Micro-Structures on Metal Alloys

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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.

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The first specimen shown above was sand casted from a super heated liquid and was allowed to cool at ambient temperature.

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The second specimen shown above was annealed at 800 ˚C for 1 hr and then was quenched rapidly with water.

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The third specimen shown above was annealed at 800˚C for 1hr., furnace cooled to 600˚C and then it was water quenched.

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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.

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