the Phase Diagrams Today s Topics

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1 MME 291: Lecture 03 Introduction to the Phase Diagrams Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Today s Topics Concept of alloying Classification of alloys Introduction to the phase diagram Classification of phase diagrams Construction of phase diagrams Reference: 1. WD Callister, Jr. Materials Science and Engineering: An Introduction, 5 th Ed., Ch SH Avner. Introduction to physical metallurgy, 2 nd Ed., Ch. 6. Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 02

2 Concept of Alloying 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) 60% Ni Monel Note the difference Alloying element and impurity element Cu Wt % Nickel Ni Alloy and alloy system Alloy systems Binary system (Fe-C system, Cu-Zn system) Ternary system (Fe-C-Mn system, Al-Si-Mg system) Property Direction Tensile strength Up Yield Strength Up % Elongation Down Electrical Conductivity Down Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 03 Classification of Alloys Classifications based on: Structure (e.g., austenitic stainless steel) Type of phase diagram (e.g., eutectic alloys) Alloy can be homogeneous or mixture. In the solid state, there are three possible phases exist: Pure metal Intermediate alloy yphase or compound Solid solution If the alloy is a mixture, then any combination of the above three phases is possible in the solid state. Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 04

3 Intermediate Alloy Phases, or Compounds Intermediate chemical composition; different crystal structure Expressed by chemical formula Having a definite melting point (like metals) Example: Mg 2 Pb, Mg 2 Sn, Fe 3 C, TiC, AgZn, Cu 3 Si, etc. Solid Solutions Solution in the solid state where two kinds of atoms are combined in one crystal lattice. Solubility of solute in solution depends on temperature and pressure of the system, and method of forming. Unsaturated, saturated, and supersaturated solutions. Substitutional and interstitial solid solutions. Substitutional Interstitial Substitutional and Interstitial Solid Solutions Hume-Rothery Rule for forming substitutional solid solution Crystal structure factor (same structure) Relative size factor (difference <15%) Chemical affinity factor (no affinity to each other; close electronegativities) Relative valence factor (metal of low valence dissolves into metals of high valence) Example: Cu Ni alloy Completely soluble Cu Zn alloy Partially soluble Cu Pb alloy Completely insoluble Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 06

4 Summary of Possible Alloy Structures Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 07 Introduction to Phase Diagrams We have seen how the properties of materials reflect their microstructures. The microstructure are controlled by the composition of the material and how it is processed. So it is important that we must know know about the structure of a material that has been developed during its manufacture, and learn how to control (and/or modify) 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) Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 08

5 Phase diagram is basically a map that presents the domains of stability of phases and the limits of stability of phases in a graphical form. Reading the map will tell you, at the state when it comes to equilibrium, 1. what phases are present, 2. the state of those phases, and 3. the relative quantities of each phase. solid liquid gas liquid-gas equilibria Temperature Typical phase diagram for one component system Reading a phase diagram will also tell what phase transformations we can expect when we change one or more parameters of the system (T, P, X). Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 09 Definitions and Basic Concepts Components Chemically recognisable species that are mixed to form the alloy. In Brass: Cu, Zn (element) In steels: Fe, C (element) In ceramics: SiO 2, Al 2 O 3 (compound) Binary alloy contains 2 components, ternary 3, etc. Phase A phase is a homogenous, physically distinct and mechanically separable portion of the material with a given chemical composition and structure. Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 10

6 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 freezes at 1538 C from its liquid state, the first solid formed has BCC structure (which is known as δ-iron) ) As it cools down to 1401 C, it changes to FCC structure (which is known as γ-iron) Upon further cooling down to 910 C, it changes again to BCC structure (which is known as α-iron) Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 11 Microstructure 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. Microstructure of Al-Cu Alloy Microstructure of Cast Iron β phase (lighter) α phase (darker) pearlite (finger print) Phase diagrams will help us to understand and predict the microstructures like the one shown in this page. graphite (grey) Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 12

7 Equilibrium state and Metastable state A system is in equilibrium if at constant T, P and X, 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 slow kinetics), another state along the path to the equilibrium may appear to be stable. This is called a metastable t state. t A system at a metastable state is trapped in a local minimum of free energy, which is not the global one. Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 13 Advantages of Using Phase Diagrams Selection of alloys having enhanced properties for a specific set of applications. Manipulation of phase transformations of materials to control their properties. Limitations of Using Phase Diagrams Phase diagrams are also known as the equilibrium diagrams; structures produced by non-equilibrium cooling cannot be explained. Rate of phase transformations is missing; TTT (Temperature- Time-Transformation) diagrams are more useful in this regard. Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 14

8 Classification of Phase Diagrams One component (unary) phase diagrams Also known as P-T diagrams. The simple case is Water. How many single-phase regions? How many two-phase regions? Is there any three-, or morephase regions? Gibb s Phase Rule: F = C P+2 F = # variables C = # components P = # phases Unary phase diagram of water Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 15 Binary (two-component) phase diagrams Type 1 (COMPLETELY SOLUBLE) Binary Phase Diagrams Ni-Cu phase diagram

9 Type 2 (COMPLETELY INSOLUBLE) Binary Phase Diagrams Au-Si phase diagram Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 17 Type 3 (PARTIALLY SOLUBLE) Binary Phase Diagrams Pb-Sn phase diagram Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 18

10 Type 4 (INERMEDIATE COMPOUND FORMING) Binary Phase Diagrams Al-Sb phase diagram Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 19 Construction of Phase Diagrams Theoretical Construction By applying thermodynamic principles Use of software like Thermocalc Experimental Methods Thermal 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 Metallographic method Heating samples of an alloy to different temperatures, and quench them after equilibrium to retain the high-temperature structure Observe the structure microscopically Rapidly cooled samples do not always retain high-temperature structures; considerable skill is required to interpret the microstructure correctly Rashid, DMME, BUET MME 291, Lec 03: Introduction to phase diagrams P 20

11 Next Class MME 291: Lecture 04 Interpretation of the Phase Diagram