Materials Science Forum Vols. 546-549

Abstract: Porous aluminum anodic oxide films fabricated by anodizing in phosphoric acid electrolyte containing organic acid were investigated by scanning electron microscope (SEM) and Atomic Force Microscopy (AFM). The results showed that the pores with circular or near elliptical honeycomb structure did not grow along the processing direction of the matrix and present an optimum ordering. It also indicated that the adding of the organic acid reduces the defects during the oxide film growing process and increases the stability and uniformity of the oxide film, and the thickness of the oxide film is significantly increased.

Abstract: The wear and corrosion protective films were synthesized on the LC4 Al-Zn-Mg-Cu alloy by microarc oxidation (MAO) technique in an alkali-silicate electrolyte. The microstructure and composition profiles were investigated by scanning electron microscopy and energy dispersive spectroscopy, and the phase structure was performed by X-ray diffraction. The corrosion resistance of coated LC4 alloy was evaluated using potentiodynamic polarization curves. The MAO film consists of γ-Al2O3, α-Al2O3 and amorphous SiO2 phases. The film up to 210 μm contains two layers. The Si element from electrolyte enriched in the outer layer film. After microarc oxidation treatment, the microhardness and corrosion resistance of LC4 alloy were improved significantly.

Abstract: Al-Si metal anode was fabricated by cold-press sintering with Al and Si as embedded powder in argon atmosphere. The anti-oxidation in the air and anti-corrosion in molten cryolite properties of Al-Si metal anode at high temperature were examined. The experimental results showed that the oxidation kinetics curve obeyed the parabolic law. The corrosion behavior Al-Si metal anode was studied in electrolysis test. The electrolyte consisted of Na3AlF6(90 wt%),CaF2(5 wt%) and Al2O3(5 wt%) which corresponded to molecular ratio of 2.4. The results indicated that the cell voltage was stable and the electric polarized corrosion rate was higher than the static corrosion rate of metal anode. SEM photographs showed an oxidation film formed in the surface of Al-Si metal anode which can obstruct the further corrosion in molten cryolite. It may be concluded that Al-Si metal anode has good combination property and can replace the carbon anode in the aluminum cell.

Abstract: The Na2SnO3 and H-N-R inhibitors effects on the high active aluminum alloy in alkaline solution (80
5mol/LNaOH) have been systematically studied by hydrogen collection and electrochemical measurement in the present investigation. The results showed that the hydrogen evolution rate was low at the value of 0.404 mL/ (min·cm2) in alkaline solution with 0.01mol/L Na2SnO3, but the electrode potential was only -1.515Vvs.SCE and it moved to the positive direction seriously; When adding 10wtH-N-R and 0.01mol/LNa2SnO3 into the alkaline solution simultaneously, the aluminum alloy anode had a lower hydrogen evolution rate of 0.342mL/ (min·cm2) and a more negative electrode potential of -1.591Vvs.SCE. The electrochemical performance of aluminum alloy anode has been improved greatly by the cooperation of H-N-R and Na2SnO3.

Abstract: The exfoliation corrosion behavior of 2524 aluminum alloy was studied by Transmission Electron Microscope (TEM), hydrogen evolution and Electrochemical impedance spectroscopy (EIS) . The results showed that 2524 aluminum alloy was very susceptible to exfoliation corrosion and the exfoliation corrosion appeared after 24 h. EIS could better explain the exfoliation corrosion than hydrogen evolution. Exfoliation corrosion kinetics included accelerated formation of pitting, quick development of pitting, retarded formation of exfoliation and steady development of exfoliation.

Abstract: Precipitation hardened nickel-based superalloys are widely used in aero and industrial gas turbine engines due to their excellent high temperature strength and remarkable hot corrosion resistance. A drawback of many of these alloys is that they are very difficult to weld due to their high susceptibility to heat affected zone (HAZ) cracking, both during welding and post weld heat treatments (PWHT). Weld cracking in many of these alloys has been attributed mostly to constitutional liquatioin of grain boundary NbC precipitates. however, HAZ cracking has been observed in carbon-free superalloys as well, Therefore, research was initiated to examine grain boundary liquation and cracking in HAZs in a variety of Ni-based superalloys. It was found that intergranular cracking of grain boundaries involved liquation of several other phases, in addition to NbC, that were present in pre-weld microstructure of the alloy. These even included the primary strengthening phase,γ’, in a very widely used superalloy, Inconel 738. In addition, segregation of melting point depressant element B was also observed at gain boundaries in other superalloys, which also caused grain boundaries in HAZ to liquate and resulting in their cracking. An overview of microstructural aspects of different liquation phenomena involved and characteristics of the liquid film contributing to the HAZ microfissuring of nickel superalloys will be discussed in this presentation

Abstract: The aeroengines have been the subject of continuous development for the past fifty years with considerable improvements in fuel efficiency, power density, exhaust emissions, decreased noise and better reliability. All these changes have been brought due to incremental improvements in engine cycle, combustion, turbine cooling, acoustics and materials technology.The main challenges for the future are not only technological and economical but also related to societal concerns. For instance, noise and emissions are becoming a major concern.In terms of materials, there is a constant need for developing new affordable materials, or materials systems such as thermal barrier coatings on single crystal superalloys in order to achieve higher TET but not at the expense of increased NOx, at least in commercial engines. The use of MMC's and CMC's in some parts of the engine will become possible due to a considerable improvement in processing and lifing methodologies.

Abstract: For several decades, strong efforts have been made in France in research and development activities dedicated to nickel-based superalloys for high-pressure disks and blades. New superalloys were designed then developed within cooperative research programmes bringing together research institutes, alloy producers and engine manufacturers. Continuous research activities were carried out on these alloys in order to perform detailed analyses of the relationships between process parameters, chemistry, microstructural features, deformation mechanisms and mechanical behavior. Typical studies are illustrated by examples outlining some recent results.

Abstract: The elemental partitioning characteristics of advanced single crystal Ni-base superalloys containing additions of both Pt and Ru have been investigated using atom probe tomography. Detailed microanalysis revealed Ru additions partitioned preferentially to the disordered matrix, whereas Pt additions tended to partition to the ordered intermetallic γ′ precipitates. The partitioning characteristics of three nominally similar alloys with systematic variations in the levels of Cr, Ru and Pt were investigated. For this particular set of experimental alloys, minor changes in the partitioning characteristics of the constituent elements could be attributed to additions of Cr and Ru. The preferential site occupancy of Pt within the L12 lattice was also statistically quantified with ALCHEMI (atomic site location by channelling enhanced microanalysis). In contrast to the atomic partitioning characteristics of Ru, Pt exhibited a tendency to occupy the Ni sublattice sites in the γ′ structure.

Abstract: The effect of Ru on the microstructure of a high content refractory elements nickel base Superalloy has been studied in the present investigation. The results showed that, the adding of 3%Ru did not prevent the precipitation of TCP phase during high temperature exposure, and even accelerated the formation of TCP phase,however the addition of Ru decreased the growth rate of μ phase significantly. In addition, the adding of Ru decreased the solving temperature of γ/γ′ eutectic by 5°C, and increased the tendency of γ′ rafting during high temperature exposure.