Sections:9'2, 9'3, 9'4, 9'5 - PowerPoint PPT Presentation

1 / 76
About This Presentation
Title:

Sections:9'2, 9'3, 9'4, 9'5

Description:

One of the reason why a knowledge and understanding of phase diagrams is ... Horizontal solidus line at TE is called the eutectic isotherm ... – PowerPoint PPT presentation

Number of Views:61
Avg rating:3.0/5.0
Slides: 77
Provided by: afma6
Category:

less

Transcript and Presenter's Notes

Title: Sections:9'2, 9'3, 9'4, 9'5


1
  • Chapter 9
  • Sections9.2, 9.3, 9.4, 9.5

2
Chapter 9 Phase Diagrams
  • Why study?
  • One of the reason why a knowledge and
    understanding of phase diagrams is important to
    the engineers related to the design and control
    of heat treating processes.
  • Some properties are functions of their
    microstructures, and, consequently, of their
    thermal histories.

3
Definitions and Basic Concepts
  • Components Pure metals and/or compounds of
    which an alloy is composed
  • Example in a copper-zinc brass, the components
    are Cu and Zn.
  • System
  • First meaning refer to a specific body of
    material under consideration ( e.g., a ladle of
    molten steel)
  • Second meaning relate to the series of possible
    alloys consisting of the same components, but
    without regard to alloy composition (e.g., the
    iron-carbon system)
  • Solid solution Consists of atoms of at least two
    different types
  • Solute ? an element or compound present in a
    minor concentration
  • Solvent ? an element or compound in greater
    amount host atoms.
  • Solute atoms occupy either substitutional or
    interstitial positions in the solvent lattice
  • Crystal structure of the solvent is maintained

4
9.2 Solubility Limit
  • Solubility Limit The maximum concentration of a
    solute atoms that may dissolve in the solvent to
    form a solid solution at some specific
    temperature.
  • The addition in excess results in the formation
    of another solid solution or compound that has a
    distinctly different composition.
  • Example Sugar-Water (C12H22O11-H2O) system
  • Initially, as sugar added to water, a solution of
    syrup forms.
  • As more sugar is added, solution becomes more
    concentrated
  • Solution becomes saturated with sugar ?Solubility
    limit is reached
  • Not capable to dissolving more ? further addition
    simply settle to the bottom
  • System now consists of two separate substances
  • A sugar-water syrup liquid solution, and
  • Solid crystals of undissolved sugar

5
(No Transcript)
6
9.3 Phases
  • Phase defined as a homogeneous portion of a
    system that has uniform physical and chemical
    characteristics.
  • Every pure material is considered to be a phase
  • Also every solid, liquid, and gaseous solution
  • e.g., syrup solution is one phase, and solid
    sugar is another
  • If more than one phase is present, it is not
    necessary that there be difference in both
    physical and chemical properties
  • A disparity in one or both is sufficient
  • e.g., water and ice in a container ( two phase,
    identical chemically)
  • When a substance can exist in two or more
    polymorphic forms (e.g. having both FCC abd BCC)
    ? each structure is a separate phase because of
    difference in physical properties.

7
  • A single-phase system is termed homogeneous
  • Systems composed of two or more phases are termed
    mixture or heterogeneous systems.
  • Most metallic alloys, ceramics, polymeric, and
    composite systems are heterogeneous.
  • Ordinarily, in multiphase systems
  • The phases interact such that the property is
    different and more attractive than individual
    phases.

8
9.4 Microstructure
  • Physical properties and mechanical behavior
    depend on the microstructure.
  • In metal alloys, microstructure is characterized
    by
  • Number of phases present
  • Their proportions
  • The manner they are distributed or arranged
  • The microstructure of an alloy depends on such
    variables as
  • Alloying elements present
  • Their concentrations
  • The heat treatment
  • Microstructure studies
  • surface preparation (Polishing and etching)
  • For two phase alloys, one phase may appear light
    and other dark

9
9.5 Phase Equilibria
  • Free energy is a function of the internal energy
    of a system, and also of the randomness or
    disorder of the atoms or molecules (or entropy).
  • A system is at equilibrium if its free energy is
    at a minimum under some specified combination of
    temperature, pressure, and composition.
  • In macroscopic sense, this means that the
    characteristics of the system do not change with
    time but persist indefinitely
  • ? The system is stable
  • A change in temperature, pressure, and/or
    composition in equilibrium ? increase in free
    energy ? another equilibrium state whereby the
    free energy is lowered.

