Nickel-based superalloys are regarded as one of the most highly developed materials created by engineers [Reed2006]. Their exceptional high-temperature stability is a prerequisite for the mechanical design of aircraft engines or gas turbines. Superalloys can resist mechanical workloads up to 1000 MPa at temperatures reaching 80 % of the melting point due to the precipitation of nano-scaled secondary phase particles. In the present study, we focus on the superalloy Inconel 718, which owes its strength to the partially coherent and ordered γ’-and γ”- precipitates. However, the quantitative description of the morphology and composition at the nanoscale is challenging and requires advanced microscopy.
A correlative, multi-scale approach is presented to reveal the processing-structure-property relationship with light optical and scanning electron microscopy, backscatter electron diffraction, transmission electron microscopy and atom probe microscopy [Gault2012]. The latter has been proven as a highly powerful technique to study nano-scaled precipitates. Crystallographic information obtained from voltage-mode atom probe allows accurate data reconstruction for often limited volumes. Laser-assisted atom probe allows to acquire larger and statistically more significant volumes, but a precise reconstruction may be challenging as crystallographic information is lost. Our combined approach to apply both modes on the same specimen promises reproducible data reconstruction of representative volumes.
For the precipitate characterization, an advanced iso-surface method is developed to refine the concentration threshold of Al + Ti, and Nb through the first derivative of the local concentration. This method is compared to the commonly applied proximity histogram method. One of our major findings is that the precipitates tend to form duplets and triplets of the γ’- and γ”-phases. While their stacking sequences are influenced by the thermo-mechanical processing, yield strength increments up to 10 % can be achieved [Krueger1989]. Therefore, the interfaces between matrix and precipitates are changed. We present a method to determine the interfacial areas of the matrix, γ’-, and γ”-precipitates from cross-sections to develop a model for the strength as a function of the thermo-mechanical history.
Our presented approaches can be applied to the advanced atom probe characterization of various precipitation hardened engineering materials such as Mg-, Al, or Fe-based alloys.