Ferrous metals and their alloys    

Ferrous metals and alloys are based upon the metal iron which is their main constituent. They get their name from the Latin word for iron which is ferrum. Iron is a soft grey metal rarely found in the pure state outside the laboratory.

The engineer usually finds it alloyed, or associated, with the non-metal carbon.  The associa­tion of iron with carbon greatly modifies the behaviour of the iron, making it harder, stronger, and of greater use to the engineer. Slight varia­tions in the amount of carbon present can make very great differences in the properties of the metal. Table 1.5 shows how the addition of varying amounts of carbon to the metal iron can produce a wide range of ferrous metals.

the effect of the carbon content upon the proper­ties of plain carbon steels up to a maximum of 1.2% carbon. The maxi­mum amount of carbon which will combine with iron to form iron-­carbide at room temperature is 1.7 % but, in actual practice, there is little or no advantage in increasing the amount of carbon present above 1.2% and there is always a possibility that as the amount of carbon approaches the maximum some carbon may precipitate out destroy­ing the properties of the steel.

  Plain carbon steels: Plain carbon steels are defined as alloys of iron and carbon in which the iron and carbon are combined at all times. Only the range of fer­rous metals with their carbon contents lying between the theoretical limits of 0.1% and 1.7% satisfy this definition. In practice an upper limit of 1.2% carbon is rarely exceeded.

 Dead mild steel: In dead mild steel the carbon content is deliberately kept as low as possible so that the steel will have a high ductility. This enables it to be pressed into complicated shapes, such as motor car body panels, even whilst it is cold. It is slightly weaker than mild steel and is not usually machined since its softness would cause it to tear and leave a poor finish.

 Mild steel:  This is a widely used material which is relatively cheap and freely available. It is soft and ductile and can be forged, pressed and drawn in the hot or cold condition. It is easily machined using high-speed steel cutting tools. Typical applications are listed.

ALLOYS AND PHASES IN ALLOYS

  Metals in their pure form are seldom used in engineering applications. Most of the useful metallic materials are combinations of metals known as alloys. An alloy is any combination of two or more elements that results in a substance possessing metallic properties.

Elements may combine in different ways to form alloys. The components of the alloys are usually completely soluble in the liquid state. In the solid state the elements may form mechanical mixtures, solid solutions or inter­mediate phases. An alloy may consist of either a single phase, two phases or more than two phases. The element which is present in the largest pro­portion is called the base metal, and all the other elements present are called alloying elements.

 Mechanical Mixtures: Mechanical mixtures are formed when the two elements are completely insoluble in the solid state. For example, lead is essentially insoluble in iron. The alloy of lead and iron is an intimate mechanical mixture of the components where each component retains its own identity, properties and crystal structure.

 Solid Solutions: When two elements mix or dissolve in the solid state, the resulting phase is called a solid solution. If element A dissolves ten per cent (by weight) of element B, then element B is said to have a solid solubility of ten per cent in A. The base metal A is also called a solvent and the alloying element B as solute.

When a solid solution forms, atoms of the solute occupy certain places in the. lattice structure of the solvent. Depending upon the type of the places occupied by the solute atoms, solid solutions could be either substi­tutional or interstitial.

(a) Substitutional Solid Solution: A substitutional solid solution is formed when some of the atoms of solvent are replaced by the solute atoms at their normal lattice points, as shown in Figure 2.2(a). In the formation of substitutional solid solutions, an element A cannot dissolve any amount of element B, its limit (known as solid solubility limit) is determined by certain factors.

The lattice structure of a solid solution is basically that of the solvent with slight changes in lattice parameter. An expansion results, if the solute atom is larger than the solvent atom and a contraction, if the solute atom is smaller.

(b)Interstitial Solid Solution: Interstitial solid solution is formed when atoms of small atomic radii fit into the empty spaces or interstices of the lattice structure of the solvent atoms. Since the empty spaces of the lattice structure are limited in size, only atoms with atomic radii less than I angstrom are likely to form interstitial solid solu­tions.

Interstitial solid solutions normally have limited solid solubility. The well known example of this group is interstitial solid solution of carbon in iron. γ-iron can dissolve upto 2 per cent carbon at 1147°C. This inter­stitial solid solution of carbon in iron is the basis for hardening in steel. Interstitial solid solution of hydrogen in iron formed during acid pickling (cleaning), plating or welding operations with steel causes a sharp decrease in ductility of steel. This harmful phenomenon is known as hydrogen embrittlement.

