Phase Transitions: FE
In-situ x-ray diffraction is a technique that is used to probe the lattice directly during shock compression. This technique is used to study a variety of materials, including silicon, copper, and iron.
A small source of x-rays is created by direct laser irradiation of a thin foil target in close proximity to a single crystal sample that is shock-loaded. X-rays from this small source are incident on the sample at a wide range of angles, resulting in diffraction from multiple lattice planes simultaneously. The x-rays diffracted from the crystal are recorded on static film (Figure 1). The pattern of x-rays diffracted from the crystal provides information on the lattice structure during shock loading.
X-ray diffraction studies of iron under shock conditions have demonstrated unambiguously the phase transformation from the bcc (a) to hcp (e) structure on a nanosecond time-scale. These real-time in-situ x- studies have observed the deformation of the iron bcc phase and subsequent transformation to the hcp phase, consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms. (Figure 1)
In-situ diffraction patterns of [001] iron were recorded at a range of loading pressures spanning the transition pressure of 13 GPa. At the driven surface, the measurements indicate the prompt elastic response of the crystal with a uniaxial compression along the [001] loading direction. The maximum uniaxial compression was approximately 6% at approximately 13 GPa. This was identified based on the shift of the diffraction lines from multiple lattice planes recorded simultaneously on film.
Above the transition pressure, two lattice configurations were observed. A uniaxial compression by 6% was evident, as well as the hcp structure. This is identified by the additional collapse along the [001] direction by 15-18% and appearance of new diffraction lines consistent with the symmetry of the hcp structure (Figure 2).
The lattice compression along the [001] direction was determined from the shift of the diffraction signal from the (002) lattice planes. This is shown plotted vs. loading pressure ((Figure 3)). The lattice spacing decreases with increasing shock pressure. Two compressions are evident above approximately 13 GPa, consistent with a two-wave structure. The higher compression is associated with the appearance of new diffraction lines consistent with the transition to the hcp structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock wave-profile analyses and static high-pressure x-ray measurements. [2-4] Because high-pressure dynamic loading can markedly affect the structural modifications of solids, such correlations traversing static and dynamic regimes are intrinsically limited in nature. The in-situ measurements provide direct evidence for this transformation under shock conditions.
This work was conducted under the auspices of the US DOE by the UC Lawrence Livermore National Laboratory and Los Alamos National Laboratory under Contract No. W-7405-Eng-48.
Experiments were conducted at the University of Rochester Laboratory for Laser Energetics under the NLUF grants program.
[1] F.M. Wang and R. Ingalls, Phys. Rev. B, 57, 5647 (1998).
[2] J.M. Walsh, Bull. Am. Phys. Soc. 29, 28 (1954).
[3] D. Bancroft, E. L. Peterson, and S. Minshall, J. Appl. Phys. 27,291 (1956). [4] John C. Jamieson and A.W. Lawson, J. Appl. Phys. 33, 776 (1962).
Additional references D. H. Kalantar, E. Bringa, M. Caturla, J. Covin, K.T. Lorenz, M. Kumar, J. Stken, A.M. Allen, K. Rosolankova, J.S. Wark, M.A. Meyers, M. Schneider, T.R. Boehly, “Multiple film plane diagnostic for shocked lattice measurement“, Rev. Sci, Instrum. 74, 1929 (2003). D. H. Kalantar, J. Belak, E. Bringa, K. Budil, M. Caturla, J. Colvin, M. Kumar, K. T. Lorenz, A. M. Allen, K. Rosolankova, J. S. Wark, M. A. Meyers, M. Schneider, “High pressure, high strain rate lattice response of materials”, Physics of Plasmas 10, 1569 (2003). D. H. Kalantar, E. A. Chandler, J. D. Colvin, R. Lee, B. A. Remington, S. V. Weber, L. G. Wiley, B. H. Failor, A. Hauer, J. S. Wark, A. Loveridge, M. Meyers, G. Ravichandran, “Transient x-ray diffraction used to diagnose shock compressed Si crystals on the Nova laser”, Rev. of Sci. Instrum. 70, 629-32 (1999). A. Loveridge-Smith, A. Allen, J. Belak, T. Boehly, A. Hauer, B. Holian, D. Kalantar, G. Kyrala, R. W. Lee, P. Lomdahl, M. A. Meyers, D. Paisley, S. Pollaine, B. Remington, D. C. Swift, S. Weber, and J. S. Wark, “Anomalous elastic response of silicon to uniaxial shock compression on nanosecond timescales“, Phys. Rev. Lett. 86, 2349-2352 (2001)
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