Somayeh Pasebani

Nickel-Chromium Alloys: Engineered Microstructure via Spark Plasma Sintering

There is a need to enhance or develop high temperature capabilities of structural materials for advanced coal‐fired power plants. These materials require a combination of high temperature strength, creep resistance and corrosion resistance in the oxygen‐rich and hydrogen‐rich high pressure environments. In this study, atomized Ni‐20Cr (wt.%) powder was mechanically milled with Y2O3 nanopowder (30‐40 nm powder size) to produce an alloy with a chemical composition of Ni‐20Cr‐1.2Y2O3 (wt.%) alloy using high energy ball milling. To minimize agglomeration during milling, 1 wt.% stearic acid was added to the powder mixture prior to milling. Microstructural characteristics of the powder were primarily characterized by the X‐ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystallite size and lattice strain were measured by XRD whereas powder morphology (powder size, shape) was studied by SEM. A milling time of 2 h was found to be optimal for the purpose that yttria particles are not dissolved yet uniformly distributed. Subsequently, the milled powder was consolidated into bulk specimens (12.5 mm in diameter) via spark plasma sintering (SPS) at 1100 °C for 30 minutes. Following SPS, the density and hardness of the specimens were measured. Microstructural characterization of the SPSed specimens was performed using SEM and TEM. The microstructural characteristics were correlated with the measured mechanical properties.

High Temperature Tensile Properties and Related Microstructural Evolution in Grade 92 Steel

Ferritic-martensitic steels with good high temperature mechanical properties have many promising applications in fossil and nuclear power plants. In this work, a F92 steel was tensile tested from room to elevated temperatures (up to 700 °C). This material exhibited higher strength than traditional P92 steels. The reasons for the observed changes in mechanical properties were investigated by studying the microstructural characteristics in undeformed and deformed specimens using transmission electron microscopy. The microstructural evolution accelerated significantly under loading as temperature increased. For instance, the deformed microstructure at 600 °C showed early stages of M23C6 precipitate formation under loading. The M23C6 precipitates exhibited more coarsening tendency whereas the MX-type precipitates retained their size. As coarsening of M23C6 precipitates progressed at elevated temperatures, the strength gradually decreased as the solid solution strengthening deteriorated by removing W and Mo from the solid solution matrix.