2nd Lecture S.Physical Metallurgy Principles.pptx

2nd Lecture S.Physical Metallurgy Principles.pptx

1. Physical Metallurgy Principles The Physical Metallurgy Principles covers crystal structures, solidification , equilibrium phase diagrams , transformation diagrams, diffusion, liquid to solid transformations , recovery and Recrystallization. Objective. To provide a basic understanding of the underlying principles that determines the evolution the evolution of structures in metals and alloys during their processing and its relation with their properties & performance in service.

2. Atomic Structure and Crystal Structure The most important aspect of any engineering material is its structure, because its properties are closely related to this feature. To be successful, a materials engineer must have a good understanding of this relationship between structure and properties. By way of illustration. In order to understand the structure of materials and its correlation to property, we have to start from the basic element of matter – The Atom An atom consists of a nucleus composed of protons and neutrons and electrons which encircle the nucleus. Protons and electrons have same and opposite charge of 1.6 x10-19 C. Atomic number (Z) = Number protons = number of electrons. Atomic mass (A) = proton mass + neutron mass Isotopes are the same element having different atomic masses . Number of protons in isotopes remains same while number of neutrons varies .

3. Atomic mass unit (amu) = 1/12 mass of Carbon 12 (12C) 1 mol of substance contains 6.023 x 10 23 (Avogadro’s number) atoms or molecules. Atomic weight = 1 amu/atom (or molecule) = 1 g/mol = Wt. of 6.023 x 1023 atoms or molecules. For example, atomic weight of copper is 63.54 amu/atom or 63.54 g/mol As we know, common to all materials is that they are composed of atoms. The properties (whether mechanical, electrical, chemical etc ) of all solid materials are dependent upon the relative positions of the atoms in the solid (in other words the atomic structure of the material) and their mutual interaction i.e. the nature of the bonding (whether e.g. covalent, ionic, metallic, van der Waals).

4. Crystal structure Metals are usually crystalline when in the solid form. The crystal structure directly influences the properties of the material, the normal metallic object consists of an aggregate of many very small crystals. Metals are therefore polycrystalline. The crystals in these materials are normally referred to as its grains. There is the basic structure inside the grains themselves: that is, the atomic arrangements inside the crystals. This form of structure is logically called the crystal structure. The unit cell of a crystal structure

5. Characterization of the crystallization Because crystals are symmetrical arrays of atoms containing rows and planes of high atomic density, they are able to act as three-dimensional diffraction gratings. If light rays are to be efficiently diffracted by a grating, then the spacing of the grating (distance between ruled lines on a grating) must be approximately equal to the wavelength of the light waves. In the case of visible light, gratings with line separations between 1000 to 2000 nm are used to diffract wavelengths in the range from 400 to 800 nm. In crystals, however, the separation between equally spaced parallel rows of atoms or atomic planes is much smaller and of the order of a few tenths of a nanometer. Fortunately, low-voltage X-rays have wavelengths of the proper magnitudes to be diffracted by crystals. An X-ray beam is reflected with constructive interference when the angle of incidence equals the angle of reflection

6. Solidification Three states of matter are distinguishable: gas, liquid, and solid. As the temperature is decreased, the motions are less vigorous and the attractive forces pull the atoms closer together until the liquid solidifies. Most materials contract upon solidification, indicating a closer packing of atoms in the solid state. Crystallization is the transition from the liquid to the solid state and occurs in two stages: Nuclei formation Crystal Growth. Atoms in a material have both kinetic and potential energy. Kinetic energy is related to the speed at which the atoms move and is strictly a function of temperature. The higher the temperature, the more active are the atoms and the greater is their kinetic energy. Potential energy, on the other hand, is related to the distance between atoms. The greater the average distance between atoms, the greater is their potential energy.

7. S olid solutions When homogeneous mixtures of two or more kinds of atoms occur in the solid state, they are known as solid solutions. The term solvent refers to the more abundant atomic form, and solute to the less abundant. These solutions are also usually crystalline. Solid solutions occur in either of two distinct types. The first is known as a substitutional solid solution, The other type of solid solution is known as an interstitial solid solution . In many alloy systems, crystal structures or phases are found that are different from those of the elementary components (pure metals). A solid solution is simply a solution in the solid state and consists of two kinds of atoms combined in one type of space lattice

8. Intermediate solid solutions and compounds When copper and zinc are alloyed to form brass, a number of new structures are formed in different composition ranges. Most of these occur in compositions which have no commercial value whatsoever, but that which occurs at a ratio of approximately one zinc atom to one of copper is found in some useful forms of brass. The crystal structure of this new phase is body-centered cubic, whereas that of copper is face-centered cubic, and zinc is close-packed hexagonal. Because this body-centered cubic structure can exist over a range of compositions (it is the only stable phase at room temperature between 47 and 50 weight percent of zinc), it is not a compound, but a solid solution . This is also sometimes called a nonstoichiometric compound or a nonstoichiometric intermetallic compound. On the other hand, when carbon is added to iron in an amount exceeding a small fraction of one-thousandth of a percent at ambient temperatures, a definite intermetallic compound is observed. This compound has a fixed composition (6.67 weight percent of carbon) and a complex crystal structure (orthorhombic, with 12 iron atoms and 4 carbon atoms per unit cell) which is quite different from that of either iron (body-centered cubic) or carbon (graphite).

