Theory Of alloys in engineering materials.pdf

Theory Of alloys in engineering materials

2. Alloys
•Alloysarecombinationsormixturesofelements.
•Metalsarealloyedtoimproveonpropertiesofpuremetalssuch
ashardness,strength,corrosionresistance,etc.
•Ex.:-Insteadofpurealuminumanalloyofaluminumhaving
combinationofAl–Zn–Mg–Cu–Mn(Afiveelementalloy)is
usedtoconstructtheaircraftbody.
•Alloysmaybehavedifferentlywhencombined,somemixeasily
whileotherswillonlybesolubletoalimitedextent.

3. Alloys
➢Analloysystemcontainsallthealloysthatcanbeformedby
severalelementscombinedinallpossibleproportions.
BinaryAlloySystem-Ifthesystemismadeupoftwoelements
itiscalledbinaryalloysystem.
TernaryAlloySystem-Ifthesystemismadeupofthree
elementsitiscalledternaryalloysystem.
➢Ineachsystemlargenumberofdifferentalloys–Seriesofalloys
possible.
➢Ex:-Carbon,Silicon,manganese,nickel,chromium,
molybdenum,tungsten,vanadium,copperandAluminum

4. Mixture Of Two Liquids:-
➢WhenAlloyareformedsolid
orliquidsolutionscanform.
➢Ex:
➢Sugaraddedintowaterforman
aqueousSugarsolution
➢Phosphorusaddedintosinglecrystalsilicontheyforma
solidsolution.
➢Whentwoliquidmetalaremixedinvaryingproportion;
I.Complete(unlimited)Solubility
II.Partial(limited)Solubility
III.Completeinsolubility
Base
Metal
Alloying
Element
Alloy

5. Complete (unlimited)Solubility
➢Weaddwater+ethylalcoholandstirthemuponstandstillonly
onephaseappear.
➢Thissolutionhasuniquepropertiesandcompositions.Itappears
likeonephasehomogeneoussolutions.
➢Otherexamples:-Cu-Ni,Pt-Au,Ge-Si,andAg-Au.
➢Asolutionisnotamixture.
➢Itcontainssoluteandsolventdissolvedintoeachother.
➢Solute:-Majorportion
➢Solvent:-Minorportion

6. Partial(limited)Solubility
➢Here,eachliquidispartiallysolubleintheother.Uptothatlimitthe
solutionwhichisformedishomogeneoussolution.
➢WhenmoreSoluteisaddedlimitofsolubilityreached.
➢Nowtwolayerformed
I.MoreDenseroneatthebottom.
II.LessDenseroneatthetop.
❑Ex:-Phenol+WaterSolution.
❖Attopwaterwithsmallamountofphenoldissolvedintoit.
❖AtbottomPhenolWithsmallamountofWaterdissolvedintoit.
❑SiliconandGermaniumDopedwithPhosphorus,Boron,Arsenic,
Etc.Toproducesemiconductingmaterials.

7. Complete Insolubility
❑EachLiquidiscompletelyinsolubleintheother.
➢Mixtureoftwoliquidsalwaysseparateintotwolayers.eachliquid
cannotbedissolvedintooneanothertheycanbedispersedintoone
another.
➢Tocompleteinsolubility,
Moredissimilararethecomponents,bothchemicallyandatomicin
size.
❑Whenoilandwateraremixedtogetherupperoneisoilandlower
oneiswater,accordingtotheirdensities.
❑Ex:-
➢AmixtureofliquidLeadandliquidaluminum.
➢AmixtureofliquidLeadandliquidcopper.

8. Mixture Of Two Liquids
Solidify
Crystallineseparately solidsolutions
➢IntheSolidsolutions,
➢Thetwocomponentsmaybecompletelyorpartlysolubleineachotherin
solidstate.
➢Insuchsolidsolutionsthesoluteatomsdistributesthemselves
throughoutthesolventcrystalsrandomly.
➢Thecrystalstructureofthesolventbeingmaintained.

9. Classification of Alloys.
➢Alloysmaybehomogeneous(uniform)ormixtures.
➢Ifthealloyishomogeneous,itwillconsistofsinglephase,andifitis
amixtureitwillbeacombinationofseveralphases.
➢Aphaseisanythingwhichishomogeneousandphysicallydistinct.
➢Thusaphaseisaregionofspace,throughoutwhichallphysical
properties(density,tensilestrength,etc.)ofamaterial,itschemical
compositionandstructureareuniform.

