Reflectance Changes during Shock-induced Phase Transformations in Metals
In performing shock wave experiments to study the characteristics of metals at high pressures, wave profiles (i.e., velocity measurements of the surface of the sample) are an established and useful way to study phase transformations. For example, a sudden change in the velocity or its slope can occur when the phase transformation induces a large volume change leading to a change in particle velocity. Allowing the shock to release into a transparent window that is in contact with the sample surface allows the study of conditions away from the shock Hugoniot. However, in cases where the wave profile is not definitive an additional phase-transformation diagnostic would be useful. Changes in the electronic structure of the atoms in the crystal offer opportunities to develop new phase-change diagnostics. We have studied optical reflectance changes for several phase transformations to see whether reflectance changes might be a generally applicable phase-transformation diagnostic. Shocks were produced by direct contact with explosives or with impacts from guns. Optical wavelengths for the reflectance measurements ranged from 355 to 700 nm. We studied samples of tin, iron, gallium, and cerium as each passed through a phase transformation during shock loading and, if observable, a reversion upon unloading. For solid-solid phase changes in tin and iron we saw small changes in the surface scattering characteristics, perhaps from voids or rough areas frozen into the surface of the sample as it transformed to a new crystal structure. For melt in gallium and cerium we saw changes in the wavelength dependence of the reflectance, and we surmise that these changes may result from changes in the crystal electronic structure. It appears that reflectance measurements can be a significant part of a larger suite of diagnostics to search for difficult-to-detect phase transformations.
- Research Organization:
- National Security Technologies, LLC (United States)
- Sponsoring Organization:
- USDOE; USDOE National Nuclear Security Administration (NA)
- DOE Contract Number:
- DE-AC52-06NA25946
- OSTI ID:
- 992608
- Report Number(s):
- DOE/NV/25946-844; TRN: US1100407
- Journal Information:
- Review of Scientific Instruments, Vol. 81, Issue 6
- Publisher:
- Review of Scientific Instruments
- Country of Publication:
- United States
- Language:
- English
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