清华大学-材料科学基础_08ppt课件.ppt

1、材料科学基础Fundamental of Materials,Prof: Tian Min Bo Tel: 62795426 ，62772851 E-mail: Department of Material Science and Engineering Tsinghua University. Beijing 100084,Lesson eight,1,Chapter ,The Structures of Alloys,2,3.1 Basic concepts of alloys,.Definition An alloy is the combination of metal(s) with

2、 other elements through chemical bonding. .Terminology,3,4,1. Component (or constituent) one component system two component system binary system three component system ternary system four component system quarternary system five component system quinary system,multi-component system,5,2. Composition

3、 It can be expressed either by atomic percentage (mol fraction) Xa or by mass percentage Xm,6,3. Phase A phase is a homogeneous part of the material in which no abrupt change in composition, structure and properties occurs. An alloy may be single phase or multi-phase material. 4. Structure Structure

4、 is a general term for the combination of atom arrangement including the types amounts and distribution of all types of material as well as grain size, defect etc.,7,. Classification of Alloy Phases 1. According to structure Solid Solution Compound In Solid Solution, atoms of different component sha

5、re a common lattice in variable proportion.,8,9,2. According to position of the alloy in phase diagram,Terminal S.S.,Intermediate S.S. or secondary S.S.,10,3.2 Factors Affecting the Structure of Alloy Phase,1. What is size factor metallic radii rA+rB =d ionic radii r+r - =d covalent radii single bon

6、d radius Van der Waals radii,Atomic radii,11,CN radii,BCC,FCC,CN of BCC:8( )+6(a) CN of FCC:12( ) r= = r= = = 0.12557nm = 0.128674nm,12,Goldschmidt atomic radii is the radii of atom in structures with CN=12,Size factor is defined as,13,2. What is Electrochemical factor Electronegativity X X represen

7、ts the ability of an atom of an element in the compond to attract electrons to itself.,Paulis empirical rule:,n: valence r(1): simple bond radius,14,EAA bonding energy between A-A atoms EBB bonding energy between B-B atoms EAB bonding energy between A-B atoms,15,3. Electron concentration ( e/a ) Ele

8、ctron concentration ( e/a ) is the number of valence electrons per atom on the average. i.g. for CuZn : e/a = 3/2 =1.5,16,3.3 Solid Solution,. Classification 1. According the position of solute atoms in the lattice of the solvent Substitutional S.S. Interstitial S.S.,17,2. According the regularity o

9、f the position occupied by solute atoms Ordered S.S. Disordered S.S.,18,13121/44112 Fe12Al4Fe3Al,19,3. According to solid solubility 0100% continuous series of S.S S.S with restricted solubility,20,summary substitutional S.S primary (terminal) interstitial S.S secondary (intermediate) ordered S.S co

10、ntinuous S.S disordered S.S S.S with restricted solubility Ex. Write out in full the coordinates of all cations and anions in nucleus, Wurtzite and CaF2 referred to a b c axes of the anions sublattice.,21,. Determination of types of S.S,is the average atomic weigh weighted by composition,22,Compare

11、n with no (atoms per unit cell of solvent) n=no : ideal substitutional S.S. nno : interstitial S.S. nno : vacant S.S.,23,. Hume-Rothery Rule for primary solid solubility 1. Size factor: Size factor 100,If size factor 15 solubility is very small.,24,NiO can be added to MgO to produce a solid solution

12、. What other ceramic systems are likely to exhibit 100% solid solubility with MgO?,FeO-MgO system will probably display unlimited solid solubility. CoO and ZnO systems also have appropriate radius ratios and crystal structures.,Example,25,26,2. Crystal structure The materials must have the same crys

13、tal structure; otherwise there is some point at which a transition occurs from one phase to a second phase with a different structure.,27,3. Electrochemical factor If the difference in X is great, the solubility is also very restricted. Formation of stable compound will restrict the solid solubility

14、. Parameter: Electronegativity (x),28,Semiemperies formulas Where: r1single bond radius n valency,29,x0,x0+0.4,x00.4,x,R,R0,0.85R0,1.15R0,Dorken-Gurry graphic,30,4. Electron concentration factor, e/a e: the number of valence electrons a: the number of atoms e/a = average number of valence electrons

15、per atom.,31,Experimental findings: Zn, Ga, Ge, As in Cu (solute-solvent),If composition is expressed in terms of e/a rather than at%, the solid solubility of all elements in Cu will be roughly the same.,32,Structure vs. e/a for CuZn alloy system (CuZn) e/a = 3/2 = 21/14 (Cu5Zn8) e/a = 21/13 (CuZn3) e/a = 7/4 = 21/12,33,. Properties of S.S 1. lattice constants properties Vagards law ass = ao + (a - ao)x for alloy S.S a = K(ZA-