Question

Refreshing questions What is the definition of material defect?? 1. 2. Explain the effect of defects on following: a) Microstructure b) Material properties 3. On what basis, defects can be classified? 4. List and show difference between the types of point defects? 5. Which type(s) of point defect distort the unit cell? 6. Which type(s) of point defects that does not affect the unit cell arrangement? 7. What is the effect of defects on material density? 8. List three types of DT? 9. List three types of NDT? 12

Answer 1

1. Material Defect

A material defect is a defect or problem that has a significant adverse impact on the appearance, safety, usability and value of a product, system or property.

2. A.

Dislocations—Linear Defects

Dislocations are abrupt changes in the regular ordering of atoms, along a line (dislocation line) in the solid. They occur in high density and are very important in mechanical properties of material. They are characterized by the Burgers vector, found by doing a loop around the dislocation line and noticing the extra interatomic spacing needed to close the loop. The Burgers vector in metals points in a close packed direction.

Edge dislocations occur when an extra plane is inserted. The dislocation line is at the end of the plane. In an edge dislocation, the Burgers vector is perpendicular to the dislocation line.

Screw dislocations result when displacing planes relative to each other through shear. In this case, the Burgers vector is parallel to the dislocation line.

Interfacial Defects

The environment of an atom at a surface differs from that of an atom in the bulk, in that the number of neighbors (coordination) decreases. This introduces unbalanced forces which result in relaxation (the lattice spacing is decreased) or reconstruction (the crystal structure changes).

The density of atoms in the region including the grain boundary is smaller than the bulk value, since void space occurs in the interface.

Surfaces and interfaces are very reactive and it is usual that impurities segregate there. Since energy is required to form a surface, grains tend to grow in size at the expense of smaller grains to minimize energy. This occurs by diffusion, which is accelerated at high temperatures.

Bulk or Volume Defects

A typical volume defect is porosity, often introduced in the solid during processing. A common example is snow, which is highly porous ice.

Atomic Vibrations

Atomic vibrations occur, even at zero temperature (a quantum mechanical effect) and increase in amplitude with temperature. Vibrations displace transiently atoms from their regular lattice site, which destroys the perfect periodicity.

2. B.

Properties of metals can be divided into intrinsic properties and extrensic properties; the first group depending only on the atoms and electrons. The second group depending on the extent and composition of the assemblies forming a real metal alloy. Via the Grüneisen constant the first group of properties shows remarkable interrelations. Hence is can be shown that the elastic constants of a metal are linked to its melting point and the coefficient of thermal expansion. So, important properties determining the metal alloys' performance are rather inaccessible for influencing via structural parameters like composition or deformation conditions. Strength is a very outspoken example of an extrensic property as it results from the role of dislocation movement: its hindering is linked to strength increase, while the opposite holds for enabling dislocation movement.

In general, vacancies enable diffusion. In case diffusion leads to precipitation, then vacancies play their role in increasing strength values. In case vacancy diffusion is the rate-controlling step for creep the result may be a deterioration of strength values.

Volume defects in crystals are three-dimensional aggregates of atoms or vacancies. It is common to divide them into four classes in an imprecise classification that is based on a combination of the size and effect of the particle. The four categories are: precipitates, which are a fraction of a micron in size and decorate the crystal; secondphase particles or dispersants, which vary in size from a fraction of a micron to the normal grain size (10-100µm), but are intentionally introduced into the microstructure; inclusions, which vary in size from a few microns to macroscopic dimensions, and are relatively large, undesirable particles that entered the system as dirt or formed by precipitation; and voids, which are holes in the solid formed by trapped gases or by the accumulation of vacancies.

      Precipitates are small particles that are introduced into the matrix by solid state reactions. While precipitates are used for several purposes, their most common purpose is to increase the strength of structural alloys by acting as obstacles to the motion of dislocations. Their efficiency in doing this depends on their size, their internal properties, and their distribution through the lattice. However, their role in the microstructure is to modify the behavior of the matrix rather than to act as separate phases in their own right.

