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J. Eng. Mater. Technol. 2017;140(1):011001-011001-6. doi:10.1115/1.4037275.

Superalloys are high temperature materials which are indispensable in many high temperature applications such as the gas turbines. IN738LC is a nickel-based superalloy that is extensively used in the hot sections of the gas turbines. The strengthening in this alloy is provided mainly by the γ′ precipitates. In this research, precipitate size and morphology of a serviced IN738LC polycrystalline turbine blade is investigated. Specimens from the trailing edge, middle, and leading edge positions of the tip, middle, and root sections on their hot (exterior) and cooled (interior) surfaces are analyzed for the precipitate size and morphology. The size and morphology are then linked to the temperature and stress/strain distribution in the blade. In general, the hot surfaces have larger precipitates that indicate a higher temperature exposure. In particular, the precipitate size is larger in the tip and middle sections than the root section, implying that the latter has a lower temperature. As the precipitates transforms to rafts at high temperature and stress/strain, the middle positions of the tip and middle sections, the trailing edge of the tip section, and the leading edge of the middle section are predicted to have high temperature–stress/strain coupling.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011002-011002-5. doi:10.1115/1.4037169.

Exfoliated graphite (EG) was prepared from commercially available natural graphite flakes (NGF), through strong acid treatment followed by thermal shock at 950 °C. The EG sheets were characterized with respect to their thermal stability via thermogravimetric analysis (TGA) and Raman spectra. Their morphology and particle size were evaluated using scanning electron microscope (SEM) and particle size analyzer. The potential of EG as reinforcement on the mechanical and thermal properties of the dynamically vulcanized polystyrene/styrene butadiene rubber (PS/SBR) composites was evaluated. The influence of EG on the electrical properties of the composites was measured as well.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011009-011009-7. doi:10.1115/1.4037393.

This paper presents a study of hydrogen diffusion for a spiral weld pipe considering the effect of weld residual stress. The results show that the hydrogen mainly gathers at heat-affected zone (HAZ). HAZ is the weakest zone where hydrogen-induced cracking (HIC) occurs. The effect of helix angle on the hydrogen diffusion is also discussed. It shows that different helix angles generate different hydrogen concentrations. As the helix angle increases, both the hydrogen concentration and residual stresses decrease. As the helix angle increases from 40 deg to 50 deg, the equivalent pressure stresses reduce a little, resulting in the change of hydrogen concentration being small. The smaller the helix angle is, the larger the diffusion rate is. The most suitable helix angle should be optimized at 40–50 deg.

Commentary by Dr. Valentin Fuster

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