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Home GATE Study MaterialCivil Engineering � Introduction to Materials Science

Introduction to Materials Science

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Introduction to Materials Science

Introduction to Materials Science

Introduction

Different materials have different properties. Think of the difference between the engine of a car and its wheels; the metal in a wire and its insulator. All these objects can only be made out of materials that have properties suited to their application.

Materials science is the study of the properties of materials. It focuses on the factors that make one material different from another. Understandably, there are many such factors, some obvious and some subtle. Examples of these factors might include elemental composition, arrangement, bonding, impurities, surface structure, length scale and so on. The ability to understand the relationships between these factors and the properties of a material has been crucial to most of mankind's technological breakthroughs. Today, materials science is a multidisciplinary subject. It draws upon just about every field of science and engineering, providing insights for other researchers to use in their field.

This book is aimed at those studying materials science at the undergraduate level in university whether as their major field or as a single module of a related engineering course.

Structure of Matter


Atomic Structure and Bonding

Fundamentally, two types of bonding exist- bonds between atoms and bonds between ions. Bonds between atoms of nonmetals are covalent, meaning that they share a pair of electrons in the space between them. These two atoms are bound together and cannot be separated by simple physical means. If these two atoms have similar electronegativity, neither atom has more pull on the electron pair than the other. This type of covalent bond is called Non Polar. Examples of non polar covalent compounds are methane, carbon dioxide and graphite. In graphite, all atoms are identical and so no atom has stronger pull than any of the others. In methane, the carbon-hydrogen bonds are very slightly polar, and the polarities are cancelled because the bonds all point to the same locus.

Crystal Structure

Defects

Defects of materials are subject to intense study. However there are some methods to determine the source of defects and, if occurred, the size, shape and position of defects in the materials. There are: destructive testing methods and Non destructive testing methods (NDT).

Thermodynamics of Material


Phase Diagrams

Phase diagrams provide a graphical means of presenting the results of experimental studies of complex natural processes, such that at a given temperature and pressure for a specific system at equilibrium the phase or phases present can be determined.

SYSTEM - Any portion of the universe which is of interest and can be studied experimentally.

PHASE- any particular portion of a system, which is physically homogeneous, has a specific composition, and can be mechanically removed or separated from any other phase in the system.

* e.g. A system containing a mixture of ol and pl in equilibrium contains two phases - ol and pl.

In petrology we generally deal with primary phases - any crystalline phase which can coexist with liquid, i.e. it formed/crystallized directly from the liquid.

EQUILIBRIUM - The condition of minimum energy for the system such that the state of a reaction will not change with time provided that pressure and temperature are kept constant.

In experimental petrology there are three practical criteria used to test for equilibrium.

1. Time - with time the system does not change its physical or chemical makeup. 2. Approach equilibrium from two directions, e.g. the melting point of Albite. * begin with a liquid of Ab composition (Na2O-Al2O3-6SiO2) and cool until Ab crystallizes - T=1100�C * begin with the same mixture of solid Albite and heat it up until liquid forms - T=1120�C Melting point of albite = 1110�C + 10�C. 3. Attainment of equilibrium by using different reactants and procedures. To determine the melting temperature of Albite * grind up a sample of pure albite * combine powdered oxides to give pure Ab composition Use both to determine Ab melting point.

One final term to be defined prior to examining phase diagrams.

COMPONENT - the smallest number of independent variable chemical constituents necessary to define any phase in the system.

* components may be oxides, elements or minerals, dependant on the system being examined.

