1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

49
1 CHAPTER 8 CHAPTER 8 Phase Diagrams Phase Diagrams and and Microstructure Microstructure Development Development

Transcript of 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

Page 1: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

1

CHAPTER 8CHAPTER 8

Phase Diagrams Phase Diagrams

andand

MicrostructureMicrostructure

DevelopmentDevelopment

Page 2: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

2

There is a strong correlation between microstructure and mechanical properties, microstructure is related to phase diagram.

Phase diagarms provide valuable information about melting, casting, crystallization, and other phenomena.

8-I. Introduction

Component : pure metals and/or compounds e.g., copper-zinc brass, the components are Cu and Zn.

System : A specific body of material under consideration or series of possible alloys consisting of the same components.

8-II. DEFINITIONS AND BASIC CONCEPTS

Page 3: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

3

Solute : atoms occupy either substitutional or interstitial positions in the solvent lattice, and the crystal structure of the solvent is maintained.

F9.1

Maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution; Solubility limit depends on the temperature

A. SOLUBILITY LIMIT

Page 4: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

4

Phase:

A homogeneous portion of a system that has uniform physical and chemical characteristics. Every pure material is considered to be a phase.

A single-phase system is termed “homogeneous” or “ a homogeneous system.”

Systems composed of two or more phases are termed “mixtures” or “heterogeneous systems.”

Most metallic alloys, ceramic, polymeric, and composite are heterogenous materials .

F10-11

B. PHASES

Page 5: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

5

Microstructure is subject to direct microscopic observation, using optical or electron microscopes

In metal alloys, microstructure: number of phases present, their proportions and the manner they are distributed or arranged.

Microstructure depends:

Elements present, their concentrations, heat treatment (i.e., the temperature, the heating time at temperature, and the rate of cooling to room temperature).

Specimen preparation (polishing and etching):

different phases may be distinguished by their appearance, light or dark,

C. MICROSTRUCTURE

Page 6: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

6

Free energy: A function of the Internal energy of a system, and also the randomness or disorder of the atoms or molecules (or entropy).

A system is at equilibrium if its free energy Gibbs energy, G is at a minimun

D. PHASE EQUILIBRIA

Phase equilibrium

A constancy with time in the phase characteristics, e-g., Sugar-water syrup at 20°C.

Equilibrium

The characteristics of the system do not change with time but persist indefinitely; that is, the system is stable.

Page 7: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

7

65wt% C12H22O11–35wt% H2O

at 100°C : 80wt% C12H22O11F9.1

In materials systems,characteristics of the microstructure:

the phases present

their compositons

relative phase Amounts

their spatial arrangement or distribution.

Free energy considerations and diagrams

Phase diagrams are made with the assumption that the systems are at equilibrium.

In practical situations, systems are often not at equilibrium.

Page 8: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

8

Nonequilibrium or metastable state.

Often, metastable structures are of more practical sighinficance than equilibrium ones.

The speed or rate at which they are established and the factors that affect the rate must be considered.

Phase diagrams do not indicate the time period necessary for the attainment of a new equilibrium state. It is often that a state of equilibrium is never completely achieved because the rate of approaching is slow.

Page 9: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

9

Phase diagram (equilibrium or constitutional diagram.) : Relationships between temperature and the compositions and the quantities of phases at equilibrium.

at least three kinds of information are available (1) the phase that are present, (2) the compositions of these phases, and (3) the percentages or fractions of the phases

Pressure is also a parameter that influences the phase structure. However, pressure remains constant in most applications

Ordinary phase diagrams (and the ones presented here) are for a constant pressure of one atmosphere (1 atm).

Ordinate: temperture; abscissa: composition (weight percent and atom percent )

F10-12E. EQUILIBRIUM PHASE DIAGRAMS

Page 10: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

10

Isomorphous System (phase Diagram) Eutectic System (phase Diagram) Phase Diagram with Intermediate phases or co

mpounds

8-III. Phase Diagram of Metallic Materials

Page 11: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

11

Liquid L : a homogeneous liquid solution composed of both copper and nickel

F9-3a

8-III-1 BINARY ISOMORPHOUS SYSTEMS F9.2a

For example: Figure 9.2

(a) Three different phase regions, or fields: An alpha ( ) field, a liquid (L) field, a two-phase +L field

Binary: a system that contains two components.