STRUCTURAL METALS

  Structural metals are those that are used in load bearing metallic frames works or structures.

Knowledge and understanding of the uses, strengths, limitations, and other characteristics of structural metals is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specification, or the sub­stitution of inferior materials, may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsman­ship. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various metals.

  Metals for aircraft structural use: Metals used in the aircraft structures and other application is both ferrous and non-ferrous including their various alloys.

The outstanding structural metals are: (i) Steels and Nickel-base alloys (ii) Aluminium alloy, (iii) Magnesium alloy and (iv) Titanium. Besides, various other non-ferrous metals like copper and its different alloys have widespread applications in aircraft.

In the following sections, ferrous and non-ferrous metals will be discussed.

STRUCTURES, PROPERTIES AND PROCESSING

As discussed in previous week that differences in properties of various materials are due to the differences in the structures of materials. Different materials possess different struc­tures. The structure of a material exhibits its internal and surface details. These details can be examined and expressed in different orders of magni­fication varying from a few times to several thousands. In order of decre­asing magnification, the structure of a solid material can be expressed as follows:

(i) Atomic structure (ii) Crystal structure (iii) Microstructure (iv) Macrostructure

Detail description of these structures started in previous week. This week continues the same.

  MICROSTRUCTURE

 The appearance of the structure of a material under a microscope is called microstructure. Optical microscopes are used for magnifications upto 1000 times while electron microscopes can produce magnifications upto several thousand times. Microstructure of a material consists of phase structure and grain structure.

The phase structure is expressed in terms of various phases present, their relative amounts, distribution and alignment. Depend­ing upon the number of phases present, microstructures are either called single phase or multiphase structures.

The grain structure of a material shows shape and size of the grains (crystals) which form the bulk material. It is characterised by grain boundaries, grain shape and grain size. Typical examples of grain structures are columnar, dendritic and equiaxed grains.

 Phases in Metals: Pure metals consist of identical atoms. These atoms combine together to form crystals. Each crystal has a definite lattice structure and represents a phase. Another crystal having the same lattice structure would have the same phase. Two crystals represent different phases if their lattice struc­tures are different. The lattice structure of a crystal is expressed in terms of lattice parameters and the number of atoms per unit cell.

Metallurgically, a phase is a substance, or a portion of matter, which is homogeneous, physically distinct and mechanically separable. It is homo­geneous in the sense that its two smallest parts cannot be distinguished from one another. Physically distinct and mechanically separable means that the phase will have a definite boundary surface.

The number of phases present in a system is the number of different sub­stances that exist in it. Each substance must be chemically and structurally homogeneous within itself and physically separable by definite boundary surfaces from all dissimilar substances.

 A phase can exist in three different states depending upon the values of a set of quantities, such as, pressure, temperature, etc. These states could be either vapour, liquid or solid.

Since all gases mix with one another in all proportions, there can be only one vapour phase in a system. Two liquids may dissolve in each other to form one phase. On the other hand, if they are essentially insoluble in each other or have limited solubility, they will separate into two distinct liquid phases. In solids, each different type of crystalline substance present forms a separate phase. A solid phase will have a definite arrangement of atoms given by its lattice structure. Each different lattice structure constitutes a different phase. The lattice structure is given by lattice parameters such as a, b, c, α, β and γ  and the arrangement of atoms in the lattice.

When a phase changes its state, it is called a phase change. A phase change is accompanied by a change in pressure or temperature. The phase changes which take place in magnesium metal by changing pressure and temperature are shown in Figure 2.1. The figure shows that solid phase can change directly to vapour phase without going into liquid phase by chang­ing temperature at low pressures.

Allotropy: Many metallic elements change their arrangement of atoms and the lattice structure due to changing external conditions of pressure and temperature. Such a change of phase in solid state is called allotropy or polymorphism. For example, iron at room temperature has body centered cubic (BCC) structure with a lattice parameter of 2.866 A. When the temperature of iron is in­creased and reaches 910⁰ C, rearrangement of atoms 1n the lattice takes place giving rise to lower free-energy to this form of iron. Above this tem­perature iron will have face-centered cubic structure (FCC). Again at 1400°C, iron changes its lattice structure and becomes BCC. At 1539°C iron changes its state and becomes liquid.

