Engineering materials and metallurgy -SOLID SOLUTIONS

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Engineering materials and metallurgy

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ALLOY  An alloy is a combination of two or more elements of which one of the element should be a metal in major proportion. Example : Brass Cu-Zn Steel Fe-C etc… Classification of alloys: iPure metal iiSolid Solution iiiIntermediate phase Dr.K.RaviKumar Dr.N.G.P .Institute of T echnology

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Solid solutions  Solid solution is an alloy in which the solute atoms are distributed in the solvent matrix and has the same structure of the solvent. The element which is present in larger amount in the alloy is called solvent and the other element is called solute.  Solid solutions are of two types namely  i Substitutional  ii Interstitial

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 In a substitutional solid solution the atoms of the solvent metal are replaced in the crystal lattice by atoms of the solute. This substitution is either ordered or disordered.  In the ordered solid solution the substitution of either atoms in solvent is by a definite order while this is not so in a disordered solid solution.  Substitutional solid solution formation is favoured when the atomic sizes of the two metals are almost equal. Au in Cu  Cu-Zn Cu2MnAl- ordered solution  Interstitial solid solutions are formed only when the atoms of the solute element are very small compared with those of the solvent thus enabling them to fit into the interstices or spaces in the crystal C in FCC iron Nitrogen in steel

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Hume Rothery’s rules of solid solubility  In the formation of solid solution the solubility limit of a solute in the solvent is governed by certain factors. These factors are known as Hume Rothery’s rules of solid solubility  Relative Size  Chemical Affinity  Relative Valency  Crystal Type

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Relative Size  If the atomic sizes of solute and solvent differ by less than 15 conditions are favorable for the formation of solid solution  If the atomic size difference exceeds 15 solid solution formation is extremely limited

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Chemical Affinity  When two elements have a high chemical affinity for each other the greater is tendency to restrict the solid solution and to form intermediate phases.  This occurs when one element is electronegative and the other is electropositive.

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Relative Valency  A metal of higher valence valence electrons dissolves only a small amount of the lower valence metal  The lower valence metal dissolves greater amount of the higher valence metal. Al High valence – 0.04 Ni- Ni low valence- 5 Al

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Crystal Type  If two metals are of the same crystal lattice and all the other factors are favorable it is possible for complete solid solubility.

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Intermediate Phases or Compounds  Formed between two dissimilar elements having widely divergent electrochemical properties.  Types  Intermetallic compounds  Electron compounds  Interstitial compounds

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Intermetallic Compounds  Obeys the valence law  Formed when one metal such as Mg having chemical properties which are strongly metallic and other metal such as Sn Pb Bi which are weakly metallic.  They have poor ductility and low electrical conductivity. Mg 3 Bi 2 Mg 2 Sn

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Electron Compounds  Does not obey valence law  There is a fixed ratio between the total number of valence electrons and total number of atoms. There are three such ratios  3:2  21:13  7:4  e.g.1 Ratio : 3:2 : CuZn ◦ Cu has valence electron 1 ◦ Zn has valence electron 2 ◦ T otal has valence electron 3 ◦ No of atoms 2

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e.g.2 Ratio : 21:13 : Cu 31 Sn 8 •Cu has valence electron 1 •Sn has valence electron 4 •T otal has valence electron 63 •No of atoms 39 •Ratio 63:39 or 21:13

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Interstitial compounds  Formed between certain transition metals and small non metallic atoms  Have metallic properties and comprise hydrides nitrides carbides borides etc and they are very hard  TiC WC TiN.

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Phase Diagrams  A phase is the chemically and structurally homogeneus portion of the microstructure.  Phase diagram is graphical representation of phases in the system at various temperature pressure and compositions.  Depending on the number of components they are called as Unary Binary and T enary phase diagrams.

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Phase Rule Gibbs Phase rule P+F C+2 P - No of phases in the system F - Degree of freedom No of variable temperature pr or composition that could be changed independently without changing the no of phases C- No of components in the system

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Phase Diagram for pure Mg

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Unary phase diagram for water

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Lever rule  The lever rule is a tool used to determine weight percentages of each phase of a binary equilibrium phase diagram. It is used to determine the percent weight of liquid and solid phases for a given binary composition and temperature that is between the liquidus and solidus.

