Metallurgy PDF

CHAPTER 6 METALLURGY 102  Chapter (6) 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 period...

CHAPTER 6 METALLURGY 102  Chapter (6) 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 (Fig. 1), could be classed as metals. The basis for a lot of metals properties is the fact that valence electrons are delocalized (unbound) in metallic solids and are free to move throughout the metal rather than remaining bound to individual atoms. From the periodic chart of elements (Fig. 1) 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. Properties of metals: 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. 3. A metallic surface exhibits a luster that is difficult to duplicate in other types of solid matter. This arises from the response of the unbound electrons to electromagnetic vibrations at light frequencies, which give the mirror-reflecting property. 103 104 4. 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. 5. 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. 6. Metals are characterized by their ductility and malleability and this is related to the crystal structure and imperfections which allow for plastic deformation as will be discussed latter. 7. On striking a metal surface, a metallic ring is given. 8. 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. Shaping of metals: a. Casting This involves melting the metal or alloy and shaping it in a mold of the required shape. 105 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 (sintering): Sintering is the process of bonding of solid particles by heat in the absence of any liquid. It is an agglomeration process that involves not only bonding of powder particles but also the elimination of the initial porosity to give a denser product. 106  Sintering occurs naturally if the temperature is high enough to allow for significant number of atoms to diffuse. The atomic diffusion can be aided by applying pressure. It is accompanied by an appreciable amount of shrinkage and decreased porosity when temperature, time and pressure are applied. d. Electroforming: Using the process of electrolysis i.e. corrosion in reverse, a metal can be plated onto a conducting surface e.g. silver and copper electroplating. 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 as shown in the figure. From the figure, temperature decreases from A to B then 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. Cooling or temperature time curve for pure metal. 107  It 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. Structure during solidification: The most accepted theoretical model proposed for solidification of metals is a two-steps mechanism involving nucleus formation and crystallization. a. Nuclear formation: When a molten 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 (homogeneous 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 (Heterogeneous nucleation) 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]. The growth is stopped when there is contact with adjacent growing crystals c. Grain boundary: It is a region of transition between different oriented crystal lattice of the two adjacent crystals [grains]. At the grain boundaries, the atoms take up position intermediate 108 between those of the atoms in the adjacent space lattices thus have higher energy. The atoms at the grain boundaries are located in distorted position to bridge the mismatch in the lattice orientation of adjacent crystals [grains]. Because of their irregular arrangement, grain boundaries affect the properties of polycrystalline solids in various ways: 1. Crystallization and formation of new nuclei in solid phase usually start at the grain boundaries, where there is enough surface energy to start the formation of new set of grains. 2. Diffusion of atoms occurs more readily along grain boundaries. 3. Impurities in metals tend to accumulate at grain boundaries. 4. Grain boundaries also play an important role in the mechanical behavior of metals and affect their corrosion resistance. Control of grain size: In general, the smaller the grain size, the better are the mechanical properties. The factors affecting grain size are: 1. Rate of cooling from the liquid state: As the number of grains is proportional to the number of nuclei of crystallization at the time of solidification, the more rapid the rate of cooling, the more the number of nuclei of crystallization, and the smaller is the grain size. Rapid cooling can be done by using molds of high thermal conductivity, with small sized casting and by heating the metal to just above its melting temperature. 109 2. 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 or may be found as impurity. Effect of Stress on Micro-Structure of Metals: 1. When a material is stressed under its elastic limit, it will deform temporarily. So elastic deformation or strain in a metal is mainly due to stretching of the interatomic bonds. Since the modulus of elasticity is the resistance for elastic deformation and it depends on the chemical composition of the material "nature of the atomic bond" it is not affected by microstructure. 