Feng Tang

Influence of Cryomilling on the Morphology and Composition of the Oxide Scales Formed on HVOF CoNiCrAlY Coatings

Commercially available, gas-atomized CoNiCrAlY powder was cryomilled to produce powder with nanocrystalline grains. The cryomilled powder and conventional gas-atomized powder were thermally sprayed using the HVOF process to prepare two coatings with fine-grain (∼15 nm) and coarse-grain (∼1 μm) microstructure, respectively. The two coatings were isothermally oxidized in air at 1000° C for up to 330 hr. The morphology and composition of the oxide scales formed on the two coatings were compared with each other. The results indicate that, while a fine-grain microstructure can promote the formation of a pure alumina layer on the coating by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability.

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al2O3 layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.

Effect of boron microalloying on microstructure, tensile properties and creep behavior of Ti–22Al–20Nb–2W alloy

Abstract The effect of 0.2 at.% boron addition on the microstructure, tensile properties, and creep behavior of Ti–22Al–20Nb–2W were investigated. A TiB phase was observed in the B-doped alloy, which shows that the solubility of boron in this alloy is very low. The addition of 0.2 at.% boron was very effective in reducing the prior β-grain size of the Ti–22Al–20Nb–2W alloy, which is thought to be due to a strong boron segregation at the grain boundaries. The decrease in grain size due to the addition of boron is effective in improving the alloys room temperature ductility. The addition of a small amount of boron was beneficial in lowering the steady-state creep rate at 650°C/310 MPa, but did not improve the creep behavior at 750°C/310 MPa.

Thermal stability in nanostructured Al-5083/SiCp composites fabricated by cryomilling

Abstract Nanostructured Al-5083/SiCp composites, which consist of an ultrafine grained Al-5083 matrix reinforced with nanosized SiC particles, were fabricated via a cryomilling plus consolidation process. The thermal stability of these composites was studied by investigating the effects of annealing temperature on tensile properties and Vickers hardness of the composites and grain growth in the Al-5083 matrixes, with the annealing temperature ranging up to 873 K. The experimental results indicate that, after annealing at temperatures up to 773 K (0·90 T m, where T m is 862 K, the melting onset temperature of the Al-5083 matrixes), the strength of the composites did not decrease significantly, while the grain size of the Al-5083 matrixes did not show a noticeable increase. The grains in some areas of the Al-5083 matrixes did not grow even after annealing at 873 K (1·01 T m). The high thermal stability is attributed to the dispersion of native oxide particles observed at the lamellar interfaces created by cryomilling.

Modulated microstructure in Ti-22Al-11 Nb-4Mo alloy

In the previous two decades, considerable effort has been made to develop the Ti{sub 3}Al({alpha}{sub 2})-based and TiAl({gamma})-based titanium aluminides for the elevated temperature applications. However, the application is limited by the low ductility of these alloys at low temperature. During the systematic investigation in the effect of the compositional modification on the physical and mechanical properties of Ti-22Al-27Nb alloy, the authors found that a modulated microstructure was formed in a Ti-22Al-11Nb-4Mo alloy when this alloy was subjected to the water-quenching from above the {beta} transus temperature and subsequent aging in the temperature range up to around 950 C. In this paper, the evolution of modulated microstructure and associated change in Vickers hardness with various heat treatment conditions are presented.

Tensile Deformation and Fracture in a Bulk Nanostructured Al-5083/SiCp Composite at Elevated Temperatures

An ultrafine-grained Al-5083 alloy reinforced with 5 vol.% nano-sized β-SiC particles was fabricated with a powder cryomilling and consolidation technique. Tensile tests were conducted at temperatures from 298 to 773 K for this composite. The mechanisms for deformation and fracture of this nanostructured composite at various temperatures are discussed.