Published on March 7, 2014
Solidification & microstructure of metals CONTENTS: 1. Introduction 2. Metals 3. History of Metals 4. Properties of Metals 5. Classification of Metals 6. Inter Atomic Bonds 7. Microscopic Structure of Metals 8. Space Lattices 9. Lattice Imperfection 10. Heat Treatment 11. Strengthening of Metals 12. References Page 1
Solidification & microstructure of metals INTRODUCTION: Metals and alloys play an important role in dentistry. These form one of the four possible groups of materials used in dentistry which include ceramics, composites and polymers. These are used in almost all the aspects of dentistry including the dental laboratory, direct and indirect dental restorations and instruments used to prepare and manipulate teeth. Although the latest trend is towards the “metal free” dentistry, the metals remain the only clinically proven material for long term dental applications.. METALS: Chemical elements in general can be classified as 1. Metals 2. Non-metals 3. Metalloids Metalloids are those elements on the border line showing both metallic and non metallic properties, e.g. carbon and silica. They do not form free positive ions but their conductive and electronic properties make them important. Metals constitute about 2/3rd of the periodic table published by DMITRI MEDELEYEV in 1868. Of the 103 elements which are categorized in the periodic table according to the chemical properties, 81 are metals. According to the metals hand book, they can be defined as “AN OPAQUE LUSTROUS CHEMICAL SUBSTANCE, THAT IS A GOOD CONDUCTOR OF HEAT AND ELECTRICITY AND WHEN POLISHED IS A GOOD REFLECTOR OF LIGHT” Page 2
Solidification & microstructure of metals HISTORY OF METALS: Metals have been used by man ever since he first discovered them. In ancient and pre-historic times, only a few metals were known and accordingly these periods were called as “COPPER AGE”, “BRONZE AGE” and “IRON AGE”. Today more than 80 metallic elements and a large number of alloys have been developed. Ore is a mineral containing one or more metals in a free or combined state. PROPERTIES OF METALS: All metals are solids except for mercury and gallium which are liquid at room temperature and hydrogen which is a gas. The properties of metals can be listed out as follows: 1. They have a metallic luster and mirror like surface 2. They make a metallic sound when struck 3. Are hard, strong and dense 4. Ductile and malleable 5. Conduct heat and electricity 6. Have specific melting and boiling points 7. Form positive ions in solution and get deposited at the cathode during electrolysis. E.g. copper in copper plating. Page 3
Solidification & microstructure of metals The outer most electrons of the atom are known as valence electrons. These are readily given up and are responsible for most of the properties. Metals are tough and this is due to the fact that the atoms of the metals are held together by means of metallic bonds. The chemical properties of metals are based upon the electromotive series which is a table of metals arranged in decreasing order of their tendency to lose electrons. The higher an element is in the series, the more metallic it is. This tendency of metals of lose electrons is known as oxidation potential. CLASSIFICATION OF METALS: They can be done in many ways like: 1. Pure metal and mixture of metals (alloys) 2. Noble metals and base metals : Noble metal is one whose compounds are decomposable by heat alone, at a temperature not exceeding that of redness. E.g. Au, Ag, and Pd. Base metal is one whose compounds with oxygen are not decomposable by heat Alone, retaining oxygen at high temperature. E.g. Zn, Fe, and Al 3. Case metal and wrought metal Cast metal is any metal that is melted and poured into the mould Wrought metal is a cast metal which has been worked upon in cold condition 4. Light metal e.g. Al and heavy metal e.g. Fe 5. High melting metal e.g. chromium and low melting metal e.g. tin 6. Highly malleable and ductile metal e.g. gold and silver Page 4
Solidification & microstructure of metals INTER ATOMIC BONDS: The atoms are held together in place by atomic bonds or forces. They may be 1. Primary 2. Secondary Primary bonds or inter atomic bonds: These are very strong bonds and may be of either type: a. Ionic - These are seen in ceramics b. Covalent - They are seen in organic compounds c. Metallic bonds - They are seen in metals and are non directional Secondary bonds or inter molecular bonds: These are weak forces and are otherwise known as Vander waal’s forces. The various types are: a. Hydrogen bonds b. Dipole bonds c. Dispersion bonds Of all these, the most important one is the metallic bond which was explained for the first time by LORENTZ, a Dutch scientist in 1916. It can be explained by using the atomic and sub atomic structures. The sub – atomic structures 1. Protons – positive charge Page 5
