D AY 40: R EVIEW –P ART II Multi-phase Alloys. Phase Diagrams Fe-C system Phase Diagram Microstructures and Properties in Steel

R EVIEW OF THREE S TRENGTHENING M ECHANISMS 1. Solute Atoms. (Alloying) 2. Grain boundaries. (Grain boundary refinement) 3. Dislocations. (Cold Work, i.e. plastic deformation done on purpose. AND 4. Phase Boundary. What’s a phase?

L ET ’ S LOOK AT SOME METALLOGRAPHS An alloy of Cu in Aluminum Al, surrounded by a mixture of Al and CuAl 2. Al and CuAl 2 mixture.

A NOTHER – A CLOSEUP OF A STEEL. The darker area is Fe with small amount of interstitial carbon. The lighter standout areas are the compound cementite, Fe 3 C. (Iron carbide.)

W HAT 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.INCREASE

C ONCEPT OF THE P HASE 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.

T HE L EAD T IN S YSTEM P HASE D IAGRAM

F E -C S YSTEM Phases 1.Ferrite,  2.Austenite,  3.Cementite, Fe 3 C

S TEEL M ICROSTRUCTURES Most steel consists of a mixture of  and Fe 3 C. 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.

D IFFERENT P HASES AND M ICROCONSTITUENTS Proeuctectoid Ferrite Pearlite. Will it be fine or coarse? Proeutectoid Cementite. Where is it found? Bainite Martensite Tempered Martensite Spheroidite

H ERE IS THE TTT C URVE FOR A E UCTECTOID S TEEL. Coarse P Fine P Upper B Lower B M (Martensite)