BAB 1

BAB 1JF302 MATERIAL TECHNOLOGY 1Material Structure and Binary Alloy System111.11 Terminologya. Atom

The smallest component of an element having thechemical properties of the element, consisting of a nucleuscontaining combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction; the number of protons determines the identity of the element.

b. Element

Consists of only one kind of atom, Cannot be broken down into a simpler type of matter by either physical or chemical means, andcan exist as either atoms (e.g. argon) or molecules (e.g., nitrogen).

21.0 Material Structure and Element Periodical Table (EPT) c. Mixture

Consists of two or more different elements and/or compounds physically intermingled, Can be separated into its components by physical means, and Often retains many of the properties of its components.

d. Compound

Consists of atoms of two or more different elements bound together, Can be broken down into a simpler type of matter (elements) by chemical means (but not by physical means),

31.12 Atomic number and atomic massAtomic number The number of protons in the nucleus of an atom. Since atoms are electrically neutral, the numbers of protons equal the number of electrons in an atom.Atomic number = Number of protons = Number of electrons

Atomic Mass The sum of the number of protons and neutrons in the nucleus of an atom.Mass number = Number of protons + Number neutrons41.1.3 Total number of atomic orbits/ShellsThe electronic structure of an atom shows/tells us how the electrons in an atom are arranged in their electron shells. The rule below applies;

5ElectronShell 2nd shell3rd shellThe 1st shell can contain a maximum of 2 electrons.The 1st shell is the shell closest to the nucleusThe 2nd shell can contain a maximum of 8 electrons.The 3rd shell can contain a maximum of 8 electrons6ELEMENT PERIODIC TABLE

1.1.4 Elements sequence in EPT

Each element in the periodic table is characterized by its atomic number.An element is composed of atoms. All the atoms in one element are identical and similar. All elements in periodic table are arranged in increasing atomic number.Horizontal columns in the periodic table are call groups. Vertical rows are called Periods. There are 8 groups (Groups I-VIII) and 7 periods.Elements in Groups I-III are metals(except for Boron) and Groups V-VIII consist mainly of non-metals. Elements in Group IV contain both metals and non-metals.All elements in the same GROUP have the same number of valence electrons.

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1.1.4 Elements sequence in EPT

7Nitrogen142:5Atomic massSymbolElement Electron configuration Atomic numberN91.15 CRYSTAL STRUCTURE

8 atom at the corner of cube x 1/8 = 1 atom

8 atom at the corner of cube x 1/8 = 1 atom1 atom at the middle of cube = 1 atomtotal = 2 atom

EXAMPLE: SC= table salt (NaCl)EXAMPLE : BCC = chromium, Molybdenum, titanium and tungsten.a. Simple Cubeb. Body Cubic Centre10

8 atom at the corner of cube x 1/8 = 1 atom6 atom at the face of cube x = 3 atomtotal = 4 atom12 atom at the corner of atom x 1/6 = 2 atom2 atom at the face of cube x 1/2 = 1 atom3 atom at the middle = 3 atomtotal = 6 atomEXAMPLE: FCC = aluminum, Cuprum, GOLD and nikel.EXAMPLE: HCP = Berilium, magnesium and zink.c. Face Cubic Center (FCC)d. Hexagonal closed packed(FCC)Crystals - A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions.Lattice The smallest group of atoms showing the characteristic lattice structure of a particular metal is known as a unit cell. It is the building block of crystal, and a single crystal can have many unit cell.Grain - A given block of metal may contain millions of individual crystals, called grains. Each grain has its own unique lattice orientation.Grain boundary - When the block is cooled from the molten state and begin to solidify, nucleation of individual crystals occurs at random positions and orientations throughout the liquid. As the crystals grow the finally interfere with each other, forming at their interface a surface defect a grain boundary.

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1.1.6 Crystallization process

12a) Ionic bond / electrovalent

One electron is completely transferred. The electrostatic attraction is what holds the iron together.NaClAtoms lose or gain electrons to attain a complete outer shell of electrons. An ionic bond is formed when electrons are lost and gained by two or more atoms. When atoms lose electrons they become positive ionsWhen atomsgain electrons they become negative ions Ionic bonds are formed between metals and non - metals. 1.1.7 Type of bonds13b) Covalent bond

(a) Cl2 . (b) O2.Cl Cl OOCovalent chemical bonds involve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons in ionic bonds. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.

