As far as I know, the shape is defined by the interface energy which shouldn't change from grain to grain, except if you apply a loading and then the orientation may have an effect. Commonly it is controlled by the misfit, and this is not changing for a cast material (or you applied a strange cooling). Could you post an image which shows three grains with different gamma' morphology?

Could you give me your e-mail address?

Best regards.

There are two main effects that play a role in the apparent morphology of your gamma prime phase : thermal history and the crystalline orientation of the plane that you observe by metallography. I assume that there was no applied loading.

I do not know the size of your sample, but anyway the surface of your sample must have been cooled more quickly than the bulk part : that may cause strong differences in the mean size and consequently a different balance between surface energy (that makes precipitates spherical) and volumic elastic energy (that makes precipitates rectangular cuboids). The same thing is true if there are two different populations in the same location of your sample : the smaller they are, the more spherical they will be.

Depending on the grain, you cut your rectangular cuboids in a different way, which can give them triangular morphologies (111 cut) or others. Of course, the spheres are always spheres, but you may have "not so round" precipitates, or even butterfly shapes for very large precipitates.

 My opinion is that the different sizes of g' particles are owing to the segregation (the g' forming elements are usually rich in the last solidification region). The different shape of g' particles can be attributed to the lattice misfit and subsequent particle growth. The non-cuboidal shape could be due to the misfit and the rectangle ones should be the results of particle growth.

Best regards,

Song

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I would have to agree with Gert and Song, however I would also have to say Mikael provides a very valid point with regard to the observation plane. 

Generally, gamma prime (y') morphology does NOT change from grain to grain. Y' morphology is a function of the lattice misfit in the microstructure which is governed by the specific chemistry of your alloy. Typically in nickel-base superalloys a lattice misfit near zero results in near-spherical y' precipitate morphologies whereas increasingly positive or negative lattice misfits result in increasingly cuboidal shapes. In some ni-base superalloys the sign of the misfit (+/-) has even been shown to influence the rafting (morphology change of y' participates) during deformation at elevated temperatures.

Therefore, if you are observing cast material that has been heat treated or subjected to some mechanical deformation (e.g. creep, tension or compression) there are a few other items which may be contributing to your observations which you should consider. If you have sectioned your sample unconventionally or in multiple orientations this may be a source of your conflicting observations as well. Lastly, if your samples are mechanical deformation-free, and you are sure you are viewing uniform cross-sections, a closer examination of your chemistry should likely reveal the contributing factors for the y' morphologies you have seen. A few papers that discuss the combined effects of chemistry and lattice misfit on y' morphology (with and without creep) are below. Hopefully these may provide you some specific insight based on the nickel-base superalloy you are studying.

- L.J. Carroll, et al. "High Refractory, Low Misfit Ru-Containing Single Crystal Superalloys" Met Trans A, (2006) vol. 37, no. 10, pp. 2927-2938

- J.S. Van Sluytman, et al. "Gamma Prime Morphology and Creep Properties of Nickel Base Superalloys with Platinum Group Metal Additions", SUPERALLOYS, (2008) pp. 499-508

- N.A. Protasova, et al. "Lattice-Parameter Misfit Between the y and y' Phases in Single Crystals of Nickel Superalloys", Struc., Phase Trans. and Diff. (2008) vol. 106, no. 5, pp. 495-502

- X.P. Tan, et al. "Measurement of y/y' Lattice Misfit and y' Volume Fraction for a Ru-containing Nickel-Based Single Crystal Superalloy" J. Mater. Sci. Technol., (2011) vol. 27, no. 10, pp. 899-905

Dear all,

I agree with Gert, Mikaël and Jonathan.

In addition, the morphology of gamma prime particles can vary significantly within one grain of a polycrystalline alloy due to residual dendritic segregations (effect on interfacial ccoherency) and, after plastic deformation (e.g. Creep), due to stress concentration close to inclusions (carbides, pores) and due to  a neighboring effect close to grain boundaries.