H. Wei

Site preference of Re in NiAl and valence band structure of NiAl containing Re: First-principles study and photoelectron spectrum

The site preference of Re in NiAl was studied using first-principles calculations. The calculation of formation energies of the NiAl alloys indicated the site preference of Re on the Ni sites. The valence band structures of the NiAl alloys were investigated by photoelectron spectroscopy. The valence band spectra of the NiAl with Re shifted away from the Fermi energy level so that the Ni d-band centroid moved to a higher energy by 0.25 eV as Re was added. Such a shift could be attributed to the Ni-Re interaction, which was supported by the photoelectron spectroscopy measurement.

Influence of Re on the Properties of a NiCoCrAlY Coating Alloy

MCrAlY can serve as stand-alone overlay coatings or bond coats in thermal barrier coating systems, and its properties play a vital role in determining the performance of these coating systems. In order to further understand the behavior of MCrAlY coatings, several NiCoCrAlY model alloys with different levels of Re (0.3 wt%, 6 wt%, and 9 wt%) were investigated. Microstructural observation showed the addition of Re promoted the precipitation of Cr-rich phases, such as α-Cr and σ. The presence of α-Cr lowered the coefficient of thermal expansion (CTE) of the coating alloys, which could reduce the CTE mismatch at the scale-metal interface. The solid solution strengthening effect of Re is responsible for an increase in Rockwell hardness of the coating alloys. But the isothermal oxidation resistance at 1100°C was deteriorated due to the precipitation of brittle α-Cr phase, a phase of inferior oxidation resistance compared with β-NiAI and γ-Ni.

Site preference of Ru in NiAl and valence band structure of NiAl containing Ru: First-principles study and photoelectron spectrum

The site preference of Ru in NiAl was studied using first-principles calculations. The calculation of formation energies indicated that the site preference of Ru was on the Ni sites. The valence band (VB) structures of the NiAl alloys were investigated by photoelectron spectroscopy. The VB spectra of the NiAl–Ru alloy was shifted away from the Fermi level so that the Ni d-band centroid moved to a higher energy by 7.77 eV as Ru was added. Such a shift revealed that the Ni–Ru interactions gave a significant contribution to the VB structure of the NiAl–Ru alloy.

Experimental and Numerical Investigation of Air Radiation in Superorbital Expanding Flow

To investigate air radiation in expanding flows and provide experimental data for validating associated computational models, experiments were conducted in the X2 expansion tunnel facility at the Centre for Hypersonics of the University of Queensland. A 54◦ turning angle wedge model was employed to generate steady expanding flows with inflow total enthalpies of 50.7, 63.4 and 75.4 MJ/kg. VUV spectra from 118 to 180 nm were acquired across the wedge at three equispaced distances away from the top of the model, as well as through its top surface. High speed filtered images were also obtained by coupling a Shimadzu 1 MHz high speed camera to a bandpass filter to obtain calibrated images of the 777 nm oxygen triplet. Both the across-wedge VUV spectra and filtered images of the 777 nm atomic oxygen were compared with NEQAIR simulations, which were performed using flow field data from two-dimensional CFD simulations with two-temperature 11-species air chemistry utilising the in-house Navier-Stokes flow solver Eilmer3. Data extracted from consecutive frames of the filtered high speed images confirmed up to 8 μs of available test time for the flow conditions tested. For the strongly radiating 149 and 174 nm atomic nitrogen lines, large disagreement between experimental data and NEQAIR predictions can be observed from the start of the expansion fan where the electron-ion recombination process commences. The spatial extent, or spans of the radiance profiles of the 149 and 174 nm N lines are significantly underpredicted by NEQAIR, and are very close to those of N, N and electron number density profiles, which follow that of flow density. The electron-ion recombination process is proposed as the main reason for these discrepancies. The comparisons between NEQAIR simulations and filtered images of the 777 nm oxygen triplet show good agreement in the post-shock compression region and the start of the expansion fan for the 63.4 MJ/kg condition, but with up to a factor of three overprediction by NEQAIR further downstream, which is attributed to electron-impact excitation. Similar trends are found with the 75.4 MJ/kg condition, with reduced level of agreement in the compression region, which can be due to uncertainties in inflow condition.