For example, experiments carried out in the H2O system, show that the phases which appear over a wide temperature and pressure range are ice, liquid water and water vapour. The composition of each phase is H2O and only one chemical parameter or component is required to describe the composition of each phase. Systems which can be defined by a single component are called Unary Systems. H2O System In this system pressures from 0 to 15 kbars seven phases, each with the same composition - H2O have been recognized:

* Ice I * Ice II * Ice III * Ice IV(actually not exist) * Ice V * Water * Steam

SiO2 System In the one component SiO2 system in the temperature range from 0 to 2,000�C and a pressure range from 0 to 30 kbars six phases of SiO2 are recognized. At pressures > 30 kbar a seventh phase, stishovite, exists. The six phases of SiO2 are:

* coesite * alpha quartz (Trigonal) * beta quartz (hexagonal) * tridymite * cristobalite * anhydrous melt

Materials Processing


In order to produce an article for any application out of a particular material there are several steps that may be required. The first step is usually to obtain the raw materials from our environment. This may involve discovering where these raw materials are located (often achieved with knowledge of geology) and developing processes to extract them from these locations (e.g. mining the ores, drilling for oil etc.). Otherwise, it may be possible to find sources of material suitable for recycling or reprocessing. Once these raw materials have been obtained they may need to undergo some initial processing to get them into a usable form. This may be some form of extractive metallurgy, chemical synthesis or some other chemical process. It may also be necessary to mix different raw materials to achieve a certain composition (e.g. alloying in metals) that is appropriate for or has been optimised the application. The application will usually require that the material be in a particular shape and a suitable shaping process or combination of process must be employed to achieve this. Often, it may be possible to produce a shape out of a material with any one of the many different shaping processes. However, there is usually one particular process that either results in particular benefits in terms of the properties of the material or the article that is produced or meets some other important criteria - such as low cost - that it is selected over the other options. Finally, it may be necessary or beneficial to process the article further, once it has been formed, in order to optimise the properties of the material.

Firstly, this chapter will present the various chemical processes that may be necessary to produce suitable materials from the raw materials in our environment. The different methods for shaping these materials will then be presented. Finally, the processes used to optimise the properties of the materials will be discussed.

Chemical Processing
Extraction of Raw Materials
Chemical Synthesis of Materials
Shaping Processes
Melt Processing
Casting
Physical Processing
Forging
Rolling
Extrusion

Powder Process

Powder processes are used in the production of metallic and ceramic parts. The use of metal powders is commonly referred to as Powder Metallurgy (P/M).

There are 4 main stages to producing products with, they are: Powder Mixing, Compaction, Sintering and final finishing.

A metal or ceramic powder is prepared, then compacted into a desired shape. This part is then heated in a furnace causing the powders to weld together forming a solid part. The part is then final processed by final shaping, minor smoothing, or drilling.

Using Powders to produce parts is viable when you require a high volume of simple parts that need to be cost efficient. All though casting can also do this, P/M offers near net shape products. This means that the part that comes out of the process needs little or no finishing done to it.

Ceramics lend themselves well to powder processing as they are very hard and brittle, thus a near net shape is highly beneficial.

Mixing

Mixing is mainly done to add waxes for the compaction, binders to temporarily strengthen the compacts and sometimes to get the right chemistry.

As most suppliers recommend lubricant for idea compaction, mixing is a very important process, so a homogenous mixture is required. Optimum mixing occurs with turbulent mixing and at low centrifugal forces.

Along with ensuring a homogenous mix, the mixing process also provides some milling of the powders. As we all know you can put more tennis balls in an area than beach balls, thus increasing the surface area of the balls. The same is true with powders, more surface area, the better the final product is.

Compaction

Compaction is the process of squishing the powders into the desired shape with enough force so as to hold its shape. This is called a green body, as it still has moisture in it and needs to be Sintered. Same basic concept as pottery, the plate or cup is considered "green" until it is fired

There are 2 categories of pressing: Isostatic and Axial.

Sintering

Sintering is simply the furnace heating of a compacted powder object, also known as a green body to form a solid part. The powders can be either metallic or ceramic. They can be in elemental form, as an alloy, or mixture of both. Most sintering processes are done in a protective atmosphere, such as nitrogen or hydrogen mixed gas, to avoid degradation of the green bodies, and at a temperature lower than the melting point, approximately 60~90% of the main elements meting point. The specific atmosphere and temperature is dependant upon the material being processed.

If the material being sintered is an alloy, it is possible that one or more of the constitutes has a melting point lower than the sintering temperature, thus causing a small amount of liquid to form. This is called Liquid Phase Sintering. Caution needs to be taken when choosing a temperature as too much liquid will result in the deformation of the part. This is referred to as slumping.

