Materials Science Forum Vols. 522-523

Abstract: The microstructures of oxide scales formed on MoSi2 at medium-high temperatures in air were observed by TEM. Based on the observation, relationships between oxidation temperature and formation of MoO3 and crystallization of amorphous SiO2 scales were investigated. At 1273 K and 1373 K, the oxide scales had a structure consisting of amorphous SiO2 with small amounts of fine MoO3 particles. The oxide scales at 1573 K and 1773 K had a structure consisting of amorphous and crystalline SiO2. Growth rate of the oxide scale formed at 1773 K appreciably increased due to crystallization of amorphous SiO2. It was thought that the increase in the oxidation rate at 1773 K was caused by a change in the diffusion mechanism from O2 diffusion to lattice diffusion of O2- through SiO2. In addition, the diffusion coefficient of oxygen was estimated from the growth rate of SiO2 scale.

Abstract: An effect of addition of 2 mass % aluminum powder on the oxidation rate of magnesia-carbon refractory, MgO-C, containing 5 mass % carbon has been investigated by measuring weight change with a thermobalance for the cubic specimens in the temperature range from 1073 to 1823 K in air. The oxidation rate of carbon was measured by analyzing the residual carbon in the specimens at some oxidation time. The variations of the weight with oxidation time at temperatures from 1273 to 1673 K showed the similar tendency. The oxidation rate of carbon in the MgO-C refractory with 2 mass % of aluminum additive was a little smaller than that of the MgO-C refractory with no additives. When the oxidation temperature increased to 1823 K, the weight change became very small. It was suggested that the gaps between MgO grains in the thin oxidation layer were bridged by MgO·Al2O3, which was identified by a X-ray diffraction method.

Abstract: The oxidation behavior of Ti-48Al-2Cr-2Nb, Ti-48Al-2Cr-2W and Ti-48Al-2Cr-2Fe was studied in a simulated combustion gas, 10O2-7CO2-6H2O-bal.N2 (vol%), at 1173 K and TEM observation was performed for understanding the initial stage of oxidation behavior. Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr-2W show excellent oxidation resistance in the test gas by forming thin and protective Al2O3-rich scales, while Ti-48Al-2Cr-2Fe shows poor oxidation resistance. The superior oxidation resistance of W-containing alloy is explained in terms of the formation of a bcc phase with low Al content in the alloy which was confirmed by TEM observation and also possible enhanced Al diffusion from the substrate to the scale in this phase. The oxidation resistance of the former two alloys in the test gas is better than in laboratory air, due to the lower O2 content in the test gas. The presence of H2O and CO2 in the test gas enhances the oxidation of Ti-50Al, while it has almost no influence on the oxidation behavior of these two alloys, indicating that these gases are influential to a TiO2-rich scale but not to Al2O3-rich scales.

Abstract: The effects of the third element on the high temperature oxidation of γ'-Ni3Al with 5at%X (X=Ti, Ta, Nb, Cu, Co and Fe) alloys were investigated at 1173K in air, and oxidation behavior could be classified into three groups. The first group, comprised of alloys with Cu and Co, showed good oxidation performance with Al2O3 formation. A second group contains Ti, Ta, and Nb as alloying elements, and showed poor oxidation performance. With Fe or Mn addition the alloy oxidation performance was intermediate between the first and second group. The effects of these elements are discussed associate with partitioning factors for each element in the γ'-phase.

Abstract: Coupon specimens of 30Nb-40Al-(30-x)Cr-xMo (mol %), with x being 0, 7.5, 15, 22.5 or 30, and of 30Nb-40Al-(22.5-y)Cr-7.5Mo-ySi, with y being 5 or 10, were oxidised in air in a temperature range 1473 to 1773 K for 72 ks. The influence of additions of 0.05 and 0.1% Hf to 30Nb-40Al-22.5Cr-7.5Mo was also examined. Conventional metallographic examinations were performed for the specimens before and after the oxidation. The major phase of the specimens is AlNbCr, and their minor phases are Cr2Al and NbAl3, except for high Mo-content specimens which form AlNbMo, Nb2Al, Mo3Al and MoAl3. The additions of Si lead to additional formation of Nb5Si3. Virtually Al2O3 scales (a dense and continuous Al2O3 layer with a thin layer consisting of AlNbO4, CrNbO4 and/or SiO2 on it) are formed at temperatures up to 1673 K, except for the high Mo-content specimens which form poorly protective scales. The additions of Hf increase the mass gain to some degree. 5Si forms protective scales at temperatures up to 1723 K, and 30Cr and 10Si up to 1773 K.

