Browsing by Author "Smialek, James L."

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    Low temperature hot corrosion screening of single crystal superalloys
    (MDPI, 2018-10-25) Smialek, James L.; Gray, Simon
    Single crystal superalloys were screened in Type II molten (Na,K)-sulfate hot corrosion re-coat tests in air +300 ppm SO2 at 700 °C. They exhibited large 20–40 mg/cm2 weight changes, repeated spallation, and non-protective, 25–50 μm thick corrosion layers after 300 h of testing. Scale cross sections revealed dual outer Ni(Co)O and inner Al(Cr)S-rich corrosion layers. This chemical differentiation was partially consistent with previous models of oxide fluxing, alloy sulfidation, NiO micro-channel diffusion, and synergistic dissolution mechanisms. Broad shallow pits or uniform attack morphologies were consistent with prior studies performed in high >100 ppm pSO2 environments. Higher Mo experimental alloys trended toward more degradation, producing 100 μm thick scales with distinct Al(Cr)S-rich inner layers or 500 μm thick NiO. The aggressive behavior in these environments supports the need for LTHC-resistant coatings for single crystal superalloys.
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    Type II hot corrosion screening tests of a Cr2AlC MAX phase compound
    (Springer, 2018-07-24) Smialek, James L.; Gray, Simon
    Low-temperature hot corrosion tests were performed on bulk Cr2AlC MAX phase compounds for the first time. This material is a known alumina-former with good oxidation and Type I high-temperature hot corrosion resistance. Unlike traditional (Ni,Co)CrAl alumina formers, it contains no Ni or Co that may react with Na2SO4 salt deposits needed to form corrosive mixed (Ni,Co)SO4–Na2SO4 eutectic salts active in Type II hot corrosion. Cr2AlC samples coated with 20K2SO4–80Na2SO4 salt were exposed to 300 ppm SO2 at 700 °C for times up to 500 h. Weight change, recession, and cross-sectional microstructures identified some reactivity, but much reduced (< 1/10) compared to a Ni(Co) superalloy baseline material. Layered Al2O3/Cr2O3 scales were indicated, either separated by or intermixed with some retained salt. However, there was no conclusive indication of salt melting. Accelerated oxidation was proposed to explain the results, and coarse Cr7C3 impurities appeared to play a negative role. In contrast, the superalloy exhibited outer Ni(Co) oxide and inner Cr2O3 scales, with Cr–S layers at the interfaces. Massive spallation of the corrosion layers occurred repeatedly for the superalloy, but not at all for Cr2AlC. This indicates some potential for Cr2AlC as LTHC-resistant coatings for superalloys.