9. A binary alloy system may contain both stoichiometric and nonstoichiometric compounds. For example , aluminum and nickel alloys contain five nickel aluminide intermediate phases designated as Al 3 Ni, Al 3 Ni 2 , AlNi and Al3Ni5, and AlNi3. The first intermediate phase, Al 3 Ni, has a fixed stoichiometric composition of 75 at. % Al and 25 at. % Ni. The other compounds, on the other hand, are nonstoichiometric . A conventional stoichiometric compound is characterized as  one that has an exact and fixed composition with a small integer ratio among its atomic components .   Non-stoichiometric compounds are defined as  any solid chemical compounds in which the number of atoms of the particular elements can not be expressed in a ratio of a small whole number . In this, the atomic ratios are represented in the form of small integers. For example, nitrides, oxides(iron oxide, nickel oxide etc ).

10. Non-stoichiometric compounds are  chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers  (i.e. an empirical formula); most often, in such materials, some small percentage of atoms are missing or too many atoms.  

11. Concept of Alloys 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. An alloy system contains all the alloys that can be formed by several elements combined in all possible proportions. If the system is made up of two elements, it is called a binary alloy system ; three elements, a ternary alloy system ; etc. in each system, a large number of different alloys are possible. If the composition is varied by 1 percent, each binary system will yield 100 different alloys.

12. Alloys may be homogeneous (uniform) or mixtures. If the alloy is homogeneous it will consist of a single phase , and if it is a mixture it will be a combination of several phases. A phase is anything which is homogeneous and physically distinct. Any structure which is visible as physically distinct microscopically may be considered as a phase. A simple system: a single metallic element, for example, copper, a so-called one-component system. Solid copper conforms to the definition of a phase, and the same is true when it is in the liquid and gaseous forms. It can be concluded that each of the three forms of copper solid, liquid, and gas constitutes a separate and distinct phase.

13. Iron and tin, are polymorphic (allotropic) and crystallize in several structures, each stable in a different temperature range. Here each crystal structure defines a separate phase, so that polymorphic metals can exist in more than one solid phase. As an example , consider the phases of iron,. Notice that there are three separate solid phases for iron, each denoted by one of the Greek symbols: alpha ( α ), gamma ( γ ), or delta ( ẟ ). Actually, there are only two different solid iron phases since the alpha and delta phases are identical; both are body-centered cubic.

14. Alloys instead of pure metals as shown before a binary alloys , two- component systems , are mixtures of two metallic elements, while ternary alloys are three- component systems : mixtures of three metallic elements. A system , as used in the sense usually employed in thermodynamics, or physical chemistry, is an isolated body of matter and The components of a system are often the metallic elements that make up the system. Pure copper or pure nickel are by themselves one-component systems , while alloys formed by mixing these two elements are two-component systems. Metallic elements are not the only types of components that can be used to form metallurgical systems; it is possible to have systems with components that are pure chemical compounds. Steels are normally considered to be two-component systems consisting of iron (an element) and iron carbide (Fe 3 C), a compound. In most alloys of commercial interest, the liquid components dissolve in each other to form a single liquid solution.

15. Phase diagrams Phase diagrams, also called equilibrium diagrams or constitution diagrams, are a very important tool in the study of alloys. They define the regions of stability of the phases that can occur in an alloy system under the condition of constant pressure (atmospheric). The coordinates of these diagrams are temperature and composition . The interrelationships between the phases, the temperature, and the composition in an alloy system are shown by phase diagrams only under equilibrium conditions. These diagrams do not apply directly to metals not at equilibrium.

16. In materials science, equilibrium refers to a state in which the properties of a material remain constant over time, assuming that the material is not subjected to any external disturbances. This can include the equilibrium state of phases within a material, such as the balance between solid, liquid, and gas phases in a substance, or the equilibrium between different crystal structures. Understanding equilibrium in materials science is crucial for predicting and controlling the properties of materials in various applications, such as in the design of new materials or in manufacturing processes . The phase diagram at any given temperature gives us the proper picture only if sufficient time is allowed for the metal to come to equilibrium .

17. The simplest of the binary systems is the isomorphous , in which only a single type of crystal structure is observed for all ratios of the components.

18. The Phase diagrams classified into: Those with composition as a variable (e.g. T vs. %Composition)

19. Those without composition as a variable (e.g. P vs. T)