Slow strain rates tests (SSRT) were conducted on hydrogen-containing specimens of PH13-8Mo maraging stainless steel. Hydrogen-assisted subcritical quasi-cleavage cracking was shown to take place during SSRT, thus accelerating material failure. Fractographic analysis showed that quasi-cleavage is composed of flat brittle areas and rougher areas. Using cross-sectional electron backscatter diffraction (EBSD) analysis of a secondary subcritically grown crack, we observed brittle cracks propagated across martensite blocks ahead the main crack tip. These cracks were stopped at high-angle boundaries. The crack direction was consistent with propagation along {100} type planes. High-resolution EBSD showed significant crystal lattice rotation, hence consequential plastic deformation, concentrated between the main crack tip and the cracks located ahead it. It is concluded that quasi-cleavage in the material investigated here consists of {100} cleavage cracks connected by ductile ridges. A discontinuous mechanism, involving re-initiation of new cleavage cracks ahead the main crack tip is suggested.

MLX19 stainless maraging steel grade exhibits a mechanical strength/fracture toughness balance within the required range for landing gear applications. However, the microstructure after the heat treatment still needs to be precisely controlled to obtain a better repeatability of the mechanical properties. This work shows that austenitizing is a critical stage. The influence of austenitization treatment parameters on the microstructure obtained after quenching was thus precisely quantified. It was first revealed that, after a standard austenitization at 850 °C and for specific heating rates and holding times, undissolved β-NiAl precipitates, reaching sizes up to 500 nm, still remain in the as-quenched state, in addition to a high retained austenite fraction. It was also found that large amounts of retained austenite are the result of local heterogeneities in the chemical composition of the austenitic phase prior to quenching, while the undissolved precipitates change the overall chemical composition of the austenitic matrix. New austenitization conditions were thus proposed, leading to a better homogeneity of the chemical composition of the martensitic matrix after quenching.

The high temperature creep properties of next generation cast and wrought AD730 superalloy have been investigated taking into consideration three microstructural parameters: the grain size, the presence of grain boundaries and the γ′ precipitates size and distribution. Definitive analysis of the influence of the grain boundaries and γ′ precipitates size distribution has been enabled by the study of single crystalline versions of the polycrystalline alloys studied. At high temperature (equal to or in excess of 850°C), the grain size controls creep properties. Comparisons between polycrystalline and single crystalline specimens indicate that the grain boundaries provide a strengthening effect, especially in the small strain regime. At intermediate temperature (700°C), the γ′ precipitates size is the main creep-rate controlling parameter. In this temperature domain, creep strength seems to be mainly controlled by dislocation motion. A very striking grain boundary strengthening mechanism is observed at small creep strain and intermediate temperature.

Two grades of Fe-Cr-Ni-Al-Ti-Mo maraging steels, with a different titanium content, were investigated. Particular attention was given to the correlation between the precipitated phases and the yield strength. Synchrotron X-ray diffraction, small-angle neutron scattering and atom probe experiments were performed to determine the crystal structure, shape, size distribution, chemical composition, particle number density and volume fraction of precipitates. Both alloys show a strong increase in strength after an aging treatment, which is attributed to the co-precipitation of two different intermetallic phases. Strengthening by a single precipitation of β-Ni (Al,Ti) particles induces a saturation of yield strength around 1600 MPa above a volume fraction of 6 %. The improvement of yield strength is then obtained by introducing a nanoscale co-precipitation of η-Ni3(Ti,Al) phase.

To reduce CO2 emissions on coal-fired power plant, A-ultra supercritical (A-USC) power plant whose steam conditions exceed 700 °C are being developed. At these elevated temperatures, the use of Ni-base superalloys becomes necessary. In this context and within the European project NextGenPower, focus is made on commercial Nimonic C-263 as a candidate material for turbine rotors. Nimonic C-263 is known to have low sensitivity to segregation, high workability and high weldability which are major properties for the manufacture of large shafts. Long-term creep strength is also required for this application and unfortunately Nimonic C-263 shows η-phase precipitation after long-time exposure between 700 °C–900 °C which is detrimental for long-term creep properties. The composition of Nimonic C-263 was thus optimised to overcome the formation of η-phase. Trial tests were made in order to study the effect of hardening contribution elements on microstructural and mechanical properties. Then, a 500 mm diameter forged rotor was made from optimised 263 alloy and shows promising properties.