Microstructural analysis of TiC reinforced ferrous surface composites processed by accelerated electron beam irradiation

The present study aims to analyze microstructures of TiC reinforced ferrous surface composites processed by accelerated electron beam irradiation. Two kinds of powder/flux mixtures, i.e., TiC and (Ti + C) powders with 40 wt% of CaF₂ flux, were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with electron beam. TiC agglomerates and pores were found in the surface composite specimen processed by irradiation of TiC powders because of insufficient melting of TiC powders. In the specimen processed by irradiation of Ti and C powders having lower melting points than TiC powders, a lot of large TiC carbides were precipitated in the melted region, together with a few TiC agglomerates or pores. This indicated the more effective TiC precipitation obtained from the melting of Ti and C powders, instead of TiC powders. The hardness of the surface composite layer was about two times higher than that of the AISI 304 substrate mainly due to the precipitation of TiC carbides.     

Simple heat treatment for fabrication of carbonaceous layer-coated microelectrodes and conductive stainless steels

A simple heat treatment was used to fabricate carbonaceous layer-coated electrodes: micro-ring electrodes and conductive stainless steel. Substrates of sharpened quartz capillaries or type-316 stainless steel plates were put in an alumina boat with powder of petroleum pitch A240F separately and heated at 1073-1273 K in a flow of nitrogen or argon. By this treatment, both of the substrates were coated with a uniform carbonaceous layer of several hundred nano-meters in thickness. The electric conductivity of the layer was improved by increases in temperature and period of the heating. The quartz glass-capillary covered with the conductive layer was modified to a needle-type microelectrode by coating with an insulating polymer and baring the tip. At least a dozen carbon micro-ring electrodes with an outer radius of about 1 μm were successfully prepared by the simple heat treatment. On the other hand, the carbonaceous layer formed on type-316 stainless steel showed relatively poor conductivity due to the formation of oxides in the layer. However, the conductivity was improved by electroplating of nickel on the substrate before the heating. The carbonaceous layer-coated stainless steel showed good corrosion resistance in sulphuric acid.     

Methods for atom probe analysis of microgradients in functionally graded cemented carbides

Methods to prepare needle-shaped specimens for atom probe field ion microscopy from near surface regions have been developed. The material used was a cemented carbide with a composition gradient towards the surface, but the method is equally applicable for other materials. The preparation technique involves dimple grinding, electropolishing and focused ion beam (FIB) milling. The use of FIB milling allows for specimen preparation of materials which due to the preferential etching of different phases are difficult to electropolish. The technique also allows for preparation of specimens at well defined depth from the sample surface, selection of phase to be analysed, and to sharpen and re-use already analysed specimens.

Atom probe analyses of the near surface zone region in a gradient sintered WC–Ti(C,N)–TaC–Co cemented carbide are presented.     

Morphological and compositional analysis of passive film on austenitic stainless steel in nitric acid medium

Passive film properties of type 304L stainless steel in nitric acid medium are investigated in both ex situ and in situ conditions. Ex situ results revealed that variation in passive film morphology occurs depending upon the concentration and time of immersion. In situ surface morphological investigation showed formation of platelet like structures at lower concentrations (0.1 M, 0.5 M), and towards higher concentration (0.6 M, 1 M) the platelet like structures got agglomerated, homogenized and started depleting from the surface leading to opening up of oxide boundaries. Compositional analysis of the passive film revealed duplex nature at lower concentration consisting of hydroxide and oxide layer, and with increasing concentration oxide layer predominates over the surface.     

Atom probe tomography analysis of radiation-induced solute clustering in austenite stainless steels

Various types of defects are produced by the irradiation of energetic particles onto a structural material. The large number of mobile vacancies and self-interstitial atoms during irradiation induce defect fluxes and the diffusion of solute atoms in the matrix. The preferential interaction between the solute atoms and radiation-induced defects leads to the enrichment/depletion or clustering of the solutes at defect sinks. In the current work, atom probe tomography (APT) was used for the analysis of radiation-induced solute clustering in ion-irradiated austenite stainless steel 316. Quantitative analysis of the localised clustering of chemical elements was implemented and a parameter selection procedure was proposed. The number density and size distribution of Si clusters in APT specimens irradiated at various temperatures were examined. At high temperature, the number density of the clusters decreased and their size increased. The localized Si atoms in variously shaped defects were clearly identified. The APT method was demonstrated to be suitable for identifying defect structures and for the quantitative analysis of clustering in irradiated specimens.