Properties of Metals
The important properties of an engineering material determine the utility of the material which influences quantitatively or qualitatively the response of a given material to imposed stimuli and constraints. The various engineering material properties are given as under.
1. Physical properties
2. Chemical properties
3. Thermal properties
4. Electrical properties
5. Magnetic properties
6. Optical properties, and
7. Mechanical properties
These properties of the material are discussed as under.
The important physical properties of the metals are density, color, size and shape (dimensions), specific gravity, porosity, luster etc. Some of them are defined as under.
1. Density: Mass per unit volume is called as density. In metric system its unit is kg/mm3. Because of very low density, aluminium and magnesium are preferred in aeronautic and transportation applications.
2. Color: It deals the quality of light reflected from the surface of metal.
3. Size and shape: Dimensions of any metal reflect the size and shape of the material. Length, width, height, depth, curvature diameter etc. determines the size. Shape specifies the rectangular, square, circular or any other section.
4. Specific Gravity: Specific gravity of any metal is the ratio of the mass of a given volume of the metal to the mass of the same volume of water at a specified temperature.
5. Porosity: A material is called as porous or permeable if it has pores within it.
The study of chemical properties of materials is necessary because most of the engineering materials, when they come in contact with other substances with which they can react, suffer from chemical deterioration of the surface of the metal. Some of the chemical properties of the metals are corrosion resistance, chemical composition and acidity or alkalinity. Corrosion is the gradual deterioration of material by chemical reaction with its environment.
The study of thermal properties is essential in order to know the response of metal to thermal changes i.e. lowering or raising of temperature. Different thermal properties are thermal conductivity, thermal expansion, specific heat, melting point, thermal diffusivity. Some important properties are defined as under.
Melting Point: Melting point is the temperature at which a pure metal or compound changes its shape from solid to liquid. It is called as the temperature at which the liquid and solid are in equilibrium. It can also be said as the transition point between solid and liquid phases. Melting temperature depends on the nature of inter-atomic and intermolecular bonds. Therefore higher melting point is exhibited by those materials possessing stronger bonds. Covalent, ionic, metallic and molecular types of solids have decreasing order of bonding strength and melting point. Melting point of mild steel is 1500°C, of copper is 1080°C and of Aluminium is 650°C.
The various electrical properties of materials are conductivity, temperature coefficient of resistance, dielectric strength, resistivity, and thermoelectricity. These properties are defined as under.
1. Conductivity: Conductivity is defined as the ability of the material to pass electric current through it easily i.e. the material which is conductive will provide an easy path for the flow of electricity through it.
2. Temperature Coefficient of Resistance: It is generally termed as to specify the variation of resistivity with temperature.
3. Dielectric Strength: It means insulating capacity of material at high voltage. A material having high dielectric strength can withstand for longer time for high voltage across it before it conducts the current through it.
4. Resistivity: It is the property of a material by which it resists the flow of electricity through it.
5. Thermoelectricity: If two dissimilar metals are joined and then this junction is heated, a small voltage (in the milli-volt range) is produced, and this is known as thermoelectric effect. It is the base of the thermocouple. Thermo -couples are prepared using the properties of metals.
Magnetic properties of materials arise from the spin of the electrons and the orbital motion of electrons around the atomic nuclei. In certain atoms, the opposite spins neutralize one another, but when there is an excess of electrons spinning in one direction, magnetic field is produced. Many materials except ferromagnetic material which can form permanent magnet, exhibit magnetic affects only when subjected to an external electro-magnetic field. Magnetic properties of materials specify many aspects of the structure and behavior of the matter. Various magnetic properties of the materials are magnetic hysteresis, coercive force and absolute permeability which are defined as under.
1. Magnetic Hysteresis: Hysteresis is defined as the lagging of magnetization or induction flux density behind the magnetizing force or it is that quality of a magnetic substance due to energy is dissipated in it on reversal of its magnetism. Below Curie temperature, magnetic hysteresis is the rising temperature at which the given material ceases to be ferromagnetic, or the falling temperature at which it becomes magnetic. Almost all magnetic materials exhibit the phenomenon called hysteresis.
2. Coercive Force: It is defined as the magnetizing force which is essential to neutralize completely the magnetism in an electromagnet after the value of magnetizing force becomes zero.
3. Absolute Permeability: It is defined as the ratio of the flux density in a material to the magnetizing force producing that flux density. Paramagnetic materials possess permeability greater than one whereas di-magnetic materials have permeability less than one.
The main optical properties of engineering materials are refractive index, absorptivity, absorption co-efficient, reflectivity and transmissivity. Refractive index is an important optical property of metal which is defined as under.
Refractive Index: It is defined as the ratio of velocity of light in vacuum to the velocity of a material. It can also be termed as the ratio of sine of angle of incidence to the sine of refraction.
Under the action of various kinds of forces, the behavior of the material is studied that measures the strength and lasting characteristic of a material in service. The mechanical properties of materials are of great industrial importance in the design of tools, machines and structures. Theses properties are structure sensitive in the sense that they depend upon the crystal structure and its bonding forces, and especially upon the nature and behavior of the imperfections which- exist within the crystal itself or at the grain boundaries. The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. The main mechanical properties of the metal are strength, stiffness, elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, formability, castability and weldability. These properties can be well understood with help of tensile test and stress strain diagram. The few important and useful mechanical properties are explained below.