Isomorphous : the components can be dissolved into each other completely without solubility limits.

Page 12: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

12

α Phase : a substitutional solid solution consisting

of both Cu and Ni atoms and having an FCC crvst

al structure α , complete solublity : both Cu and Ni

have the same crystal structure (FCC) , nearly ide

ntical atomic radij and electronegativities, similar

valences ⇒ Isomorphous

Solid solutions: desinated by lowercase Greek letters ( etc.).

Two-phase region

Liquidus line

Solidus line

Page 13: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

13

Pure component

solid-to-liquid transformation takes place at the melting temperature, and no further heating increase in temperature is possible until this transformation has been completed

Solid solution

for any composition of solid solution, melting phenomenon will occur over the range of temperatures between the solidus and liquidus lines, both solid and liquid phases will be in equilibrium within this temperature range

(b) Melting temperaturesF9.3a

F9.3b

Page 14: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

14

PHASES PRESENT F9.3a

For example : Figure 9.3

F9-3b

A. INTERPRETATION OF PHASE DIAGBAMS

at least three kinds of information are available (1) the phase that are present, (2) the compositions of these phases, and (3) the percentages or fractions of the phases

60wt% Ni-40wt% Cu at 1100C : point A , single phase

35wt% Ni-65wt% Cu alloy at 1250C : point B , and liquid phases at equilibrium.

Page 15: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

15

(a) one phase region : same as the overall composition

e.g., point A ,α phase having a composition of 60wt% Ni-40wt% Cu

(b) two-phase region

DETERMINATION OF PHASE COMPOSITIONS

tie line : horizontal lines (isotherm) across the two-phase region and terminate at the phase boundary lines

tie line-liquidus intersection : 31.5wt% Ni-68.5wt% Cu composition of the liquid phase CL :

solidus-tie line ntersection :C , 42.5wt% Ni-57.5wt% Cu

F9.3a

F9.3b

Page 16: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

16

(a) single-phase region : e.g., point A

only the phase is present , 100%

(b) two-phase region

The tie line must be utilized : lever rule (or the inverse lever rule)

The fraction of one phase is computed by taking the length of tie line from the overall alloy composition to the phase boundary for the other phase , and dividing by the total tie line length.

For example : point B in Figure 9.3b 35 wt% Ni-65 wt% Cu alloy overall alloy composition : C0

mass fractions WL and W

DETERMINATION OF PHASE AMOUNTS

F9.3b

Page 17: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

17

LL CC

CCW

0

68.05.315.42

355.42

LW

32.05.315.42

5.31350

L

L

CC

CC

SR

RW

(9.1b)

(9.2a)

(9.2b)

(9.1a)SR

SwL

Page 18: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

18

B. Development of Microstructure in Isomorphous Alloys

Equilibrium Cooling: Cooling occurs very slowly, phase equilibrium is continuously maintained.

(microsfructure equilibrium cooling;

nonequilibrium noneqiilibrium cooling)

Process of Solidification(1) Nucleation: the formation of initial solid phase

(very beginning step of solidification)

(2) growth of the nuclei

(3) maturity of microstructure: grain, grain boundary (polycrystalline)

initial solid phase: in the form of small particles nuclei

Page 19: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

19

Example: copper-nickel system (Figure 9.3a), 35wt% Ni-65 wt% Cu is cooled from 1300 . ℃

at point b, ~1260 , ℃ first solid begins to form, 46wt%

Ni-54wt% Cu, noted as (46Ni) .

The overall alloy composition (35wt% Ni-65wt% Cu)

remains unchanged during cooling.

The solidification process is complete at about 1220 , ℃point d, last remaining liquid: 24wt% Ni-76wt% Cu.

