Microstructure Investigation of Superalloys and Ceramic Coatings Using Small-Angle Neutron Scattering

Superalloys and ceramics are both processed and used at high temperatures, which brings two strong reasons to investigate their microstructure in-situ. SANS can detect evolution of precipitates and pores in these materials. Examples taken from the research of Recontaining Ni-base superalloys and ceramic thermal barrier coatings (TBC) are presented. Introduction Materials for high-temperature applications e.g. superalloys and plasma-sprayed ceramics are conventionally investigated by a number of techniques (TEM, SEM, XRD). Nevertheless, the application of neutron scattering is not only complementary. In certain cases, it can be indispensable as it provides information not accessible by the other methods. Due to the low absorption of neutrons by a majority of materials, it is a suitable method for bulk characterization as well as for in-situ studies at extreme conditions (e.g. high temperatures). Small-Angle Neutron Scattering (SANS) [1] provides information on the microstructure through measuring angular dependence of neutron intensity scattered by sample to low angles (<15o). In case of neutron scattering, the microstructure is represented by the scattering length density ρ(r) which is analogous to the electron density in the case of XRD. Inhomogeneities like ’ precipitates in superalloys or pores in ceramics have sufficient contrast [∆ρ(r)]=[ρ(r)ρ ] to give strong scattering intensity. Examples taken from the research of Re-containing Ni superalloys (carried out at SINQ, PSI Villigen) and ceramic thermal barrier coatings (BENSC, HMI Berlin) demonstrate the applicability of SANS to these fields. They were focused on the solution treatment of superalloy Re31 and on the porosity evolution in TBC layer at operational conditions. P. Strunz, D. Mukherji, G. Schumacher, R. Vassen, R. Gilles, J. Rosler, A. Wiedenmann 2 In-situ SANS study of polycrystalline alloy Re31 Nickel-base superalloys [2] are multicomponent alloy systems which are used at severe working conditions (rotors, discs or blades in turbines). The structural stability and creep resistance of superalloys depend on the morphology and volume fraction of precipitates, mainly ’, which strengthen the -phase matrix . Modern cast superalloys have additions of heavy elements, which have tendency to segregate. Consequently, a non-homogeneous element distribution after casting is obtained. It results in a non-uniform precipitate distribution between the dendritic and interdendritic regions. Such segregation can be minimized by solution heat treatment in the single-phase region, which is a standard first step of any heat treatment of Ni-base superalloys. In Re-rich superalloys, complete solutionizing is sometimes difficult as incipient (localized) melting can occur at temperatures lower than the equilibrium solidus of the alloy in the Re-lean interdendritic regions. A SANS investigation of the solution window and the high-temperature precipitate microstructure [3] of an experimental superalloy designated Re31 is reported. The alloy has large amount of Re and exhibits thus a strong tendency for segregation. Two different solution treatment procedures were applied to as-cast samples during in-situ scattering experiments. The thermal histories can be seen in Fig. 1 together with the total volume fraction times scattering contrast determined from SANS data. The most important feature in Fig. 1a is the decrease at 1400°C for the sample which was held at 1290°C for relatively long time. It indicates that all the ' precipitates, including the largest ones, were dissolved and the superalloy is in the most homogeneous state. This solution treatment procedure (Fig. 1a) thus enables the full dissolution of precipitates without causing incipient melting. The sample without hold at 1290°C (Fig. 1b) contains still at 1400°C a lot of inhomogeneities. The cause of the scattering probability increase during holding at 1290°C can be explained by compositional changes in the matrix surrounding the remaining large precipitates in the interdendritic region. The holding enables diffusion between the dendritic (Re-rich) and the interdendritic (Re-lean) regions, resulting thus in homogenization of phase. It causes the observed increase of the scattering contrast between the matrix and the remaining Re-lean precipitates. a 00:00 06:00 12:00 18:00 24:00 30:00 36:00 0 90