      Dispersants are larger particles that behave as a second phase as well as

influencing the behavior of the primary phase. They may be large precipitates, grains, or polygranular particles distributed through the microstructure. When a microstructure contains dispersants such properties as mechanical strength and electrical conductivity are some average of the properties of the dispersant phase and the parent.            

      Inclusions are foreign particles or large precipitate particles. They are usually undesirable constituents in the microstructure. For example, inclusions have a deleterious effect on the useful strength of structural alloys since they are preferential sites for failure. They are also often harmful in microelectronic devices since they disturb the geometry of the device by interfering in manufacturing, or alter its electrical properties by introducing undesirable properties of their own.

      Voids (or pores) are caused by gases that are trapped during solidification or by vacancy condensation in the solid state. They are almost always undesirable defects.Their principal effect is to decrease mechanical strength and promote fracture at small loads.

3.

Defects, Materials and Products

General Classification of Defects

Crystal lattice defects (defects in short) are usually classified according to their dimensions. Defects as dealt with in this course may then be classified as follows:

0-dimensional defects

1. We have "point defects" (on occasion abbreviated PD), or, to use a better but unpopular name, "atomic size defects".
2. Most prominent are vacancies (V) and interstitials (i). If we mean self-interstitials (and you should be careful with using the name interstitials indiscriminately), these two point defects (and if you like, small agglomerates of these defects) are the only possible intrinsic point defects in element crystals.
3. If we invoke extrinsic atoms, i.e. impurity atoms on lattice sites or interstitial sites, we have a second class of point defects subdivided into interstitial or substitutional impurity atoms or extrinsic point defects.
4. In slightly more complicated crystals we also may have mixed-up atoms (e.g. a Ga atom on an As site in a GaAs crystal) or antisite defects.

1-dimensional Defects

1. This includes all kinds of dislocations; for example:
2. Perfect dislocations, partial dislocations (always in connection with a stacking fault), dislocation loops, grain boundary and phase boundary dislocations, and even.
3. Dislocations in quasicrystals.

2-dimensional Defects

1. Here we have stacking faults (SF) and grain boundaries in crystals of one material or phase, and
2. Phase boundaries and a few special defects as e.g. boundaries between ordered domains.

3-dimensional Defects

1. This includes: Precipitates, usually involving impurity atoms.
2. Voids (little holes, i.e. agglomerates of vacancies in three-dimensional form) which may or may not be filled with a gas, and
3. Special defects, e.g. stacking fault tetrahedra and tight clusters of dislocations.

4, 5 and 6. (Answer)  Point defects

Point defects are defects that occur only at or around a single lattice point. They are not extended in space in any dimension. Strict limits for how small a point defect is are generally not defined explicitly. However, these defects typically involve at most a few extra or missing atoms. Larger defects in an ordered structure are usually considered dislocation loops. For historical reasons, many point defects, especially in ionic crystals, are called centers: for example a vacancy in many ionic solids is called a luminescence center, a color center, or F-center. These dislocations permit ionic transport through crystals leading to electrochemical reactions. These are frequently specified using Kröger–Vink Notation.

• Vacancy defects are lattice sites which would be occupied in a perfect crystal, but are vacant. If a neighboring atom moves to occupy the vacant site, the vacancy moves in the opposite direction to the site which used to be occupied by the moving atom. The stability of the surrounding crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. In some materials, neighboring atoms actually move away from a vacancy, because they experience attraction from atoms in the surroundings. A vacancy (or pair of vacancies in an ionic solid) is sometimes called a Schottky defect.
• Interstitial defects are atoms that occupy a site in the crystal structure at which there is usually not an atom. They are generally high energy configurations. Small atoms in some crystals can occupy interstices without high energy, such as hydrogen in palladium.

Schematic illustration of some simple point defect types in a monatomic solid

• A nearby pair of a vacancy and an interstitial is often called a Frenkel defect or Frenkel pair. This is caused when an ion moves into an interstitial site and creates a vacancy.

