Gong Shengkai

Effects of Re and Ru Additions on Solidification Partition Coefficients and Solidification Characteristic Temperatures of Nickel Base Single Crystal Superalloys

In order to understand the effects of Re and Ru additions on solidification characteristics of nickel base single crystal superalloys, four nickel base single crystal superalloys containing different contents of Re and Ru, were produced by dual heating zone melting and liquid-metal cooling (LMC) directional solidification technique at the withdraw rate of 2 μm/s, and a planar solid/liquid interface was obtained. The equilibrium partition coefficients of each element and solidification characteristic temperatures of experimental alloys were also obtained. Results show that with the increasing of Re content, the segregation degree of high segregation elements Re, W and Ta decreases, the liquidus temperature decreases initially and then increases, while the solidus temperature decreases continuously. The addition of Ru has little influence on the partition of other elements, and slightly lowers the liquidus temperature and solidus temperature of superalloys. In addition, we can effectively forecast the segregation degree of elements and solidification defects of non-equilibrium directional solidified superalloys on the basis of equilibrium partition coefficients and solidification characteristic temperatures.

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.

INTER-DIFFUSION BEHAVIOR OF RECOATED CoCrAlY COATING/DZ125 DIRECTIONALLY SOLIDIFIED SUPERALLOY

The thermal barrier coatings (TBCs) are widely used in gas turbines which could lower the surface operating temperature and improve the service life of the coated part component. When repair or refurbishment of these coated components, it is necessary to remove the coating and replace it with a new coatings. The CoCrAlY coatings were deposited by EB-PVD on the directionally solidified nickel-based superalloy DZ125. The coatings were removed via physical methods in varying thicknesses and recoated subsequently. The results showed that the depth of the removal had a significant impact on the inter-diffusion behavior as well as the oxidation resistance. The thickness of Al-depleted zone formed in the recoated coatings after oxidation are thinner than the first coated coatings. Moreover, the more the coating was removed, the thinner the Al-depleted zone was formed. The recoated coatings deposited on the substrate removed the entirely inter-diffusion zone (IDZ) showed the best oxidation resistance, and the IDZ formed after oxidation was similar to the first coated coatings.