Xu Huibin

Hot Corrosion Behavior of Double-ceramic-layer LaTi2Al9O19/YSZ Thermal Barrier Coatings

Abstract LaTi2Al9O19 (LTA) exhibits promising potential as a new kind of thermal barrier coating (TBC) material, due to its excellent high-temperature capability and low thermal conductivity. In this paper, LTA/yttria stabilized zirconia (YSZ) TBCs are produced by atmospheric plasma spraying. Hot corrosion behavior and the related failure mechanism of the coating are investigated. Decomposition of LTA does not occur even after 1 458 hot corrosion cycles at 1 373 K, revealing good chemical stability in molten salt of Na2SO4 and NaCl. However, the molten salt infiltrates to the bond coat, causing dissolving of the thermally grown oxide (TGO) in the molten salt and hot corrosion of the bond coat. As a result, cracking of the TBC occurs within the oxide layer. In conclusion, the ceramic materials LTA and YSZ reveal good chemical stability in molten salts of Na2SO4 and NaCl, and the bond coat plays a significant role in providing protection for the component against hot corrosion in the LTA/YSZ TBCs. LTA exhibits very promising potential as a novel TBC material.

Effects of Shot Peening Process on Thermal Cycling Lifetime of TBCs Prepared by EB-PVD

Abstract Conventional two-layered thermal barrier coatings (TBCs) are prepared by electron beam physical vapor deposition (EB-PVD) with ZrO 2 -8 wt% Y 2 O 3 (8YSZ) as top coat and CoCrAlY as bond coat on disk-shaped Ni based super-alloy. In this paper, three kinds of shot peening process with different lengths of operating time were adopted for bond coating. As a result, changes took place in its surface roughness and the surface micro-hardness. A thermal cycling test at 1 273 K× 55 min and another at room temperature for 5 min were performed to study the effects of shot peening process on the thermal cycling lifetime of TBCs. It is found that a moderate shot peening process will be able to prolong the life time. The oxidation dynamic of the as-processed TBCs basically accords with the parabolic rule, and the oxidation test also attests to the spallation between YSZ and thermal growth oxide (TGO) responsible mainly for the failure of TBCs.

Effects of Nb Content on Yield Strength of NiTiNb Alloys in Martensite State

Abstract Two near single-phase NiTiNb alloys—Ni50Ti48Nb2 and Ni49.5Ti46.5Nb4—are prepared and studied by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), differential scanning calorimetry (DSC) and tensile tests in order to unearth the effects of Nb-atom solid solution in NiTi phase on the yield strength induced by self-accommodation of martensite variants. The results show that the yield strength of near single-phase NiTiNb alloys varies inversely with the amount of Nb-atoms solid-dissolved in NiTi phase. From the results out of the prior and current studies, it can be surmised that the effects of Nb content on the yield strength of NiTiNb alloys in martensite state depend on the coaction. Nb solid solution weakening mechanism and β-Nb phase composite strengthening mechanism. This inference might be a satisfactory explanation to the fact that the yield strength of (NiTi)50–0.5xNbx alloys in martensite state begins with decline and then rises when the Nb content increases.

Effect of rapid solidification on the site preference of Heusler alloy Mn2NiSb

The site preference of Mn atoms in Heusler alloy Mn2NiSb can be influenced obviously by different preparing methods. Mn atoms enter the A and B sites after arc-melting and subsequent annealing, and form an Hg2CuTi-type of structure. However, after melt-spinning, the Mn atoms tend to occupy the (A, C) sites and form the Cu2MnAl-type of structure. The electronic structure calculations suggest that the Hg2CuTi-type of structure is lower in energy than the Cu2MnAl one and more stable. The lattice constant of the former is a bit larger than the latter one, agreeing with experimental results. Ferromagnetism is observed in Mn2NiSb with both structures. Calculations give a total moment of 4.21 μB for the Hg2CuTi-type of structure and 3.93 μB for the Cu2MnAl-type one. These results fit the saturation magnetization at 5 K quite well. The difference between the Curie temperatures of the bulk and ribbon samples is about 77 K.

Phase Equilibria in Nb-Si-Mo Ternary Alloys at 1 273 K and 2 073 K

Phase equilibrium in Nb-Si-Mo ternary alloys (<37.5 at.% Si) at 1 273 K and 2 073 K is investigated by using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The partial isothermal section at 1 273 K, which contains four single-phase regions, five two-phase regions and two three-phase regions, is basically the same as that at 1 973 K. However, when the temperature increases to 2 073 K, the three-phase region of Nbss+α-(Nb(Mo))5Si3+β-(Nb, Mo)5Si3 obviously moves towards the Nb-rich corner. This suggests that Nb-Si-Mo ternary alloys remain stable at least up to 1 973 K.

