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We study the electron relaxation processes in nanoporous Au structures using pump-probe thermoreflectance. Using a modified two temperature model, we determine that the electron Fermi relaxation time is unaffected by the ligament size and is an athermal process that is constant with laser pulse excitation, yet the electron-phonon relaxation is affected by boundary scattering and is dependent on electron temperature. The increased, temperature dependent electron-phonon coupling measurements can be explained by electron-boundary scattering and electron-electron scattering that alters the rate of electron equilibration with the surrounding media. These results lend insight into electron relaxation processes in high surface area nanostructures. [ABSTRACT FROM AUTHOR]

The effect of strain rate on mechanical properties at room temperature (RT) and a cryogenic temperature (77 K) was studied in a nanocrystalline Ni–20%Fe alloy (grain size: 22 nm). The activation volume, strain rate sensitivity, strain hardening rate, and microstructural changes were also characterized. The results show that this nanocrystalline alloy has a combination of high strength and large plastic strain at both temperatures. At 77 K, the strength increased dramatically without a loss in the plasticity. Calculations demonstrate that the effect of temperature on yield strength, activation volume, and strain rate sensitivity is strongly dependent on the grain size. The high strain rate sensitivity and strong yield strength dependence on temperature are due to small activation volumes at fine grain sizes. In addition, at 77 K, both the activation volume and strain rate sensitivity exponent decreased significantly. The effect of temperature on the strain hardening capability in this nanograined alloy is opposite to what was observed for the large-grained metals. Comprehensive analysis suggests that the deformation at 20 nm is still dominated by dislocations. However, the dislocation generation models may be different at RT and 77 K. [ABSTRACT FROM AUTHOR]

In situ time-of-flight neutron diffraction was performed to investigate the martensitic phase transformation during quasistatic uniaxial compression testing of 304L stainless steel at 300 and 203 K. In situ neutron diffraction enabled the bulk measurement of intensity evolution for various hkl atomic planes during the austenite (fcc) to martensite (hcp and bcc) phase transformation. Based on the neutron diffraction patterns, the martensite phases were observed from the very beginning of the plastic deformation at 203 K. However, at 300 K, no newly formed martensite, except a small amount of preexisting hcp phase, was observed throughout the test. From the changes in the relative intensities of individual hkl atomic planes, the grain-orientation-dependent phase transformation was investigated. The preferred orientation of the newly formed martensite grains was also investigated for the sample deformed at 203 K using neutron diffraction. The results reveal the orientation relationships between the austenite and the newly formed martensites. The fcc grain family diffracting with {200} plane normal parallel to the loading axis is favored for the fcc to bcc transformation and the bcc {200} plane normals are primarily aligned along the loading direction. For the fcc to hcp transformation, the fcc grains with {111} plane normals at an angle in between about 10° and 50° to the loading direction are favored. [ABSTRACT FROM AUTHOR]