Hydrogen gas forms the simplest covalent bond in the diatomic hydrogen molecule. The halogens such as chlorine also exist as diatomic gases by forming covalent bonds14c) Metallic bond

The properties of metals suggest that their atoms possess strong bonds, yet the ease of conduction of heat and electricity suggest that electrons can move freely in all directions in a metal. The general observations give rise to a picture of "positive ions in a sea of electrons" to describe metallic bonding. The metallic bond accounts for many physical characteristics of metals, such as strength, malleability, ductility, conduction of heat and electricity.151.2 Solidification of Metal and Alloy(a)Necleus(b)Dendrite(d)Grain(c)Dendrite

When the temperature of the liquid metal has dropped sufficiently below its freezing point, stable aggregates or nuclei appear spontaneously at various points in the liquid. As cooling continues, more atoms tend to freeze, and they may attach themselves to already existing nuclei or form new nuclei of their own. Each nucleus grows by the attraction of atoms from the liquid into its space lattice. Crystal growth continues in three dimensions, the atoms attaching themselves in certain preferred directions, usually along the axes of a crystal. This gives rise to a characteristic treelike structure which is called dendrite. Since each nucleus is formed by chance, the crystal axes are pointed at random and the dendrites will grow in different directions in each crystal. Finally, as the amount of liquid decreases, the gaps between the arms of the dendrite will be filled and the growth of the dendrite will be mutually obstructed by that of its neighbors.

1.2.1 Solidification phase1.2.2 Differentiation between metal and alloy

Pure Metal is made up of only one element ( Periodic table of elements ).Alloy is a mixture of metals, or metal and non-metal, for example Brass is an alloy which is made up of copper and zinc, two different elements. Both alloy and metal are good conductors of heat, but alloy has a relatively high melting point when compared to that of metals

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1.2.3 Formation of solid solution

A solid solution is formed when two metals are completely soluble in liquid state and also completely soluble in solid state. In other words, when homogeneous mixtures of two or more kinds of atoms (of metals) occur in the solid state, they are known as solid solutions. The more abundant atomic form is referred as solvent and the less abundant atomic form is referred as solute.For example sterling silver (92.5 percent silver and the remainder copper) is a solid solution of silver and copper. In this case silver atoms are solvent atoms whereas copper atoms are solute atoms. Another example is brass. Brass is a solid solution of copper (64 percent) and zinc (36 percent). In this case copper atoms are solvent atoms whereas zinc atoms are solute atoms.171.2.4 Substitutional and Interstitial Solid Solution

Substitutional solid solutions.If the atoms of the solvent or parent metal are replaced in the crystal lattice by atoms of the solute metal then the solid solution is known as substitutional solid solution. For example, copper atoms may substitute for nickel atoms without disturbing the F.C.C. structure of nickel.

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1.2.4 Substitutional and Interstitial Solid Solution

(b) Interstitial solid solutions.

In interstitial solid solutions, the solute atom does not displace a solvent atom, but rather it enters one of the holes or interstices between the solvent atoms. An excellent example is iron-carbon system19

Solidification of pure metals

A pure metal solidifies at one fixed temperature, a fact which can be checked by plotting a cooling curve. A cooling curve may be obtained by melting a small amount of a metal and recording the temperature drop at suitable time intervals as this metal solidifies (the metal must be allowed to cool very slowly i.e. under equilibrium conditions) . We can then plot a graph of temperature against time to give us the cooling curve for that particular metal201.2.5 Solidification of pure metal and alloy according to the cooling curveSolidification of pure metals

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1.2.5 Solidification of pure metal and alloy according to the cooling curveSolidification of pure metals

At temperatures above and below line "ab" the curve falls smoothly.When the solidification temperature is reached, the temperature remains constant for some time thus giving rise to the step "ab" in the curve. Down to "a"the temperature of the liquid drops in a regular manner as heat is being lost to the surroundings at a nearly constant rate.The step "ab" is due to latent heat. This leads to zero change in temperature until the last drop of liquid has solidified. After no more latent heat is available the solid continues to cool in a regular manner giving the smooth curve "bc". 221.2.5 Solidification of pure metal and alloy according to the cooling curveb) Solidification of Alloy

Unlike pure metals alloys solidify over a range of temperatures.Below the temperature at which the alloy begins to solidify and the temperature when it is completely solidified the alloy gradually becoming stiffer as the lower limit of the solidification range is approached. Therefore for any alloy there is a definite temperature at which solidification begins and an equally definite point where it ends. These two points are known as the "arrest points". As two metals may be alloyed in many different compositions i.e. you could have 80% A and 20% B or 60%A and 40% B it stands to reason that the cooling curves for all these alloys will be different. Shown here are a selection of cooling curves for an alloy of Lead and Tin. 231.2.5 Solidification of pure metal and alloy according to the cooling curveb) Solidification of Alloy

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1.2.5 Solidification of pure metal and alloy according to the cooling curve25

1.2.6 Phase diagram and cooling curve of Solid solutionThe phase fields in equilibrium diagrams depend on the particular systems being depicted. Liquidus - Set of solubility curves that represents locus of temperatures above which all compositions are liquid.Solidus - Represents set of solubility curves that denotes the locus of temperatures below which all compositions are solid.Equilibrium phase diagrams - Represent the relationships between temperature and the compositions and the quantities of phases at equilibrium. Composition - the percentage content of certain ingredients put in a substance. With the presence of percent composition of these materials can cause changes in phases, properties and the micro structure of materials.261.2.6 Phase diagram and cooling curve of Solid solution