Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (VS) and high Poisson’s ratio (σ) in the solid inner core. To date, neither VS nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present VS and σ derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to ~230 GPa and ~5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low VS and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease VS and increase σ. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.

Using dynamic compression technique, the equation of state for Fe‐8.6 wt% Si was measured up to 240 GPa and 4,670 K. A least squares fit to the experimental data yields the Hugoniot parameters C0 = 4.603±0.101 km/s and λ = 1.505±0.037 with initial density ρ0=7.386±0.021 g/cm3. Based on the Hugoniot data, the calculated isothermal equation of state is consistent with static compression data when the lattice Grüneisen parameter γl =1.65(7.578/ρ) and electronic Grüneisen parameter γe=1.83. The calculated pressure‐density data at 300 K were fitted to a third‐order Birch‐Murnaghan equation of state with zero pressure the parameters K0=192.1±6.3 GPa, K0'=4.71±0.27 with fixed ρ0ε =7.578±0.050 g/cm3. Under the conditions of Earth's core, the densities of Fe‐8.6±2.0 wt% Si and Fe‐3.8±2.9 wt% Si agree with preliminary reference Earth mode (PREM) data of the outer and the inner core, respectively. These are the upper limits for Si in the core assuming Si is the only light element. Simultaneously considering the geophysical and geochemical constraints for a Si‐S‐bearing core, the outer core may contain 3.8±2.9 wt% Si and 5.6±3.0 wt% S.

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Expanding upon the effort to redesign Alloy 718 in order to provide microstructural and mechanical stability beyond 65O"C, six modified compositions have been studied after a precipitation hardening treatment and then after aging for 534 hr at 730°C (1350°F). The variations in Al, Ti and Nb content provided an (Al+Ti)/Nb ratio between 0.66 and 0.92, an Al./Ti ratio between 0.88 and 1.69 and total hardener (Al+Ti+Nb) content between 5.80 and 6.80 at pet; two of the alloys also contained 0.76 or 1.77 at pet W as a solid solution strengthening element. Even though the rate of y ' coarsening was faster after the aging treatment, the coarsening rate of y " and the decline in strength occurred more slowly in the four alloys with a higher (Al+Ti)/Nb ratio, and with less transformation to 6 phase, than was the case in the two lower (Al+Ti)/Nb ratio alloys. The two alloys having the highest (Al+Ti)/Nb and Al/Ti ratios and a W addition provided increasing strength and a slower rate of y " coarsening. Hardness and tensile strength after the conventional heat treatment exceeded the published results for high strength processed 718 and, after 534 hours at 730°C and testing at room temperature, these mechanical properties were retained to the highest degree in these same alloys. Under stress rupture conditions at 700°C (1300"F), these alloys survived from 35 to 53 hours under a stress of 92.5 ksi whereas conventional 718 has a published rupture life of 18 hours when stressed at 85 ksi. At this stress level, one of the alloys having both ratios and total hardening element content on the high side of the range survived for 74.3 hours. These preliminary results suggest that the basic composition of Alloy 718 can be modified to a minor degree in order to improve temperature capability beyond 650°C.