The final product then is a polycrystalline -phase solid

solution .F9.4

Page 20: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

20

Nonequilibrium cooling

Equilibrium solidification : readjustments in the compositions of the liquid and solid phases must occur (by diffusion)

Nonquilibrium solidification:

In virtually all practical solidification situations, cooling rates are much too rapid to allow compositional readjustments in the solid phases: nonequilibrium solidification (but assumed that diffusion rates in the liquid phase are sufficiently rapid such that equilibrium is maintained in the liquid).

Example : Fig.9.4

F9.5

F9.4

Page 21: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

21

The solidus line has been shifted to higher Ni content

s-to the average compositions of the phase. At point

e’ (~1250 ) , the last ℃ phase: 31wt% Ni , average 35

wt% Ni.

Point c’ (about 1240 ), liquid composition 29wt% Ni-℃71wt% Cu; phase that solidified is 40wt% Ni-60wt%

Cu [(40 Ni)]. The composition of the grains has

continuously changed with radial position, from 46wt

% Ni at grain centers to 40wt% Ni at the outer grain

perimeters. Average composition : 42wt% Ni-58wt%

Cu [(42 Ni)].

Page 22: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

22

The distribution of the two elements within the grains is nonuniform:

nonuniform composition: segregation (concentration gradients, the center of each grain, is rich in the high-melting element: cored sturcture).

Cored structure:

Grain boundary regions will melt first as they are richer in the low-melting component cored structure has a lower melting point. This produces a sudden loss in mechanical integrity.

Furthermore, this melting may begin at a temperature below the equilibrium solidus temperature of the alloy. Coring may be eliminated by a homogenization heat treatment at a temperature below the solidus point for the particular alloy composition.

Page 23: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

23

Mechanical Properties Of Isomorphous Alloys

Solid-solution strengthening (Section 7.9): an increase in strength and hardness by additions of the other component.

8-III-2. Binary Eutectic Systems

For exampleFor example: the copper-silver system.

three single-phase regions , , and liquid.

phase : a solid solution rich in copper, silver as the solute FCC crystal structure.

-phase : solid solution FCC structure, copper is the solute.

The and phases are considered to include pure copper and pure silver, respectively.

F9-5

F9-6

F9.7

Page 24: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

24

The solubility limit for the phase has a maximum

[8.0 wt% Ag] at 779°C (1434 °F ), point B.

Solubility limit line separating the and + phase regio

ns : solvus line. Boundary AB between the and +L fiel

ds: solidus line.

Maximum solubility in the Phase: point G .

Horizontal line BEG, a solidus line, the lowest temperatu

re at which a liquid phase may exist.

Three two-phase regions: + L, + L and + .

Page 25: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

25

The melting temperature of copper is lowered by silver additions. The same may be said for silver.

Point E is called an invariant point: the composition CE and temperature TE; (71.9 wt% Ag and 779 ) (9-8)℃

Eutectic reaction (eutectic means easily melted), CE and TE the eutectic composition and eutectic temperature, respetctively; (CE and CE)

L(71.9 wt% Ag) (8.0 wt% Ag) + (91.2 wt% Ag)

cooling

heating

The horizontal solidus line at TE : eutectic isotherm (eutectic phase diagrams , eutectic system)

L(CE) (CE) + (CE)cooling

heatingF 9.7

Page 26: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

26

In general, one or at most two phases may be in equilibrium within a phase field.

for a eutectic system, three phases (, , and L) may be in equilibrium, but only at points along the eutectic isotherm.

single-phase regions are always separated from each other by a two-phase region.

B. Another common eutectic system: lead and tin

Low-melting-temperature alloys : having near-eutectic compositions, example: 60-40 solder, 60 wt% Sn and 40 wt% Pb.