The mechanism of sintering is the diffusion of the atoms across the particle boundaries of compacted powders. As the atoms diffuse, all voids are filled and the material forms one solid part. As the voids between particles are no longer present, the part increases in density, and experiences shrinkage. However, due to the nature of this process, only 93%-98% theoretical density can be achieved, thus further mechanical processing is needed to obtain 100% dense material.

The resultant microscopic structure resembles the starting green compact. The starting particle boundaries eventually turn into the final grain boundaries.

As the voids between the powder particles are filled during the sintering process, the gases need to be expelled from the compact. These gases are; air trapped between powders and gasses from additives added during the mixing and/or compaction process. These gases are expelled through capillaries formed by the particle boundaries. If the compacts are hated to fast, these capillaries can be �pinched� off and if these gasses are not expelled, the part will have defects such as warpage, porosity, or even holes.

A typical industrial sintering process is done on a traveling grate furnace with a 2 stages of heating. The green bodies are placed on a conveyor which travels into the furnace which has a positive pressure protective atmosphere blown onto the conveyor belt. The parts travel into the first temperature zone to vaporize and wax and degas. The second temperature zone is to do the actual sintering of the material. After the appropriate sinter time, the parts travel through a cooling zone to allow the parts to be handled, or to lock properties for continued processing. Degas and sinter times vary based on material.

Finishing Machining Welding

Materials Optimisation

Heat Treatment:It is defined as combination of heating and cooling cycles given to a particular material of interest to achieve desired properties.

Surface Engineering

Materials Characterisation


An important aspect of materials science is the characterisation of the materials that we use or study in order to learn more about them. Today, there is a vast array of scientific techniques available to the materials scientist that enables this characterisation. These techniques will be introduced and explained in this section.

Macroscopic Observation

The first step in any characterisation of a material or an object made of a material is often a macroscopic observation. This is simply looking at the material with the naked eye. This simple process can yield a large amount of information about the material such as the colour of the material, its lustre (does it display a metallic lustre), its shape (whether it displays a regular, crystalline form), its composition (is it made up of different phases), its structural features (does it contain porosity) etc. Often, this investigation yields clues as to what other tests could be performed to fully identify the material or to solve a problem that has been experienced in use.

Microscopic Observation

Microscopy is a technique that, combined with other scientific techniques and chemical processes, allows the determination of both the composition and the structure of a material. It is essentially the process of viewing the structure on a much finer scale than is possible with the naked eye and is necessary because many of the properties of materials are dependent on extremely fine features and defects that are only possible to observe using one of the following techniques in this field.

Optical Microscopy

Optical microscopes are formed of lenses that magnify and focus light. This light may have been transmitted through a material or reflected from a material's surface and can be used to ascertain a great deal of information about that material under evaluation. This can include whether the material is dense or contains porosity, what colour the material is, whether the material is composed of a single phase or contains multiple phases etc.

A common practice performed in conjunction with optical microscopy is that of targeted and controlled chemical attack of the material using one of many chemical reagents available. For metallic materials, this technique combined with optical microscopy is know as optical metallography. The basis of this combined technique is that regions of different composition within a material as well as entirely different materials are affected differently when exposed to certain chemicals. These chemical effects are catalogued in various works (for example the ASM Metals Handbook or Metallographic Etching by G. Petzow) and through an understanding of these effects and a systematic experimental process they can be used to determine material composition and structure.

There are several limitations to the usefulness of optical microscopy. The first is that the maximum resolving power is limited by diffraction effects to approximately 0.2 micrometres at a magnification of around 1500X (see reference). Many of the defects and structural features important in determining material properties, and therefore of interest to materials scientists, are of atomic scale. (for reference, the diameter of a helium atom is approximately 100 picometers) The second major limitation in optical microscopy is limited depth of field. This limitation means that surfaces with features at different heights - such as the rough surfaces of a fractured specimen for example - cannot be seen in sharp focus at the same time. This means that flat or polished surfaces are preferred for this technique. Furthermore, the chemical techniques required for identifying different phases within a structure are destructive. Thus, if a only a small amount of a certain portion of the sample is present then this may be destroyed by the process by the etching technique.