Solidification & microstructure of metals 2. Neutrons – neutral charge 3. Electrons - negative charge The center or the nucleus of an atom consists of proton and neutrons and are therefore positively charged. This is balanced by the revolving electrons which are negatively charged and arranged in concentric shells with progressively increasing energy. The electrons in the outer most shell are known as VALENCE ELECTRONS. These are loosely bound and are therefore readily given up by the atom to form positive ions. The cations thus formed behave like hard spheres and the electron cloud formed by the freed valence electrons roam about freely in the interstices formed by the arrangement of the solid spheres. The electrons act like glue to hold all atoms together and are known as INTER ATOMIC CEMENT. Because of this, the metals are strong, hard, malleable, ductile and good conductors of heat and electricity. MICROSCOPIC STRUCTURE OF METALS: In the solid state, most metals have crystalline structure in which atoms are held together by metallic bonds. This crystalline array extends for many repetitions in 3 dimensions. In this array, the atomic centers are occupied by nuclei and core electrons. The ionisable electrons float freely among the atomic positions. The space lattice is a 3 dimensional pattern of points in space and hence called as point lattice. In this the simplest repeating unit is called as the UNIT CELL. The size and shape of the unit cell are described by three vectors. They Page 6
Solidification & microstructure of metals are a,b,c, and known as crystallographic axes. The length and angle between them are known as LATTICE CONSTANTS AND LATTICE PARAMETERS. When a molten metal is cooled the solicitation process is one of crystallization. These are initiated at specific sites called nuclei. These in the molten metal are present as numerous unstable atomic aggregates or clusters that tend to form crystal nuclei. These temporary nuclei are known as EMBRYOS. These are generally formed from impurities within the molten metal. In the case of pure metals, the crystals grow as dendrites which can be defined as a three dimensional network which is branched like a tree. The critical radius is the minimal radius of the embryo at which the first permanent solid space lattice is formed. The crystals are otherwise known as grains since they seldom exhibit the customary geometric forms due to interference from adjacent crystals during the change of state. The grains meet at grain boundaries which are regions of transition between differently oriented crystals. These are regions of importance as they are sites of: 1. Less resistance to corrosion 2. High internal energy and non crystalline 3. Collection of impurities 4. Barriers for dislocations The nuclei can be homogeneous or heterogenous based upon whether they are developed from the molten liquid or formed as a result of foreign bodies incorporated into the molten metal. When the crystals meet at the grain boundaries Page 7
Solidification & microstructure of metals they stop growing further. The grain boundaries are about 1-2 atomic distances thick. Grain boundaries can be high angles (>10-15 degrees) or low angled (< 10 degree). The grain structure can be fine where in, it contains numerous nuclei as obtained during the rapid cooling process (quenching) or refined when foreign bodies are added to obtain the fine grain structure. EQUALIXED GRAINS When cooling occurs and grains are formed, the grains start growing from the nuclei peripherally. This takes the shape of a sphere and are equalized in structure meaning that they have the same dimensions in any direction. COLUMNAR AND RADIAL GRAINS In a square mould, crystals grow from the edges towards the centre to form columnar grains whereas in the cylindrical mould the grains grow perpendicular to the wall surface and form radial grains. Columnar grains are weak due to interferences in the converging grains. Sharp margins have columnar grains. Page 8
