Carbides Evolution in a Ni-16Mo-7Cr Base Superalloy during Long-Term Thermal Exposure
The effect of long-term thermal exposure on the carbide evolution in a Ni-16Mo-7Cr base superalloy was investigated. The results show that M12C carbides are mainly precipitated on the grain boundaries during thermal exposure, and the primary massive M6C carbides can be completely transformed to M12C carbides in situ at temperatures above 750 °C for long-term thermal exposure. The transformation from M6C carbides to M12C carbides is attributed to the release of C atoms from M6C, which results in the morphology changes of massive carbides, and stabilization of the sizes of M12C carbides precipitated on the grain boundaries.
Corrosion behaviors of the UNS N10003 alloy in Fenton's reagent
The corrosion behavior of the UNS N10003 alloy in Fenton's reagent was investigated at room temperature for 600 hr by the static immersion method. The results showed that the alloy suffered obvious pitting corrosion. The lattice parameter of this alloy has a slight decrease after corrosion. This is mainly attributed to the depletion of Mo‐rich carbides dissolving into solution caused by the hydroxyl radical with strong oxidization, which is very different from the result of the UNS N10003 alloy under the molten salt condition. The results provide an insight into the corrosion of the alloy under hydroxyl radical conditions (i.e., Fenton's reagent), and will offer a realistic instruction in related fields.
M23C6 precipitation behavior and grain boundary serration in Ni-based Alloy 690
Abstract A study on the Cr-rich M23C6 precipitation and grain boundary serration in Ni-based Alloy 690 was conducted through various heat treatments. Fine granular carbides and discontinuous precipitates were evolved during isothermal treatments at 720 °C for 0.1–100 h, and their precipitation morphologies and behaviors were considerably varied depending on the grain boundary characteristic. Carbide precipitation was retarded on the low angle grain boundaries, compared to that on the random high angle grain boundaries. Needle-like carbides were precipitated on the incoherent twin boundaries. The discontinuous precipitates were initiated even by a heat treatment of 0.1 h, and covered the entire grain boundary region within a heat treatment of 10 h. Grain boundaries were serrated by a slow cooling process, and it was revealed that the serration occurred in a limited temperature range during controlled cooling. As the average grain size with more carbon content increased and/or the cooling rate decreased, the degree of serration increased with a precipitation of coarse planar intergranular M23C6 carbides. The grain boundaries had a convex shape into the incoherent grain. It is suggested that the grain boundary migration during serration is closely correlated with the precipitation of intergranular Cr carbides in this alloy.