Kaka Ma

Influence of cryomilling on the microstructural features in HVOF-sprayed NiCrAlY bond coats for thermal barrier coatings: Creation of a homogeneous distribution of nanoscale dispersoids

Previous research has revealed that thermal barrier coatings with cryomilled bond coats exhibit improved thermal cycling lifetime by growing a continuous and uniform oxide layer at a slower rate; yet the mechanism controlling the ultimate failure remains unclear. In an effort to provide a foundation for understanding the improved behavior, the influence of cryomilling on the microstructure of the NiCrAlY bond coat material is investigated in this article. Rather than focusing on the alumina scale formation, the microstructural features and their evolution within the high-velocity oxy-fuel (HVOF)-sprayed NiCrAlY bond coats themselves, prepared from conventional powder and cryomilled powder, were carefully compared through extensive scanning electron microscope/energy-dispersive X-ray spectroscopy characterization. In addition, the as-cryomilled NiCrAlY powder is characterized to provide evidence of the direct influence of cryomilling and to exclude the impact from the HVOF spraying. It is found that the essential change in microstructural features resulting from the cryomilling is the creation of a homogeneous distribution of ultrafine (nanoscale) Al-rich oxide/nitride dispersoids, which remain thermally stable even after exposure at 1100°C for 100 h. The TEM study on the as-cryomilled powder, prior to the HVOF spraying, indicates that some Al and Y-rich oxides are already present within the material as a direct result of the cryomilling process.

An integrated approach for probing the structure and mechanical properties of diatoms: Toward engineered nanotemplates

The wide variety of diatom frustule shapes and intricate architectures provide viable prototypes to guide the design and fabrication of nanodevices and nanostructured materials for applications ranging from sensors to nanotemplates. In this study, a combined experimental-simulation method was developed to probe the porous structure and mechanical behavior of two distinct marine diatom species, Coscinodiscus sp. (centric) and Synedra sp. (pennate), through ambient nanoindentation and finite element method analysis. These diatom frustule dimensions differed largely depending on diatom species with pore diameters d ranging from 0.3 to 3.0 μm. Youngs modulus E and hardness H measurements of the diatom frustules were obtained via nanoindentation experiments. These values varied depending on diatom species (E between 1.1-10.6 GPa, H between 0.10-1.03 GPa for the Coscinodiscus sp.; and E between 13.7-18.6 GPa, H between 0.85-1.41 GPa for the Synedra sp.). Additionally, the mechanical response of diatom structures to uniform compression was examined. Predictive simulations were performed on the aforementioned diatom frustules, as well as another diatom structure (pennate Fragilariopsis kerguelensis), to correlate the mechanical response with specific morphology variables (e.g., pore or slit sizes). Results from calculated von Mises stress and displacement distributions unveil unique information on the effect that uniform loads have on these frustules, which can aid the design of tailored nanotemplates. A correlation between mechanical properties and porosity was established for selected frustules, and reported for the first time in this study.

Atom-Probe Tomographic Study of Precipitation in an Ultrafine-Grained Al-Zn-Mg-Cu Alloy (Al 7075)

Precipitation-hardened alloys can be further strengthened by reducing the grain size down to the ultrafine regime (<1 μm) and thereby incorporating significant grain boundary (GB) strengthening [1, 2]. The ultrafine-grained (UFG) structure is expected to induce an influence on the precipitation behavior, because of the significantly reduced length scale. Nevertheless, there have been very limited studies on the effect of length scale on precipitation [3]. The present study was undertaken to obtain quantitative information on the differences in precipitation behavior between UFG structure and the traditional coarse-grained (CG) counterpart, and to provide fundamental insights into the underlying mechanisms. Al 7075 alloy, with major alloying elements of Zn, Mg and Cu, was selected for study because of its technological importance and the existing extensive studies on precipitation phenomena in CG Al 7075.

Mechanistic investigation into the role of aluminum diffusion in the oxidation behavior of cryomilled NiCrAlY bond coat

High temperature oxidation behavior of the bond coat layer is a critical factor that controls the failure mechanism of thermal barrier coatings (TBCs). Previous work reveald that TBCs with cryomilled NiCrAlY bond coats exhibited an improved oxidation behavior compared to equivalent TBCs with conventional bond coats. The cryomilled NiCrAlY bond coats contributed to a slower growth rate of thermally grown oxides (TGO) with a final thinner thickness and enhanced homogeneity in TGO composition. To better understand the improved oxidation behavior, a mechanistic investigation based on diffusion theory and quantum mechanics is performed to elucidate the role of aluminum diffusion in the oxidation behavior and how the microstructural features of the cryomilled NiCrAlY bond coats, i e, the creation of a thermally stable, uniform distribution of ultrafine Al-rich oxide dispersoids, affect the diffusion kinetics of Al and the migration of free electrons. It is revealed that these Al-rich oxide dispersoids result in a uniform diffusion of Al and slow migration of free electrons within the NiCrAlY bond coat, consequently leading to the improved oxidation behavior.