Review: Binary Phase Diagrams

1
Review Binary Phase Diagrams
all solutions
when entropy is only configurational
ideal solutions
N0z?XAXB
regular solutions
? ?AB ½ (?AA ?BB)
regular, positive enthalpy
?, ? gt 0 ? ?AB is large ? avoid A-B bonds ?
clustering, unmixing, phase separation
2
Regular solutions, positive enthalpy
solid phase is a regular solution, W gt 0 liquid
phase is a an ideal solution
Binary Eutectic
Liquid
TMA
TMA
Liquid
liquidus
liquidus
? L
? L
liquidus
solidus
TMB
TMB
solidus
? L
solidus
?
?
?
?
??
?
?
??
increasing W
3
Binary Eutectic Phase Diagram
For W gtgt 0, no longer useful to think of A and B
as forming a continuous solution
T1
XB
XB
4
Binary Eutectic Phase Diagram
For W gtgt 0, no longer useful to think of A and B
as forming a continuous solution
At the eutectic temperature
XB
XB
three phases in equilibrium
5
Regular solutions, negative enthalpy
N0z?XAXB
regular solutions
? ?AB ½ (?AA ?BB)
regular, negative enthalpy
?, ? lt 0 ? ?AB is small ? favor A-B bonds ?
ordering, compound formation
both terms lt 0 and symmetric about XA 0.5
6
Regular solutions, negative enthalpy
solid phase is a regular solution, W lt 0 liquid
phase is a an ideal solution
at high T, TDSmix dominates, at low T, DHmix
becomes important
DGmix
maximize AB bonds? AB compound
AB
a AB
a AB
a
a
XB
7
Regular solutions, negative enthalpy
solid phase is a regular solution, W lt 0 liquid
phase is a an ideal solution
W is small so liquid region unaffected
a
low T
a
a AB
TMA
a AB
liquid
AB
L S(a)
DGmix
a
TMB
high T
AB region narrows at high T
XB
XB
W ? will impact liquid region
8
Regular solutions, negative enthalpy
solid phase is a regular solution, W lt 0 liquid
phase is a an ideal solution
solid with AB (solid) bonds preferred over random
liquid
TMA
T
liquid
L S(a)
L S(a)
L S(a)
TMA
TMB
a
TMB
a
also moves up
AB
a
a
XB
XB
a AB
a AB
increasing W
9
Regular solutions, negative enthalpy
solid phase W ltlt 0, AB compound forms (g), no
longer meaningful treat as a solution liquid
phase is a an ideal solution
liquid
TMg
T
TMA
L S(a)
L S(a)
TMB
TMA
TMB
a
really moves up
AB
a
a
XB
a AB
a AB
XB
increasing W
10
Regular solutions, negative enthalpy
solid phase W ltlt 0, AB compound forms (g), no
longer meaningful treat as a solution liquid
phase is a an ideal solution
consider a, b, g as distinct
liquid
L
TMg
TMA
G
TMB
T1
XB
XB
11
Shape of G curves
wide composition range, anti-site defects
narrow composition range, few defects
G
narrow
12
Now consider non-ideal liquid
Wsolid and Wliquid gt 0, DHmix gt 0 ? tendency to
cluster (unmix)
Peritectic
XB
XB
increasing W
13
Now consider non-ideal liquid
Wsolid and Wliquid gtgt 0, A hates B and B hates A,
? phase separation even in the liquid
Monotectic
or
XB
XB
14
Now consider structure change in solid
Eutectoid
Liquid ideal g solid ideal a/b not a
continuous solution (or W gtgt 0)
L g
T
g b
a g
a b
15
Invariant Reactions