Evolution of the premartensitic state in the NiAl phase of a NiCoCrAlY bond coat during thermal cycling

Abstract

Transmission electron microscopy (TEM) is used to investigate the evolution of the premartensitic state in the NiAl-based β-phase in NiCoCrAlY bond coats during thermal cycling. Bright-field imaging and diffraction analysis confirm that the lattice modulation is in the form of {110} ⟨110⟩ transverse displacement waves. High resolution electron microscopy (HREM) revealed an increase in modulation wavelength during thermal cycling due to the β-phase becoming progressively richer in Ni through superalloy–bond coat interdiffusion. Transverse modulations extending over large regions of the crystal were observed in the HREM images, inconsistent with previous theories attributing the origin of the premartensitic state to point defects that couple to the dynamic instability of the lattice.

Acknowledgements

This study was supported by AFOSR under the MEANS-2 program (Grant No. FA9550-05-1-0173). The authors are grateful to M. Maloney and D. Litton of Pratt and Whitney and B. Tryon of the University of Michigan for providing the specimens. We would also like to thank Professor I. P. Jones (University of Birmingham, UK) for useful comments on the ALCHEMI technique.