Papers by Author: Shigeji Taniguchi

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: 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: 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 scale failure temperature (Tf) during cooling from 1173 K of low carbon steels containing Si of up to 2.1% was assessed by in-situ acoustic emission measurements and analyses including wavelet transform. In general, Tf lowers with an increase in the Si content for steels without S or P, indicating that the scales on higher Si-content steels are more resistant to thermal stress. This tendency becomes larger for higher cooling rate. Contrarily, Tf rises with an increase in the S, P or (S+P) content for 1% Si steels. This means that the scale failure is enhanced by the additives. S segregates at the scale/substrate interface and seems to enhance the partial scale separation. P is incorporated in the (FeO+SiO2) layer on the substrate and forms microspores at the interface to the FeO layer, and thus enhanced the crack initiation by providing the sites for stress concentration. Wavelet transform showed that the scale failure mode is mainly the following; local separation of the scale over a small area takes place first, and then cracking in the scale follows. For steels containing S or/and P scale cracking is the initial failure in many cases, probably because the stress concentration sites are already formed during the scale growth.

Abstract: Isothermal oxidation behavior of a 4th generation Ni-base single crystal superalloy with Pt-modified and Ru-modified aluminide coating was examined in a temperature range 1223 to 1373 K in air. Both Pt and Ru modification improve the oxidation resistance of a simple aluminide coating, especially above 1273 K. They allow thin protective and continuous Al2O3 scales to be intact for at least 500 h at temperatures up to 1323 K. However, the Pt modification drastically accelerates the formation of a secondary reaction zone (SRZ). This suggests that Pt promotes the formation of a topologically close-packed phase by lowering the solubility of refractory elements in γ-Ni. In contrast, the Ru modification reduces the SRZ, and is expected to enhance the phase stability under the coating by preventing the depletion of Ru due to its outward diffusion.

Abstract: Ru coating prior to aluminizing is one of the effective methods to reduce the harmful intermediate layer that forms under the coating (SRZ) on a 4th generation Ni-base SC superalloy. This study examined the short-term isothermal oxidation behavior of this Ru-modified coating at 1373 K in air. Surface observation by SEM showed that the scale becomes flat and uniform in comparison to simple aluminide coating. XRD and cross-sectional analysis results also showed that phase transformation from β-NiAl to γ’-Ni3Al seldom occurs in the Ru-modified coating layer leading to the prevention of local oxidation.