10. Classification of Alloys.
❑In the solid state there are three possible phases:
I.Pure metal
II.Compound or intermediate alloy phase and
III.Solid solution
➢Thusifanalloyishomogeneous(composedofasinglephase)inthe
solidstate,itcanbeonlyasolidsolutionoracompound.
➢Ifthealloyisamixture,itisthencomposedofanycombinationof
thephasesavailableinthesolidstate.Itmaybemixtureoftwopure
metals,ortwosolidsolutions,ortwocompounds,orapuremetal
andasolidsolution,andsoon.

11. A summary of possible alloy structures is shown in the
figure given below.

12. Pure metal
Characteristics of a pure metal are discussed in the crystallization.
Under equilibrium conditions, all metals show a definite melting or
freezing point.
If a cooling curve is plotted for a pure metal, it will show a horizontal
line at the melting or freezing point as shown in above figure.

13. Intermediate alloy phase or Compounds
➢Inmanybinaryalloysystems,whenthechemicalaffinityofelements
isgreat,theirmutualsolubilitybecomeslimitedandcompounds
(alsocalledintermediatephases)areformed(ratherthansolid
solutions).
➢Mostordinarychemicalcompoundsarecombinationof+ve&-ve
valenceelements.
➢ExpressedbychemicalformulaEx:-H2O,NaCl,H2SO4etc….
➢Whencompoundisformed,theelementsloosetheirindividual
identityandcharacteristicpropertiestoalargeextent.

14. Intermediate alloy phase or Compounds
➢Example:-
➢Water(H
2O)iscomposedofelementsthatarenormallygasesat
roomtemperature,yetthecompoundisliquidatroomtemperature.
➢InNaCl,
❖sodiumisveryactivemetalthatoxidizeveryrapidly&soitisstored
inkerosene,Chlorineispoisonousgas.Butthecombinationofthese
twoelementsgivestheelementusedasfood.

15. Intermediate alloy phase or Compounds
➢Themostcommonintermediatealloyphasesare:-
I.Intermetallic Compounds or Valence Compounds
II.Interstitial Compounds
III.Electron Compounds
❖Intermetallic Compounds or Valence Compounds
➢They are generally formed between chemically dissimilar metals.
➢They usually show poor ductility and poor electrical conductivity and
may have a complex crystal structure.
➢Examples:-Mg
2Pb, Cu
2Se, etc.

16. Intermediate alloy phase or Compounds
❖Interstitial Compounds
➢Thewordinterstitialmeansbetweenthespaces.
➢TheyareformedbetweenthetransitionmetalssuchasSc,Ti,Ta,W
andFewithhydrogen,oxygen,carbon,boronandnitrogen.
➢Examples are Fe
4N, Fe
3C, W
2C, CrN, etc.
❖ElectronCompounds
➢Inalloysofcopper,gold,silver,ironandnickelwiththemetals
cadmium,magnesium,tin,zinc,andaluminium,anumberof
compoundsareformed.
➢Example–AgCd,Ag
5Cd
8andAgCd
3
➢Theyhaveadefiniteratioofvalenceelectronstoatomsandare
thereforecalledelectroncompounds.

17. Intermediate alloy phase or Compounds
❖Electron Compounds
Compound Electron-Atom
Ratio
Crystal Structure
AgCd 3:2 Body centered cubic
Ag
5Cd
8 21:13 Complex cubic
AgCd
3 7:4 Close packed hexagonal

18. Solid solution
❑Solubilityis the property of a solid, liquid or gaseous chemical
substance where solute (minor part) dissolves in a solvent (major part)
to form a homogeneous solution.
❑The solventis a chemical substance and can be in a solid, liquid or
gaseous state. Thus solution can exist in a gaseous, liquid or solid state.
❑Therearethreepossibleconditionsforasolution:unsaturated,
saturatedandsupersaturated.

19. Solid solution
❑figureshowsthecoolingcurveforasolidsolutionalloycontaining
50%Sb(antimony)and50%Bi(bismuth).Itmaybenotedthat
thisalloybeginstosolidifyattemperaturelowerthanthefreezing
pointofpureantimony(1170°F)andhigherthanthefreezingpoint
ofpurebismuth(520°F).

