents Covered 1. Flow chart showing classification of phases N 2. Substitutional solid solutions n ru 3. Interstitial solid solutions Ta 4. Conditions for formation of interstitial solid solutions r. D Dr. Tarun Nanda, MED, TIET, Patiala 2 CLASSIFICATION OF PHASES (based on properties) d a an N n ru Ta r. D 3 Figure 11.1* Various categories of phases in alloys based on properties of phases  SUBSTITUTIONAL SOLID SOLUTIONS SUBSTITUTIONAL SOLID SOLUTION defined: In a solid solution, if the atoms of the solute element a substitute (i.e. replace) the atoms of the solvent element in the crystal structure of the solvent element, the solid d solution is called ‘substitutional solid solution’. The substituted or replaced solvent atoms join the lattice an elsewhere. N n ru Ta r. D 4 Figure 11.2 Solute atoms substituting for a few solvent atoms in the crystal lattice of solvent element in a substitutional solid solution  INTERSTITIAL SOLID SOLUTIONS INTERSTITIAL SOLID SOLUTION defined: In a solid solution, if the atoms of the solute element do NOT substitute a (i.e. do NOT replace) the atoms of the solvent element but occupy randomly, interstitial or interatomic spaces in-between the d solvent atoms in the crystal structure of the solvent element, the solid solution is called ‘interstitial solid solution’. The an substituted or replaced solvent atoms join the lattice elsewhere. N n ru Ta r. D 5 Figure 11.3 Solute atom occupying interstitial position/void in crystal lattice of solvent element in an interstitial solid solutionA  INTERSTITIAL SOLID SOLUTIONS Conditions for formation of interstitial solid solutions: a  Since the atoms of the solute elements have to occupy interstitial spaces, their atomic size ought to be very small. As a d general rule, atomic radii of solute atoms should be less than 1 A°. For this reason, only a few elements can act as solutes for an interstitial solid solutions. N  Boron, carbon, nitrogen etc. have small sized atoms and can act as interstitial solutes for solvent metals. Examples include steels, where carbon is interstitially dissolved in iron. n ru Ta r. D Figure 11.4 Example of an interstitial solid solution Figure 11.5 Example of a solid solution in multi-component alloy  d a an N n ru Ta r. D 7 Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) d a an N Module: ALLOY SYSTEMS n ru Ta r. D Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Dr. Tarun Nanda, MED, TIET, Patiala 1  ALLOY SYSTEMS Session 13 d a Contents Covered an 1. Flow chart showing classification of phases N 2. Intermediate phases n 3. Interstitial compounds ru 4. Hume Rothery compounds Ta 5. Intermetallic compounds r. D Dr. Tarun Nanda, MED, TIET, Patiala 2 CLASSIFICATION OF PHASES (based on properties) d a an N n ru Ta r. D 3 Figure 11.1* Various categories of phases in alloys based on properties of phases  INTERMEDIATE PHASES INTERMEDIATE PHASE explained: a Suppose an alloy is made from two elements, M (solvent) and N (solute). d an Elements M and N are such that they form a solid solution. Further, suppose that amount of solute N taken is more in quantity than its solid solubility is in N solvent M. n ru In such a case, in addition to the solid solution of N in M (the first phase), a Ta second phase will also appear. In a few cases, this second phase is again a solid r. solution, but now, solid solution of M in N (means M acting as solute and N D solvent). However, generally this second phase is a chemical compound called Intermediate phase or Transitional phase. 4  INTERMEDIATE PHASES - Interstitial Compounds Interstitial compounds (Size factor compounds) a d Formation: In the formation of an interstitial solid solution, if the amount of solute taken is more than an its solid solubility in the solvent, the excess of solute combines with the solvent to form an intermediate phase called interstitial compound (this compound precipitates out of interstitial solid solution). N Composition: Formed between the transition metals (Sc, Ti, W, Fe) with small sized atom based n elements of B, C, N etc. ru Crystal Structure Ta  Crystal structure is different from their parent interstitial solutions. r. 𝑹𝑿 D  Depends on the ratio of ; here, RX is radius of solute atom and RM is radius of solvent atom) 𝑹𝑴 𝑹𝑿  If ≤ 𝟎. 