Solid Solutions

Definition, Classification,Hume Rothery Rules

Reference: Material Science and Engineering, William Callister

1. CHAPTER 2 (CONTD.)
Solid Solutions
Substitutional Solid Solution
Hume-RotheryRules
Interstitial Solid solution
Intermediate Phases
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2. SOLID SOLUTIONS
SolidSolutionoranAlloyisaphase,wheretwoormoreelementsarecompletelysolubleineach
other.
Solidsolutionshaveimportantcommercialandindustrialapplications,assuchmixturesoftenhavesuperior
propertiestopurematerials.
Manymetalalloysaresolidsolutions.
Ex:Cu-Ni,Au-Agetc.
Inasolidsolution,themetalinthemajorproportioniscalledthesolvent(hostorparentormatrix)
andthemetalintheminorproportioniscalledthesolute
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3. There are two types of Solid Solutions:
i.SubstitutionalSolid Solution
ii.Interstitial Solid Solution
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4. Substitutional Solid Solution
Inthistypeofsolidsolution,thesoluteatoms
substitutetheatomsofsolventinthecrystal
structureofthesolvent.
Thesubstitutionalsolidsolutionaregenerallyorderedat
lowertemperaturesanddisorderedathigher
temperatures.
Temperatureisthedecidingfactor.
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5. There are twotypes of substitutionalsolid solutions:
i.Ordered SubstitutionalSolid Solution (OSSS)
ii.Disordered SubstitutionalSolid Solution (DSSS)
OSSS:Inthistype,thesoluteatomssubstitutethesolventatomsin
anorderlymanner,takingupfixedpositionsofsymmetryinlattice.
Thissolidsolutionhasuniformdistributionofsoluteandsolvent
atoms.
DSSS:Inthistype,thesoluteatomsdonotoccupyanyfixed
positionsbutaredistributedatrandominthelatticestructureof
solvent.Theconcentrationofsoluteatomsvaryconsiderablythrough
outlatticestructure.
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6. HUME-ROTHARYRULES:
ThesearetheruleswhichgoverntheformationofSolidSolutions.
Inotherwords,onlywhentheserulesaresatisfied,asubstitutionalsolidsolutionisformed.
1.CrystalStructureFactor:
Forcompletesolubilityoftwoelements,theyshouldhavethesametypeofcrystallattice.
Forexample,Au-Agsolution,bothshouldhaveFCCstructure.
2.RelativeSizeFactor:
Theatomsofthesoluteandsolventshouldhavethesameatomicsizeapproximately.
Thisfactorissatisfiedifthedifferenceofatomicradiioftwoelementsislessthan15%.
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RULES FOR FORMATION OF SOLID SOLUTION

7. 3.Chemical-AffinityFactor:
Forasubstitutionalsolidsolutiontobeformed,twometalsshouldhavelesschemicalaffinity.
Greateristhechemicalaffinity,lesseristhechanceofformingasolidsolution.
Iftwoelementsarefartherapartinaperiodictable,chemicalaffinityismore.
4.Electro-negativity:
Highertheelectro-negativity,greateristhechanceofforminganintermediatephaseratherthanasolidsolution.
Electro-negativityisthetendencytoacquireelectrons.
5.RelativeValenceFactor:
Amongtwometals,whichhavesatisfiedalltheaboverules,themetalwithlowervalencytendstodissolvemoreofa
metalofhighervalencyandvice-versa.
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8. Interstitial Solid Solution
Theseareformedwhenatomsofsmallatomicradiifitintothe
interstitialspacesoflargersolventatoms.
Atomsofelementssuchascarbon,nitrogen,boron,hydrogen,etc.
whichhaveradiilessthan1A
o
arelikelytoforminterstitialsolute
atomsandmaydissolvemorereadilyintransitionmetalssuchas
Fe,Ni,Mn,Cr,etc.thaninothermetals.
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9. INTERMEDIATE PHASES
Intermediatephasesarethosephaseswhosechemicalcompositionsareintermediatebetweenthetwopure
metalsandgenerallyhavecrystalstructuredifferentfromthoseofthebase(parent)metals.
Analloycanbemadeupofasolidsolutionphaseentirelyorcanexistalongwithanintermediatephase.
Anintermediatephasehereisnothingbutacompoundandismadeupoftwoormoreelements
ofwhichatleastoneofthemisametal.
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10. Acompoundisachemicalcombinationofpositiveandnegativevalenceelements.i.e.,atomsofdifferentelements
arecombinedindifferentproportionsandareexpressedbychemicalformulaelikeH
2O,NaCl,etc.
Whenacompoundorintermediatephaseisformed,theelementslosetheirindividualidentityandpropertiesto
agoodextentandthecompoundwillhaveitsowncharacteristicphysical,mechanicalandchemicalproperties.
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11. There are threemost common intermediate alloy phases:
i. Intermetallic or ValencyCompounds
ii. Interstitial Compounds
iii. Electron Compounds
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12. Intermetallic or Valence Compounds
Whenalloyphaseareexclusivelymetal-metalsystems,theyarecalledintermetalliccompounds.
Theseareformedbetweenchemicallydissimilarmetalsandarecombinedbyfollowingtherulesofchemical
valence.
Thecombinationisusuallynon-metallicandshowpoorductilityandpoorelectricalconductivityandhave
complexcrystalstructure.
Examples for intermetallic compounds: Mg
2Pb, Mg
2Sn, CaSe, Cu
2Se
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13. Interstitial Compounds
These are similar to interstitial solid solutions except that they have more or less a fixed composition.
Example: Fe
3C.
The interstitial compounds are metallic in nature, have high melting points and are extremely hard.
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14. Electron Compounds
Theseareofvariablecompositionsanddonotobeythevalencelaw,buthaveadefiniteelectrontoatomratio.
Example:Cu
9Al
4
EachCuatomhas1valenceelectronandeachAlatomhas3valenceelectrons.
So13atomswhichmakeupthecompoundhave21valenceelectronswithelectrontoatomratiobeing21:13
Electroncompoundshavepropertiessameasthoseofsolidsolutions–widerangeofcompositions,highductilityand
lowhardness.
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