Among the non-metallic elements polymorphism is found in phospho­rous (white, black and red forms) and carbon (diamond, graphite, etc.). The diamond structure of carbon is very different from the layer structure of graphite.

CRYSTAL IMPERFECTION

CRYSTAL IMPERFECTION

 Real crystals deviate from the perfect periodicity of atoms which is assumed in an ideal crystal. This deviation is chiefly responsible for the changes in the mechanical and electrical properties of the real crystals. This deviation of atoms from an orderly array of lattice points is termed as defect or imperfection. An understanding of these lattice defects is very important to explain the mechanical behaviour of metals. For example the actual streng­th of polycrystalline material is about 10³ to 10⁵ times lower than the the­oretical strength of an ideal crystal.

Crystal defects or imperfections could be of three types--point defects, line defects and area defects.

 Point Defects: When the deviation from the periodic arrangement of the lattice is locali­zed to the vicinity of only a few atoms, it is called a point defect, or point imperfection. The point defects in crystals could be of three types a vacan­cy, an interstitial and an impurity atom. These defects are . A vacancy or vacant lattice site, exists when an atom is missing from a normal lattice position, (a). In pure metals, a defi­nite number of vacant lattice sites exist at temperatures greater than abso­lute zero. This number increases rapidly with increase in temperature. Rapid quenching from a higher temperature to room temperature increa­ses the number of vacancies than that possible under equilibrium conditions. Number of vacancies can also be increased by extensive plastic deforma­tion (cold working) or as a result of bombardment with high energy nuclear particles. When the density of vacancies becomes relatively large, they may cluster together to form voids.

A point defect is called interstitial when an atom is trapped inside the crystal at a point intermediate between normal lattice positions,  (b).

The presence of a different type of atom either at a lattice position or an interstitial position, the point defect is called an impurity atom.

 Line Defects: If the crystal defect extends through microscopic regions of the crystal, it is called a line defect. The line detects obtain their name because they pro­pagate as lines or as a two-dimensional net in the crystal. Typical examples of line defects are edge and screw dislocations.

The most important line defect is the dislocation. Dislocations exist in all real crystals. Depending upon its nature, the dislocation is termed as either edge or screw type dislocation. An edge dislocation, which is the edge of an incomplete plane of atoms within a crystal, is represented in the cross-section in Figure 2.9(c). In this illustration, the incomplete plane extends part way through the crystal from the top down, and the edge dislocation (which is indicated by the standard symbol T) is its lower edge.

The defect `dislocation' is mainly responsible for the observed low stren­gth in pure metals. A dislocation moves easily on the application of a small amount of force and results in plastic deformation through the phenomenon of slit).

Area Defects: if the crystal defect appears in two dimensions through the microscopic regions of the crystal, it is called an area defect. Typical examples of area defects are stacking faults, twin interfaces and grain boundaries.

Stacking faults are planes where there is an error in the normal sequence of stacking of atom layers. These may be formed during the growth of a crystal or may result from motion of partial dislocations. A partial dis­location produces a movement that is less than a full distance.

Twins are portions of a crystal that have certain specific orientations with respect to each other. The twin relationship may be such that the lattice of one part is the mirror image of that of the other. Twins may occur fre­quently during crystallization from the liquid or the vapour state, by growth during annealing or during phase transformation. -

CRYSTAL STRUCTURE

CRYSTAL STRUCTURE

 Properties of an individual atom are determined by its atomic structure. In this respect, valence electrons play an important role in producing most of its chemical, electrical and optical properties. These atoms combine toge­ther to form crystals. The arrangement of atoms in the interior of a crystal is called crystal structure. This structure is determined by: (a) grouping of the atoms, (b) bonding between them, (c) type of the space lattice formed, (d) parameters of the unit cell, and (e) the number and position of atoms per unit cell. Let us try to understand these factors.

  Grouping of Atoms: Metals are aggregate of atoms. Metallic properties depend, not only on the nature of the atoms but also on the manner in which atoms have been assembled. Depending upon the type of their grouping, materials can be classified into three categories, namely: (i) molecular structures, (ii) crystal struc­tures and (iii) amorphous structures.

(i) Molecular structures: These structures are formed when a limited number of atoms come together and get strongly bonded to one another. The resulting groups are called molecules, for example, H₂O, C0₂, CCl₄, 02, N2, etc. Within these molecules, the atoms are held together by strong attractive forces that usually have covalent or ionic bonds. Similar group of atoms have rela­tively weak bonds among themselves. The groups have no net charge.