A. Consturction of Binary Phase Diagrams

F9.8

F9.9

F 9.7

Page 27: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

27

C. Development of Microstructure In Eutectic Alloys

(a) The first case : between a pure component and the maximum solid solubility at room temperature (0 – ~ 2wt% Sn)

(b) The second case : between the room temperature solubility limit and the maximum solid solubility at the eutectic temperature. (2wt% Sn–18.3wt% Sn)

Upon crossing the solvus line, the solid solubility is exceeded, which results in the formation of small -phase particles (nucleation) , these particles will grow in size because the mass fraction of the phase increases with decreasing temperature.

F9.9 F9.11

F9.12

Page 28: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

28

(c) The third case : the eutectic composition.

Upon crossing the eutectic isotherm, the liquid transforms to the two and phases

L(61.9 wt% Sn) (18.3 wt% Sn) + (97.8 wt% Sn)cooling

heating

(9.9)

By atomic diffusion : alternating layers (lamellase )of the and phases (eutectic structure) , atomic diffusion of short distances

(d) The fourth case :

Eutectic α, primary α. microconstituent , two microconstituents : primary and the eutectic structure.

Fraction of the eutectic microconstituent we and fraction of liquid wL

6.43

3.18

3.189.61

3.18 44

CC

Qp

pww Le (9-10)

F9.13

F9.14 F9.15

F9.16F9.18

F9.17

Page 29: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

29

fraction of primary , W’

6.43

9.61

3.189.61

9.61 44 CC

Qp

Qw

(9.11)

Fraction of total , W (both eutectic and primary) , total , W

5.79

8.97

3.188.97

8.97 44 CC

RQP

RQw

(9.12)

5.79

3.18

3.188.97

3.18 44

CC

RQP

Pw (9.13)

Page 30: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

30

RQP

RQ

RQP

R

QP

P

QP

Q

W

fraction of primary + fraction of eutectic

When, for case 4, conditions of equilibrium are not maintained :

(1) grains of the primary microconstituent will be cored ,

(2) the fraction of the eutectic microconstituent will be

greater than for the equilibrium situation.

Page 31: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

31

8-III-3. Equilibrium Diagrams Having Intermediate Phases or Compounds

Terminal solid solutions: the solid solutions at each end.

Intermediate solid solutions (or intermediate phases):

the solid solutions in between:

There are six different solid solutions-two terminal ( and ) and four Intermediate (, , , and ∈). (the ’ phase: a specific and ordered arrangement). Dashed phase boundary lines : not determined.

The commercial brasses are copper-rich copper-zinc alloys.

the copper-zinc systemFor example:F9.19

Page 32: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

32

For some systems: intermetallic compounds.

(1) Mg2Pb melts at 550 (M) ,℃

(2) The solubility :

(3) This phase diagram may be thought of as two simple eutectic diagrams joined back to back: Mg-Mg2Pb system and Mg2Pb-Pb; Mg2Pb is really considered to be a component.

F9.20

the magnesium-lead system compound,

Mg2Pb (19wt% Mg-81wt% Pb): a vertical

line.

For example,

Page 33: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

33

A. Eutectoid and Peritectic Reactions

Invariant points (involving three different phases):eutectoid

eutectoid (or eutectic-like) reaction

eutectoid isotherm

distinguishing “eutectoid” from “eutectic” : one solid phase instead of a liquid transforms into two other solid phases at a single temperature.

A eutectoid reaction is found in the iron-carbon system (section 9.17) that is very important in the heat treating of steels.

(9.14)

F9.21cooling

heating F9.19

Page 34: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

34

Peritectic reaction (another invariant reaction):one solid phasetransforms into a liquid phase and another solid phase.

Phase transformations without compositional alterations: congruent transformations, e.g., melting of pure materials and intermetallic compounds; with a change in composition : incongruent tansformations ,e.g., eutectic and eutectoid reactions.

cooling

heating+L

B. CONGRUENT PHASE TRANSFORMATIONS

(9.15) F9.19

Page 35: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

35

(a)Pure iron, upon heating, experiences two changes in crystal structure before it melts. At room tempeature: ferrite, or αiron, has a BCC crystal structure; At 912 ° C, FCC austenite, or γ iron; at 1394 ° C, BCC phase, δferrite; finally melts at 1538 ° C (2800 ° F ).

