The effect of wall thickness on the microporosity and stress-rupture properties of a second-generation singlecrystal nickel-based superalloy was investigated using optical microscopy, scanning electron microscopy, energydispersive spectroscopy, and X-ray computed tomography. Specimens with various thicknesses (0.8, 1, 1.5, and 3 mm) were subjected to stress-rupture experiments at 980 degrees C and 250 MPa. The stress-rupture lives of the 0.8- and 1-mm-thick samples were shorter than those of the 1.5- and 3-mm-thick samples. The results indicate that increasing wall thickness results in an increased eutectic fraction, primary dendrite arm spacing, and degree of dendritic segregation of the as-cast alloys, which promotes porosity growth during solution heat treatment. The reduction in the real load-bearing cross-sections and a discontinuous Al2O3 layer caused by the oxidation behavior significantly influence the fracture mechanism of the thin-walled specimens, whereas preexisting micropores significantly affect the stress-rupture properties of the thicker specimens.

The recovery of microstructure and creep properties of a directionally solidified superalloy after long-term thermal exposure was investigated. After reheat treatment, successful restoration of γ′ microstructure is achieved including morphology, size and chemical composition. However, carbides degradations within interior grains and at grain boundaries are irreversible through the reheat treatment. Short and long-term creep rupture properties are effectively improved by the reheat treatment, but cannot be fully recovered compared to that of the original material. During prolonged thermal and creep exposure, grain boundary carbides degradation occurred rapidly in comparison with that in the original material, which is regarded as the main reason leading to the reduction of stress rupture lifetime.

To help our understanding of the structural and superconducting transitions in ferropnictides, partial phonon density of states (PDOS) of iron in a single-crystal SrFe2As2 pnictide have been investigated from both out-of-plane and in-plane polarizations with respect to the basal plane of the crystal structure using nuclear resonant inelastic x-ray scattering in a high-pressure diamond anvil cell at ambient temperature. The partial PDOS of iron in the pnictide crystal changes dramatically at approximately 8 GPa, which can be associated with the tetragonal (T) to collapsed tetragonal (CT) isostructural transition as evidenced in high-pressure x-ray diffraction measurements and theoretical calculations. Across the T-CT phase transition, analysis of the PDOS spectra shows a rapid stiffening of the optical phonon modes and a dramatic increase of the Lamb-Mossbauer factor (f(LM)) and mean force constant which can be associated with the rapid decrease of the c axis and the anomalous expansion of the a axis. Theoretically calculated Fe partial PDOS and lattice parameters of SrFe2As2 further reveal the strong correlation between the lattice parameters and phonons. Our results show that the T-CT transition can induce significant changes in the vibrational, elastic, and thermodynamic properties of SrFe2As2 single crystal at high pressure.