Nickel based superalloy: dislocation structure

H. K. D. H. Bhadeshia

Nickel based superalloys are particularly important in the manufacture of aeroengines and industrial gas turbines.

The transmission electron micrograph below is taken from a nickel-based superalloy, known commercially as 'C263'. The alloy is used in the manufacture of gas turbine combustion chambers. It was heat treated at 800oC for 8 h, with an initial average $\gamma'$ size of about 22 nm. The fraction of $\gamma'$ is 0.1. The alloy was creep tested at 800oC at a stress of 160 MPa for about 2.5 days. The creep strain recorded was about 0.7%. The micrograph shows the dislocation structure following creep, with dislocations looping around the $\gamma'$ particles.

TEM image

Notice that the dislocations in the image are paired-loops (i.e. superdislocation loops).

In all nickel based superalloys, the presence of the $\gamma'$ phase hinders the motion of dislocations even though it is coherent with the matrix. This is because it has an ordered crystal structure. In $\gamma$ the Burgers vector of a dislocation is ${a\over 2}<1~1~0>$; this is, of course, a lattice vector so that slip does not alter the crystal structure. However, for $\gamma'$ ${a\over 2}<1~1~0>$ is not a lattice vector - ${a}<1~1~0>$ is the lattice vector for a primitive cubic lattice. It follows that the motion of an ${a\over 2}<1~1~0>_\gamma$ dislocation into the $\gamma'$ will disrupt the order, leaving behind an anti-phase domain boundary. However, the passage of a second such dislocation through $\gamma'$ on the same slip plane restores the order.

As a consequence, the penetration of $\gamma'$ has to occur by pairs of $\gamma$ dislocations. Such pairs are called `superdislocations'. The requirement for pairing makes it more difficult for dislocations to penetrate the $\gamma'$ and hence improves the resistance to creep deformation.

Lattice vectors along <110> directions in γ and γ', lying on { 111 } planes. The γ on the left has a random distribution of Ni, Al and Ti atoms, whereas the γ' on the right has its nickel atoms located at the face-centres.

Download high resoultion TIF file of transmission electron micrograph.


The micrograph has been provided for teaching purposes by Dr Yanhui Zhang.

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