Very high temperature creep behavior of a single crystal Ni-based superalloy under complex thermal cycling conditions

Very high temperature thermal cycling has been performed on the single crystal superalloy MC2 to evaluate the effect of periodic overheating on creep behavior. The experiments consist of alternately performing 1 min dwell time at 1100°C and 1150°C for every 15 min during creep test at 1050°C/120 MPa. Both thermal cycling and prior γ′-rafting appear to be deleterious to the cyclic creep properties. The observed non-isothermal creep behavior is correlated with γ′-dissolution/coalescence processes, especially during overheatings where γ′ micro-rafts seem to play a significant role.     

Indentation behaviour of ion-irradiated X-750 Ni-based superalloy

The mechanical properties of an ion-irradiated Ni-based superalloy have been evaluated using nano-indentation hardness tests. A high-energy Ni ion beam was employed for irradiation at room temperature up to different doses. Nano-indentation tests were carried out in the range of 100–1000 nm to measure hardness changes of the experimental alloy. The results show softening of the irradiated material compared to the un-irradiated material, with the softening being more pronounced at the higher dose. Cross-sectional transmission electron microscopy characterisation reveals that γ′-precipitate instability (disordering/dissolution) is the main reason for the softening behaviour and this outweighs the hardening effect of irradiation-induced defects.     

Evidence of the climb motion of a Shockley partial during glide at room temperature

A dissociated dislocation introduced by compression at room temperature in a Ag-15at.%Al alloy was studied comprehensively by the weak-beam method. In addition to the two Shockley partials of the dissociated dislocation, faint contrasts were observed near the Shockley partial with pure edge orientation. Contrast experiment was carried out on the faint contrast, and we conclude that this is evidence for the interaction between an edge-orientated Shockley partial moving under the action of an applied stress and vacancies at room temperature.     

Microstructural characterization of the M23C6 carbide in a long-term aged Ni-based superalloy

The microstructural features of M₂₃C₆ carbide in a long-term aged heat- and corrosion-resistant Ni-based superalloy have been investigated in detail using various kinds of transmission electron microscope (TEM) techniques. It is found that TEM contrast, which is related to structural and chemical inhomogeneities inside the grains, always exists in the interior of grains in the alloy. The structure of these inhomogeneous regions has been determined to be the same as that of the γ′ and t-M₂₃C₆ phases, where t-M₂₃C₆ indicates a transitional and metastable phase. Although possessing the same structure as the M₂₃C₆ phase, the chemical composition of the t-M₂₃C₆ is different from that of the M₂₃C₆ phase. Compared with M₂₃C₆, t-M₂₃C₆ is richer in Ni, Co, Al and Ti but poorer in W, Mo and Cr. This phenomenon of structural and chemical inhomogeneity demonstrates that pristine M₂₃C₆ carbide (p-M₂₃C₆) precipitated in standard heat-treated samples is unstable. Therefore, upon long-term ageing treatment, Ni, Co, Al and Ti may locally enrich inside the p-M₂₃C₆ phase, finally forming the γ′ phase, which can be described by the decomposition reaction p-M₂₃C₆ → M₂₃C₆ + γ′.     

Static and dynamical ageing processes at room temperature in a Fe25Ni0.4C virgin martensite: effect of C redistribution at the nanoscale

The work-hardening behaviour of virgin martensitic steel has been investigated in a strictly un-aged state and after various ageing conditions. At room temperature (RT), the un-aged alloy shows astonishing tensile performances (ultimate tensile stress?=?1600?MPa/uniform elongation?=?15%) but unexpected serrations. These serrations can be suppressed by static ageing (at RT or higher) while maintaining the initial work-hardening rate (ageing at RT). Parallel investigations using atom probe tomography reveal that the distribution of carbon at the atomic scale evolves from purely homogeneous for virgin martensite to partly segregated at a very fine scale (5–10?nm) after static ageing. This particular mechanical behaviour can therefore be associated with a very local decrease in available carbon in solid solution due to redistribution and segregations on defects (nanotwins) that occurs rapidly, even after few days at RT.