Solidification & microstructure of metals GRAIN SIZE: The grain size can be altered by heating. When the metal is heated above the solidus temperature to the molten state and rapidly quenched, small grains are formed whereas, when they are allowed to cool slowly to room temperature the grains tend to grow due to atomic diffusion and this results in an increased grain size and subsequent decrease in the number. The more fine the grain structure, the more uniform and better are the properties. ANISOTROPHY: Alloys with uniform properties due to the presence of fine grain structure are said to be anisotropic. METHODS OF FABRICATION OF METALS AND ALLOYS 1. CASTING: It is the best and most popular method. 2. WORKING ON THE METAL: They can be worked in the hot or cold conditions. They are known as wrought metals. They can be pressed, rolled, forged or hammered. 3. EXTRUSION: A process in which a metal is forced through a die to form metal tubing. 4. POWDER METALLURGY: It involves the pressing of the powdered metal into the mould of desirable shape and heating it to a high temperature to cause a solid mass. Page 9
Solidification & microstructure of metals SPACE LATTICES: The structure of the crystal can be determined using the BRAGG’S LAW OF X-RAY DIFFRACTION. There are 14 lattices known as BRAVIS LATTICES and these are grouped under six families. These vary depending upon the crystallographic axes and lattice constants which are the length of the vertices and the angle between them. The six families are: 1. Cubic Simple Body centered Face centered 2. Triclinic 3. Tetragonal Simple Body centered Rhombohedral 4. Orthorombic 5. Hexagonal Simple Body centered Face centered Base centered 6. Monoclinic Simple Page 10
Solidification & microstructure of metals Base centered Page 11
Solidification & microstructure of metals The arrangement of atoms in the crystal lattice depends on the atomic radius and charge distribution of atoms. The most commonly used metals in dentistry have one of the following space lattices: body centered cubic, face centered cubic or hexagonal lattice. Page 12
Solidification & microstructure of metals SIMPLE CUBIC LATTICE SYSTEM LATTICE IMPERFECTIONS AND DISLOCATIONS Crystallization from the nucleus does not occur in a regular fashion, lattice plane by lattice plane. Instead, the growth is likely to be more random with some lattice positions left vacant and others overcrowded with atoms being deposited interstitially. These are called defects and can be classified as: A. POINT DEFECTS OR ZERO DIMENSIONAL DEFECTS 1. Vacancies or equilibrium defects: Absence of an atom from its position. This can be: Vacancy Divacancy Trivacancy 2. Interstitialcies: Presence of extra atoms in the interstitial spaces. Page 13
Solidification & microstructure of metals 3. Impurities 4. Electronic defects Point defects are responsible for increased hardness, increased tensile strength, electrical conductance, and phase transformations. B. LINE DEFECTS OR SINGLE DIMENSIONAL DEFECTS: These can be 1. Edge dislocation 2. Screw dislocation The planes along which a dislocation moves is called as slip planes and when this occurs in groups it is called as slip bands. The crystallographic direction in which the atomic planes move is called as the slip direction and the combination of slip plane and slip direction is called as slip system. Page 14
Solidification & microstructure of metals These are responsible for ductility, malleability, strain hardening, fatigue, creep and brittle fracture. The face centered cubic consists of large number of slip systems and therefore is very ductile. This is seen in gold. The hexagonal closely packed system seen in zinc possesses relatively few slip systems and is therefore very brittle. In between these is the body centered cubic with intermediate properties. The strain required to initiate movement is the elastic limit. The method of hardening of metals and alloys is based on the impedance to the movement of dislocations. Page 15
Solidification & microstructure of metals C. SURFACE DEFECTS OR PLANE DEFECTS OR TWO DIMENSIONAL DEFECTS: 1. Grain boundaries 2. Twin boundaries: These are seen in the NiTi wires responsible for transformation between the austenitic and martensitic phases. These are important for the deformation of the α titanium alloys. The atoms have a mirror relationship. 3. Stacking fault 4. Tilt boundaries Page 16
