Jason R. Trelewicz

The Hall–Petch breakdown at high strain rates: Optimizing nanocrystalline grain size for impact applications

The breakdown of Hall–Petch strength scaling is investigated in nanocrystalline Ni–W alloys at high strain rates, using dynamic hardness testing at grain sizes ranging from 3to150nm. Whereas quasistatic tests show a strength plateau below about 15nm, high-rate tests (indentation strain rate of ∼103s−1) exhibit a pronounced strength peak and a regime of “inverse Hall–Petch” weakening. This effect is shown to be the result of a grain size-dependent rate sensitivity that exhibits a maximum at a grain size near 10–20nm. High strain rates are also shown to promote shear banding at nanocrystalline grain sizes.

Heat Flux Measurements in a Scramjet Combustor Using Direct Write Technology

In this experimental study, heat flux measurements were made at four locations within the isolator/combustor/nozzle region of a direct-connect gaseous hydrocarbon-fueled scramjet combustor. Combustor conditions were fixed and representative of a Mach-5 flight condition. The heat flux measurements were made using sensors deposited directly on the engine thermal barrier coating (TBC) using Direct Write Technology. Flush-wall fuel injection locations were fixed and a cavity on the body side wall provided the necessary flame holder. Overall equivalence ratios were varied from 0.6 to 1.0, but were held constant for a given run. Preliminary results show heat flux values rising with combustor equivalence ratio at all measurement locations. Heat flux values ranging from 400 to 1000 kW/m 2 (35 to 88 BTU/ft 2 -s) were measured in the location downstream of the cavity flame holder. Heat flux values ranging from 600 to 2000 kW/m 2 (53 to 176 BTU/ft 2 -s) were measured in the cavity flame holder location. Heat flux values ranging from 200 to 600 kW/m 2 (18 to 53 BTU/ft 2 -s) were measured in the locations upstream of the cavity flame holder. Measured local heat flux values were consistent with area-averaged heat flux measurements over a section of the combustor.

Softening due to Grain Boundary Cavity Formation and its Competition with Hardening in Helium Implanted Nanocrystalline Tungsten

The unique ability of grain boundaries to act as effective sinks for radiation damage plays a significant role in nanocrystalline materials due to their large interfacial area per unit volume. Leveraging this mechanism in the design of tungsten as a plasma-facing material provides a potential pathway for enhancing its radiation tolerance under fusion-relevant conditions. In this study, we explore the impact of defect microstructures on the mechanical behavior of helium ion implanted nanocrystalline tungsten through nanoindentation. Softening was apparent across all implantation temperatures and attributed to bubble/cavity loaded grain boundaries suppressing the activation barrier for the onset of plasticity via grain boundary mediated dislocation nucleation. An increase in fluence placed cavity induced grain boundary softening in competition with hardening from intragranular defect loop damage, thus signaling a new transition in the mechanical behavior of helium implanted nanocrystalline tungsten.

High-Temperature Calibration of Direct Write Heat Flux Sensors From 25 °C to 860 °C Using the In-Cavity Radiation Method

Heat flux sensors fabricated using Direct Write Thermal Spray are thin, surface-based devices that can operate at high temperatures. In this paper, Direct Write heat flux sensors are calibrated over a temperature range of 25 °C-860 °C. A substitution-based quartz lamp configuration is used to measure the steady-state sensitivity and transient response of Direct Write sensors at ambient temperatures, with repeatability confirmed over a 10-month period and after thermal aging. A matched heat flow approach is used to characterize the sensors at operating temperatures up to 860 °C. The sensitivity is found to increase by a factor of two from 25 °C to 650 °C, after which it plateaus up to the maximum tested temperature of 860 °C. A cubic polynomial calibration function captures the temperature dependence of the sensitivity and provides a good agreement for the measured data.