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The percent weight of element B at the Liquidus is given by w l The percent weight of element B at the solidus is given by w s . w o is the percent weight of element B for the given composition

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Binary Alloy Phase Diagrams  Types i Components completely soluble in liquid state and a completely soluble in solid state Isomorphous system b Partially soluble in solid stateEutectic Reaction I c Insoluble in solid stateEutectic Reaction II d Peritectic reaction ii Components partially soluble in liquid state and a completely soluble in solid state b Partially soluble in solid state iii Components completely insoluble in liquid state and Insoluble in solid state

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 Transformations in solid state ◦ Eutectoid reaction ◦ Peritectoid reaction

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Isomorphous system: soluble in liquid state and completely soluble in solid state: Cu-Ni Au-Ag Au-Cu Au-Ni

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Eutectic Phase I Completely insoluble in solid state :Bi-Cd Pb-As Au-Si

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Eutectic phase II- Partially soluble in solid state Ag-Cu Pb-Sn Al-Si

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Peritectic reaction: Ag-Pt

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Solid State Reactions: Eutectoid: Cu-Sn Zn-Al Al-Mn Peritectiod: Ni-Mo Fe-Nb

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Cooling curve for pure iron

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Iron-Iron Carbide Phase Diagrams

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Microstructures

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Ferrite α :  Iron which contains no carbon  Very soft and ductile known as α iron.  Maximum solubility of carbon is 0.02 at 723° Ferrite δ : Maximum solubility of carbon is 0.1 at 1492° Austeniteγ :  Solid solution of Fe-C which is stable only within a particular range of composition and temperature.  Maximum solubility of carbon is 2.08 at 1147° and decreases to 0.8 at 723°  Soft ductile and malleable phase

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Cementite:  Carbide of iron  Extremely hard of low tensile and high compressive strength  Fixed carbon of 6.67 Pearlite:  Alternate layers of ferrite and cementite  Contains about 0.8 carbon  87.5 ferrite and 12.5 cementite Ledeburite:  Eutectic mixture of austenite and cementite  Contains 4.3 carbon  Formed at about 1130°

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Martensite:  Brittle mass of fibrous or needle like structures  Normally is a product of quenching  Chief constituent of hardened steel T oorsite:  Mixture of radial lamellae of ferrite and fine cementite  Produced on tempering below 450° Sorbite:  Mixture of ferrite and fine cementite  Produced on tempering below 450° Bainite Bainite is an acicular microstructure not a phase that forms in steels at temperatures from approximately 250-550°C A fine non-lamellar structure bainite commonly consists of cementite and dislocation-rich ferrite.

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Classification of steel Types Properties Ultimate Tensile Strength N/m2 Carbon Application Dead Mild Steel good formability very low strength 320-400 0.05 – 0.15 Chain rivets wires nails Low carbon steels good formability and weld ability low strength 400-600 0.15 – 0.20 Screws deep drawing parts pipe some machine parts 0.20-0.30 Gears shafts levers forgings Medium carbon steels good toughness and ductility relatively good strength may be hardened by quenching 550-850 0.30 – 0.40 Connecting rods shafts axels 0.40 – 0.50 crankshafts axles gears shafts 0.50 – 0.60 Laminated springs rails wire ropes heat treated machine parts High carbon steels high strength hardness and wear resistance moderate ductility 650-1400 0.7 – 1.0 rolling mills rope wire screw drivers hammers wrenches band saws Tool carbon steels very high strength hardness and wear resistance poor weld ability low ductility 900-1600 1.0 – 2.1 punches shear blades milling cutters knives razors dies drills taps mill cutters etc

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Alloying Elements  NiCrC Metal Percentage Nickel 0.4-0.7 Silicon 0.3-0.6 Chromium 0.4-0.6 Molybdenum 0.15-0.3 Manganese 0.5-1.0 Tungsten Silicon Copper

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Carbon content and mechanical properties in steel

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Classification of Cast Iron  White cast irons  Grey cast irons  Malleable cast iron  Nodular ductile cast irons  Chilled cast Irons

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White cast irons  Chemical composition: C2.0-3.6 Mn0.2-0.8 Si0.5-2.0 Ni+Cu1.5 Cr1P0.15 S0.15 Mo0.5  Si content is low 1 in combination with faster cooling rates  White cast irons – hard and brittle highly wear resistant cast irons consisting of pearlite and cementite.  White cast irons are produced by chilling some surfaces of the cast mold. Chilling prevents formation of Graphite during solidification of the cast iron.  Applications of white cast irons: brake shoes shot blasting nozzles mill liners crushers pump impellers and other abrasion resistant parts.

