Research On Microstructure And Hot Corrosion Behaviors Of The Ni-base Superalloy Manufactured By Plasma Deposition

In this thesis, the microstructural and hot corrosion behaviors of Ni-based superalloy manufactured by plasma transferred arc-assisted deposition have been systematically investigated by using optical microscope(OM), X-ray diffraction(XRD), scan electronic microscope(SEM), transmission electron microscopy (TEM) and Electron probe X-ray microanalyzer (EMPA).In this first part of the thesis, a historic progress of Ni-based superalloy over past 50 years has been overviewed, which includes composition, phases and structure of superalloy, as well as the traditional techniques for manufacture of different superalloys. Following that, the principle and processes of the plasma transferred arc-assisted deposition technique has been also introduced as to its application in superalloy forming.The superalloy, GH163, was employed in the paper, and the samples were prepared by plasma transferred arc-assisted deposition and vacuum-casting technology.The GH163 superalloy powders with size of 80-200 mesh were used in the study. From the powders, the bulk superalloy samples was fabricated by a homemade setup of plasma transferred arc-assisted deposition using two preparation processes, i.e., single-path multi-layer and multi-path multi-layer. The structural investigations using OM and XRD indicates that the two deposited samples exhibit a fine, dense and directional solidification-like structure with <001> orientation as long as proper processing parameters were used. The crystallization of the remelted zone between deposition layers followed quite well the crystallographic orientation of the afore-layer, indicating a well inheritance of the structure during plasma deposition. Composition analysis using EPMA revealed that the composition distribution in whole samples was pretty uniform and no significant segregation occurred in the interface regions between deposited layers. In addition, no much difference in microhardness in interface regions between layers was found in the sample with multi-path multi-layer, demonstrating that deposition layers were strongly constructed and no interface effect between them. The structural feature of the deposited samples can be well explained by the solidification theory proposed by Winegard. After solid-solution and ageing treatments, a large number of fineγ′particles with a size of about 20-30 nm precipitated inside grains of the alloy. Electron diffraction using TEM indicates that theγ′phase precipitated has a fcc structure and exhibited a completely coherent relation with the matrix. The MC has also been found inside grains and has cubic-on-cubic relation with the matrix. In addition, a few fine carbide particles were formed at grain boundaries, which are believed to be good for creep strength of the superalloy.The hot corrosion behavior of a nickel base superalloy, GH163, manufactured by vacuum-casting and plasma transferred arc-assisted deposition, has been comparatively studied at 800℃in the melted Na₂SO₄+NaCl wt.25% mixed salt. The results show that the samples formed by plasma deposition have better resistance to hot corrosion in comparison with the casting one. SEM and EPMA examinations indicate that the preferential formation of the most compact and highly corrosion- resistant Cr₂O₃ scale on the surface of plasma deposited samples may account for its excellence of the hot corrosion resistance.