𝟓𝟗, the crystal structure is simple (B.C.C, F.C.C, or H.C.P) 5 𝑹𝑴  INTERMEDIATE PHASES - Interstitial Compounds Interstitial compounds (Size factor compounds) a Crystal Structure: d 𝑹𝑿 an  If > 𝟎. 𝟓𝟗, the crystal structure is complex 𝑹𝑴 𝑹  For example, Fe3C is an interstitial compound. The 𝑿 ratio for this phase is 0.63. Since, it is more than N 𝑹𝑴 0.59, iron carbide (cementite) has a complex crystal structure. The crystal structure is basically orthorhombic with 4 carbon atoms and 12 iron atoms per unit cell. n ru Characteristics: True alloys having metallic properties.  High melting points  Exceptionally high hardness Ta r.  Always formed at a fixed composition of their constituents like conventional compounds D Examples: Fe3C, Fe4N, TiC, TiN etc. 6 INTERMEDIATE PHASES – Hume Rothery Compounds Hume Rothery compounds (Electron compounds) a Formation: Hume Rothery compounds are intermediate phases which are formed at a d an definite value of electron ratio. Electron ratio is defined as the ratio of number of valence electrons to the number of atoms. Electron ratio can have three specific values only which N 𝟑 𝟕 𝟐𝟏 are , ,. 𝟐 𝟒 𝟏𝟑 n ru Crystal Structure 𝟑 Ta  Crystal structure depends on the electron ratio.  Electron ratio of generally results in B.C.C. but can also result in H.C.P or complex crystal structure. r. 𝟐 𝟕 D  Electron ratio of results in H.C.P crystal structure. 𝟒 𝟐𝟏  Electron ratio of results in complex crystal structure. 𝟏𝟑 7 INTERMEDIATE PHASES – Hume Rothery Compounds Hume Rothery compounds (Electron compounds) a d Examples an  Ag Zn compound. Total number of atoms is ‘1+1=2’. Total number of valence electrons is ‘1+2=3’. 𝟑 N ER = 𝟐 𝟐𝟏 n  Cu9Al4. Total number of atoms is ‘9+4=13’. Total number of valence electrons is ‘9×1+4×3=21’. ER = 𝟏𝟑 ru Ta Characteristics: True alloys having metallic properties.  Exceptionally high ductility r.  Low hardness D  NOT bound to obey the rules of chemical valence. Can get formed over a wide range of compositions of the same constituents. 8  INTERMEDIATE PHASES - Intermetallic Compounds Intermetallic compounds (Valency compounds) a Formation: Intermetallic compounds are intermediate phases which are formed between d chemically dissimilar metals. The metals combine according to the rules of chemical valence. an  Since get formed between chemically dissimilar metals, so generally have strong ionic and covalent bonds. N Crystal Structure n  Crystal structure is also complex. ru Ta Characteristics: Essentially non-metallic properties.  Because of the strong ionic and covalent bonds, these have very high melting points (higher than the constituent r. elements).  Poor ductility D  Poor electrical conductivity Examples: CaSe, Cu2Se, Mg2Sn, Mg2Pb 9  d a an N n ru Ta r. D 10 Dr. Tarun Nanda, MED, TIET, Patiala Online Sessions: Industrial Metallurgy (UME733) d a an N Module: ALLOY SYSTEMS n ru Ta r. D Dr. Tarun Nanda Associate Professor, MED Thapar Institute of Engineering and Technology, Patiala Dr. Tarun Nanda, MED, TIET, Patiala 1  ALLOY SYSTEMS Session 14 d a Contents Covered an 1. Crystallization defined N 2. Mechanism (steps/stages) of crystallization n 3. Nuclei formation ru 4. Grain growth Ta r. D Dr. Tarun Nanda, MED, TIET, Patiala 2  Mechanism of Crystallization Crystallization defined a Crystallization is defined as the process of transition of a metallic system from the liquid state d to the solid state (in simple words, the solidification process of metallic systems). an Stages of crystallization N 1. Nuclei formation n 2. Crystal growth (grain growth) ru Mechanism of crystallization Ta When a materials system is in the liquid state, the atoms do not have any definite arrangement. But, still r. there are some atoms at any instant which are present in positions which exactly correspond to the space lattice positions, they would occupy after solidification. These groups of atoms are called ‘chance groups’ or D ‘chance aggregates’. Life of chance groups depends on temperature of materials system and size of chance groups. 3  Metallic system in the molten state containing randomly oriented atoms da an N Chance group n ru Chance group Ta r. D 4 Mechanism of Crystallization d a an N n ru Ta r. D Figure 14.1 Presence of chance groups in metall