Abstract: TiAl-based alloys have attractive properties as light weight heat-resisting material. In the present work, the influence of Cu, Zn, Ag and Se on the oxidation behavior of TiAl was investigated by ion implantation at acceleration voltage of 50 kV and ion doses of 1019 to 2x1021 ions/m2. The oxidation behavior was assessed by a cyclic oxidation test at 1200 K in a flow of purified oxygen under atmospheric pressure. The oxidation products were analyzed by conventional methods including X-ray diffractometry, SEM and EPMA. The implantation of Zn and Cu improves the oxidation resistance significantly by forming virtually Al2O3 scales, while Ag and Se enhance the oxidation. The improvement by Zn is attributable to the formation of complex oxide of Zn in the initial stage of oxidation. The oxygen partial pressure under the layer seems to be very low, resulting in the formation of alumina scale due to a selective oxidation of Al. The influence of Cu is not certain. The influence of Ag and Se is explained in terms of Al depletion in the implanted layer.

Abstract: High-temperature oxidation of sintered β-FeSi2 doped with Mn and Co was evaluated at 800°C in air. Amorphous SiO2 was developed as an oxide scale. Granular ε-FeSi also appeared below the SiO2 scale as a result of consumption of Si in β-FeSi2. Growth of the oxide scale on doped FeSi2 followed a parabolic law and its rate was similar to oxidation of undoped samples. Thermoelectric properties of sintered β-FeSi2 were also evaluated before and after oxidation at 800°C for 7 days. There was no significant change in thermoelectric properties after high-temperature oxidation on β-FeSi2 sintered bodies.

Abstract: TiAl-Mn-(0,5,10) wt.%Y2O3 alloys were prepared by the MA-SPS process, and their oxidation behavior was studied between 800 and 1000oC in 1 atm of air. The added Y2O3 particles were segregated along the matrix grain boundaries. The addition of (5~10) wt.%Y2O3, viz. (0.9~1.8) mol%Y2O3 increased the tendency to form the rutile phase along the grain boundaries, resulting in the increment of the oxidation rate and decrement of scale adherence. Mn was oxidized to Mn2O3. The primary mode of scale growth was the outward diffusion of Ti and Mn cations for the outer TiO2 scale, and the inward transport of oxygen anions for the inner (TiO2+Al2O3) mixed scale.

Abstract: Metal matrix composites (MMCs) consisting of an Al matrix reinforced with initially added BN particles and newly formed AlN dispersoids were fabricated using the pressureless infiltration method. This method led to the incorporation of Mg and nitrogen into the Al matrix. In the retained BN dispersoids, the depletion of nitrogen and enrichment of Al and Mg occurred. The more Mg available, the more AlN formed in the MMCs. The oxidation resistance of the prepared MMCs at 773 and 823 K in air was not satisfactory, owing to the formation of MgO-rich scale. Mg, which was used as an infiltration enhancer precursor, oxidized preferentially, and thereby decreased the oxidation resistance of the prepared MMCs.

Abstract: The effects of coatings on the creep and oxidation behavior of Ti-50Al alloy were investigated at 1173K in air at a constant loading of 30MPa. The coating was formed by a two-step Cr/Al diffusion treatment and consisted of an outermost TiAl2 layer, an outer Al-rich γ layer, an intermediate γ, Laves and β mixture layer, and a Cr diffusion zone. Creep tests were also carried out with sole Cr or Al coated TiAl and also of uncoated TiAl. The oxide scales formed on the uncoated TiAl and the sole Cr coated specimens were a mixture of TiO2 and Al2O3, which displayed several exfoliations. Both the two-step Cr/Al coated TiAl and the sole Al coated specimens formed a protective Al2O3 layer and little oxide exfoliation was observed here. Significant cracks were observed in the sole Al coated TiAl, while no cracks were observed in the sole Cr coated TiAl; the two-step Cr/Al coated TiAl showed a number of cracks in the coatings. Low creep rates in the two-step Cr/Al coated TiAl could be due to the Laves phase with a hexagonal C14 structure in the intermediate, γ, β and Laves phase mixture, and the high creep rates of the sole Cr coated TiAl may originate in the major β phase component with a B2 structure in the γ, β, and Laves phase mixture.