Sans from low volume fraction of plate-like precipitates present in two single-crystal Ni-base superalloys

Nickel-base superalloys for gas turbine applications are strengthened by a high volume fraction of gamma-prime precipitates. The SC16 alloy (17.5 at. pct Cr, 1.8 at. pct Mo, 7.35 at. pct Al, 4.15 at. pct Ti, 1.1 at. pct Ta, balance Ni) was developed for applications in land-based gas turbines. Two-dimensional small-angle neutron scattering (SANS) profiles from SC16 specimens show strong streaks along low-index crystallographic directions, which are superimposed over the anisotropic intensity pattern from the cuboidal gamma-prime precipitates oriented parallel with 100-line crystallographic direction. SANS data analysis and complementary measurements by SEM, TEM, and XRD have suggested that the streaks are caused by the presence of a small number of plate-like precipitates having very low volume fraction, which were identified as sigma phase (Gilles et al., 1998). This paper presents SANS results from a single-crystal alloy, called SCA, having composition similar to SC16. The SCA alloy also contains plate-like precipitates. The paper discusses why scattering from these precipitates in an SCA alloy specimen is not detectable by SANS. (AIAA)