1. Elasticity: It is defined as the property of a material to regain its original shape after deformation when the external forces are removed. It can also be referred as the power of material to come back to its original position after deformation when the stress or load is removed. It is also called as the tensile property of the material.
2. Proportional limit: It is defined as the maximum stress under which a material will maintain a perfectly uniform rate of strain to stress. Though its value is difficult to measure, yet it can be used as the important applications for building precision instruments, springs, etc.
3. Elastic limit: Many metals can be put under stress slightly above the proportional limit without taking a permanent set. The greatest stress that a material can endure without taking up some permanent set is called elastic limit. Beyond this limit, the metal does not regain its original form and permanent set will occurs.
4. Yield point: At a specific stress, ductile metals particularly ceases, offering resistance to tensile forces. This means, the metals flow and a relatively large permanent set takes place without a noticeable increase in load. This point is called yield point. Certain metals such as mild steel exhibit a definite yield point, in which case the yield stress is simply the stress at this point.
5. Strength: Strength is defined as the ability of a material to resist the externally applied forces with breakdown or yielding. The internal resistance offered by a material to an externally applied force is called stress. The capacity of bearing load by metal and to withstand destruction under the action of external loads is known as strength. The stronger the material the greater the load it can withstand. This property of material therefore determines the ability to withstand stress without failure. Strength varies according to the type of loading. It is always possible to assess tensile, compressive, shearing and torsional strengths. The maximum stress that any material can withstand before destruction is called its ultimate strength. The tenacity of the material is its ultimate strength in tension.
6. Stiffness: It is defined as the ability of a material to resist deformation under stress. The resistance of a material to elastic deformation or deflection is called stiffness or rigidity. A material that suffers slight or very less deformation under load has a high degree of stiffness or rigidity. For instance suspended beams of steel and aluminium may both be strong enough to carry the required load but the aluminium beam will “sag” or deflect further. That means, the steel beam is stiffer or more rigid than aluminium beam. If the material behaves elastically with linear stress-strain relationship under Hooks law, its stiffness is measured by the Young’s modulus of elasticity (E). The higher is the value of the Young’s modulus, the stiffer is the material. In tensile and compressive stress, it is called modulus of stiffness or “modulus of elasticity”; in shear, the modulus of rigidity, and this is usually 40% of the value of Young’s modulus for commonly used materials; in volumetric distortion, the bulk modulus.
7. Plasticity: Plasticity is defined the mechanical property of a material which retains the deformation produced under load permanently. This property of the material is required in forging, in stamping images on coins and in ornamental work. It is the ability or tendency of material to undergo some degree of permanent deformation without its rupture or its failure. Plastic deformation takes place only after the elastic range of material has been exceeded. Such property of material is important in forming, shaping, extruding and many other hot or cold working processes. Materials such as clay, lead, etc. are plastic at room temperature and steel is plastic at forging temperature. This property generally increases with increase in temperature of materials.
8. Ductility: Ductility is termed as the property of a material enabling it to be drawn into wire with the application of tensile load. A ductile material must be strong and plastic. The ductility is usually measured by the terms, percentage elongation and percent reduction in area which is often used as empirical measures of ductility. The materials those possess more than 5% elongation are called as ductile materials. The ductile material commonly used in engineering practice in order of diminishing ductility are mild steel, copper, aluminium, nickel, zinc, tin and lead.
9. Malleability: Malleability is the ability of the material to be flattened into thin sheets under applications of heavy compressive forces without cracking by hot or cold working means. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice in order of diminishing malleability are lead, soft steel, wrought iron, copper and aluminium. Aluminium, copper, tin, lead, steel, etc. are recognized as highly malleable metals.
10. Hardness: Hardness is defined as the ability of a metal to cut another metal. A harder metal can always cut or put impression to the softer metals by virtue of its hardness. It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc.
11. Brittleness: Brittleness is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. The materials having less than 5% elongation under loading behavior are said to be brittle materials. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Glass, cast iron, brass and ceramics are considered as brittle material.
12. Creep: When a metal part when is subjected to a high constant stress at high temperature for a longer period of time, it will undergo a slow and permanent deformation (in form of a crack which may further propagate further towards creep failure) called creep.
13. Formability: It is the property of metals which denotes the ease in its forming in to various shapes and sizes. The different factors that affect the formability are crystal structure of metal, grain size of metal hot and cold working, alloying element present in the parent metal. Metals with smal1 grain size are suitable for shallow forming while metal with size are suitable for heavy forming. Hot working increases formability. Low carbon steel possesses good formability.
14. Castability: Castability is defined as the property of metal, which indicates the ease with it can be casted into different shapes and sizes. Cast iron, aluminium and brass are possessing good castability.
15. Weldability: Weldability is defined as the property of a metal which indicates the two similar or dissimilar metals are joined by fusion with or without the application of pressure and with or without the use of filler metal (welding) efficiently. Metals having weldability in the descending order are iron, steel, cast steels and stainless steels.