• Due to fundamental limitations of material purification methods, materials are never 100% pure, which by definition induces defects in crystal structure. In the case of an impurity, the atom is often incorporated at a regular atomic site in the crystal structure. This is neither a vacant site nor is the atom on an interstitial site and it is called a substitutional defect. The atom is not supposed to be anywhere in the crystal, and is thus an impurity. In some cases where the radius of the substitutional atom (ion) is substantially smaller than that of the atom (ion) it is replacing, its equilibrium position can be shifted away from the lattice site. These types of substitutional defects are often referred to as off-center ions. There are two different types of substitutional defects: Isovalent substitution and aliovalent substitution. Isovalent substitution is where the ion that is substituting the original ion is of the same oxidation state as the ion it is replacing. Aliovalent substitution is where the ion that is substituting the original ion is of a different oxidation state than the ion it is replacing. Aliovalent substitutions change the overall charge within the ionic compound, but the ionic compound must be neutral. Therefore, a charge compensation mechanism is required. Hence either one of the metals is partially or fully oxidised or reduced, or ion vacancies are created.
• Antisite defects^[6][7] occur in an ordered alloy or compound when atoms of different type exchange positions. For example, some alloys have a regular structure in which every other atom is a different species; for illustration assume that type A atoms sit on the corners of a cubic lattice, and type B atoms sit in the center of the cubes. If one cube has an A atom at its center, the atom is on a site usually occupied by a B atom, and is thus an antisite defect. This is neither a vacancy nor an interstitial, nor an impurity.
• Topological defects are regions in a crystal where the normal chemical bonding environment is topologically different from the surroundings. For instance, in a perfect sheet of graphite (graphene) all atoms are in rings containing six atoms. If the sheet contains regions where the number of atoms in a ring is different from six, while the total number of atoms remains the same, a topological defect has formed. An example is the Stone Wales defect in nanotubes, which consists of two adjacent 5-membered and two 7-membered atom rings.

Schematic illustration of defects in a compound solid, using GaAs as an example.

• Also amorphous solids may contain defects. These are naturally somewhat hard to define, but sometimes their nature can be quite easily understood. For instance, in ideally bonded amorphous silica all Si atoms have 4 bonds to O atoms and all O atoms have 2 bonds to Si atom. Thus e.g. an O atom with only one Si bond (a dangling bond) can be considered a defect in silica. Moreover, defects can also be defined in amorphous solids based on empty or densely packed local atomic neighborhoods, and the properties of such 'defects' can be shown to be similar to normal vacancies and interstitials in crystals. Complexes can form between different kinds of point defects. For example, if a vacancy encounters an impurity, the two may bind together if the impurity is too large for the lattice. Interstitials can form 'split interstitial' or 'dumbbell' structures where two atoms effectively share an atomic site, resulting in neither atom actually occupying the site.

7. Effect of defect on Density

You already know that to obtain a perfectly pure substance is almost impossible. Purification is a costly process. In general, analytical reagent-grade chemicals are of high purity, and yet few of them are better than 99.9% pure. This means that a foreign atom or molecule is present for every 1000 host atoms or molecules in the crystal.

Perhaps the most demanding of purity is in the electronic industry. Silicon crystals of 99.999 (called 5 nines) or better are required for IC chips productions. These crystal are doped with nitrogen group elements of P and As or boron group elemnts B, Al etc to form n- aand p-type semiconductors. In these crystals, the impurity atom substitute atoms of the host crystals.

Presence minute foreign atoms with one electron more or less than the valence four silicon and germanium host atoms is the key of making n- aand p-type semiconductors. Having many simiconductors connected in a single chip makes the integrated circuit a very efficient information processor. The electronic properties change dramatically due to these impurities. This is further described in Inorganic Chemistry by Swaddle.

In other bulk materials, the presence of impurity usually leads to a lowering of melting point. For example, Hall and Heroult tried to electorlyze natural aluminum (aluminium) compounds. They discovered that using a 5% mixture of Al₂O₃ (melting point 273 K) in cryolite Na₃AlF₆ (melting point 1273 K) reduced the melting point to 1223 K, and that enabled the production of aluminum in bulk. Recent modifications lowered melting temperatures below 933 K.

Some types of glass are made by mixing silica (SiO₂), alumina (Al₂O₃), calcium oxide (CaO), and sodium oxide (Na₂O). They are softer, but due to lower melting points, they are cheaper to produce.

