Novel Ni-based nanocomposites, consisting of nanocrystalline Ni matrix and different dispersed nanoparticles (Cr, Al2O3 or CeO2 particles), have been synthesized by electrodeposition. The thermal stability of a Ni-Cr nanocomposite, the plasma nitridation of Ni-Cr nanocomposites with various contents of codeposited Cr nanoparticles, as well as, the pack chromization of Ni-Al2O3 and Ni-CeO2 nanocomposites were investigated by differential scanning calorimetry (DSC), scanning electron microscopy with an energy-dispersive X-ray analysis (SEM/EDX), electron probe microanalysis (EPMA), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The main results are presented below:
1. Thermal stability of Ni-Cr nanocomposite:
During linear heating at a rate of 10K/min in the DSC system used, significant coarsening of the nanocrystalline Ni matrix (mean grain size: 60 nm) of Ni-10.9 mass%Cr did not occur before 550 oC. But at 570 oC, TEM investigation showed that abnormal grain growth of Ni nanocrystals occurred in the particle-free area due to the nonuniform distribution of Cr nanoparticles; meanwhile, significant solid solution of Cr nanoparticles was also observed at the temperature. Thereafter, the mean grain size of Ni(Cr) matrix was coarsened to ~220 nm at 590 oC and to ~300 nm at 610 oC.
2. Plasma nitridation of Ni-Cr nanocomposite:
A double-layered nitriding zone was formed on the Ni-10.8 mass%Cr nanocomposite after plasma nitridation at 560 oC for 10 h. The outer layer (~50 m thick) precipitated nanometer-sized CrN (<100 nm), which increased in size but decreased the number with increasing the nitridation depth (following Böhm-Kahlweit’s mode). The inner layer (~5 m thick) exhibited larger-coarsened nitride precipitates (100-200 nm) which almost linked each other. However, a compositionally-similar but microstructurally different Ni-10Cr alloy (mean grain size: 30 m) only achieved a 5 m-thick nitriding layer in the same condition. The greatly enhancement of nitriding kinetics of the nanocomposite is mainly associated with that the numerous grain boundaries dramatically increases the nitrogen permeability, according to the treatment using a classical Wagner’s approach. This outer nitrided layer having the typical structure yielded a nanohardness profile with a decrease gradient from ~ 7.5 GPa in the near-surface area to ~ 6.0 GPa in the innermost area. The inner nitrided layer has the maximum hardness of ~12.0 GPa, because the layer is almost composed of larger-coarsened nitride precipitates.
With increasing the content of the codeposited Cr nanoparticles, the continuous CrN inner layer was shifted to a place closer to the surface. It means that higher Cr content favors the transition of the nitridation of the nanocomposite from “internal” to external”. When the Cr content was 30 mass%, external nitridation of the nanocomposite occurred. The result demonstrates that the nitrided layers with different microstructures and properties can be achieved by adjusting the Cr content of the Ni-Cr nanocomposite.
3. Oxidation behavior of chromized Ni-Al2O3 and Ni-CeO2 nanocomposites:
The electrodeposited Ni-Al2O3 and Ni-CeO2 nanocomposites were chromized at 1120 oC for 4 hours. It was found that the oxidation resistance at 900 oC of the chromized coating on the Ni-Al2O3 nanocomposite, although it had a fine-grained structure, was not improved intrinsically compared to that of the chromized coating on the electrodeposited Ni. In contrast, the oxidation resistance of the fine-grained chromized coating on the Ni-CeO2 nanocomposite with respect to that on the Ni was significantly increased. The result confirms that the added CeO2 exerted a so-called reactive element effect (REE) during oxidation.
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