2. Plastic deformation involves the slip of layers of atoms over each other in certain planes in the metal crystals. Slip dose not occur by the movement of an entire plane of atoms over the next layer in a single movement, which requires enormous stress. Instead, the slip occurs by a localized region of shear, which passes progressively through the length of the slip plane. This localized shear zone is called a dislocation and the movement of the localized zones is called movement of dislocation. Wrought metals: These are metals that had been formed from cast or grain structure by cold working to attain a microscopically fibrous structure. Hammering, rolling or drawing into a wire transforms the grain structure into fibrous structure. 110 Cold working and strain hardening: A wrought structure or fibrous structure is plastically formed structure that was subjected to stresses above its yield point at ambient temperature. As mentioned previously plastic deformation occurs by moving of a dislocation through slip planes and become difficult if it meets other type of lattice discontinuity. Greater stress is required to produce further slip. Therefore, the metal becomes stronger and harder by cold work. With further increase in cold working, fracture occurs. In conclusion, cold worked structures are highly stressed structures with increased hardness, strength and proportional limit. On the other hand, these structures have lower ductility and corrosion resistance. The effect of cold working can be reversed simply by heating the cold worked structure and this is termed heat treatment annealing. The heating of a cold worked metal may lead to the following three stages: i. Stress-relief anneal or recovery. ii. Recrystallization. iii. Grain growth. i. Stress-relief recovery: It involves No visible change in the fibrous structure Very slight decrease in strength No change in ductility 111  Recovery of electrical conductivity Any cold worked structure should be annealed before insertion in the oral cavity to provide: - Relief of internal stresses to prevent warpage or fracture during service. - Increase in corrosion resistance. ii. Recrystallization If the temperature is held for longer time, the following will occur: Change from fibrous structure to fine cast structure, because new grains nucleate from the highly distorted grain boundaries The metal will be characterized by low strength, low hardness, and high ductility. iii. Grain growth: If the temperature is held for longer time, the following will occur: Further grain growth occurs Change from fine to coarse crystal [grain] structure The metal will be characterized by detrimental decrease in strength and hardness and very high ductility. Effect of annealing heat treatment on the wrought structure 112  Effect of annealing on the mechanical properties 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 alloy (2-constituents), ternary alloy (3- constituents), quartinary alloy (4-constituents). For simplicity, only binary alloy 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. A solid solution alloy may be formed in which the atoms are distributed randomly in a common space lattice. Because the structure is homogenous, the grains of' solid solution alloys resemble those of pure metals. The metals are said to be soluble in each other in the solid state. 113  2. The two metals may be completely insoluble or partially soluble in the solid state, the solid alloy contains two phases (Eutectic alloy). 3. A new chemical compound can be formed in the solid state (Intermetallic compound) when there is a chemical affinity between the two metals. (1) Solid Solution Alloys Definition: Metals which are completely soluble in each other in both liquid and solid and solid states. Examples: Most dental restorations are solid solutions. Gold and copper, silver and palladium, silver and gold, iron and carbon, cobalt and chromium, nickel and chromium. Types of solid solutions: 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. b. Interstitial solid solutions: Where very small atoms can be accommodated in the spaces between larger atoms e.g. carbon in iron. 114 Conditions for substitutional solid solution: Conditions for the formation of substitutional solid solutions between two metals: 1. They have the same type of space lattice. 2. Metals have the same valence. 3. Their atomic sizes differ by less than about 15%. 4. They do not react to form intermetallic compounds i.e. they do not exhibit a higher degree of chemical affinity. Phase Diagrams: These diagrams are temperature-composition graphs, obtained from a collection of cooling curves of an alloy system. The cooling curves of pure metal compared to an alloy are shown. The graph shows that the pure metal has a melting point, while an alloy has a melting range. The line ABC is a curve passing through all the points (b) and the lower line ADC represents the location of points (c). Because the alloys are molten above all (b) temperatures the alloys must be liquid at all points above the line ABC. For this reason the line ABC is called the liquidus line. The alloys below the line ADC are solid so the line ADC is called the solidus line. In the area between the curves ABC and ADC, the alloy, are partly liquid and partly solid. 