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Micro structure of white cast Irons

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Grey cast irons  Chemical composition: C2.7-4 Mn0.4-1.0 Si1.8- 3 S0.07 max P0.2 max  Alloying addition of Si 1-3wt. is responsible for decomposition of cementite and also high fluidity  Grey cast irons – cast irons produced at slow cooling and consisting of ferrite and dispersed graphite flakes.  Grey cast irons possess high compressing strength fatigue resistance and wear resistance. Presence of graphite in grey cast irons impart them very good vibration damping capacity.  Applications of grey cast irons: gears flywheels water pipes engine cylinders brake discs gears.

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Micro structure of gray cast Irons

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Malleable cast iron  Chemical composition: C2-2.7 Mn0.25-1.25 Si1- 1.75 S0.03-0.18 P0.05max  Malleable cast iron – cast irons produced by heat treatment of white cast irons and consisting of ferrite and particles of free graphite.  Malleable cast irons have good ductility and machinability. Ferritic malleable cast irons are more ductile and less strong and hard than pearlitic malleable cast irons.  Applications of malleable cast irons: parts of power train of vehicles bearing caps steering gear housings agricultural equipment railroad equipment.

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 Micro structure of malleable cast Irons

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Nodular ductile cast irons  Chemical composition: C3.5-3.9 Mn0.15- 0.35 Si2.25-2.75 S0.01-0.025 Mg-.06 P0.05max  Nodular ductile cast irons – grey cast iron in which graphite particles are modified by magnesium added to the melt before casting. Nodular cast iron consists of spheroid nodular graphite particles in ferrite or pearlite matrix.  Ductile cast irons possess high ductility good fatigue strength wear resistance shock resistance and high modulus of elasticity.  Applications of nodular ductile cast irons: automotive engine crankshafts heavy duty gears military and railroad vehicles.

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 Micro structure of Nodular cast Irons

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Name Nominal composition by weight Form and condition Tensile strength ksi Elongation in 2 inches Hardness Brinell scale Grey cast iron C2.7-4 Si 1.8 Mn 0.5 Cast 25 0.5 180 White cast iron C2.0-3.6 Si 0.7 Mn 0.6 Cast as cast 25 0 450 Malleable iron C 2-2.7 Si 1.0 Mn 0.55 Cast annealed 52 12 130 Ductile or nodular iron C3.5-3.9 P 0.1 Mn 0.4 Ni 1.0 Mg 0.06 Cast 70 18 170

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Wrought Iron  Wrought iron is an iron alloy with a very low carbon content in comparison to steel and has fibrous inclusions known as slag.  0.025 C 0.13 Mn 0.10 Si 0.13 P and 0.01 S.  Applications includes rivets nails wire chains railway couplings water and steam pipes nuts bolts horseshoes handrails straps for timber roof trusses and ornamental ironwork

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Chilled Cast Iron  When localized area of a gray cast iron is cooled very rapidly from the melt cast iron is formed at the place that has been cooled. This type of white cast iron is called chilled iron. A chilled iron casting can be produced by adjusting the carbon composition of the white cast iron  Chromium is used in small amounts to control chill depth. Because of the formation of chromium carbides chromium is used in amount of 1 to 4 percent in chilled iron to increase hardness and improve abrasion resistance.  Chilled cast iron is used for railway-car wheels crushing rolls stamp shoes and dies and many heavy-duty machinery parts.

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Spheroidal Graphite Cast Iron  The chemical composition of the cast iron is similar to that of the grey cast iron but with 0.05 wt of magnesium. All samples are etched using 2 nitalalcohol and nitric acid.  Spheroidal graphite cast iron usually has a pearlitic matrix.  However annealing causes the carbon in the pearlite to precipitate on to the existing graphite or to form further small graphite particles leaving behind a ferritic matrix.  This gives the iron even greater ductility

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Compacted graphite Iron • Produced when molten iron is desulphurised and treated at 1400 0 C • Alloy containing approximate amounts of MgTi • Sometimes called as Vermicular graphite • Contains short graphite flakes and have round edges • Intermediate between gray and nodular cast iron • Applications-Gear pumps fluid and air cylinders vehicle brake compoundsengine blocks cylinder heads

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