Gamma prime may form on the grain boundary it is obvious that nucleation barrier is lower and diffusion coefficient is higher on grain boundary. For sure gamma prime will be coherent with one of the grains that provides lowest energy. What I observed was very narrow precipitate free zone in the vicinity of gamma prime that was because grain boundary gamma prime was much larger than in grain interior. Same thing may happen on carbide/gamma boundaries gamma prime may decorate carbides. In some compositions gamma prime on grain boundary may be very large and be in much different shape than in grain interior.

Dear Dongsoo

The volume fraction, size, and distribution of γ' are important parameters for control of properties. The volume fraction of γ' increases with the addition of aluminum and titanium, but the amounts of each must be carefully controlled. Gamma prime contents above approximately 45% render the alloy difficult to deform by hot or cold working. In the iron-nickelbase alloys, the volume fraction of γ' phase is less than 0.2%, and it is usually spherical. Optimum strength results when the γ' is in the size range of 0.01 to 0.05 μm, much too small to be seen using the optical microscope. If the aluminum/titanium ratio is equal to or greater than 1, extended high-temperature exposure results in replacement of γ' by Ni2AlTi, NiAl, or Ni(Al,Ti). These phases overage rapidly at moderately high temperatures, forming massive platelike

precipitates.

Alloys with γ' contents below 20%, such as Nimonic 80A, are heat treated using a simple two-step process of solution annealing and aging. The solution anneal recrystallizes the austenite matrix and dissolves any γ' and M23C6 carbides present. Aging precipitates γ' uniformly throughout the matrix and precipitates M23C6 carbides at grain and twin

boundaries. Alloys with γ' contents of approximately 30%, such as Waspaloy or Udimet 500, are solution treated, then given two aging treatments. Alloys with 40 to 45% γ', such as Udimet 700, are solution treated, then given three aging treatments.

Positive identification of γ' is usually performed by x-ray diffraction of the residue of bulk extractions or by electron diffraction using extraction replicas. Some of the electrolytes that selectively attack γ' can be quite useful, because the γ' will be recessed below the matrix, and the other second phases will be in relief or plane with the surface depending on the preparation procedure. (Metals handbook Volume 9 - Metallography and microstructure)

Grzegorz Cios well addressed the case. I would like to add that gamma prime phase exist at two main sizes, primary gamma prime and secondary gamma prime. The primary is much larger than the secondary in size and you can observe it at grain boundaries while the secondary gamma prime is uniformly dissipated within the gamma phase matrix.

The lower surface energy of the primary gamma prime, due to their higher surface to volume ratio, minimize their interfacial coherent energy with the gamma phase matrix  therefore moving to the grain boundaries of higher energy side is due for stability. The main advantage of the primary gamma prime at grain boundaries is to prevent the migration of these boundaries and controlling grains size.

Dear Mr.Mohammed,

Thanks for kind answer.

One thing that I could not understand is, 

how the lower surface energy of primary gamma prime minimize the interfacial coherent energy between G and G'?

  Dear Mr Dongsoo , fully coherent interfaces should have very low interfacial energy between the particle and the matrix (i.e. low surface energy of the particle).

I would like to say sorry for inconvenient of second paragraph of my answer as you see. Therefore, I would like to revise the paragraph to be:

The higher surface energy of the primary gamma prime, due to their higher surface area, maximizes their interfacial coherent energy with the gamma phase matrix. Therefore, moving to the grain boundaries is due for stability. The main advantage of the primary gamma prime at grain boundaries is to prevent the migration of these boundaries and controlling grains size.

 Best regards

What is your alloy? Is it heat treated?

What i observed in case of alloy IN713LC which is used as-cast, you can see smaller particles of g' in dedrites and larger particles in interdendritic regions. That is a result of dendritic solidification of an alloy. If you consider binary Ni-Al alloy, solidification starts in dendrites with lower amout of Al. Interdendritic regions solidify later with higher amount of Al, therefore larger g' particles. Grain boundaries are extrapolation of such example. Grain boundaries are the last part of casting which solidifies, therefore containing a lot of Al, thus resulting in a lot of g'.

Similar way is in case of interdendritic and grain boundary carbides. Result is both carbides and g' particles can be found at grain boundaries. Morphology can be different, depending on cooling rates during solidification of the casting, or different part of the casting and heat treatment performed. g' phase particles morphology is easily altered by heat treatment.

Sometimes g' phase  envelopes were reffered to form around grain boundary carbides.