Solidification & microstructure of metals D. VOLUME DEFECTS These include cracks ALLOTROPHY AND ISOMORPHOUS STATE: ALLOTROPHY This ability to exist in more than one stable crystalline form is called as allotrophy. The various forms have the same composition but different crystal structure. ISOMORPHOUS STATE The ability to exist as a single crystal at any atomic composition of binary alloys is known as iomorphous state e.g. Au-Ag, Au-Cu. HEAT TREATMENT OR SOLID STATE REACTIONS: Heat treatment of meals (non-melting) in the solid state is known as solid state reactions. This is a method to cause diffusion of atoms of the alloy by heating a solid metal to a certain temperature and for a certain period of time. This will result in changes in the microscopic structure and physical properties. Important criteria are: 1. Composition of the alloy 2. Temperature to which it is heated Page 17
Solidification & microstructure of metals 3. Time of heating 4. Method of cooling slowly or quenching. The purpose of heat treatment is: 1. Shaping and working on the appliance in the laboratory is made easy when the alloy is soft. This is the first stage and called as softening heat treatment. 2. To harden the alloy to withstand high oral stresses, it is again heated and this is called hardening heat treatment. i. ANNEALING OR SOFTENING HEAT TREATMENT This is done for structures that are cold worked. These cold worked structures are characterized by: 1. Low ductility 2. Distorted and fibrous grains When cold work is continued in these, they will eventually fracture. This may be: 1. Transgranular – through the crystals and occur at room temperature 2. Intergranular – in between the crystals and occurs at elevated temperature These can be reversed by annealing. The various phase are: 1. Recovery Page 18
Solidification & microstructure of metals 2. Recrystallization and 3. Grain growth Technique: The alloy is placed in an electric furnace at a temperature of 700° C for 10mins and then rapidly quenched. Annealing temperature should be half that necessary to melt the metal in degrees Kelvin. Recovery During this phase, the cold work properties begin to disappear. There is a slight decrease in tensile strength and no change in ductility. The tendency for warping decreases in this stage. Recrystallization There is a radical change in the microstructure. The old grains are replaced by a set of new strain free grains. These nucleate in the most severely cold worked regions in the metal. The temperature at which this occurs is the recrystallization temperature. During this the metal gets back to the original soft and ductile nature. Grain growth If the fine grain structure in a crystallized alloy is further heated, the grains begin to grow. This is essentially a process in which the larger grains consume the Page 19
Solidification & microstructure of metals smaller grains. This process minimizes the grain boundary energy. This does not progress until the formation of a coarse grain structure. Properties of an annealed metal: 1. There is an increase in ductility 2. Makes the metal tougher and less brittle Stress relief annealing is a process which is done after cold working a metal to eliminate the residual stress. This is done at relatively low temperatures with no change in the mechanical properties. Page 20
Solidification & microstructure of metals ii. HARDENING HEAT TREATMENT This is done for cast removable partial dentures, saddles, bridges but not for inlays. This is done for clasps after the try in stage so that adjustments can be carried out during the try in when the metal is soft. Technique The appliance is heat soaked at a temperature between 200-450° C for 15-30 minutes and then rapidly quenched. The result is: 1. Increased strength 2. Increased hardness 3. Increased proportional limit 4. Decreased ductility Microscopic changes Diffusion and rearrangement of atoms occur to form an ordered space lattice. Therefore this is called as order hardening or precipitations hardening. iii. SOLUTION HEAT TREATMENT OR SOLUTION HARDENING Page 21