10
  • Phase equilibrium ? refers to equilibrium as it
    applied to systems in which more than one phase
    may exist.
  • Example
  • Sugar-water syrup is contained in a closed vessel
  • solution is in contact with solid sugar at 20oC
  • If system is in equilibrium,
  • Composition of syrup is 65wt C12H22O11-35wt H2O
    (Fig 9.1)
  • Amount and composition of syrup and sugar will
    remain constant
  • If temperature is raised to 100oC
  • Equilibrium is temporarily upset
  • Solubility limit of sugar has increased to 80 wt
  • Some of the solid sugar will dissolve until new
    equilibrium is reached

11
  • Metastable state
  • Nonequilibrium state
  • A state of equilibrium is never completely
    achieved because the rate of approach to
    equilibrium is extremely slow
  • Common in many metals or solid solutions
  • Persist indefinitely with imperceptible changes
    with time.
  • Metastable structure
  • More practical than equilibrium
  • Some steel and aluminum rely on this for heat
    treatment designing

12
  • Chapter 9
  • Sections 9.6

13
Equilibrium Phase Diagrams
  • Equilibrium Phase diagram
  • Represents the relationships between temperature
    and the compositions and the quantities of phases
    at equilibrium.
  • Also known as phase, equilibrium or
    constitutional diagram
  • A binary alloy is one that contains two
    components.
  • Temperature and composition are the variable
    parameters for binary alloys.
  • Of more than two components, phase diagrams
    become extremely complicated and difficult to
    represents

14
9.6 Binary Isomorphous systems
  • Phase diagram of the copper-Nickel system is
    shown in Fig 9.2a.
  • Ordinate ? Temperature
  • Abscissa ? composition
  • Composition ranges from 0 wt Ni (100 wt Cu) to
    100 wt Ni (0 wt Cu)
  • Three different phase regions, or fields, appear
  • An alpha (a) field
  • A liquid (L) field
  • A two-phase (aL) field

15
  • Liquid L homogeneous liquid solution composed of
    both copper and nickel
  • a phase a substitutional solid solution
    consisting of both Cu and Ni atoms, and having an
    FCC crstal structure.
  • Isomorphous complete liquid and solid solubility
    of two components
  • Copper-Nickel system is Isomorpous
  • At temperatures below about 1080oC, mutually
    soluble in solid state for all compositions
  • Complete solubility is due to same crystal
    structure (FCC), nearly identical atomic radii
    and electronegativities, and similar valences

16
  • Nomenclature
  • For metallic alloys, solid solutions are
    designated by a, b, g, etc.
  • Liquidus line liquid phase at all temperature
    and composition above this line
  • Solidus line solid phase below this line at all
    temperatute and composition
  • Liquidus and solidus lines intersect at two
    extreme points
  • Correspond to melting temperature of pure
    components
  • Copper (1085oC) and Nickel (1453oC)
  • Heating of pure copper
  • Moving vertically on left-temperature axis
  • Remains solid until its melting temperature is
    reached
  • No further heating possible, until this
    transformation is complete

17
  • For any composition other than pure components
  • Melting phenomenon occurs over the range of
    temperature between the solidus and liquidus
    lines
  • Both solid a and liquid will be in equilibrium
    within this range

18
Interpretation of Phase Diagrams
  • For binary system of known composition and
    temperature that is in equilibrium, at least
    three kinds of information are available
  • The phases that are present
  • The composition of these phases
  • The or fraction of the phases
  • 1.0 Phases present
  • Relatively simple
  • Example (refer to Fig 9.2a),
  • 60 wt Ni-40 wt Cu at 1100oC Point A
    a phase
  • 35 wt Ni-65 wt Cu at 1250oC Point B
    a liquid phases

