Metallurgy PDF

Metallurgy It is the study of metals and alloys. I. Metals: A metal is any element that ionizes positively in solution. About 80 of the 103 elements currently listed in the periodic table of elements could be classed as metals. Fro...

Metallurgy It is the study of metals and alloys. I. Metals: A metal is any element that ionizes positively in solution. About 80 of the 103 elements currently listed in the periodic table of elements could be classed as metals. From the periodic chart of elements, non-metals occupy the right side with a stepwise transition group of elements called metalloids. Metalloids or semiconductors are at the boundary between metals and nonmetals and they have properties incoming with both or midway between both, e.g. Carbon, Silicon and Boron. 1  Properties of metals: Generally, the properties of metals result from both their crystalline structure and Metallic bond where valence electrons are delocalized and free to move throughout the metal rather than the remaining bounded electrons. 1. Ionize positively in solution. 2. In a normal environment, they are crystalline solids with the exception of mercury and gallium which are liquids at room temperature and another exception is hydrogen, which is a very reactive metal; it exists as a gas at room temperature. 3. Opacity results from the ability of the valence electrons to absorb light. 4. A metallic luster arises from remission of light by free electrons 5. Metals are good electrical and thermal conductors. This is because free electrons are efficient carriers of thermal as well as electrical energy along a potential gradient. 6. Metals are characterized by high hardness, melting and boiling points. These properties are due to the strength of the primary interatomic bonding within the crystalline solid. 7. Metals are characterized by their ductility and malleability because they have many slip planes allowing dislocation movement. 8. On striking a metal surface, a metallic ring is given. 9. Most metals are white with slight differences in tint. Two metals are non white, gold and copper, both happen to be rather important in dentistry.  Pure Metals in dentistry: 1- Gold foil as posterior direct restorative material 2- Mercury and Gallium in amalgam restorations 3- Titanium in implant and prosthetic appliance 2 Shaping of metals: a. Casting: This involves melting the metal or alloy and shaping it in a mold of the required shape b. Cold Working: - A solidified block of a cast metal can be formed by mechanical working to produce a rod, wire, tube or other shapes. - During the process of cold working, stresses are applied above the yield strength of the material where the mechanism of plastic deformation is through slip along crystal planes involving dislocation movements. c. Powder metallurgy: - It involves bonding of metallic solid particles by pressing and heating below the melting temperature of the metal in non- oxidizing atmosphere. - It is accompanied by shrinkage and elimination of porosity. - E.g. amalgam tablets 3 Solidification of metals: - If a metal is melted and then allowed to cool, its temperature during cooling can be plotted as a function of time. - From the figure, temperature decreases from A to B at which temperature is constant until time C. After C, the temperature decreases steadily to room temperature. - Temperature Tf indicated by the straight or plateau portion BC, is the freezing point or fusion temperature. - During freezing, heat is evolved as the metal changes from the liquid to the solid state; this heat is the latent heat of fusion which is equal to the heat of fusion and equals to the number of calories of heat liberated from one gram of a substance when it changes from the liquid to the solid state. - At all temperatures above Tf, the metal is molten and at all temperatures below Tf, it is a solid. - Once the crystals begin to form, the latent heat of fusion causes the temperature to rise to temperature Tf where it remains until the crystallization is completed. 4   Mechanism of solidification Solidification of metals occurs through two-steps mechanism a. Nuclear formation: When a molten metal or an alloy is cooled and approaches its freezing temperature, the atoms try to aggregate forming initial starting points of crystallization [nuclei of crystallization] at supper cooling point.  