8. Three types of Destructive Testing (DT)

In destructive testing (or destructive physical analysis, DPA) tests are carried out to the specimen's failure, in order to understand a specimen's performance or material behaviour under different loads. These tests are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive testing. Destructive testing is most suitable, and economic, for objects which will be mass-produced, as the cost of destroying a small number of specimens is negligible. It is usually not economical to do destructive testing where only one or very few items are to be produced (for example, in the case of a building). Analyzing and documenting the destructive failure mode is often accomplished using a high-speed camera recording continuously (movie-loop) until the failure is detected. Detecting the failure can be accomplished using a sound detector or stress gauge which produces a signal to trigger the high-speed camera. These high-speed cameras have advanced recording modes to capture almost any type of destructive failure.^[1]After the failure the high-speed camera will stop recording. The capture images can be played back in slow motion showing precisely what happen before, during and after the destructive event, image by image.

Some types of destructive testing:

• Bend test
• Break test
• Tensile test
• Hardness test
• Impact test
• Macro examination
• Micro examination
• Stress tests
• Crash tests
• Hardness tests
• Metallographic tests

9. Three types of Non-Destructive Testing (NDT)

Nondestructive testing or non-destructive testing (NDT) is a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage.^[1] The terms nondestructive examination (NDE), nondestructive inspection (NDI), and nondestructive evaluation (NDE) are also commonly used to describe this technology.^[2] Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research. The six most frequently used NDT methods are eddy-current, magnetic-particle, liquid penetrant, radiographic, ultrasonic, and visual testing.^[3] NDT is commonly used in forensic engineering, mechanical engineering, petroleum engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and art.^[1] Innovations in the field of nondestructive testing have had a profound impact on medical imaging, including on echocardiography, medical ultrasonography, and digital radiography.

Various national and international trade associations exist to promote the industry, knowledge about non-destructive testing, and to develop standard methods and training. These include the American Society for Nondestructive Testing, the Non-Destructive Testing Management Association, the International Committee for Non-Destructive Testing, the European Federation for Non-Destructive Testing and the British Institute of Non-Destructive Testing.

NDT methods rely upon use of electromagnetic radiation, sound and other signal conversions to examine a wide variety of articles (metallic and non-metallic, food-product, artifacts and antiquities, infrastructure) for integrity, composition, or condition with no alteration of the article undergoing examination. Visual inspection (VT), the most commonly applied NDT method, is quite often enhanced by the use of magnification, borescopes, cameras, or other optical arrangements for direct or remote viewing. The internal structure of a sample can be examined for a volumetric inspection with penetrating radiation (RT), such as X-rays, neutrons or gamma radiation. Sound waves are utilized in the case of ultrasonic testing (UT), another volumetric NDT method – the mechanical signal (sound) being reflected by conditions in the test article and evaluated for amplitude and distance from the search unit (transducer). Another commonly used NDT method used on ferrous materials involves the application of fine iron particles (either suspended in liquid or dry powder – fluorescent or colored) that are applied to a part while it is magnetized, either continually or residually. The particles will be attracted to leakage fields of magnetism on or in the test object, and form indications (particle collection) on the object's surface, which are evaluated visually. Contrast and probability of detection for a visual examination by the unaided eye is often enhanced by using liquids to penetrate the test article surface, allowing for visualization of flaws or other surface conditions. This method (liquid penetrant testing) (PT) involves using dyes, fluorescent or colored (typically red), suspended in fluids and is used for non-magnetic materials, usually metals.

Analyzing and documenting a nondestructive failure mode can also be accomplished using a high-speed camera recording continuously (movie-loop) until the failure is detected. Detecting the failure can be accomplished using a sound detector or stress gauge which produces a signal to trigger the high-speed camera. These high-speed cameras have advanced recording modes to capture some non-destructive failures.^[4] After the failure the high-speed camera will stop recording. The capture images can be played back in slow motion showing precisely what happen before, during and after the nondestructive event, image by image.

Non destructive testing

• Visual testing
• Radiographic testing
• Ultrasonic testing
• Penetrant testing
• Magnetic particle testing
• Eddy current testing
• Physical testing of soil
• Physical testing of buildings and building constructions.