Solidification & microstructure of metals When the alloy is soaked at 700°C for 10 minutes and then rapidly quenched like that for a softening treatment, any precipitation formed during the earlier heat treatment will become soluble in the solvent metal. iv. AGE HARDENING This is a process in which following solution heat treatment; the alloy is once again heated to bring about further precipitation as a finally dispersed phase. This causes hardening of the alloy and it is known as age hardening because the alloy will maintain the quality for many years. E.g. Duralium. METHODS OF STRENGTHENING METALS AND ALLOYS : All metals possess an inherent barrier to dislocations. This is relatively small and known as pearls stress. This is imposed by the bonds associated with the arrangement of atoms in a given crystal structure. Thus to improve the mechanical properties, other methods of hardening are used. These are: 1. GRAIN BOUNDARY HARDENING OR GRAIN REFINEMENT HARDENING A poly crystalline metal contains numerous grains or crystals. These meet at the grain boundaries. The grain boundary is non –crystalline and contains impurities. These act as barriers to dislocations as it moves by slip planes from one grain to another. Finely grained structure contains large grain boundaries and hence the obstacle to motion of dislocations is higher. Therefore dislocation density rises Page 22
Solidification & microstructure of metals rapidly due to plastic deformation. These dislocations at the grain boundaries increase and therefore the stress necessary to continue the plastic deformation also increases. Therefore, there is an increase in the yield strength and ultimate tensile strength. The yield strength varies inversely with the square root of grain size (hall petch equation). Grain refinement can be done by: 1. Heat treatment 2. Addition of grain refiners which act as nucleating agents. Grains refiners are metals or foreign bodies of high melting temperature. They crystallize out at high temperature and act as nuclei or seeds for further solidification. e.g. iridium, rhodium. The best method to improve properties of alloys and metals is by the addition of grain refiners. Finely reined grains structure contain grain size >70µm. 2. SOLUTION HARDENING OR SOLID SOLUTION STRENGTHENING An alloy is a solid solution; it has a solute and a solvent. The atomic diameter of a solute and solvent will never be the same. The principle of solid solution hardening is by introducing either tensile or compressive strain depending on whether the solute atom is smaller or larger than the solvent respectively and finally distorting the grain structure. This solute can be either: Page 23
Solidification & microstructure of metals - Substitutional - Interstitial 3. PRECIPITATION HARDENING Another method of strengthening alloys is by means of this technique. In this, the alloy is heated so that precipitates are formed as a second phase which blocks the movement of dislocations. The effectiveness is greater if the precipitate is part of the normal crystal lattice which is known as coherent precipitation. 4. DISPERSION STRENGTHENING It is a means of strengthening a metal by adding finely divided hard insoluble particles in the soft metal matrix as a result of which, the resistance to dislocations is increased. This increases hardness and tensile strength. The ideal properties are seen when the particles range from 2-15% by volume with spacing at 0.1 – 1.0µm intervals and particle size from 0.01 – 0.1µ. The ideal shape of the dispersed particle is a needle like LAMELLAR SHAPE which can intersect with the slip planes. Powdered metallurgy makes use of this method for strengthening. 5. STRAIN HARDENING OR WORK HARDENING Page 24
Solidification & microstructure of metals This is seen in wrought metals. The metals are worked after casting to improve their mechanical properties. They may be forged, hammered, drawn as wires, etc. All this is done below the re-crystallization temperatures. This working causes vast number of deformations within the alloys or metals. These interact with each other mutually, impeding the movements. The increased stress required for further dislocation movement to achieve permanent deformation provides the basis for work hardening. This result is distorted grain structure with the grains being fibrous. REFERENCES: 1. Anderson’s Applied Dental Materials – John F.Mc. Cabe 2. Dental Materials – Craig. O’Brien – Powers 3. Essentials of Dental Materials – S.H. Soratur 4. Material and Metallurgical Science – S.R.J. Shantha Kumar 5. Phillips Science of Dental Materials (Eleventh Edition) – Anusavice 6. Restorative Dental Materials (Eleventh Edition) – Robert G. Craig and John. M. Powers 7. Restorative Dental Materials – Floyd. A. Peyton 8. J.P.D. April 2002 Volume 87 No.4 Page 351 – 363. Page 25
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