Site Preference and Alloying Effect of Excess Ni in Ni–Mn–Ga Shape Memory Alloys

The formation energies and electronic structures of Ni-rich Ni–Mn–Ga alloys have been investigated by first-principles calculations using the pseudopotential plane wave method based on density functional theory. The results show that the alloying Ni prefers to occupy the Mn site directly in Ni9Mn3Ga4 and to occupy the Mn site and drive the displaced Mn atom to the Ga site in Ni9Mn4Ga3, which is in accordance with the experimental result. According to the lattice constants and the density of states analyses, these site preference behaviours are closely related to the smaller lattice distortion and the lower-energy electronic structure when the excess Ni occupies the Mn site. The effect of Ni alloying on martensitic transformation is discussed and the enhancement of martensitic transformation temperature by Ni alloying is estimated by the calculated formation energy difference between austenite and martensite phases.

Effect of Thermal Treatment on the Grain Growth of Nanostructured YSZ Thermal Barrier Coating Prepared by Air Plasma Spraying

Abstract A nanostructured thermal barrier coating is prepared by air plasma spraying using the 8wt% Y 2 O 3 partially stabilized zirconia nano-powder with an average grain size of 40 nm. The microstructure and phase composition of feedstock nano-powder and coating are investigated using SEM, TEM and XRD. It is found that the as-sprayed zirconia coating has an average grain size of 67 nm and mainly consistes of metastable tetragonal phase, together with some monoclinic phase and tetragonal phase. Thermal treatment results show that the grains of the nanostructured coating grow slightly below 900°C, whereas over 1000°C the gains grow rapidly and monoclinic phase noticeably appeares.

Interface conjunction factors of thermal barrier coatings and the relationship between factors and composition

In thermal barrier coatings (TBC), ceramics is covered on the metal matrix as coatings in order to raise its temperature endurance. Today most of the TBCs are of the double-layer-structure of Ni base heat-resistant alloy matrix+the bonding layer of MCrAIY alloy (M=Ni, Co, Ni+Co) +ZrO2. In this paper, the concept of interface conjunction factor (ICF) in the biphase interface of alloys is expanded to coatings. The ICFs of the interface between the ceramics and the bonding layers with various compositions, such as the electron density ρ, the electron density difference Δρ, and the number of atom state group which keeps the electron density continuous δ are calculated. From the calculation results, the following estimations can be deduced. When Al content is less than 6 wt% it improves the mechanical properties of the coatings; when the content is 6 wt%–12 wt% it will not worsen the properties; when the content is greater than 12 wt% it will have disadvantageous effect. The estimations accord well with the experiment results of the properties and the service time of the coatings. Therefore the concept of ICF has the same important meaning in coatings, and the valence electron structure of the interface can be a possible theoretical guide for the content optimization of TBCs.

Preparation and Properties of the Ni2MnGa Magnetic Shape Memory Alloy

The polycrystalline Ni48.2Mn22.4Ga29.4 alloys were prepared by means of melting in argon arc furnace. X-ray diffraction pattern indicates a fcc cubic structure with a lattice parameter of 0.5822 nm at room temperature. The phase transformation temperatures were: 1°C for martensite start temperature, -11°C for martensite finish temperature, 3°C for austenite start temperature and 11°C for austenite start temperature, respectively. The results showed that the effect of magnetic field on the strains increases with the decrease of temperature. A strain of 2.1 × 10-4 was obtained with an applied magnetic field of 5 kOe at -66°C.

Interlayer Coupling of Co/NM/FM(NiFe and Co) Nano-Sandwich Films

Abstract Cu/Co, Cu/NiFe, Ta/NiFe bilayers and Co/Cu/Co, Co/Cu/NiFe, Co/Ta/NiFe sandwich films were deposited by a magnetron sputtering method. Magnetic properties were evaluated by VSM and spin valve magnet oresistance was investigated by a four-probe method to study the interlayer coupling of the two magnetic layers. It has been found that the interlayer coupling depended not only on the layer thickness of the nonmagnetic spacer but also on the nature of the spacer. The interlayer coupling was reduced as the spacer layer thickness increased. The result was consistent with those from observations of the magnetic domain for the trilayers by means of Lorentz Electron Microscope. The trilayers with Cu spacer layer have shown a stronger coupling than those with Ta spacer layer.