Metal and Alloys PDF

Chapter 6 Metals and Alloys  Metals and alloys have many uses in dentistry.  Steel alloys are commonly used for the construction of instruments and of wires for orthodontics.  Gold alloys and alloys containing chromium are used for making crowns, inlays and denture...

Chapter 6 Metals and Alloys  Metals and alloys have many uses in dentistry.  Steel alloys are commonly used for the construction of instruments and of wires for orthodontics.  Gold alloys and alloys containing chromium are used for making crowns, inlays and denture bases  whilst dental amalgam, an alloy containing mercury, is the most widely used dental filling material shaping of metals  The shaping of metals and alloys for dental use can be accomplished by one of three methods, namely, casting, cold working or amalgamation.  Casting involves heating the material until it becomes molten, when it can be forced into an investment mould which has been prepared from a wax pattern.  Cold working involves mechanical shaping of the metal at relatively low temperatures, taking advantage of the high values of ductility and malleability possessed by many metals.  Some alloys can be mixed with mercury to form a plastic mass which gradually hardens by a chemical reaction followed by crystallization(amalgamation). Structure and properties of metals Crystal structure  Metals usually have crystalline structures in the solid state.  When a molten metal or alloy is cooled, the solidification process is one of crystallization and is initiated at specific sites called nuclei.  The nuclei are generally formed from impurities within the molten mass of metal’  Crystals grow as dendrites or spherulites, which can be described as three-dimensional, structures emanating from the central nucleus  Each crystal is known as a grain and the area between two grains in contact is the grain boundary.  After crystallization, the grains have approximately the same dimensions in each direction, measured from the central nucleus.  They are not perfectly spherical or cubic however, nor do they conform to any other geometric shape.  They are said to have an equiaxed grain structure  Change from an equiaxed structure to one in which the grains have a more elongated, fibrous structure can cause important changes in mechanical properties.’  The atoms within each grain are arranged in a regular three-dimensional lattice.  There are several possible arrangements such as cubic, body-centred cubic and face-centred cubic   Although there is a tendency towards a perfect crystal structure, occasional defects occur Such defects are normally referred to as dislocations and their occurrence has an effect on the ductility of the metal or alloy  The plane along which the dislocation moves is called a slip plane and the stress required to initiate movement is the yield stress  The concentration of grain boundaries increases as the grain size decreases.  Metals with finer grain structure are generally harder and have higher values of yield  stress than those with coarser grain structure  The proof stress is the stress required to produce a certain level of permanent strain.  For example the 0.2% proof stress indicates the stress required to produce a strain of 0.002.  A fine grain structure can be achieved by rapid cooling of the molten metal or alloy following casting.  This process, often referred to as quenching, ensures that many nuclei of crystallization are formed, resulting in a large number of relatively small grains  Slow cooling causes relatively few nuclei to be formed which results in a larger grain size  Some metals and alloys are said to have a refined grain structure.  This is normally a fine grain structure which is achieved by seeding the molten material with an additive metal which forms nuclei for crystallization Cold working  For an applied tensile force the maximum degree of extension is a measure of the ductility of the metal or alloy.  For an applied compressive force the maximum degree of compression is a measure of malleability   These changes occur when the stress is greater than the yield stress and at relatively low temperatures  The grains are no longer equiaxed but take up a more fibrous structure’  The properties of the material are altered, becoming harder and stronger with a higher value of yield stress’  The ductility or malleability is decreased because the potential for further cold working is reduced.  Cold working is sometimes referred to as work hardening due to the effect on mechanical properties  The temperature below which work hardening is possible is termed the recrystallization temperature. Examples of cold working in dentistry include the following 1) The formation of wires, in which an alloy is forced through a series of circular dies of gradually decreasing diameter. The resulting fibrous grain structure is responsible for the special springy properties possessed by most wires. 2) The bending of wires or clasps during the construction and alteration of appliances. 3) The swaging of stainless steel denture bases  If a cold-worked metal or alloy with a fibrous grain structure is heated to above its recrystallization temperature it gradually reverts to an equiaxed form and becomes softer with a lower value of yield stress but a higher ductility  Hence, recrystallization can be used as a softening heat treatment  If the material is maintained above the recrystallization temperature for sufficient time, diffusion of atoms across grain boundaries may occur, leading to grain growth  Cold working may cause the formation of internal stresses within a metal object  For certain metals and alloys the internal stresses can be wholly or partly eliminated by using a low temperature heat treatment referred to as stress relief annealing Alloy  It the is mixture of two or more metals  When molten mixture is cooled below the MP these four things occurs 1) Solid solution  Component metal may remain soluble in eachother forming a solid solution , it my take form of  Random solid solution= metal atom occupy random site in common crystal structure  Ordered solid solution=metal atom occupy specific site in crystal structure  Interstitial solid solution= for binary alloy the primary lattice site are occupied by one metal and the atom of 2nd component doesnot occupy the site  Solid solution = are generally harder , stronger and have high value of elastic limit 2) Component metals  Completely insoluble in solid state → susceptible to electrolytic corrosion 3) May be partially soluble in eachother 4) Two metals show particular affinity for each other and form intermetallic compounds Cooling curves  The material is heated till molten then allow to cool then plot a temp against time is recorded  For pure metals we get plateau phase , means temperature remains constant over a period of time during crystallization Phase diagrams  Temperature range over which alloy crystal can be obtained from cooling curves Liquidus line = temperature in region above the line T1 is called liquidus line , alloy remains totally liquid Solidus line = the temperature in region below the line T2 is called solidus line , alloy remains solid  Temperature in region between the T1 and T2 alloy consists of mixture of solid and liquid PERITECTIC ALLOYS  Another example of the limited solid solubility of two metals is the peritectic transformation. Coring  defects in alloy when heated alloy is cooled to fast for diffusion to occur →exterior surface cools and solidifies before the interior portion which remains hot and soft. Homogenization heat treatment  It is used to eliminate cored structure  Alloy is heated just below solidus line so that diffusion may occur  The alloy is then quenched in order to prevent grain growth from occurring  An example of solid solution is gold-silver system