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Phase Diagrams

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Peritectic L a b. Monotectic L1 L2 a. Eutectoid a b g. Peritectoid a b g. 22 ... Peritectic. L d g. Eutectic. L g Fe3C. Eutectoid. g a Fe3C. Steel ... – PowerPoint PPT presentation

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Title: Phase Diagrams


1
Chapter 9
  • Phase Diagrams

2
Phase Diagram Vocabulary
3
Unary Phase Diagrams H2O
1 atmosphere
4
Unary Phase Diagram Pure Fe
5
Gibbs Phase Rule (Section 9.17)
  • Tells us how many phases can exist under a given
    set of circumstances.
  • PFC2
  • P number of phases
  • F number of degrees of freedom number of
    variables that can be changed independently of
    all other variables in the system
  • Cnumber of components
  • The number two indicates the ability to change
    temperature and pressure these are
    non-compositional variables that affect the
    phases.
  • Modified Gibbs phase rule
  • Most engineering systems function at a pressure
    of 1 atmosphere, i.e. we have picked the pressure
    as one of our degrees of freedom. Therefore,
  • PF C1

6
Binary Isomorphous System
  • Two components are completely soluble in each
    other in both solid and liquid phases
  • Hume-Rotherys Rules (Section 4.3 text 7th
    edition)
  • Atomic size difference not greater than 15
  • Crystal structure is the same for both components
  • Similar electronegativity (i.e. no ionic bonding)
  • Elements have a similar valance
  • Example Cu-Ni System
  • rCu 0.128 nm rNi 0.125 nm
  • Both have a face centered cubic (fcc) structure
  • Electronegativity Cu 0.19 Ni 0.18
  • Valance Cu and Cu Ni

7
Cooling Curves during Solidification
Solidification occurs at constant temperature
while latent heat of fusion is released
8
Cooling curves for a binary isomorphous alloy
  • Features
  • Solidus locus of temperatures below which all
    compositions are solid
  • Start of solidification during cooling
  • Liquidus locus of temperatures above which all
    compositions are liquid
  • Start of melting during heating

9
(No Transcript)
10
Modified Gibbs Phase Rule
  • In the liquid or solid phase
  • P1, C2
  • PFC1
  • F2
  • Both composition and temperature can be varied
    while remaining in the liquid or solid phase
  • In the La region
  • P2, C2
  • PFC1
  • F1
  • If we pick a temperature, then compositions of L
    and a are fixed
  • If we pick a composition, liquidus and solidus
    temperatures are fixed

TL
TS
11
Tie Line and Lever Rule
  • At point B both liquid and a are present
  • WLR WSS

WL
WS
R
S
12
Equilibrium Cooling
13
  • Non-equilibrium cooling results in
  • Cored structure
  • Composition variations in the solid phase as
    layers of decreasing Ni concentration are
    deposited on previously formed a phase
  • Solidification point is depressed
  • Melting point on reheat is lowered
  • Homogenization or reheating for extended times at
    temperature below e

14
Effect on Mechanical Properties
Due to solid solution strengthening, alloys tend
to be stronger and less ductile than the pure
components.
15
Binary Eutectic System
  • The two components have limited solid solubility
    in each other
  • Solubility varies with temperature
  • For an alloy with the Eutectic composition the
    liquid solidifies into two solid phases

16
Binary Eutectic System
  • Apply Modified Gibbs Phase Rule
  • Phases present L, a and b (P3)
  • Components Pb and Sn (C2)
  • PFC1
  • F0 ? no degrees of freedom
  • Therefore, three phases can coexist in a binary
    system only at a unique temperature and for
    unique compositions of the three phases
  • Upon cooling, there is a temperature arrest
    during the solidification process (eutectic
    reaction)

17
Microstructures in the Eutectic System
  • Depending on the system, eutectic solidification
    can result in
  • Lamellar structure alternating plates
  • Rod-like
  • Particulate

18
Microstructures in the Eutectic System
Solvus Line
19
Microstructures in the Eutectic System
20
Amounts of Phases at different temperatures
  • At Teutectic DT
  • At Teutectic - DT

21
Other Reactions in the Binary System
  • Upon Cooling the following reactions are also
    possible
  • Peritectic L a ? b
  • Monotectic L1 ? L2 a
  • Eutectoid a ? b g
  • Peritectoid a b ? g

22
Copper-Zinc System
  • Terminal phases
  • Intermediate phases
  • Several peritectics
  • Eutectoid
  • Two phase regions between any two single phase
    regions

23
Mg-Pb System
  • Intermediate Compound Mg2Pb
  • Congruently melting
  • Mg2Pb ? L
  • heating

24
Portion of the Ni-Ti System
  • Congruently melting intermediate phase g
  • g ? L
  • heating

25
Iron-Carbon System
  • Reactions on cooling
  • Peritectic
  • L d ? g
  • Eutectic
  • L ? g Fe3C
  • Eutectoid
  • g ? a Fe3C

Cast Iron
Steel
26
Iron-Carbon or Iron-Fe3C
  • In principle, the components of the phase diagram
    should be iron (Fe) and carbon/graphite (C).
  • Fe and C form an intermediate compound Fe3C,
    which is very stable
  • There isnt anything of interest at carbon
    contents greater than 25 at. or 6.7 wt. C.
  • Fe3C is considered to be a component, and the
    binary phase diagram is drawn using Fe and Fe3C.
  • Names of phases
  • Ferrite - a iron bcc structure
  • Austenite g iron fcc structure
  • High temperature d iron bcc structure
  • Cementite Fe3C
  • Steels have carbon contents lt2, usually lt1.2
  • Cast irons have carbon contents gt2

27
Phase Transformations in Steels
Eutectoid Composition 0.76wt C
Pearlite Alternating plates (lamellae) of Fe and
Fe3C
Austenite ? Ferrite
Cementite (at 727ºC upon cooling) 0.76wt.C
0.022wt.C 6.7wt. C
28
Phase Transformations in Steels
  • Hypoeutectoid composition lt0.76 wt C
  • Proeutectoid ferrite nucleates and spreads along
    austenite grain boundaries at Tgt727ºC
  • Remaining austenite converts to pearlite during
    eutectoid transformation

29
Phase Transformations in Steels
  • Hypereutectoid composition gt0.76 wt C
  • Proeutectoid cementite nucleates and spreads
    along austenite grain boundaries at Tgt727ºC
  • Remaining austenite converts to pearlite during
    eutectoid transformation

30
Phase Transformations in Steels
Hypereutectoid
Hypoeutectoid
Proeutectoid ferrite
Pearlite
Proeutectoid cementite
31
Effect of Alloying Elements
  • Addition of an alloying element increases the
    number of components in Gibbs Phase Rule.
  • The additional degree of freedom allows changes
    in the eutectoid temperature or eutectoid Carbon
    concentration
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