F9.24

C. THE IRON-CARBON SYSTEM

Both steels and cast irons, are essentially iron-carbon alloys.

THE IRON-IRON CARBIDE (Fe-Fe3C)PHASE DIAGRAM

Page 36: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

36

The composition extends only to 6.70 wt% C, an intermediate compound: iron carbide, or cementite (Fe3C)

Carbon is an interstitial impurity in iron.

In the BCC α ferrite, only small concentrations of carbon are soluble; (explained by the shape and size of the BCC interstitial positions.)

Relatively soft, may be made magnetic.

The austenite, or γ phase of iron, solubility is approximately 100 times greater than for BCC ferrite since the FCC interstitial positions are larger. Austenite is nonmagnetic.

Mechanically, cementite is very hard and brittle; the strength of some steels is greatly enhanced by its presence.

F9.24

Page 37: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

37

From the γ region into the α + Fe3C phase field with eutect

oid composition (0.76wt% C): lamellae, relative layer thickness ( α to Fe3C): 8 to 1: pearlite ( because it has the app

earance of mother of pearl), with grains termed “colonies”.

Pearlite has properties intermediate between the soft, ductile ferriteand the hard, brittle cementite.

The alternating α and Fe3C layers in pearlite form by carb

on diffusion from the 0.022 wt% ferrite regions and to the 6.7 wt% cementite layers.

(b). DEVELOPMENT OF MICROSTRUCTURE IN IRON-CARBON ALLOYS

F9.26 F9.27 F9.28

Page 38: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

38

In cooling to point d, most of the small α particles (proeutectoid ferrite) will form along the original γ grain boundaries.

As the temperature is lowered just below the eutectoid, all the γ (having the eutectoid composition) will transform to pearlite (eutectoid ferrite ) ferrite. The fraction of pearlite, Wp:

74.0

022.0'

022.076.0

022.0'0

O

UT

Tp

CC

W

F9.29 F9.30

F9.31

(9.18)

HYPOEUTECTOID ALLOYS

0.022 — 0.76 wt% C : hypoeutectoid (less than eutectoid) alloy.

Page 39: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

39

Ferrous alloys are those in which iron is the prime component, but carbon as well as other alloying elements may be present. Based on carbon content, there are three types: iron, steel, and cast iron. Pure iron: less than 0.008 wt% C, ferrite phase; Steels: between 0.008 and 2.14 wt% C (rarely exceed 1.0 wt%) , consists of both α and Fe3C phases ; Cast irons: 2.14 —6.70 wt% C( normally less than 4.5 wt% C).

3 (9.16)cooling

heatingL Fe C

eutectic reaction (at 4.30 wt% C and 1147°C) :

3(0.76 % ) (0.22 % ) (6.7 % ) (9.17)cooling

heatingwt C wt C Fe C wt C

eutectoid reaction ( at 0.76wt% C and a temperature of 727 °C) :

(fundamental to the heat treatment of steels)

Page 40: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

40

Containing between 0.76 and 2.14 wt% C.

Proeutectoid cementite as the temperatue is lowered through the eutectoid to point i , all remaining austenite of eutectoid composition is converted into pearlite ; resulting pearlite and proeutectoid cementite as microconstituents

74.0

'76.0

022.076.0

'76.0 0

'

O

UT

T

CC

W

F9.29 F9.30

F9.32 F9.33

The fraction of proeutectoid α , W α, (9.19)

Fractions of both total α ( eutectoid and proeutectoid) and cementite are determined using the lever rule.