Electron Microscopy
Scanning Electron Microscopy
Transmission Electron Microscopy
Chemical Analysis in Electron Microscopy
Diffraction Techniques
Principles of Diffraction
X-Ray Diffraction
Neutron Diffraction
Electron Diffraction
Spectroscopic Techniques
Energy Dispersive X-Ray Spectroscopy
Wavelength Dispersive X-Ray Spectroscopy
Electron Energy Loss Spectroscopy
X-Ray Photoelectron Spectroscopy
Auger Electron Spectroscopy
Infra-red and Raman Spectroscopy
Ultra-violet and Visible Spectroscopy
Electrical and Magnetic Techniques
Electrical Resistance
Impedance Spectroscopy
Thermal Techniques
Thermogravimetric Analysis (TGA)
Differential Scanning Calorimetry (DSC)
Mechanical Testing

Strength

Hardness

Hardness is defined as the resistance of a material to penetration by an indentor. The Mohs scale of hardness has ten level and diamond is the material with the highest level of hardness ever known. There are several methods used to determine material's hardness, such as: Brinell, Rockwell, Vickers and Poldy.

Hardness Brinell (HB)

Is the method used for raw metallic materials. It uses a spherical ball indentor in order to stamp a print in the material. An external force transmitted through the indentor over the surface of the material determines the material's penetration.

Hardness Rockwell (HRB/HRC)

Is the method used for heat treated metallic materials. It has two variants regarding the indenter shape (ball or cone).

Hardness Vickers (HV)

Is a method used for the determination of hardness of special metallic materials, such as high alloyed materials, characterized by a very high degree of hardness.

Non destructive testing (NDT)

Some of the NDT methods available are: ultrasonic method, radiation penetration method.

Metals


Metals are materials made of elements on the left hand side of the periodic tables 'stair step' border starting on the left of Boron and going down and right and finishing at polonium. These elements can be mixed and combined with other elements (metals or non-metals) to create materials called alloys. Alloys are just a mix of elements and materials to create a new material with favorable properties.

Metals can be generally identified by a set of few physical properties (these a very general and there are plenty of exceptions). The main definition of a metal is an element that readily loses electrons and forms positive ions. The general bulk properties that are used to simply identify metals is that they tend to be lustrous (shiny when not oxidised), they are malleable (so can be beaten into a shape and not break), they are ductile (they can be drawn out into a wire) and that they conduct electricity; this rises from the fact that they readily lose electrons so there is a free electron 'gas' where the electrons can move around and this means that a charge can flow when an electric field is placed across the metal.

The metal that has changed the way the whole world functions and takes up a huge majority of the industry even now after over a century of its discovery and use (in terms of its modern production and composition). This metal is steel and is an alloy of mainly iron (Fe) and carbon (C) with many other constituent elements added depending on the type of steel wanted and the properties required.

Steel can be produced in a number of ways. Traditional methods utilise integrated steel processes which use energy intensive blast furnaces (to produce iron) sand basic oxygen steelmaking (to convert iron to steel). More modern methods use electric arc furnaces in which scrap steel is melted using electric currents and then formed into slabs or ingots for further processing. When the steel slab or ingot has cooled, a variety of forming operations such as rolling or extrusion are used to form the metal into flat sheet for use in cars, fridges, filing cabinets or radiators, or into beams, and heavy plate for use in construction and ship building

Ceramics

inorganic and non-metallic materials bimevox

Polymers

Polymer is a group of substances that has large molecules consisting of at least five repeated chemical units bonded together with a same type of linkage, like beads on a string.Polymer usually contains more than five repeated units and some polymers contain hundreds or thousands of monomers in each of their polymer chains. Polymer materials can be natural or synthetic. Polymer material is a large group of materials whereby they can be further classified specifically into plastics, elastomers and composites!



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