(ii) Crystal structures: Atomic arrangements which have a repetitive pattern in all the three dimensions of space are called crystal structures or crystals. In such struc­tures, a fundamental unit of the arrangement repeats itself at regular inter­vals in three dimensions, throughout the interior of the crystal. Most of the metals are crystalline and consist of crystals.

(iii) Amorphous structures are formed when atoms do not have the long range repetitive pattern of arrangement and the pattern breaks at different places. Common examples of this group are glasses and polymers. Most glasses consist primarily of silicate ions, SiO₂, to which an appreciable number of large-sized atoms such as sodium have been added. The added sodium and other atoms, since they do not fit into the silicate structure very well, make it more difficult for crystallization to occur when the melt is cooled. Glass is thus a supercooled liquid having a very high viscosity.

Structures of polymers greatly differ from those of metals. Metals and ceramics are aggregates of atoms which can be regarded as arrays of hard and spherical atoms in three directions of space, while the structure of poly­mers is composed of molecules of extremely high molecular weight which cannot be regarded as hard spheres. Most polymers have the form of long, flexible chains in which molecules are twisted up and inter-twined^- with each other. A typical molecule of polyethylene, for example, might be represented by a chain with a length of about 5 X 10^-2 micron and a dia­meter of less than 5 X 10^-6 micron. A molecule such as this, in a mass of material, can readily become knotted and entangled with the surrounding molecules.

  Binding in Solids: Since most of the metals are solid at room temperature, we shall consider only solids. Even in solid state, materials can have a very wide range of properties. This is partly due to the fact that atoms have different types of bindings among themselves. All the atoms of a crystal have definite types of attractive forces among themselves which keep the atoms bonded toge­ther. The attractive forces could be of the following types.

(a) Metallic Bond: This type of bond results when each atom of the metal contributes its valence electrons to the formation of an electron cloud that spreads throughout the solid metal. A characteristic of the metallic bond is that the conduction of electricity and heat are produced by the free move­ment of valence electrons through the metal. All metal conductors show this type of bond. The resistance to the free movement of the electron occurs     if it collides with other electrons. This collision of the electrons interferes with the flow of the electrons and accounts for the resistivity in metals. Metallic crystals are malleable and have variable hardness and melting point.

(b) Ionic Bond: This bond exists between two unlike atoms. If an electron is transferred from a metallic atom to a non-metallic atom, the two result­ing ions are held together by electrostatic attraction. Examples are the sodium and chlorine atoms. Sodium atom gives away its valence electron and becomes a positive ion, while chlorine atom takes the electron to fill its last orbit and becomes a negative ion. Ionic crystals have poor electrical conductivity, high hardness and high melting point.

(c) Covalent Bond: This bond is formed by sharing of electrons between adjacent atoms. An excellent example of covalent bonding is found in the chlorine molecule. Here, the outer shell of each atom possesses seven elec­trons. Each chlorine atom would like to gain an electron, and thus form a stable octet. This can be done by sharing of two electrons between pairs of chlorine atoms. Each atom contributes one electron for the sharing process. The diamond form of carbon is an another example of this bond where four valence electrons are shared between four neighbouring atoms. Other examples of this type of bond are hydrogen, nitrogen etc. Covalent crystals are characterized by poor electrical conductivity and high hardness.

(d) Van der Waals Bond: Inert gases and molecules like methane, which have no valence electrons available for crystalline binding, obtain a weak attractive force as a result of polarization of electrical charges. Polarization is displacement of the centres of positive and negative charges in an electri­cally neutral atom or molecule when as it is brought close to its neighbouring atom. Its neighbours also become polarized. The resulting weak electrical attraction between neighbouring atoms or molecules is the Van der Waals force. This force can be overcome by the disrupting effect of thermal mo­tion of atoms and molecules at higher temperatures.

Members of the halogen family-fluorine, chlorine, bromine and iodine all form stable diatomic molecules with covalent bonds. The additional forces whⁱch hold the molecules together are of the Van der Waals type. Elements possessing Van der Waals bond are soft, have poor electrical conductivity and low melting point.

Schematic representation of the various types of bonds in solids is shown in Figure 2.1.

