On the microstructural instability of an experimental nickel-based single-crystal superalloy

  • PDF / 1,936,058 Bytes
  • 13 Pages / 612 x 792 pts (letter) Page_size
  • 89 Downloads / 206 Views

DOWNLOAD

REPORT


URING the past 50 years, superalloy compositions have become ever more complex as the search for better turbine blade materials has continued. Modern superalloys contain significant concentrations of refractory elements such as Mo, W, Ta, and Re,[1,2,3] and it has been found that these can enhance the high-temperature creep-rupture properties. There are a number of reasons why this is the case. Undoubtedly, these elements act as solid-solution strengtheners, although the role of Re is still the subject of some controversy.[4] However, there is also a considerable influence on the lattice mismatch (␦), i.e., the difference between the lattice parameter of the coherent ␥ ⬘ precipitates and that of the ␥ matrix. There is now considerable evidence (e.g., References 5 and 6) that optimum creep resistance, particularly at high temperatures where any dislocation activity is confined to the ␥ phase, is conferred only by a suitable choice of lattice misfit. By a suitable selection of ␦, the morphological instability of the ␥ ⬘ precipitates, i.e., the rafting effect, can be controlled. Given these arguments, one might wonder whether there are limits to the concentrations of the refractory elements which can be tolerated. In practice, these are set by the appearance of topologically close-packed phases (TCPs) such as sigma (␴), mu (␮), P, or R, which are known to impair the creep properties.[7,8,9] Ideally, the alloy designer would like a predictive capability for the onset of the formation of the TCPs, so that their presence could be avoided. M.S.A. KARUNARATNE, Graduate Student, C.M.F. RAE, Senior Research Associate, and R.C. REED, Assistant Director of Research, are with the Department of Materials Science and Metallurgy, University of Cambridge/Rolls-Royce University Technology Centre, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted December 6, 2000.

METALLURGICAL AND MATERIALS TRANSACTIONS A

Traditionally, this has been provided by tools based on relationships in the periodic table, e.g., PHACOMP methods, which are based upon electron-vacancy numbers.[10,11,12] Here, the “residual matrix composition” is estimated after allowing for precipitation of phases such as carbides, borides, and ␥ ⬘. For the resulting matrix composition, an average electron-vacancy concentration (N␷ ) is then assigned by considering the electron-vacancy number of each element. This is related to the average number of electron vacancies in the bonding orbitals of each of the transition elements, scaled by their atomic fractions. In this scheme, if N␷ is found to be above a certain empirically determined value, the alloy is considered to be TCP-prone. Nowadays, computer-based thermodynamic models (e.g., Reference 13) represent a viable alternative to the PHACOMP method for the prediction of such microstructural instabilities in particular, but also of phase equilibria in general. However, while some success has been achieved (e.g., References 14 through 16), it should be remembered that this approach can only be successful if an accurate and