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1Chapter 8
Chapter 8
Phase Diagrams
Chapter 8 in Smith & Hashemi
Additional resources: Callister, chapter 9 and 10A phasein a material is a region that differ in its microstructure and
or composition from another region • homogeneous in crystal structure and atomic arrangement • have same chemical andphysical properties throughout • have a definite interfaceand able to be mechanically separated from its surroundings Al 2 CuMg Al H 2O(solid, ice) in H
2 O (liquid) 2 phases2Chapter 8
Phase diagram and "degrees of freedom"
A phase diagramsis a type of graph used to show the equilibriumconditions between the thermodynamically-distinct phases; or to show what phases arepresent in the material system at various T, p, andcompositions• "equilibrium" is important: phase diagrams are determined by using slow cooling
conditions no information about kineticsDegree of freedom (or variance) F is the numberof variables (T, p, and
composition) that can be changed independently without changing the phases of the systemPhase diagram of CO
23Chapter 8
8.1 Phase Diagram of Water
• Field - 1 phase • Line - phase coexistence, 2 phases • Triple point - 3 phases3 phases
: solid, liquid, vapourTriple point
4.579 Torr
(~603Pa),0.0098
o C4Chapter 8
8.2 Gibbs Phase Rule
F + P = C + 2
F is # of degrees of freedom or
varianceP is # of phases
C is # of components
H 2 O C=1 (i) P=1, F=2; (ii) P=2, F=1; (iii) P=3, F=0 Gibbs' phase rule describes the possible # of degrees of freedom (F)in a closed systemat equilibrium, in terms of the number of separate phases (P) and the number of chemical components (C)in the system (derived from thermodynamic principles by Josiah W. Gibbs in the 1870sComponentis the minimum # of
species necessary to define the composition of the system5Chapter 8
8.3 How to construct phase diagrams? -
Cooling curves
Cooling curves
• used to determine phase transition temperature • record T of material vs time, as it cools from its molten state through solidification and finally to RT (at a constant pressure!!!)The cooling curve of a pure metal
BC:plateaueor region of
thermal arrest; in this region material is in the form of solid and liquid phasesCD: solidification is
completed, T drops6Chapter 8
Cooling curve for pure iron @ 1atm
As T : melted iron (liquid) bccFe, (solid) fccFe, (solid) bccFe, (RT)7Chapter 8
8.4 Binary systems (C = 2)
F + P = C + 2 = 4 F = 4 - P
1. Two components are completely mixablein liquid and solid phase (form a
solid state solution), and don't react chemically2. Two components (A and B) can form stable compoundsor alloys (for
example: A, A 2 B, A 3 B, B)Degrees of freedom (F):
p, T, composition p T compositionAt p = const (or T=const)
T0 weight % of B 100%
100%A100% B
F = 3 - P
8Chapter 8
Binary Isomorphous Alloy System (C=2)
Isomorphous: Two elements are completely soluble in each other in solid and liquid state; substitutional solid state solution can be formed; single type of crystal str. existReminder: Hume-Rothery rules: (1) atoms have similar radii; (2) both pure materials have same crystal structure; (3) similar electronegativity (otherwise may form a compound instead); (4) solute should have higher valence Example: Cu-Ni phase diagram (only for slow cooling conditions)Liquidus line: the line connecting
Ts at which liquid starts to solidify
under equilibrium conditionsSolidus: the temperature at which
the last of the liquid phase solidifiesBetween liquidus and solidus: P =2
9Chapter 8
53 wt% Ni - 47 wt% Cu at 1300
o C • contains both liquid and solid phases neither of these phases can have average composition 53 wt% Ni - 47 wt% Cu •draw a tie line at 1300 oCfrom the graph: composition of liquid phase w
L45% and solid phase w
S = 58% at 1300 o C P = 1F = 3 - P = 2
P = 2 ; F = 3 - P = 1
10Chapter 8
8.5 The Lever Rule
The weight percentages of the phases in any 2 phase region can be calculated by using the lever rule Let x be the alloy composition of interest, its mass fraction of B (in A) is C Let Tbe the temperature of interest at T alloy xconsists of a mixture of liquid (with C L -mass fraction of B in liquid) and solid (C S- mass fraction of B in solid phase)Consider the binary equilibrium phase diagram of elements A and B that are
completely soluble in each other C oMass fraction of B
11Chapter 8
Lever Rule (cont.)
12Chapter 8
Q.:A Cu-Ni alloy contains 47 wt % Cu and 53% of Ni and is at 1300 oC. Use Fig.8.5 and
answer the following: A. What is the weight percent of Cu in the liquid and solid phases at this temperature? B. What weight percent of this alloy is liquid and what weight percent is solid?13Chapter 8
8.6 Nonequilibrium Solidification of Alloys
constructed by using very slow cooling conditionsAtomic diffusion is slow in solid state; as-cast
microstructures show "core structures" caused by regions of different chemical compositionAs-cast 70% Cu - 30% Ni alloy
showing a cored structure14Chapter 8
Nonequilibrium Solidus
Solidification of a 70% Ni-30%Cu alloy
Fig. 8.9, Smith
Schematic microstructures at T2 and T4Fig.8.10, Smith • each core structure will have composition gradient 1 7 •additional homogenizationstep is often required (annealing8.7 Binary Eutectic Alloy System
• Components has limitedsolid solubility in each other • Example: cooling 60%Pb - 40%Sn systemTeutectic
Liquid
_ a solid solution+ b solid solutionThis eutecticreaction is called an invariant
reaction occurs under equilibrium conditions at specific T and alloy composition F=0 at eutecticpoint16Chapter 8
Solubility Limit: Water-Sugar
70 80 1006040200
Temperature (°C)
Co=Composition (wt% sugar)
L liquid solution i.e., syrup) A (70, 20)2 phases
B (100,70)1 phase
2010 0
D (100,90)2 phases
406080
0 L (liquid) S (solid sugar) • Changing T can change # of phases: path Ato B. • Changing C o can change # of phases: path Bto DAdapted from Callister
17Chapter 8
Binary Eutectic Alloy System
Figure 8.13, Smith
18Chapter 8
Q: A lead-tin (Pb - Sn) alloy contains 64 wt % proeutectic () and 36% eutectic at 183o C -T. Using Figure 8.13, calculate the average composition of this alloy.