19
PHASE DIAGRAMS
Tell us about phases as function of T, Co, P.
For this course --binary systems just
2 components. --independent variables T and
Co (P 1atm is always used).
Phase Diagram for Cu-Ni system
Adapted from Fig. 9.2(a), Callister 6e. (Fig.
9.2(a) is adapted from Phase Diagrams of Binary
Nickel Alloys, P. Nash (Ed.), ASM International,
Materials Park, OH (1991).
5
20
PHASE DIAGRAMS and types of phases
Rule 1 If we know T and Co, then we know
--the and types of phases present.
Examples
Cu-Ni phase diagram
Adapted from Fig. 9.2(a), Callister 6e. (Fig.
9.2(a) is adapted from Phase Diagrams of Binary
Nickel Alloys, P. Nash (Ed.), ASM International,
Materials Park, OH, 1991).
6
21
PHASE DIAGRAMS composition of phases
Rule 2 If we know T and Co, then we know
--the composition of each phase.
Cu-Ni system
Examples
Adapted from Fig. 9.2(b), Callister 6e. (Fig.
9.2(b) is adapted from Phase Diagrams of Binary
Nickel Alloys, P. Nash (Ed.), ASM International,
Materials Park, OH, 1991.)
7
22
PHASE DIAGRAMS weight fractions of phases
Rule 3 If we know T and Co, then we know
--the amount of each phase (given in wt).
Cu-Ni system
Examples
27wt
Adapted from Fig. 9.2(b), Callister 6e. (Fig.
9.2(b) is adapted from Phase Diagrams of Binary
Nickel Alloys, P. Nash (Ed.), ASM International,
Materials Park, OH, 1991.)
8
23
THE LEVER RULE A PROOF
Sum of weight fractions
Conservation of mass (Ni)
Combine above equations
A geometric interpretation
9
24
  • Composition need to be specified in terms of only
    one of the constituents
  • For example, composition of Ni is used
  • Identical results if composition of Cu is used
  • Co 35 wt Ni
  • Ca 42.5 wt Ni
  • CL 31.5 wt Ni
  • WL ( 42.5 35) / (42.5 31.5) 0.68
  • Wa (35 31.5) / (42.5 31.5) 0.32
  • Volume fraction See equations 9.5 9.7

25
Volume Fractions
26
Development of Microstructure in Isomorphous
alloys-- Equilibrium Cooling
  • 35 wt Ni-65wt Cu
  • as cooled from 1300oC
  • Cooling very slowly ? phase equilibrium is
    maintained
  • Cooling ? Moving down
  • At 1300oC, completely liquid
  • At b (1260oC), solidification starts
  • At d (1220oC), solidification completes

27
Development of Microstructure -- Non-Equilibrium
Cooling
  • Extremely slow cooling not valid
  • Temperature change ? readjustment in composition
    ? diffusional processes
  • Diffusion rates are low for the solid phase and,
    for both phases, decrease with diminishing
    temperature
  • Practical solidification processes, cooling rates
    are much too rapid to allow these compositional
    readjustments and maintenance of equilibrium ?
    different microstructure develops

28
(No Transcript)
29
  • At b, a phase begin to form a(46Ni)
  • At c,
  • liquid composition 29wt Ni-71 wt Cu
  • Solid phase 40 wt Ni-60 wt Cu a(40Ni)
  • Since diffusion in solid is relatively slow, a
    phase formed at b has not changes composition
    appreciably ? still a(46Ni)
  • Composition of a grains continuously changes
    radially from 46 wt Ni at center to 40 wt Ni at
    the outer grains ? average composition (say 42
    wtNi)
  • Solidus line has shifted

