Advanced material and technologies, MSc Structure of alloys, thermodinamic background and its effect on physical properties Advanced material and technologies, MSc 2017 1

Melting of two different metal or a metal with a non metallic element: 1. Thermodynamic background of alloy formation Formation of alloys: Melting of two different metal or a metal with a non metallic element: no solution of the two component → 2 phases of the pure metals, correlated and unlimited solution → 1 phase in all concentration region, limited solution → 1 or 2 phases (depend on concentration), compound with more or less constant composition and lattice structure. What does determine the solubility and formation of compounds? How does the phys. properties change if an other metal or metalloid is dissolved in the host metal? What is the character of the intermetallic compounds? What is the difference from the properties of the components? 2

Halloy= Galloy + TSalloy 1. Thermodynamic background of alloy formation Effect of chemical composition to thermodynamic functions – Configuration enthalpy Enthalpy of the alloy: Halloy= Hi + ∆HM Configuration enthalpy (heat effect caused by solution) Hi= (1-x)HA + xHB Weighted average of the component’s enthalpies x: composition Entropy of the alloy: Salloy= Si + ∆SM Si= (1-x)SA + xSB Free (Gibbs) enthalpy of the alloy: Halloy= Galloy + TSalloy Galloy= Gi + ∆GM Gi= (1-x)GA + xGB Enthalpy change caused by solution: ∆GM= ∆HM - T∆SM 3

∆GMid= - T∆SM id = TR(xAlnxA + xBlnxB) 1. Thermodynamic background of alloy formation Formation of solid solution: ∆GM= ∆HM - T∆SM<0 At ideal solid solution: ∆HM=0 ∆GMid= - T∆SM id = TR(xAlnxA + xBlnxB) Ideal character at metallic solid solutions is very rare. Heat effect at solution mainly depends on energy of the chemical bond: At ideal solid solution: 4

Elegyedési entalpia H= Hi + HM H= Hi + HM Hi= (1-x)HA + xHB 1. Thermodynamic background of alloy formation Elegyedési entalpia H= Hi + HM H= Hi + HM Configuration enthalpy Hi= (1-x)HA + xHB Weighted average of the component’s enthalpies 5 composition

Entropy 1. Thermodynamic background of alloy formation where in system with N atoms the number of B atoms nB=xN 6

GM=HM - T·SM T·SM T·SM HM HM GM GM negative conf. enthalpy 1. Thermodynamic background of alloy formation GM=HM - T·SM T·SM T·SM HM HM GM GM negative conf. enthalpy positive conf. enthalpy 7

GM=HM - T·SM HM T·SM GM 1. Thermodynamic background of alloy formation GM=HM - T·SM HM T·SM GM 8

GB GA A B Two phases, no solution One phase, unlimited solution 1. Thermodynamic background of binary phase diagrams Two phases, no solution GB One phase, unlimited solution GA Two phase, with limited solution A B 9

1. Thermodynamic background of alloy formation The phase diagram and mechanism of solidification of solid solutions. G T2 Golv Gszil A B 10 10

Conditions of solid solution formation (Hume—Rothery-rules) 1. Thermodynamic background of binary phase diagrams Conditions of solid solution formation (Hume—Rothery-rules) Conditions of unlimited solid solution formation: difference in atomic diameter: maximum 15% same valence electron configuration near the same electronegativity solidification in the same type of lattice Solid solutions substitutional, interstitial, ordered, disordered. 11

Which effect determines the solubility Which effect determines the solubility? Hume-Rothery: atomic size, electron configuration (difference in valence electron structure), electronegativity, type of lattice Ag(Cd,,Sn, Sb): e/a= const, limit in solubility is occure! 12

Changes in resistivity (/at.%) vs. dissolved elements Changes of the properties in solid solution Changes in resistivity (/at.%) vs. dissolved elements 13

El. resistivity vs. concentration Changes in properties in solid solution El. resistivity vs. concentration S = AC(1-C) where C: concentration A: const. Characterised by the alloying system Mott-rule: AB = BA 14

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Changes in resistivity () vs. dissolved elements Changes of the properties in solid solution Changes in resistivity () vs. dissolved elements Please be surprised.! (But why?) Shielding effects in electron structure (effect of impurities), stress field produced (but not mechanically, but by the disruption of the electron structure) El. resistivity vs. concentration 16

The relationship between the shape of the phase diagrams and the electrical conductivity 17

Changes of the properties in solid solution 18

Changes of the properties in solid solution 19

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What kind of strength-enhancing mechanisms are there? Increase of strength What kind of strength-enhancing mechanisms are there? Strain hardening (work hardening) plastic deformation, increase of dislocation density (recently: radiation damages also) Solution hardening (properties of dissolved elements, connection with the Hume—Rothery-rules!) Precipitation hardening Dispersion hardening Quench hardening: high density of lattice defect, non-equiulibrium constituents Grain refinement 21