Transcript Slide 1

DAY 40: REVIEW –PART II
Multi-phase Alloys. Phase Diagrams
 Fe-C system Phase Diagram
 Microstructures and Properties in Steel

REVIEW OF THREE STRENGTHENING
MECHANISMS
1.
2.
3.
4.
Solute Atoms. (Alloying)
Grain boundaries. (Grain boundary refinement)
Dislocations. (Cold Work, i.e. plastic
deformation done on purpose. AND
Phase Boundary. What’s a phase?
LET’S LOOK AT SOME METALLOGRAPHS

An alloy of Cu in Aluminum
Al, surrounded by a mixture
of Al and CuAl2.
Al and CuAl2 mixture.
ANOTHER – A CLOSEUP OF A STEEL.
The darker area is Fe
with small amount of
interstitial carbon.
The lighter standout
areas are the
compound cementite,
Fe3C. (Iron carbide.)
WHAT HAVE WE SEEN?
Multiphase materials, or alloys. Phases are
separate, they are clearly different materials.
But they are mixed together, at times very finely.
 We do not always have multiphase alloys. There
are many useful single phase alloys. BUT
 The presence of the second phase is very
important to…

BLOCK DISLOCATIONS! INCREASE STRENGTH.
CONCEPT OF THE PHASE
Phase: “A distinct state of matter in a system;
matter that is identical in chemical composition
and physical state and separated.” (Google)
 Examples
1. Ice in water
2. Sugar and water
3. Alloys in metals can have a phase rich in each
metal component.

THE LEAD TIN SYSTEM PHASE DIAGRAM
FE-C SYSTEM
Phases
1. Ferrite, a
2. Austenite, g
3. Cementite,
Fe3C
STEEL MICROSTRUCTURES
Most steel consists of a mixture of a and Fe3C.
The questions are:
1. How much of each?
2. How finely are they mixed?
3. Size and shape of the cementite?
 Some of these questions can be answered with
the phase diagram.


Others require the TTT Diagram.
DIFFERENT PHASES AND
MICROCONSTITUENTS
Proeuctectoid Ferrite
 Pearlite. Will it be fine or coarse?
 Proeutectoid Cementite. Where is it found?
 Bainite
 Martensite
 Tempered Martensite
 Spheroidite

HERE IS THE TTT CURVE FOR A
EUCTECTOID STEEL.
Coarse P
Fine P
Upper B
Lower B
M (Martensite)