Computer simulation of the initial rafting process of a nickel-base single-crystal superalloy
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TION
COARSENING of the ␥ ⬘ phase of two-phase nickelbase superalloys is a phenomenon that has been investigated repeatedly. The effect of coarsening can be divided into stress-free coarsening during annealing and coarsening under the action of an external load. The latter leads to a directionality of the coarsened morphology consisting either of ␥ /␥ ⬘ plates or of ␥ ⬘ needles.[1–5] This process is called rafting. For the sake of simplification, the rafted morphologies are divided into two groups: (1) plates aligned perpendicular to the stress axis are called N-type rafts (normal to the stress axis), and (2) plates or needles aligned parallel to the stress axis are called P-type rafts (parallel to the stress axis).[6] The type of rafting is determined by several factors such as the sign and magnitude of the relative lattice mismatch and of the external force. Thus, in the case of a negative lattice mismatch and tensile deformation, N-type rafting is found, whereas P-type rafting is found for either positive mismatch alloys after tensile or for negative mismatch alloys after compressive deformation. A recent review on the effects of rafting is given by Nabarro.[7] A. Significance Rafting of nickel-base superalloys has been reported for deformation in laboratory experiments as well as for turbine blades after service (see, e.g., References 8 through 14). In most technically relevant superalloys, the lattice mismatch is negative at least at the high service temperatures. Because during service the turbine blades are loaded in tension due to the centrifugal forces, N-type rafting is observed in the blades. In the literature, there is a controversy on the question of whether the rafted microstructure is beneficial or detrimental for the creep and fatigue properties (see the review of Mughrabi[15]). According to present understanding, there HUA FENG, Process Engineer, formerly with the Institut fu¨r Werkstoffwissenschaften, Universita¨t Erlangen-Nu¨rnberg, is with Formet Industries, St. Thomas, ON, Canada N6H 4P5. HORST BIERMANN, Senior Staff Scientist, and HAE¨L MUGHRABI, Professor, are with the Institut fu¨r Werkstoffwissenschaften, Universita¨t Erlangen-Nu¨rnberg, D-91058 Erlangen, Federal Republic of Germany. Manuscript submitted May 4, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
seem to be different regimes of temperature and stress in which either the rafted state of the materials or the initial state with cuboidal ␥ ⬘ particles has superior high-temperature strength properties. In most cases of interest, rafting has a negative effect on the relevant strength properties.
B. Literature Status Due to the importance of the effect of rafting in the technical application of superalloys, there have been several attempts to develop models that predict and explain the types of rafting and their dependence on several factors. The different models can be classified into one group that is based on energy considerations in a purely elastically loaded regime and one group in which plastic deformation is supposed as a nece
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