Nucleus formation can occur by: - Homogenous nucleation: The atoms of the metal itself form the nuclei of crystallization. - Heterogeneous nucleation Foreign solid metallic particles e.g. iridium, which has a higher melting temperature than that of the liquid metal are added to the liquid metal. b. Crystallization: - The metals can solidify in single crystal [grain] which is very rare, or polycrystalline. - As cooling continues the nuclei of crystallization grow independently in three dimensions [tree like structure] to form crystals [grains]. - growth continue until contact is made with adjacent growing crystals - The atoms at the grain boundaries are located in distorted positions intermediate between those atoms in the adjacent space lattices thus having higher energy 5   Factors affecting grain size are: 1. Rate of cooling from the liquid state: 2. Rapid cooling  increase number of nuclei of crystallization  increase numbers of grains  smaller is the grain size. 3. Nucleating agents "grain refiners": Addition of certain nucleating agents, during solidification will act as nuclei of crystallization producing castings of small grain size i.e. increase the number of nuclei of crystallization. These are called grain refiners which may be added intentionally eg: Irridium or may be found as impurity. II. ALLOYS Pure metals are not suitable for dental applications because they are too soft and ductile. The mechanical properties are improved by using mixtures of metals. These are called alloys. An alloy is a combination of two or more metals which are soluble (miscible) in the molten condition. Classification of Alloys: 1) According to the number of alloying elements: Binary alloys (2-constituents), ternary alloys (3-constituents), quaternary alloys (4-constituents), etc. For simplicity, only binary alloys will be studied. 2) According to the miscibility of the atoms in the solid state: When two molten metals are mixed, they usually form a solution in the molten condition. A solution is defined as a perfectly homogenous mixture. On cooling such a solution, one of three things may happen: 1- Solid solution alloy: The two metals are soluble in each other in the liquid and solid state. homogenous, one phase. 2- Eutectic alloy: The two metals are soluble in liquid state and may be completely insoluble or partially soluble in the solid state, the solid alloy contains two phases. 3- Intermetallic compound: A new chemical compound can be formed in the solid state when there is a chemical affinity between the two metals. 6  (1) Solid Solution Alloys: Definition: Metals which are completely soluble in each other in both liquid and solid states. Types of solid solution: Solid solutions may be of two types: substitutional or interstitial. a. Substitutional solid solutions: Where two different types of atoms occur in different positions in the same space lattice, Ordered Disordered Conditions for substitutional solid solution: a) They have the same type of space lattice. b) Metals have the same valence. c) Their atomic sizes differ by less than about 15%. d) No chemical affinity; otherwise, intermetallic compounds may be formed. b. Interstitial solid solutions: Where very small atoms can be accommodated in the spaces between larger atoms e.g. carbon in iron.  Properties of solid solution alloys: 1. They have melting ranges. 2. They are homogenous (their solid is one phase) and so resist tarnish and corrosion. 3. The strength, and hardness are increased, whereas the ductility is usually decreased 7  (2) Eutectic Alloys: Definition: - Metals which are completely soluble in the liquid state but either insoluble or partially soluble in the solid state. - Eutectic is a Greek word that means lowest melting. - For an alloy system that has a eutectic composition there is a eutectic temperature which has a melting point not a melting range. Properties of eutectic alloys : 1. They have a melting point. 2. They have poor tarnish and corrosion resistance due to their heterogeneous structure (two phases system). 3. They are brittle because of the presence of insoluble phases that inhibit dislocation movement. 4. The strength and hardness are higher than those of the constituent (parent) metals because of the heterogeneous nature of the alloy. (3) Intermetallic compound: Upon solidification, both metals have chemical affinity towards each other forming intermetallic compounds with certain composition and below certain temperature. Properties of intermetallic compound: 1. They are usually very hard and brittle. 2. Their properties commonly differ from those of metals making up the alloy. 8 Differences between solid solution and eutectic alloys: General differences between solid solution alloys and eutectic alloys may be summarized as follow: Solid solution Eutectic Solubility in the solid Soluble Partial or incomplete state solubility One phase Two phases Phases “homogenous” “heterogeneous” Melting Melting range Melting point “very low” increase in strength Properties and hardness and Strong, hard, brittle decrease in ductility Resistance to tarnish and corrosion High Low Most of metallic dental Used only: restorations are solid - In soldering solution alloys e.g. Uses base metal alloys and gold alloys 9