MICROSTRUCTURAL CHARACTERIZATION OF SINGLE-CRYSTAL NICKEL-BASE SUPERALLOYS BY SMALL-ANGLE NEUTRON SCATTERING

Pavel Strunz¹, Ralph Gilles², Debasish Mukherji², Albrecht Wiedenmann², Rajeshwar P. Wahi² and Jozef Zrník³

¹Nuclear Physics Institute, 25068 Rez near Prague, Czech Republic (phone: 420-2 66172034, fax: 420-2 6857003, e-mail: strunz@ujf.cas.cz)
²Hahn-Meitner-Institut, Berlin, Germany
³Technical University, Košice, Slovakia

Keywords: single-crystal nickel-base superalloys, precipitate morphology, anisotropic small-angle neutron scattering,

The two-phase microstructure consisting of g' precipitates growing in the g-phase matrix is the basic feature which determines a high creep-resistance of nickel-base superalloys, the materials frequently used in aircraft and land-based turbines. Outstanding high-temperature mechanical properties of these materials strongly depend on the morphology of the precipitates and thus also on the applied heat treatment. The morphology is conventionally studied by transmission electron microscopy, however, this and other standard methods of materials science do not usually characterize the microstructure completely.

Small-angle neutron scattering (SANS), which provides bulk-averaged information, has been found to be a powerful tool for investigation of microstructural inhomogeneities in single-crystal alloys [1]. A number of studies (see e.g. [2-3]) documents an applicability of SANS particularly to investigation of precipitation in single-crystal Ni-base alloys. An overview of the recent investigation of precipitate microstructure in ZS26, SC16 and SCA superalloys on V4 pin-hole SANS facility of BENSC in HMI Berlin is presented. Anisotropic 2d SANS curves provide information on the average shape of the ordered cuboidal g'-precipitates [4] (the asymptotic region of a SANS curve) as well as on the precipitate dimension and distances between them [5] (central part of the scattering curve). Consequently, the original data evaluation procedure [6] (numerical modeling and fitting) allows - in many cases - to estimate well the volume fraction of the precipitates even without the knowledge on the scattering contrast. Finally, the results are related to the used heat treatment procedures. Moreover, in the case of SC16, the SANS measurement revealed the presence of well oriented additional phase which could not be effectively characterized by another experimental method [7]. The figure displays one of the measured 2d scattering curves from this superalloy.

Figure: Scattering pattern from solution treated sample (1350°C/3h/WQ); orientation: [320] vertical, no other low-index crystallographic direction in the displayed plane.

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