Materials Science Forum Vols. 654-656

Abstract: An assessment of a creep-fatigue crack growth for Mod. 9Cr-1Mo steel wide plates have been carried out based on an extended French high temperature design code, RCC-MR A16 guide. The defect assessment guide of the A16 provides assessment procedures on creep-fatigue crack growth for an austenitic stainless steel, but no guidelines are available yet for a Mod. 9Cr-1Mo steel. In this study, assessments of a creep-fatigue crack growth at defects of Grade 91 steel wide plates have been carried out based on the extended A16 method for austenitic stainless steel.

Abstract: The fretting wear performance of surface wear-resistant Cr3C2/Ni3Al composite deposited onto IC6SX alloy by manual argon tungsten-arc welding was investigated in this study. The results showed that the welded substrate exhibits a very good performance in comparison to the unwelded substrate at 500°C and 800°C, with 51.5% and 57.8% decrease of the fretting sectional area respectively. However, the fretting sectional area of the welded substrate is larger than that of the unwelded substrate at RT indicating poor fretting resistance as the reinforce phases of the welding layer is harder and more harmful to the sample surface. Examination of the fretting surfaces indicated that both the welded and unwelded samples undergo gross slip at RT and 500°C, and partial slip at 800°C. Coefficient of friction (COF) results were given by the SRV-4 tester which shows that as the temperature goes up, the COF of samples became lower and more stable. The results showed that the COF of welded substrate is lower than that of the unwelded substrate most of the time.

Abstract: The isothermal oxidation behaviour under static atmosphere of a new directionally solidified Ni-base superalloy was investigated. The results showed that the oxidation kinetics curves of the alloy follow parabolic law in the temperature range of 750-950°C. The diffusion activation energy Q is to be about 244.86 kJ•mol-1 and the oxidation within this temperature range is mainly controlled by Cr3+ diffusion among Cr2O3 oxidation film. The oxidation resistance grade of the alloy is perfect anti-oxidation within 750-850°C, and anti-oxidation within 900-950°C. The oxidation film can be divided into TiO2, Cr2O3+TiTaO4+NiCr2O4, Al2O3 and TiN layers, from the surface to inside. Priority oxidation and the inside oxidation of Al are considered with the oxidation of Ti and the depletion of Cr, and it is the key point for the improving of oxidation resistance.

Abstract: High temperature oxidation behavior of Si-Mo ferritic ductile cast iron was investigated in the point of the effect of chromium and vanadium addition. The addition of Cr promoted the formation of as-cast pearlite around carbide which exists in cell boundary, which was eliminated during annealing process. The addition of vanadium promoted the precipitation of tiny carbide and reduced the grain size of ferrite. As the test temperature increased, the change of volume increased, on the other hand, the change of weight decreased above 1173K. In the case of Cr added specimen, the change of weight decreased with the increase of test temperature because of the presence of Cr oxide layer. The vanadium added specimens showed higher increase in the weight and volume change. The oxide layer of vanadium added specimen had very porous structure and showed severe internal oxidation. It is due to the catastrophic oxidation characteristic of vanadium alloyed ferrous alloy.

Abstract: The mechanical properties and the oxidation resistance of -TiAl at elevated temperatures have to be improved to be used in the severe environmental conditions. It has become clear that the addition of more than 4at.%Ta in TiAl demonstrates a superior oxidation to the other TiAl-X compounds, according to the weight gain results of cyclic oxidation experiments at 1173 and 1273K. Oxidation behaviors are strongly influenced by the Ta concentration in TiAl. XRD, SEM-EDS, and TEM-EDS observations have been carried out to determine the microstructures and the surface compositions of multi-layered oxide scales. It was revealed that a protective intermediate phase simultaneously formed between the substrate γ-TiAl and the oxide scale layer. The Ti53Al32Ta15 ternary compound exists as an equilibrium phase at 1373K, according to the published Ti-Al-Ta ternary phase diagram. This ternary compound can work as a barrier to some extent. It contributes to decelerating the diffusion of Ti and Al atoms and to decreasing the oxidation rate. The formation mechanism of the intermediate phase has been discussed in conjunction with diffusion in TiAl.

Abstract: The isothermal oxidation behavior and thermal stability of a cobalt base alloy was investigated up to a period of 312 hr in air from 1000 to 1200°C. A comparison of oxidation behavior of this alloy with a conventional nickel-base superalloy (Inconel 713C) has been conducted in detail. This experimental alloy oxidizes by forming layers of Al2O3, Cr2O3, TiO2, CoO and traces of SiO2 with WO2 oxides on the surface of the specimen in contact with air. Scanning electron microscopy (SEM) was used to study the microstructure, morphology and compositions of oxides formed after the exposure. Thermal stability of the alloy after extended periods of exposures to air at 1000, 1100 and 1200°C was studied using transmission electron microscopy (TEM).

Abstract: The mechanical property, hot corrosion resistance and phase stability of a new-type directional solidification nickel-base superalloy named DZ468 that was developed independently by Institute of Metal Research, Chinese Academy of Sciences were investigated in this paper.The rupture properties of DZ468 alloy are nearly the same as those of DZ4125 alloy and its hot corrosion resistance property nearly the same as that of IN738 alloy under the same conditions. There is no finding TCP phase in the DZ468 alloy after aging at 900°C for 1000h. DZ468 alloy displays excellent phase stability.

Abstract: Sufficient oxidation stability of advanced carbon based materials is a key feature for their application in fusion, fission, aerospace and chemical engineering. These materials were examined as part of the EU research projects ExtreMat and Raphael-ML. This paper discusses the presently available oxidation model, outlines its limits on basis of respective experiments and proposes improvements. Methods for oxidation protection of advanced carbon based materials as doping and coating with different components are discussed. Experimental oxidation results on EX10, a carbon fibre in SiC matrix, in air at high flow rates and high temperatures are presented.

Abstract: There are several ways to improve the oxidation resistance of TiAl alloys. One of them is alloying with elements such as Nb, Ta, W, Si, Ag, Zr or Hf. However, bulk alloying influences the mechanical properties. Surface treatment of TiAl alloys by the small amounts of halogens leads to the formation of the protective alumina scale (“halogen effect”). The halogens can be applied by ion techniques (ion implantation, plasma immersion implantation) but also by spraying or dipping with halogen-containing inorganic and organic compounds. Deposition of the fluorine-containing resin on the surface of TiAl alloys is the easiest way to achieve the best results. SO2 impurity in air significantly influences oxidation behavior of TiAl alloys. In this work the results of the oxidation of a Ti-48Al-2Cr alloy coated with a fluorine-containing resin in the synthetic air and air containing 1% SO2 were presented. The oxidation runs were carried out in the temperature range 800-1000°C.

Abstract: The authors present studies of behavior of Crofer steel in H2 polluted by H2S. The high temperature corrosion behavior of Crofer 22APU steel pre-oxidized samples and not pre-oxidized ones were studied in the temperature range of 600 – 900°C in H2/H2S atmospheres in sulphur vapor pressures in the range of 10-7-10-1 Pa. Kinetics depend on the temperature and sulphur pressure, however the process of sulfurization for pre-oxidized samples was slower.