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t z 4, noo " , a, > + " " g 3,000 ? 2, non a, 1 , 0 0 0 L1 Effect of Oxide Defect Structure on the I I I I t bure energy in all environments where moisture was present.the energy for refracture was as much as 3 times that needed for initial fraoture.The defect structure of monoclinic ZrO, was studied by measuring the transfer numbers and electrical conductivity as

The effect of quaternary additions of 0.5% Y, 0.5 and 1.0% Th to a base alloy of Ni-lOCr-SAJ. on the oxidation behavior and mechanism was studied during oxidation in air over the range of 1000 to 1200°C. The presence of yttrium decreased the oxidation kinetics slightly, whereas, the addition of thorium caused a slight increase. Oxide scale adherence was markedly improved by the addition of the quaternary elements.Although a number of oxides formed on yttrium-containing alloys, quantitative X-ray diffraction clearly showed that the rate-controlling step was the diffusion of aluminum through shortcircuit paths in a thin layer of alumina that formed parabolically with time. Mixed oxides containing both aluminum and yttrium formed by the reaction of Y 2 0, to form YA£0 3 initally, and Y 3 AZ 5°12 ( YAG ) after longer times. Although the scale adherence of the yttrium-containing alloy was considerably better than the base alloys, spalling did occur that was attributed to the formation of the voluminous YAG particles which grew in a "mushroom"-like manner, lifting the protective scale off the substrate locally. The YAG particles formed primarily at grain boundaries in the substrate in which the yttrium originally existed as YNi 9 . This intermetallic compound reacted to form Y 2 0 3 , liberating metallic nickel that subsequently reacted to form NiO and/or NiAX^O^ spinel. The Y 2 0 3 reacted with aluminum to ultimately form the YAG "mushrooms."Thorium did not form any mixed oxides; the only oxide involving thorium was Th0 2 , which existed as small particles at the oxide-metal interface. A highly beneficial effect of the thoria particles in reducing film spalling was observed* Scale spalling in the base alloy was attributed to void formation at the oxide-metal interface, the voids forming by condensation of excess vacancies from the Kirkendall effect associated with slow back-diffusion of nickel into the substrate as aluminum was preferentially oxidized and diffused rapidly outward. The mechanism of improved scale adherence in the quaternary alloys was the elimination of voids by annihilation of the Kirkendall vacancies at vacancy sinks introduced by the non-coherent interfaces between yttrium and thorium-containing intermetallics and/or oxides.17.

The results of a comprehensive characterisation study of different phase transformations that take place upon heating and cooling in some low carbon, 9 wt-%Cr steels with varying concentrations of microalloying additions are presented in this paper. The steels investigated include: standard 9Cr–1Mo grade, V and Nb added modified 9Cr variety, controlled silicon added versions of plain 9Cr variety, (Ni+Mn) content controlled modified 9Cr welding consumables and one composition of W, Ta added reduced activation steel. The various on-heating diffusional phase changes up to the melting range and subsequent rapid cooling induced martensitic transformations are investigated in a controlled manner using differential scanning calorimetry under different heating and cooling rates, in the range 1–100 K min−1. In addition to the accurate determination of A c1, A c3, M23C6, MX carbide dissolution and δ-ferrite formation temperatures upon heating, the melting range and the associated fusion enthalpy have also been established for these steels. The effect of prolonged thermal aging at temperatures of 823–873 K on austenite formation characteristics has also been investigated for standard and modified 9Cr–1Mo steels. The critical cooling rate for the formation of martensite on cooling from single phase austenite region is estimated to be about 4–5 K min−1 for all 9Cr steels investigated in this study. The effect of holding at 1273 K in the austenite region on martensite start temperature Ms, has also been evaluated as a part of this study. The experimental results are discussed in the light of the prevailing understanding of the physical metallurgy of high chromium low carbon steels.

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