30
  • Chapter 9
  • Sections 9.7

31
9.7 Binary Eutectic Systems
  • Binary Eutectic Phase Diagram
  • Another type of common and relatively simple
    phase diagram
  • Figure 9.6 shows for the copper-silver system
  • Features of Binary Eutectic Phase Diagram
  • Feature 1 Three single-phase regions ( a, b, and
    liquid )
  • The a phase solid solution rich in copper,
    silver as solute, FCC
  • The b phase solid solution rich in silver,
    copper as solute, FCC
  • Solubility in each of these solids phases is
    limited
  • Solubility limit for a phase
  • Line ABC ( Increases with temperature, maximum,
    decreases to minimum)
  • Solvus line (BC)
  • Solidus line (AB)

32
  • Solubility limit for b phase
  • Line FGH ( Increases with temperature, maximum,
    decreases to minimum)
  • Solvus line (GH)
  • Solidus line (FG)
  • Line BEG is also solidus line
  • Maximum solubility in both a and b phases occur
    at 779oC
  • Feature 2 Three two-phase regions
  • a L
  • b L
  • a b

33
(No Transcript)
34
  • As silver is added to copper,
  • The melting temperature of copper is lowered by
    silver additions.
  • Line AE the liquidus line
  • Same is true as copper is added to silver
  • Point E is called the invariant point (CE 71.9
    wt Ag, TE 779oC)
  • At E, an important reaction occurs
  • Upon cooling, a liquid phase is transformed into
    a and b solid phases
  • The opposite reaction occurs upon heating
  • This is called eutectic reaction ( Eutectic means
    easily melted)
  • CE and TE represents eutectic composition and
    temperature
  • Horizontal solidus line at TE is called the
    eutectic isotherm

35
  • The eutectic reaction, upon cooling, is similar
    to solidification of pure components
  • Reaction proceeds to completion at a constant
    temperature
  • Isothermal at TE
  • Solid products of eutectic solidification is
    always two solid phases
  • Another common eutectic system is that for lead
    and tin
  • The phase diagram is shown in Figure 9.7
  • Example 9.2
  • Example 9.3

36
Development of Microstructure in Eutectic Alloys
  • Depending on composition, several different types
    of microstructures
  • These possibilities considered in terms of the
    lead-tin phase diagram
  • Figure 9.7

37
  • First case Composition C1
  • Range
  • Composition ranging between a pure metal and the
    maximum solid solubility for that component at
    room temperature (20oC)
  • Lead-rich alloy (0-2 wt Sn)
  • Slowly cooled down

38
  • Second Case Composition C2
  • Range
  • Composition ranging between the room temperature
    solubility and the maximum solid solubility at
    the eutectic temperature.
  • Corresponds 2 wt Sn to 18.3 wt Sn

39
  • Third Case Composition C3
  • Solidification of the eutectic composition
  • Corresponds 61 wt Sn
  • The microstructure at i is known as eutectic
    structure.

40
  • Lamellae
  • The microstructure of a solid consisting of
    alternating layers
  • Shown in Figure 9.12

41
  • Fourth Case Composition C4
  • All composition other than the eutectic
    composition
  • At m, a phase will be present in both
  • Eutectic a
  • Primary a

42
  • Microconstituents
  • An element of the microstructure having an
    identifiable and characteristic structure.
  • At m, two microconstituents ( primary a and the
    eutectic structure )

43
  • Relative amounts of both eutectic and primary a
    microconstituents
  • Eutectic microconstituents forms from liquid
    having eutectic composition (61 wt Sn, Fig
    9.11, point i)
  • Apply lever rule using tie line
  • Eutectic fraction
  • We WL P / (PQ)
  • Primary a fraction
  • Wprimary a Q / (PQ)
  • Total a fraction (primary plus eutectic)
  • W a (QR) / (PQR)
  • Total b fraction (primary plus eutectic)
  • W b P / (PQR)

44
(No Transcript)
45
  • Chapter 9
  • Sections 9.8, 9.9, 9.13

46
9.8 Equilibrium Diagrams Having Intermediate
Phases or Compounds
  • Terminal solid solutions
  • Solid phases exist over the composition ranges
    near the concentration extremities of the phase
    diagram
  • Examples Copper-Silver system (Figures 9.6)
  • Lead Tin system (Figure 9.7)
  • Intermediate solid solutions
  • Intermediate phases
  • Solid phases at other than the two composition
    extremes
  • Example Cupper-Zinc system (Figure 9.17)

