Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Concept of alloying Classification of alloys Introduction to the phase diagram Uses and limitations of phase diagrams Classification of phase diagrams Construction of phase diagrams Reference: 1. Avner. Introduction to Physical Metallurgy, Ch. 5 and 6. Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 02 1
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 Alloy and alloy system Cu Wt % Nickel Ni Alloying element and impurity element 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. 2009 MME 291, Lec 03: Introduction to phase diagrams P 03 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. 2009 MME 291, Lec 03: Introduction to phase diagrams P 04 2
Intermediate Alloy Phases, or Compounds Intermediate chemical composition; different crystal structure Expressed by chemical formula Congruently melting, exhibiting definite melting point (like metals) Intermetallic compounds (Mg 2 Pb, Mg 2 Sn,...) formed between chemically dissimilar metals have strong ionic/covalent bond nonmetallic properties (poor ductility, poor conductivity) complex strucutre Interstitial compounds (Fe 3 C, Fe 4 N, TiC,...) formed between transition metals and H/O/C/N/B small atoms of nonmetals go into the interstitices of metals metallic properties, narrow composition range, extremely hard, high melting point Electron compounds (AgZn, FeAl, Cu 3 Si,...) form structure similar to constituting elements have a definite ratio of valence electrons properties similar to solid solutions (wide composition range, high ductility, low hardness Solid Solutions Solution in the solid state where two kinds of atoms are combined in one crystal lattice. Distortion resulted during formation of solution causes an increase in strength of the alloy. This is the primary basis for strengthening of a metal by alloying. Solubility of solute in solution depends on temperature and pressure of the system, and method of forming. Unsaturated, saturated, and supersaturated solutions Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 06 3
Solid Solutions Substitutional and interstitial solid solutions Substitutional Interstitial Hume-Rothery Rule for forming substitutional solid solution Crystal structure factor Relative size factor Chemical affinity factor Relative valence factor Example: Cu Ni alloy Completely soluble Cu Zn alloy Partially soluble Cu Pb alloy Completely insoluble Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 07 Summary of Possible Alloy Structures Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 08 4
We have seen how the structure of materials control their properties. So it is important that we must know about the structure of a material that has been developed during its manufacture, and the method of controlling (and/or modifying) the structure to enhance its properties. Phase diagram is an important tool for materials scientists that tells which phases are stable in a system under specified conditions (e.g. of temperature, overall composition, pressure) Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 09 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. solid gas liquid liquid-gas equilibria 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. 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 of the parameters of the system (T, P, X). Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 10 5
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. 2009 MME 291, Lec 03: Introduction to phase diagrams P 11 What and how many phases materials possess? Solid, liquid, or gas, (and plasma)? Is it possible to have more than one solid phases? Iron, being an allotropic material, has more than one solid phases: When iron first freezes from its liquid state, it is BCC (δ-iron) As it cools it changes to FCC (γ-iron) Upon further cooling it changes to BCC (α-iron) Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 12 6
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. Phase diagrams will help us to understand and predict the microstructures like the one shown in this page Microstructure of Al-Cu Alloy Microstructure of Cast Iron β phase (lighter) α phase (darker) pearlite (finger print) graphite (grey) Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 13 Equilibrium state and Metastable state A system is at equilibrium if at constant T, pressure and composition the system is stable, not changing with time. The equilibrium state always has the minimum free energy. Equilibrium state requires sufficient time to achieve. When this time is too long g( (due to slow kinetics), another state along the path to the equilibrium may appear to be stable. This is called a metastable state. A system at a metastable state is trapped in a local minimum of free energy, which is not the global one. Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 14 7
Selection of alloys showing enhanced characteristics in: Brazing and soldering Diffusion problems Corrosion Electrical resistivity Manipulation of phase transformations of materials to control their properties Phase diagrams are also known as the equilibrium diagrams. Rate of phase transformations is missing. TTT (Temperature-Time-Transformation) diagrams are a complement to phase diagrams. Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 15 One component (unary) phase diagrams Also known as P-T Tdiagrams. The simple case is Water. How many single-phase regions? How many two-phase regions? Is there any three-, or more-phase regions? Gibb s Phase Rule: F = C P+2 F = # variables C = # components P = # phases Unary phase diagram of water Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 16 8
How does mixing of A into B effect the bond energies and the melting temperature t of the resultant t alloy? Interaction of A and B resulted three bonds: A-A, B-B and A-B bonds. In ideal cases: (A-B) = x (A-A) + (1-x) (B-B) T Alloy = T A + x (T B T A ) where x is mole fraction of A in B Must follow Hume Rothery Rule Example: Copper - Nickel, Silicon - Germanium Completely miscible/soluble phase diagrams Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 17 Ni-Cu phase diagram (completely miscible) 9
Ge-Si phase diagram (completely miscible) When A-B < 0.5 (A-A + B-B) then T Alloy < T A, T B Example: Lead - Tin, Gold - Silicon, Copper - Silver Eutectic phase diagrams Completely immiscible/insoluble phase diagrams Partially miscible/soluble phase diagrams Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 20 10
10/8/2009 Au-Si phase diagram (Completely insoluble) Cu-Ag phase diagram (Partially soluble) 11
Pb-Sn phase diagram (Partially soluble) Au-Ge phase diagram (Partially soluble) 12
When A-B > 0.5 (A-A + B-B) then T Alloy > T A, T B Example: Gallium - Arsenic, Aluminium i - Antimony Intermetallic compound formation Rashid, DMME, BUET. 2009 MME 291, Lec 03: Introduction to phase diagrams P 25 Al-Sb phase diagram 13
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. 2009 MME 291, Lec 03: Introduction to phase diagrams P 27 14