Expansion Tube Magnetohydrodynamic Experiments with Argon Test Gas

In this study an expansion tube is used to generate an experiment to study and evaluate the concept of magnetohydrodynamic aerobraking for planetary entry spacecraft. An expansion tube can theoretically generate the required hypersonic flowfield within which ionisation is confined to the shock layer; this is an important characteristic of the true flight case which previous experiments with arcjets have failed to achieve. The first part of this paper explores the operating envelope for The University of Queensland’s X2 expansion tube facility, to identify flow conditions which should produce a significant interaction between the shock layer and an applied magnetic field. Argon test gas was selected for initial experiments due to its simple chemistry and low ionisation enthalpy, and equilibrium chemistry was used to compute the expected properties of the shock layer around a blunt body model. A candidate flow condition was identified for further analysis with finite rate reacting argon CFD, which indicated that relatively short shock layer length scales for the X2 expansion tube would limit the degree of argon ionisation that was generated, however, sufficient ionisation was still expected. Three sphere models were tested: a steel ball, a plain magnet, and a magnet coated with an electrically insulating ceramic paint. It was found that shock stand-off is unchanged between the steel ball and the plain magnet, but increases by approximately 10% for the coated magnet. Spectroscopy revealed that radiance along the stagnation streamline for some, but not all, of the relevant argon wavelengths approximately doubles when a magnet is used, either coated or uncoated. This increase in radiance does not appear to be associated with changes to shock stand-off. Further work is now required to explain the observed phenomena, and also to address some key experimental challenges, which include accurate measurement of shock stand-off and reducing the significant shot-to-shot variability observed for the coated magnet experiments.

Preliminary experimental investigation of air radiation in super-orbital expanding flow

Radiative heating dominates forebody thermal loads on re-entry capsules travelling at superorbital speeds and has also recently been identified as a major component in afterbody heating. Although subjected to less severe heating, the afterbody heat shield is typically designed with a larger structural safety factor, limited by the fidelity of computational models used for the rapid flow expansion. To improve the understanding of afterbody flows, an experimental campaign was launched to interrogate the radiation from a rapidly expanding flow of 11.8 km/s air in the X2 expansion tunnel at the University of Queensland. Spectral measurements of the flow were successfully recorded from the ultraviolet to the near infrared, and two-dimensional images of atomic oxygen emission were also taken. The latter agreed well, in trend, with numerical simulations of the flow.

Local structure of NiAl compounds investigated by extended X-ray absorption fine-structure spectroscopy

The local structures of pure NiAl and Ti-, Co-doped NiAl compounds have been obtained utilizing extended X-ray absorption fine-structure (EXAFS) spectroscopy. The results provide experimental evidence that Ni antisite defects exist in the Ni-rich NiAl compounds. The site preference of Ti and Co has been confirmed. Ti occupies the Al sublattice, while Co occupies the Ni sublattice. The structure parameters obtained by EXAFS were consistent with the X-ray diffraction results. Owing to the precipitation of α-Cr, the local structure of NiAl-Cr has not been obtained, making the site preference of Cr unclear.

Valence band structure of the NiAl-Mo alloy from the photoelectron spectrum

The valence band structures of the NiAl–Mo alloy was investigated by photoelectron spectroscopy. The valence band spectra of the NiAl–Mo alloy was shifted away from the Fermi level so that the Ni-d-band centroid moved to a higher energy by 0.22 eV as Mo was added. A possible explanation lied in the overlap of Ni-d bands in the energy with Mo-d and Al-p bands. The participation of Mo-d bands was correlated with the site preference of Mo in NiAl alloys.