47
(No Transcript)
48
(No Transcript)
49
(No Transcript)
50
  • Intermetallic compounds
  • Discrete intermediate compounds rather than solid
    solutions
  • These compounds have distinc chemical formulas

51
9.9 Eutectoid and Peritectic Reactions
  • Eutectoid Reaction
  • Invariant point E, Figure 9.19
  • Upon cooling, a solid phase transforms into two
    other solid phases
  • Reverse reaction occurs on heating
  • Horizontal line at 560oC eutectoid or eutectoid
    isotherm
  • Eutectic ? liquid on cooling transforms into two
    solids
  • Importance iron-carbon diagram
  • Peritectic Reaction
  • Another invariant reaction (Point P, Figure 9.19)
  • Upon heating, one solid phase transforms into a
    liquid phase and another solid phase
  • (d L) ?on cooling ?e

52
(No Transcript)
53
THE IRON-CARBON SYSTEM9.13 The iron-iron
carbide (Fe-Fe3C) phase diagram
  • Of all binary alloys, the most important is the
    iron-carbon phase diagram.
  • A portion is shown in Figure 9.22
  • Practically all steels and cast irons have less
    than 6.70 wt C.
  • Pure iron
  • Upon heating, experiences two changes in crystal
    structure before it melts
  • At room temperature, the stable form ( ferrite or
    a iron ) has a BCC.
  • At 912oC, Ferrite experiences polymorphic
    transformation FCC austenite, or g iron.
  • At 1394oC, reverts back to a BCC phase ( d
    ferrite )
  • At 1538oC, d ferrite melts

54
(No Transcript)
55
  • At 6.7 wt C
  • Intermediate compound iron carbide, or cementite
    (FE3C), is formed.
  • 6.7 wt C corresponds to 100 wt Fe3C
  • Carbon is an interstitial impurity in iron
  • Forms a solid solution with each of a and d
    ferrites and g austenite
  • BCC a ferrite
  • Small concentration of carbon are soluble (0.022
    wt at 727oC)
  • Even though small concentration, significantly
    influences mechanical properties
  • Relatively soft, magnetic at temperature below
    768oC, density of 7.88 g/cm3
  • Figure shows photomicrograph

56
(No Transcript)
57
  • Austenite or g phase iron
  • Not stable below 727oC
  • FCC structure
  • Maximum solubility of carbon in austenite 2.14
    wt C at 1147oC.
  • Figure 9.23b shows photomicrograph
  • BCC d ferrite is virtually same as a ferrite
  • Stable only at relatively high temperatures ? no
    technological importance
  • Cementite (Fe3C) forms when the solubility limit
    of carbon in a ferrite is exceeded below 727oC
  • Coexist with g phase between 727 and 1147oC
  • Cementite is very hard and brittle ? strength of
    steel is enhanced by its presence

58
  • One eutectic reaction for iron-carbon system
  • At 4.30 wt C and 1147oC
  • L ? on cooling ? g Fe3C
  • L ? on heating ? g Fe3C
  • Eutectoid invariant point at 0.76 wt C and 727oC
  • g (0.76 wt C) ? on cooling ? a (0.022 wt C)
    Fe3C(6.7 wtC)
  • g (0.76 wt C) ? on heating ? a (0.022 wt C)
    Fe3C(6.7 wtC)

59
  • Chapter 9
  • Sections 9.14, 9.15

60
9.14 Development of Microstructures in
Iron-Carbon Alloys
  • Microstructure depends on both the carbon content
    and heat treatment.
  • Discussion confined to very slow cooling ?
    equilibrium is continuously maintained.
  • Phase change from g austenite region into the a
    Fe3C phase field
  • Relatively complex, similar to eutectic system
  • Consider cooling of an alloy of eutectoid
    composition
  • (Point a at 0.76 wt C and 800oC)
  • No changes until the eutectoid temperature
    (727oC)
  • At b, pearlite microstructure
  • Figure 9.25, photomicrograph of eutectoid steel
    showing the pearlite.