Space lattice and unit cell: A crystalline substance is one which is made of crystals or parts of crystals. In a crystal, the atoms are arranged in a periodic and regular geometric pattern in space. The arrangement of atoms in a crystal can be described with respect to a three-dimensional net of straight lines, called a space lattice, as shown in Figure 2.2. The intersections of the lines are points of a space lattice. These points may be occupied by the atoms in crystals or they may be the points about which several atoms are clustered. The impor­tant characteristic of a space lattice is that every point has identical sur­roundings.

ATOMIC STRUCTURE

ATOMIC STRUCTURE

  An atom has a very complex structure which resembles a miniature solar system. For simplicity, it may be considered to be a tiny sphere.

An atom of any element consists of the nucleus and the electrons. The nucleus is a stationary mass, situated at the centre and carrying a net posi­tive charge. It consists of heavy particles-protons and neutrons. Each of these particles are 1836 times heavier than an electron. Each proton carries a positive charge, while the neutron carries no charge.

The electrons revolve around the nucleus in definite orbits. They are bound to the nucleus by different energy levels. An electron has a very small mass, and carries a net negative charge. In its normal state an atom carries equal number of electrons and protons, and is therefore electrically neutral.

 Energy levels and quantum numbers:

The electrons in the atom are arranged in different shells, known as K-­shell, L-shell, M-shell, etc. Each shall includes a fixed number of orbits. These shells are further divided into subshells depending upon the total number of electrons in each shell. Each subshell (orbit) is at a definite dis­tance and the nucleus exerts a definite force on the electrons in this sub­shell. This force is known as the energy of the orbit. Each subshell which can be occupied by the electrons is called energy level in the atom. Each electron of the atom belongs to a particular orbit and hence occupies a definite energy level. The total energy of the atom is the sum of the energy levels occupied by the different electrons. The arrangement of electrons in different orbits is such that it produces a minimum total energy in the atom. This arrangement of electrons becomes the most stable state of the atom. If an electron occupies a higher energy orbit, it is called an excited state of the atom.

Various shells, the subshells and the number of electrons which can occupy a given subshell  .

METALLIC STRUCTURES

Materials differ from one another because of the differences in their pro­perties. Iron differs from a plastic material due to its higher density, stren­gth and electrical conductivity. Similarly gold differs from iron because of its colour, density and corrosion resistance.

Differences in properties of various materials are due to the differences in the structures of materials. Different materials possess different struc­tures. The structure of a material exhibits its internal and surface details. These details can be examined and expressed in different orders of magni­fication varying from a few times to several thousands. In order of decre­asing magnification, the structure of a solid material can be expressed as follows:

(i) Atomic structure (ii) Crystal structure (iii) Microstructure (iv) Macrostructure

All solid materials consist of a large number of particles called molecules which are bonded together to form the bulk material. Each molecule is fur­ther composed of tiny particles called atoms. Individual properties of atoms and their arrangement in the molecule determine the properties of the material. Therefore, to understand the properties and structure of materials, it is necessary to start with the structure and characteristics of individual atoms.

Equivalent Circuits

    Equivalent Circuits

In a series ac circuit, the total impedance of two or more elements in series is often equivalent to an impedance that can be achieved with fewer elements of different values, the elements and their values being determined by the frequency applied. This is also true for parallel circuits. 

Admittance and Susceptance

          Admittance and Susceptance

The discussion for parallel ac circuits will be very similar to that for dc circuits. In dc circuits, conductance (G) was de­fined as equal to 1/R. The total conductance of a parallel circuit was then found by adding the conductance of each branch. The total resistance R_T was then simply 1/G_T.

In ac circuits, we define admittance (Y) as equal to 1/Z. The unit of measure for admittance as defined by the SI system is siemens, which has the symbol S. For many years it was mhos, which had the inverted ohm symbol Ω. Admittance is a measure of how well an ac circuit will admit or allow current to flow in the circuit. The larger its value, therefore, the heavier the current flow for the same applied source of emf. The total admittance of a circuit can also be found by finding the sum of the parallel admittances. The total impedance Z_T of the circuit is then 1/Y_T; that is, for the network  

SINGLE PHASE AC CIRCUIT THEORY: RLC COMPONENTS

In this week, phasor algebra will be used to develop a quick, direct method for solving the parallel ac circuits. The close relationship that exists be­tween this method for solving for unknown quantities and the approach used for dc circuits will become apparent after a few simple examples are considered. Once this asso­ciation is established, many of the rules (current divider rule, and so on) for dc circuits can be readily applied to ac circuits.