HYPEREUTECTOID ALLOYS

Page 41: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

41

Fractions of pearlite Wp and proeutectoid cementite WFe3C‘

(9.20)

(9.21)

94.5

'70.6

76.070.6

'7.6 11 CC

W XV

Xp

94.576.0'

76.070.676.0' 11

3 '

CC

W XV

Ve CF

3e

Page 42: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

42

In this discussion, it has been assumed that conditio

ns of metastable equilibrium have been continuousl

y maintained; thatis, sufficient time has been allowe

d at each new temperature for any necessary adjust

ment in phase compositions and relative amounts a

s predicted from the Fe-Fe3C phase diagram. In mo

st situations these cooling rates are impractically slo

w and really unnecessary; in fact, on many occasion

s nonequilibrium conditions are desirable.

(c). NONEQUILIBRIUM COOLING

Page 43: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

43

Alterations of the positions of phase boundaries and the

shapes of the phase fields

One of the important changes : shift in position of the

eutectoid with respect to temperature and to carbon

concentration

Two nonequilibrium effects of practical importance are (1) t

he occurrence if phase changes or transformations at temp

eratures other than those predicted by phase boundary line

s on the phase diagram, and (2) the existence at room tem

perature of nonequilibrium phases that do not appear on th

e phase diagram.

(d). THE INFLUENCE OF OTHER ALLOYING ELEMENTS

F9.34 F9.35

Page 44: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

44

8-IV. Ceramic Phase Diagrams

Binary phase diagrams, frequently two components share a common element, e.g., Oxygen in binary oxide ceramics.

A. The Al2O3-Cr2O3 system

Isomorphous, similar to copper-nickel phase diagram (Figure 9.2a).

The solid solution is a subsitiutional one: both aluminum and chromium ions have the same charge, similar radii (0.053 and 0.062 nm, respectively,and both Al2O3 and Cr2O3 have the same crystal structure(HCP).

F12.24 F9.2

Page 45: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

45

Similar to the lead-magnesium diagram (Figure9.20), there exists an intermediate phase, or better a compound called spinel(尖晶石 ), which has the chemical formula MgAl2O4 (or MgO-Al2O3) (鋁鎂尖晶石 )

There is a range of compositions over which spinel is a stable compound.

Spinel is nonstoichiometric for other than the 50 mol% Al2O3-50 mol% MgO composition.

Limited solubility of Al2O3 in MgO due primarily to the differences in charge and radii of the Mg2+ and Al3+ ions (0.072 versus 0.053 nm)

F12.25 F9.20B. The MgO-Al2O3 System

#88

Page 46: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

46

C. The ZrO2-CaO system

The compound CaZrO3 contains 31wt% CaO (50 mol%

CaO ).

One eutectic(2250 and 23wt% CaO) and two ℃ eutectoid

(1000 and 2.5wt% CaO, and 850 and 7.5wt% CaO) ℃ ℃reactions are found for this system

Three different crystal structures exist: tetragonal,

monoclinic, and cubic.

F12.26

Page 47: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

47

Pure ZrO2 experiences a tetragonal-to-monoclinic phase transformation at about 1150℃. A relatively large volume change resulting in the formation of cracks that render a ceramic ware useless. This problem is overcome by ‘stabilizing’ the zirconia by adding between about 3 and 7 wt% CaO: above about 1000 ℃ both cubic and tetragonal phases will be present. Upon cooling to room temperature under normal cooling conditions, the monoclinic and CaZr4O9 phases do not form (as predicted from the phase diagram); consequently, the cubic and tetragonal phases are retained, and crack formation is circumvented. Partially stabilized zirconia, or PSZ.

Page 48: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

48

Yttrium oxide (Y2O3) and magnesium oxide are also used as stabilizing agents.

For higher stabilizer contents, only the cubic phase may be retained at room temperture; such a material is fully stabilized.

D. The SiO2- Al2O3 system

The polymorphic form of silica at these temperatures: cristobalite.

Silica and alumina are not mutually soluble: absence of terminal solid solutions

Intermediate compound: mullite, 3Al2O3-2SiO2, mullite melts incongruently at 1890 .℃

A single eutectic exists at 1587 and 7.7 wt% Al℃ 2O3

F12.27

Page 49: 1 CHAPTER 8 Phase Diagrams and Microstructure MicrostructureDevelopment.

51