61
(No Transcript)
62
  • The pearlite exists as grains
  • Often termed colonies
  • Layer orientation is same in each colony
  • Thick light layers ? ferrite phase
  • Thin lamellae, mostly dark ? cementite
  • Ferrite ? soft and ductile
  • Cementite ? hard and brittle
  • Pearlite ? intermediate between ferrite and
    cementite

63
(No Transcript)
64
Hypo-Eutectoid Alloys
  • Consider a composition Co to the left of
    eutectoid
  • Between 0.022 and 0.76 wt C
  • Termed a hypoeutectoid (less than eutectoid)
    alloy
  • Cooling is shown in Figure 9.27
  • At d, about 775oC, a g phase
  • Composition of ferrite (a iron) changes along MN
  • Slight changes
  • Composition of austenite (g iron) changes
    dramatically along MO
  • At f, just below the eutectoid
  • All g-phase having eutectoid composition
    transforms to pearlite
  • No change in a-phase (ferrite )
  • Ferrite exist in two phases
  • Eutectoid ferrite ? ferrite that is present in
    pearlite
  • Proeutectoid ferrite ? pre- or before eutectoid
    formed above Te

65
(No Transcript)
66
  • Figure 9.28 photomicrograph of a 0.38 wt C
    steel
  • Large white regions proeutectoid ferrite
  • Pearlite
  • Dark regions
  • Spacing between a and Fe3C layers vary from grain
    to grain

67
Hyper-Eutectoid Alloys
  • Right side of eutectoid ( between 0.76 and 2.14
    wt C)
  • At g, only g phase (austenite ) of composition C1
  • Upon cooling at h, g ? g Fe3C phase field
  • Proeutectoid cementite ? that forms before the
    eutectoid reaction
  • Cementite composition remains constant as the
    temperature changes
  • Austenite composition changes along line PO
    towards eutectoid
  • Below eutectoid temperature at i,
  • All remaining austenite of eutectoid composition
    ? pearlite (aFe3C)
  • Microconstituents of resulting microstructure
  • ? pearlite and proeutectoid cementite (Fig
    9.30)

68
(No Transcript)
69
Photomicrograph of hypereutectoid alloys
  • Photomicrograph of 1.4 wt C steel is shown in
    Fig. 9.31
  • Consists of pearlite and proeutectoid cementite
  • Proeutectoid cementite appears light
  • Same appearance as proeutectoid ferrite
  • ? difficulty in distinguishing between
    hypoeutectoid and hypereutectoid steels on the
    basis of microstructure

70
Photomicrograph of hypereutectoid alloys (Contd.)
  • Comparison with hypoeutectoid alloys
    photomicrograph

71
Relative amounts for hypoeutectoid steel alloys
  • Using lever rule
  • Tie line extends between 0.022 and 0.76 wt C
  • Fraction of pearlite,
  • Wp T / (TU) (Co 0.022) / (0.76
    0.022)
  • Fraction of proeutectoid a,
  • Wa U / (TU) (0.76 - Co) / (0.76
    0.022)

72
(No Transcript)
73
Relative amounts for hypereutectoid steel alloys
  • Using lever rule
  • Tie line extends between 0.76 and 6.7 wt C
  • Fraction of pearlite,
  • Wp X / (VX) (6.70 C1) / (6.70 - 0.76)
  • Fraction of proeutectoid cementite (Fe3C)
  • WCementite V / (VX) (C1 - 0.76 ) /
    (6.70 - 0.76)

74
9.15 The Influence of Other Alloying Elements
  • Other alloying elements (Cr, Ni, Ti, etc.) bring
    about dramatic changes
  • Changes in the position of phase boundaries and
    shapes
  • One important change ? shift in eutectoid
    position w.r.t temperature and composition
  • These effects are illustrated in Figures 9.32 and
    9.33

75
(No Transcript)
76
(No Transcript)
Write a Comment
User Comments (0)
About PowerShow.com