1. Classification( Depenting on dominant phase system)
   1. Isomorphous/ Solid solution/ single phase Eg; Gold alloys
      1. Only one phase is formed during solidification Structure is homogenous Metals are mutually soluble in solid state- hence the name
      2. Harder stronger and more ductile than either of the metals that make up the alloy Can be easily polished and burnishable Melt below the melting point of highest fusing metal. Have melting range than melting point
   2. Eutetctic Eg; Silver copper system
      1. Binary alloy exhibit complete liquid solubility but only liquid solubility Temperature at which eutectic occurs is lower than the fusion temperature of either of the metals
      2. Low melting point Brittle, hard Poor tarnish and corrosion resistance Strength and hardness surprass than those of some of the constituent metal
      3. There is no solidification range, Instead they have definite temperature which is characteristic of an eutectic composition like pure metals Inferior to solid solution Used when lower fusion temperature is desired Eg; soldering
         1. Pure components are insoluble or partially soluble in solid state In liquid state they are completly soluble Thus two metals are partially insoluble at a given temperature and composition- Eutectic composition
         2. Eutectic composition is called invariant transformation Because it occurs at a single composition and temperature
   3. Peritectic
      1. Limited solid solubility This type of reaction occurs when there is a big differences in the melting points of the components
      2. Not Of Much Use In Dentistry Except For Silver Tin System
   4. Intermetallic system Eg; silver tin in copper rich amalgam alloy
      1. Metals in solution in the liquid state that has tendency to unite and form definite compounds on solidifying Has wide range of solubility
      2. Hard Very brittle High melting point Physical properties cannot be accurately predicted
2. hardening METHODS
   1. Work hardening/ Strain hardening/ Cold working
      1. Material is strained past its yield point. Takes place at a temperature below the material's recrystallization temperature
      2. Increases hardness, Yield strength, Ultimate tensile strength Decreases ductility
      3. Distorted grains Internal stress Eg; Orthodontic appliances produced by bending wires have distorted grains and internal stresses, which will gradually relieve and may cause warpage of appliance which will lead to loss of fit
   2. Solution hardening
      1. Solid solution strengthening
         1. An alloying element is added to the desired material to be strengthened- together they form solid solution Substitutional or interstitial solid solution can be made Eg; Gold alloy contains Copper and Silver- Harder, stronger and ductile than pure gold
      2. Age hardening
         1. Done in precious metals, stainless steel and NiTi Carried out in temperature below the annealing range and cooled slowly Increases hardness, resistance to deformation and improve strength
      3. Precipitation hardening
         1. Most commonly used technique Impure particles are distributed throughout the metal Increases strength and hardness Reduces ductility
         2. Procedure Heating the alloy to 745-760 oC Holding at this temperature for 15minutes Quenching rapidly on cold water The alloy is now in softened condition, and can be re-hardened by heating to 316 oC for 30-50 minutes and then air cooling
   3. Order hardening
      1. Alloy is maintained at an elevated temperature for sufficient time to permit diffusion of atoms
      2. I f 50% Gold and 50% copper alloy is cooled rapidly from 450 oC the lattice structure id random or distorted Slow cooling permits formation of regions of an ordered substitutional solution That is ordered lattice has three copper atoms to each gold atom (Cu3Au)
3. Sofetnening method
   1. solution heat treatment/ heat softening method
      1. The process in which alloy or metal is heated to a suitable temperature, is held at that temperature and rapidly quenched
      2. Softens the metal and increases the ductility
   2. Stress relieving/ Stress relief anneal recovery
      1. Releases internal stresses incorporated into metal due to cold working procedure which can come as a cracking and distortion
      2. Recovers its original ductility Decreases hardness, elastic limit and ultimate strength
   3. tempaering
      1. Process done subsequent to quench hardening Always follows never precedes hardening operation Done in stainless steel
      2. Its a balance between strength and ductility Increases toughness Eliminates brittleness Softens the steel so some amount of hardness is lost
4. Anneling/ heat soft treatment
   1. Low temperature heat treatment process given to cold worked metals to remove internal stresses and distorted grains
      1. The process of controlled heating and cooling that is designed to produce desired properties in a metal.
      2. Soften metals, to increase their ductility, stabilize shape, and increase machinability
   2. requirements
      1. Higher the melting point higher is the annealing temperature Usually Annealing temperature= 1/2 ( Melting point)
      2. Time of annealing depends on - Degree of cold work done - Size of the particle
   3. Stages
      1. Recovary
         1. Stage at which properties due to cold work begin to disappear Slight decrease in tensile strength Electrical conductivity recovered No change in ductility
            1. No significant change in microscopic structure
      2. Recrystallization
         1. On completion material attains its original soft and ductile condition
            1. Rapid change in microscopic structure Fibrous structure in cold worked condition transformed into small equal sized grains and the metal is said to be recrystallized
      3. Grain growth
         1. Larger grain structure can lead to a decease in strength, hardness and proportional limit Increase in ductility
            1. Grain growth may occur
5. Other key points
   1. Basic unit of metal is grain or crystal
   2. Heat treatment
      1. Metal is heated for a length of time above room temperature but below solidus temperature
      2. Result depends on the metal, temperature and previous history of metal May harden or soften the metal Change its grain size and corrosion resistance
   3. Quenching
      1. Rapid cooling of a metal to room temperature or below Done by immersing the hot metal in water , hence the name
      2. Purpose To preserve at room temperature a phase that is stable only at an elevated temperature To rapidly terminate a process that occurs only at elevated temperature
   4. Welding
      1. Process of fusing two or more metal parts through the application of heat, pressure, or both, with or without a filler metal, to produce a localized union across an interface between the workpieces.
   5. Soldering
      1. Process of building up a localized metal area with a molten filler metal or joining two or more metal components by heating them to a temperature below their solidus temperature and filling the gap between them using a molten metal with a liquidus temperature below 450 °C.
   6. Flux
      1. Compound applied to metal surfaces that dissolves or prevents the formation of oxides and other undesirable substances that may reduce the quality or strength of a soldered or brazed area
   7. Antiflux
      1. A substance such as graphite that prevents flow of molten solder on areas coated by the substance.