20. Solid solution
➢Asolidsolutionissimplyasolutioninthesolidstateandconsistsof
twokindsofatomscombinedinonetypeofspacelattice
➢Dependingontheatomsizeandsoluteandsolventelements,two
typesofsolidsolutionsmaybeformed–
1.substitutional
2.interstitial.

21. Solid solution
❑Substitutionalsolidsolutions
➢Thesoluteatomssubstitutethesolventatomsinthelatticeof
solvent.
➢Ex.
➢SilveratomsmaysubstituteforgoldatomswithoutloosingtheFCC
structureofgoldandgoldatommaysubstitutesforsilveratomson
regularnormalsitesintheFCClatticeofsilver.
➢Hereallalloyofsilverandgoldbinarysystemareformedbyrandom
distributionofthesetwotypesofatomsintheFCClatticeof
structure.

22. Solid solution
❑Substitutionalsolidsolutions
➢Asshownintheabovefigure,inthistypeofsolution,
someofthesolventatomsaresubstitutedbyatomsofthe
solute(alloyingelement)atoms.

24. Solid solution
❖Hume-RotheryRules.
➢Thesolidsolubilityinalloysystembysubstitutionsiscontrolledby
severalfactorsknownasHume-RotheryRules.
▪CrystalStructureFactor:-
➢Completesolidsolubilityoftwoelementsisneverobtainedunless
theelementshavethesametypesofthecrystalstructure.
➢Ex.Copper-Nickel(FCC),Silver-gold-platinum(FCC)complete
solubility.
Copper-Zinc(FCC-HCP)partialsolubilitywith35%solubilityofzinc
incopper.

25. Solid solution
❖Hume-RotheryRules.
▪RelativeSizeFactor:-
➢Foreextensivesolubilityatomicdiametershallbesimilar.
➢Greatersizeatomcannotbefitinthesamestructureasa
substitutionalsolidsolutionwithoutproducingexcessivestrainand
correspondinginstability.
➢Extensivesolidsolubilityisencounteredonlywhenthetwodifferent
atomdiffersinsizebylessthan15%calledafavourablesizefactor.
➢Ex.Cu-Ni,Au-Pt

26. Solid solution
❖Hume-RotheryRules.
▪Chemicalaffinityfactor:-
➢Thegreaterthechemicalaffinityoftwometalsthemorerestrictedis
theirsolidsolubility.
➢Whentheirchemicalaffinityisgreatthetendencytowards
compoundformationsorintermediatephaseismore.
➢Generallythefurtheraparttheelementsareinperiodictablethe
greateristheirchemicalaffinity.

27. Solid solution
❖Hume-RotheryRules.
▪Relativevalencefactor:-
➢Ifthesolutemetalatomhasdifferentvalancefromthatofthesolvent
atomthenumberofvalenceelectronperatomiscalledtheelectron
ratio,willbechanged.
oEx.Al-Ni
✓relativesizefactoris14%.
✓(NiislowerinvalancethanAl).
✓TheNidissolve5%AlwhileAldissolve0.04%Ni.

28. Solid solution
❑Interstitialsolidsolutions
➢Theseareformedwhenatomsofsmallatomicradiifit
intothespacesorintersticesofthelatticestructureofthe
largersolventatomsasshowninfiguregivenbelow.

29. Solid solution
❑Interstitialsolidsolutions
Since the spaces of the lattice structure are restricted in size, only
atoms with radii less than one angstrom are likely to form interstitial
solid solutions. These are hydrogen, boron, carbon, nitrogen and
oxygen.
Interstitial solid solutions normally have very limited solubility and
generally are of little importance. Carbon in iron is a notable exception
and forms the basis for hardening steel.
➢Inbothtypeofsolidsolutions,distortionofthelattice
structurewillexistintheregionofthesoluteatoms.

30. Solid solution
❑Interstitialsolidsolutions
This distortion will interfere with the movement of dislocations
on slip planes and will therefore increase the strength of the
alloy.This is the primary basis for the strengthening of a metal
by alloying.
The properties of an alloy can be manipulated by varying its
composition. For example steel formed from iron and carbon can
vary substantially in hardness depending on the amount of
carbon addedand the way in which it was processed.