MAT E202 Lecture Notes - Lecture 19: Eutectic System, Phase Diagram, Martensite

• School: University of Alberta
• Department: Materials Engineering
• Course: MAT E202
• Professor: Thomas Etsell

Document Summary

Freezing and most solid state reactions occur by two steps: nucleation and growth. Chance grouping of several atoms to form the solid or the new phase. Driven by the higher stability and therefore lower energy of the new phase. Nuclei increase in size by diffusion of atoms to their surfaces. Segregation recall from phase diagrams that the proeutectic or proeutectoid solid is continuously changing composition as cooling proceeds. Only solid state diffusion can serve to keep the composition uniform but generally insufficient time is available. Only short atom movements are needed and therefore not dependent on diffusion. Important reaction in metal technology - an alloy is heated into a single phase region (solution heated), quenched and reheated (aged) to produce controlled precipitation known as precipitation hardening or age hardening. In some al alloys, precipitation of the second phase will occur at room temperature. (natural aging)