Consider the phase diagram of Iron-Carbon alloy
A composite material consists in two/some distinct materials mixed together. Only for the base of the mixture (metal or alloy) we can discuss about phase equilibrium diagram (PED). The included particles, in most cases, does not change mass (dissolve) in the base of composite, so it's tricky to study PED for them.
It depends on what you are after. In some composites, e.g. Al/Al2O3 the phases present are stable phases (once a very small amount of oxygen has dissolved in the matrix). The structure of the composite is inherently off-equilibrium because of the interface energy stored in the system, something that is not captured by EPDs.
Other composites like Al/graphite are off-equilibrium because the most stable situation would involve aluminum carbide to some extent which is kinetically excluded or kept below equilibrium quantity by the processing route.
In all generality, EPDs can indicate compositional areas where you will in fact get a composite the latter being understood as a mixture of two phases, e.g. Al-Si eutectic.
In practice such composite material that is formed in situ (in the Al-Si case by solidification) is not really meant by the term composite, the latter referring to a material that is made by mixing two component phases into one material.
An equilibrium phase diagram ( temperature versus an addition of solute composition into a pure solvent) was originally developed to reflect the equilibrium state of different phases that were present in the fabricated alloy of the solvent & the solute, at a particular composition of the fabricated alloy & a specific temperature.
Since composites don't comply with " the equilibrium state of different phases that were present in the fabricated alloy of the solvent & the solute, at a particular composition of the fabricated alloy & a specific temperature". Therefore, it is impossible to form equilibrium phase diagram for composites.
The question defies the basic definition of metal matrix composite itself. There will not be any sharing of electrons between reinforcement particles and metallic matrix to form equilibrium compounds.Further reinforcement phase will not dissolve into the matrix. If such sharing takes place and compounds form during processing, we call it inter-facial reaction which deteriorates the load bearing capacity of the composite.
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