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The phenomena of hydrogen induced cracking (HIC) in 304 stainless steels was considered in a hydrogen pressure and stress corrosive atmosphere. Microstructures with chloride pits and stress corrosion cracks around the HIC were analyzed by SEM/EDS. Abnormal phase transformations induced by the hydrogen were analyzed using TEM and diffraction. In the hydrogen pressure atmosphere, pits and pores were observed on the surface of the 304 stainless steels. In addition, it was determined that Cl, an etchant component, was concentrated at a high concentration in the pits. SCC (stress corrosion cracking) was induced in the Cl atmosphere by stress caused by the abrasive embedded in the pits. It was assumed that the SCC mechanism is similar to HIC in that it occurs in the surface tensile stress and Cl atmosphere and is accompanied by grain boundary cracks similar to IGSCC (inter-granular SCC). The deformation induced phase transformation accompanied by planar slip should be related to the main cause of HIC in the hydrogen pressured atmosphere. Abnormal forbidden spots between the main diffraction spots were induced by the HIC in the hydrogen attacked area, where the microstructure was hardened. Understanding the HIC mechanism related to chloride corrosion can be used to assess the fitness of austenitic stainless steels for uses where there is a possibility of various susceptible cracking in hydrogen and chloride atmospheres.

Yttria (Y 2 O 3 ) stabilized zirconia (ZrO 2 ) containing 8 wt% yttria, 8YSZ, was fabricated with an ingot shape with different powder compositions. Ingot shape 8YSZ is commercially used as a source to fabricate thermal barrier coatings (TBCs) by an electron beam (EB) evaporation process. Powder mixtures with different mixing ratios using small (mean dia. of ~40 nm) and large (40–80 µm, granulated) raw material powders were used to form the ingots in the present study. In addition, the tetragonal and monoclinic phases of raw powders were used selectively. The ingots from various powder mixtures showed a bimodal structure of small and large grains with 55–60% relative density (3.0–3.5 g/cm 3 ). The ingots formed from the mixtures using nanometer size powder with a large content of the monoclinic phase and several tens of micrometer size powder with the tetragonal phase showed better thermal shock resistance compared to the ingot from a simple mixture with a bimodal particle distribution. Furthermore, for these ingots, it appears that an additional cold isostatic pressing (CIP) process between uni-axial pressing and heat treatment processes might provide mechanical enhancement to form a more stable melt spot during the EB irradiation process compared to other ingot samples.