Day 40: Review –Part II

1. Day 40: Review –Part II • Multi-phase Alloys. Phase Diagrams • Fe-C system Phase Diagram • Microstructures and Properties in Steel

2. Review of three Strengthening Mechanisms • 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?

3. 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.

4. 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.)

5. 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.

6. 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 • Ice in water • Sugar and water • Alloys in metals can have a phase rich in each metal component.

7. The Lead Tin System Phase Diagram

8. Fe-C System Phases Ferrite, a Austenite, g Cementite, Fe3C

9. Steel Microstructures • Most steel consists of a mixture of a and Fe3C. The questions are: • How much of each? • How finely are they mixed? • Size and shape of the cementite? • Some of these questions can be answered with the phase diagram. • Others require the TTT Diagram.

10. Different Phases and Microconstituents • Proeuctectoid Ferrite • Pearlite. Will it be fine or coarse? • ProeutectoidCementite. Where is it found? • Bainite • Martensite • Tempered Martensite • Spheroidite

11. Here is the TTT Curve for a Euctectoid Steel. Coarse P Fine P Upper B Lower B M (Martensite)