Plot Pattern & Behaviour

It's important to understand the plot patterns of TTT diagrams as they tell you a lot about how phase transformations are affected by different rates of cooling and what properties the material will have.

For example, here is a TTT diagram for an iron-carbon alloy of eutectoid composition:

Figure 1 : Isothermal transformation diagram for Iron Carbon alloy of eutectoid composition.[1]

Firstly, in the diagram, there is a eutectoid temperature shown as a straight line. Phase transformations can only occur below the line as above the straight line you only have stable austenite. The diagram has two curves representing the start and finish of the reaction and the behaviour of the curve is due to how temperature affects the rate. The general behaviour of the curve is of a C shape where the fastest phase transformation occurs on the nose of the curve.

The high temperatures below the eutectoid temperatures have a long reaction rate, therefore, are on the right-hand side of the graph where reactions can start from 10^3 s and above. At this range of temperature, the layers of the alpha phase and

are thick which results in coarse grains of pearlite. This is due to the small number of nuclei forming with a fast rate resulting in larger grains.

The bend in the curve is due to phase transformations occurring near 540 degrees resulting in quicker transformation rates forming fine grains of pearlite. The nose of the curve is where the rate of phase transformation is at its maximum[1].

Below 540 degrees the austenite has a phase transformation into bainite instead of pearlite. The rate of the phase transformations also begins to decrease resulting in the curve to bend.

This specific curve stops near 220 degrees as martensite is formed due to austenite being rapidly cooled resulting in no diffusion occurring. This type of transformation is known as an athermal transformation [1] as it does not depend on time but only the temperature so it is shown as a straight line on the diagram. Although the transformation is instantaneous, the different temperatures you cool the austenite to will result in different amounts of martensite forming.

With the diagram, you can obtain the rate of the phase transformation by

is the time required for 50% of the phase transformation to be completed. So for a longer time taken for the reaction to reach 50% would result in a lower rate.

Here is an example of another TTT diagrams:

Figure 2: Isothermal transformation diagram

for an alloy of steel.[1]

As you can see, this particular TTT diagram has a different shape with different phase transformations due to its different composition. This is one of many compositions. CRAZY.

To learn more about:

Introduction

Non-Equilibrium Cooling

Applications of TTT Diagrams

Practice Questions

or

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References:

[1] W.D. Callister and D.G. Rethwisch, "Isothermal Transformation Diagrams" in Materials Science and Engineering an Introduction, 8th ed.[Online]. Available: https://www.academia.edu/9444719/Materials_Science_and_Engineering_By_Callister