Physical Science Research Frontiers 2018 68 How to make materials more resistant to extreme deformation Understanding of the fracture phenomena of a material under extreme conditions of pressure and/ or temperature is crucial for a wide variety of scientific fields ranging from applied science and technological developments to fundamental science such as laser- matter interactions and geology. This universal process is particularly important for the development of new materials. Indeed, the properties related to the fracture of materials depend on the way forces are applied to materials as the properties are difficult to define physically. Such properties include the bulk modulus, young’s modulus, and spall strength. A method of directly estimating spall pressure may facilitate the efficient evaluation of the spall property to explore novel materials. As an example, there is a large amount of debris around the Earth traveling at an average velocity of ~10 km/s and can hit spacecraft and satellites. If one can test and develop new materials that have different behaviors of dynamic fracture, one can make them more robust or with specific properties (e.g., void size). We have been attempting to bridge the gap between the fundamental study of dynamic fracture and the needs of engineers in materials design, system certification, and manufacturing as discussed in [1]. Several experimental techniques have been developed over the last few decades to study the dynamical damage of a material using macroscopic information, such as the evolution of the free surface velocity and/or information obtained from postmortem examination of the sample. However, a gap exists between the information retrieved at the macroscopic scale from experiments and that obtained from large- scale simulations performed at the atomic scale. In Ref. 2, a new experimental technique is presented, which allows the direct ultrafast real-time monitoring of dynamic fracture (spallation) at the atomic scale with picosecond time resolution. This is achieved in coupling an optical high-power laser ( I ~2.5 × 10 12 W·cm –2 ), which generates a shock wave inside the sample (5- μ m-thick polycrystalline tantalum), with an X-ray beam (10 keV photon energy) used as a probe. The experimental setup is displayed in Fig. 1 and the experiment has been performed at SACLA BL3 . Experimental results are presented in Fig. 2. We were able to directly measure an extension of the tantalum lattice of ~8 to 10% just before fracture occurred at an ultrahigh strain rate of ~2 × 10 8 - 3.5 × 10 8 s –1 using X-ray diffraction. The spall strength has also been determined to be approximately –16.8 GPa. These results have been directly compared with large-scale molecular dynamics simulations and are in good agreement with simulated data (see Fig. 3). This not only paves the way toward the direct measurement of the spall strength of materials as a function of strain rate but also highlights the usefulness of these facilities for investigating various physical problems such as high-speed crack dynamics, uncommon stress-induced solid-solid phase transitions, and so forth. Fig. 1. Pump-probe experiment at SACLA BL3. (a) Experimental configuration. (b) Experimental results. (a) (b) Detector 2 θ axis Compression t = 0 ps t = 1425 ps t = 1525 ps t = 1625 ps t = 1725ps t = 1825 ps t = 1925 ps t = 2025 ps t = 2125ps t = 2225 ps t = 2425 ps t = 2625 ps Expansion Debye Scherrer ring Probe XFEL: 7 fs FWHM 10 keV Pump Laser: 660 ps FWHM ~ 1 J on target FSSR Spectrometer ANDOR CCD 20° Power Target t Spherical crystal z y x Spatial axis 5 μ m polycrystalline Ta Power t Research Frontiers 2018 69 As a conclusion, it is interesting to note that the repetition rate of the SACLA platform is non-negligible. This means that it is possible, during one experimental campaign, to test many different materials. This makes it easy to investigate the atomic structures of new materials and select those having the desired macroscopic properties over a wide range of strain rates and deformations. Then, in the next cycle of development, the obtained knowledge about specific relationships between these mechanical properties and atomic structures can be used in the design of the next generation of materials. In this way, XFEL facilities may accelerate the development of new materials by bridging the gap in the understanding of the relationships between atomic structures and material properties. References [1] B.N. Cox et al. : J. Mech. Phys. Solids 53 (2005) 565. [2] B. Albertazzi, N. Ozaki, V. Zhakhovsky, A. Faenov, H. Habara, M. Harmand, N. J. Hartley, D. Ilnitsky, N. Inogamov, Y. Inubushi, T. Ishikawa, T. Katayama, M. Koenig, A. Krygier, T. Matsuoka, S. Matsuyama, E. McBride, K. Migdal, G. Morard, T. Okuchi, T. Pikuz, O. Sakata, Y. Sano, T. Sato, T. Sekine, T. Seto, K. Takahashi, H. Tanaka, K. A. Tanaka, Y. Tange, T. Togashi, K. Tono, Y. Umeda, T. Vinci, M. Yabashi, T. Yabuuchi, K. Yamauchi and R. Kodama: Sci. Adv. 3 (2017) e1602705. Bruno Albertazzi a, *, Norimasa Ozaki b and Vasily Zhakhovsky c a LULI, École Polytechnique, CNRS, France b Photon Pioneers Center, Osaka University c Dukhov Research Institute of Automatics, ROSATOM, Russia *Email: bruno.albertazzi@polytechnique.edu Fig. 2. Experimental profiles of stretching and postspallation compression in a Ta sample. (a) Observed stretching (blue curve) of the (002) plane of Ta in the experiment. (b) Observed compression wave (purple curve) due to the relaxation of tension after spallation. Fig. 3. Comparison between experimental results and those obtained in large-scale atomistic simulation. (a) Direct comparison between the diffraction signal obtained in the experiment (black) and simulation (red). (b) Dynamical comparison of the position of the peak in the experiment and simulation. (a) (b) Number of Count (arb. units) Diffraction Angle (degrees) Density (g · cm –3 ) X-ray probe depth X-ray probe depth Density (g · cm –3 ) 41 1475 ps 1525 ps 1625 ps 2625 ps 2425 ps 2225 ps 2025 ps 1925 ps t = 2125 ps t = 2125 ps 1825 ps 1925 ps t = 1725 ps t = 1725 ps Laser side Laser side Cold Ta bcc (002) Cold Ta bcc (002) 0 500 1000 1500 2000 2500 3000 3500 Number of Count (arb. units) 500 1000 1500 2000 2500 3000 3500 42 43 44 45 46 Diffraction Angle (degrees) 41 42 43 44 45 46 Spall shock Spall shock Rarefaction wave (a) (b) Number of Counts Position of the Diffraction Peak (degrees) Diffraction Angle 2 θ (degrees) Time (ps) 0 42.0 42.5 43.0 43.5 44.0 44.5 45.0 200 400 600 800 41 42 43 44 45 46 1400 1600 1800 2000 2200 2400 2600 2800 Compression dominant region Spall shock Cold bcc (002) Ta Expansion t = 1725 ps MD simulation experiment preshock MD simulation Experiment
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