115 Coring: From the silver - palladium phase diagram, it is evident that. the composition of the grain is not uniform. It can be recognized therefore that a cored structure results with the core consisting of the higher melting alloy constituents and the matrix containing the lower melting components. This is called coring. It is undesirable particularly in relation to the corrosion resistance of the alloy. A cored structure is formed when: a. The alloy is rapidly cooled after solidifying and b. When the range between the liquidus line and the solidus line is great. Homogenization: It is the process used to eliminate coring by elimination of the compositional differences. This is done by heating the cored structure below its solidus temperature to allow atomic diffusion to take place. 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. They are of high ductility but lower strength and hardness than eutectic alloys. 116  (2) Eutectic Alloys Definition: Metals which are completely soluble in the liquid state but either insoluble or partially soluble in the solid state. Examples: a. Lead and tin: used in soldering but not in dentistry. b. Silver and copper: used in dental soldering and also in admixed amalgam to remove gamma two phase which is the weakest phase in some amalgam fillings. Phase diagram : A phase diagram for a binary system where there is complete solid insolubility (if this is possible in practice) is shown the next figure for lead - tin system. Eutectic alloys have two features of interest: 1. The eutectic alloy is the lowest melting alloy of the system. 2. The temperature-time curve for the eutectic alloy has a horizontal plateau like that of a pure metal (has melting point). Properties of eutectic alloys: 1. They have a melting point. 2. They have poor tarnish and corrosion resistance due to their heterogenous 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. 117 ABC = Liquidus line ADBEC = Solidus line B = Eutectic point D = Eutectic temperature F = Eutectic composition (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: - They are usually very hard and brittle. - Their properties commonly differ from those of metals making up the alloy. Solid State Reactions It allows for atomic diffusion in the solid state by heating the solid metal below its solidus temperature. The gold copper system: The gold copper alloy system exhibits this phenomenon at certain compositions. 118  From the diagram, it could be seen that, the range is very narrow for all compositions, and that the liquidus and solidus actually touch at 80.1 percent gold. The gold copper alloy, whatever the composition, when cooled rapidly from solidus temperature to room temperature, forms a disordered substitutional solid solution (face centered cubic). But if gold copper alloy is allowed to cool slowly: - At temperature below 424 °C with a composition of gold 64-88%, a super lattice AuCu, an ordered face centered tetragonal, is formed within the disordered substitutional solid solution (face centered cubic) - At temperature below 396 °C with a composition of gold 39-64%, a super lattice AuCu3, an ordered face centered cubic, is formed within the disordered substitutional solid solution (face centered cubic) 119  The larger amount of copper in the Au Cu3 phase is not compatible with dental applications. Also, this super lattice AuCu3 is of little aid in changing the mechanical properties of the alloy because its lattice has essentially the same dimensions as those of the disordered lattice. The composition of most of the casting gold alloys is within the compositional limits (64-88% gold) for the Au Cu phase. If a gold - copper alloy is cooled rapidly (quenched) from the solidus temperature, the disordered solid solution is retained at room temperature, and the alloy is relatively soft. Slow cooling allows diffusion to occur and the alloy would partially transform to the ordered phase and would be harder than the quenched alloy. Methods of altering the mechanical properties of alloys: 1) Work hardening: The properties of an alloy can be altered by cold working where there are: 1. Increase in harndess 2. Greater yield stress and ultimate strength 3. Less ductility. 2) Solution hardening: In a substitutional solid solution, (lie atoms of the two metals are of different sizes, though the size difference is usually less than 15%. Consequently the crystal lattice of such an alloy is distorted by the presence of either smaller or larger atoms. These distortions stop the movement of dislocations and raising the strength. Thus, gold alloys containing copper or silver are harder, stronger and less ductile than pure gold. 120 3) Order hardening: As in the gold-copper system where the ordered superlattice Au cu is formed. The formation of a (tetragonal lattice within cubic structure involves contraction of one of the crystal axes. This sets up strains which interfere with the movement of dislocations. Hence, the yield stress, ultimate Strength and hardness are raised. 4) Precipitation hardening: Atomic diffusion in the solid state added by temperature for sufficient time may cause precipitation of certain phases within the structure of the parent alloy. This creates localized lattice distortion in the parent alloy inhibiting the dislocation movement. - Strength and hardness are increased while ductility is decreased. - New precipitated phases increase the heterogeneity of the alloy decreasing its corrosion resistance 121