1. Phase Diagrams Continued
Class 5
Phase Diagrams Continued

2. 1085C
420C

3. Substitutional Solid Solubilty
How much one element will dissolve in another is determined by the Hume Rothery rules
Atomic radii should be within 15% of each other
Crystal structure should be the same for each element for good solubility
Electronegativities should be similar.
The valences of the atoms should be similar.
Good solubility Cu –Ni, Cu-Au; Cu r=0.128A, Ni r=0.125, Au r=0.144
Crystal structure Ti- HCP, Al - FCC
Poor soluility Na-Cl Na electronegativity 0.9, Cl 3.0
Valences – Zn 2+, Cu 1+
Only indicate solubility from these rules.
DOES NOT APPLY TO THE ELEMENTS H,C,O,N,B THESE FORM INTERSTITIAL
SOLID SOLUTIONS.

4. Iron Carbon Phase Diagram
Liquid
1538C
3367 SUBLIMES
Fe - a then g then d
912C 1394C
g +L
Steels
Eutectoid
S1 -> S2 + S3
-> a + F e3C
Peritectic
S1+L -> S2
d + L -> g
Fe3C- cementite
A compound
g + Fe3C
a + Fe3C

5. Stainless Steel Phase Diagram
Ternary phase diagram for stainless steels. In this case an isothermal section at a constant
temperature is used.

6. Lever Arm Rule
Determine the AMOUNTS of each phase use the Inverse Lever Arm Rule.
Amount of solid = wa-wl/ws-wl
Amount of liquid = ws-wa/ws-wl
Amount of solid at 1300C is therefore / = 8/13 = 0.615= 61.5%
Amount of liquid at 1300C is therefore / = 5/13 = = 38.5%

7. Change Average Composition 50%
Determine the AMOUNTS of each phase use the Inverse Lever Arm Rule.
Amount of solid = wa-wl/ws-wl
Amount of liquid = ws-wa/ws-wl
Amount of solid at 1300C is therefore / = 5/13 = 0.385= 38.5%
Amount of liquid at 1300C is therefore / = 8/13 = = 61.5%

8. Microstructures and Composition
Pro-eutectic phase is formed before the eutectic reaction, eg a in the a+L region of phase diagram.

9. Lead Tin Microstructures
38.1%Pb 61.9% Sn
90 %Pb 10%Sn
70% Pb 30%Sn
50%Pb 50%Sn

10. Lead Tin Microstructures
15%Pb 85%Sn
Equiaxed Single Phase Grain Structure

11. Homeworks.
From the Pb­Sn phase diagram, for a 70% Pb, 30%Sn alloy, at what temperature does solid first form.  At the eutectic temperature, how much pro­eutectic phase is present and how much eutectic phase?  What are the compositions at 100oC?
For a steel containing 0.4% C:-
a) At what temperature does austenite start to transform on cooling from 950oC? 
b) How much proeutectoid phase is present at the eutectoid temperature 
c) What is its composition?
d) Upon quenching the steel from 950oC to ­50oC what phases will be present and what will be the composition?