Peritectic Reaction |Iron Cementite Phase Diagram Part 4| Heat Treatment Lecture1|Invariant Reaction

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The Peritectic Reaction
The Third and the highest temperature invariant reaction is the peritectic reaction.
In the alloy containing 0.15 percent carbon, the initial crystals of Delta-Solid solution and the whole liquid phase are completely transformed to form austenite on cooling at 1492oC.
To understand the complete transformation mechanism let's take a liquid, and cool it to just below the peritectic. The moment, the liquid start solidifying, it gives out delta.
So initially delta comes out of the liquid.
As you continue the solidification to lower and lower temperatures, more and more delta comes out.
At the peritectic temperature,
Liquid plus delta should react with each other and 100% Austenite should form by consuming all the liquid and delta phases.
But what happened here?
As the Delta forms at the interface as a thin layer, at this moment there is no more interface between the Liquid and delta phases.
As this delta phase is completely covered, the liquid phase cannot further react with the delta phase to form the Gamma phase.
At this stage, this reaction can't proceed.
So, how can this peritectic reaction proceed?
To understand this let's take a finite element.
So, in principle, the liquid wants to transform into the gamma phase.
but the number of carbon atoms in the liquid phase is higher than what a gamma phase can take.
So, the liquid would give us the gamma phase by rejecting carbon out of the liquid.
Because there is a gradient inside the gamma phase, the C atoms will flow towards the gamma delta interface.
Once these C atoms get rejected out of Liquid, a small volume of liquid has got converted into the gamma phase of the same composition as that of the gamma phase.
And the Gamma phase has got extended into the liquid.
Okay, If we look at the delta phase, the delta has smaller carbon content than the gamma phase.
If it has to convert into the gamma phase, then it needs more carbon atoms.
As there is a composition difference between both components, the delta phase with a BCC crystal structure takes away carbon atoms, increasing the carbon content in the delta phase to that of the gamma phase.
In this condition, at this temperature, according to the equilibrium, suddenly it would covert into the game phase that has FCC crystal structure.
A small amount of the delta phase has got converted into the gamma phase.
Peritectic Reaction is taking place in two steps.
This two-step reaction is called Peritectic Transformation ( Not Peritectic Reaction).
Peritectic transformations are very slow because these transformations occur by the diffusion of carbon atoms through the solid gamma phase layer.
At the start of this transformation, reaction rates are fast, but as the transformation proceeds, reaction rates decrease because of two reasons.
One of the main reason is the decrease in concentration gradient and the second reason is that the solid gamma phase thickness keep on increasing through carbon atoms has to diffuse.
That's why Peritectic Transformations are really slow.
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