The binary phase diagram for a Cu-Zn alloy is shown in the figure above. A phase diagram of an alloy is a useful tool that allows us to find the relative amount of the phases present and their respective composition for a fixed thermodynamic state of the system at equilibrium. However, the time scales required to achieve thermodynamic equilibrium can be very large (especially at low temperatures). Therefore, non-equilibrium structures are often encountered.
For example, let’s look at the grain micro-structure of four specimens of a copper-42% zinc alloy which have been obtain through different cooling paths.
The first specimen shown above was sand casted from a super heated liquid and was allowed to cool at ambient temperature.
The second specimen shown above was annealed at 800 ˚C for 1 hr and then was quenched rapidly with water.
The third specimen shown above was annealed at 800˚C for 1hr., furnace cooled to 600˚C and then it was water quenched.
The forth specimen shown above was annealed at 800˚C for1hr followed by a slow furnace cooled to room temperature.
From these pictures, one can easily see that the grain micro-structure of an alloy can be very different even though the composition is fixed. This is due to non-equilibrium structures that are captured through heat treatment and rapid cooling of the alloy (as often done in the metal industry). Non-equilibrium structures can significantly affect the properties of metal alloys. Therefore, it is important to understand the kinetics involved in non-equilibrium processes.
Structural Microstructures on metal alloys are a rapidly growing area of research and manufacturing. Microstructures are small, conical shapes, or "walls" that form with grain structures in diverse elements, such as steel, iron, carbon, and other alloys. Understanding microstructures allows designers to incorporate them into their designs, and even to make them the structural base for structural components, such as joists, beams, trusses, and plates. Understanding how they work allows designers to incorporate these small structures into their designs with greater ease.
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