Superalloys are characterized by high-temperature strength, creep, oxidation and corrosion resistance, and are the key materials in aircraft engines and industrial gas turbines. High creep strength at elevated temperatures is usually achieved by means of precipitation hardening and /or by solid solution hardening. Commercial superalloys are mostly Ni-based and consist of ten or even more alloying elements, each of which with a specific role. Hence, superalloys represent one of the most complex and sophisticated material classes which mankind has ever developed for real-world engineering application. The aim of this research is to explore and develop new alloy systems based on cobalt, which show superior high-temperature properties and are promising alternatives to Ni-based alloys.
(I. Povstugar, P. Choi et al., Acta Materialia 78 (2014) 78–85)
(O. Dmitrieva, P. Choi et al., Acta Materialia 59 (2011) 364–374)
Partitioning at phase boundaries of complex steels is important for their properties. The above figure shows the atomic distribution across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel. The system reveals compositional changes at the phase boundaries, which can be explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium.
(M. Herbig, D. Raabe, Y. J. Li, P. Choi et al, Phys. Rev. Lett. 112 (2014) 126103)
Grain boundary segregation leads to nanoscale chemical variations that can alter a material’s performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. This goal can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography.