In-situ Studies of the Martensitic Transformation in Pure Ti Thin Films using the Dynamic Transmission Electron Microsco
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In-situ Studies of the Martensitic Transformation in Pure Ti Thin Films using the Dynamic Transmission Electron Microscope (DTEM) Thomas LaGrange, Geoffrey H. Campbell, Jeffrey D. Colvin, Wayne E. King, Nigel D. Browning, Michael R. Armstrong, Bryan W. Reed, Judy S. Kim, and Brent C. Stuart University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550 ABSTRACT We have measured the transient events of the α-β martensitic transformation in nanocrystalline Ti films via single shot electron diffraction patterns with 1.5 ns temporal resolution. This was accomplished with a newly constructed dynamic transmission electron microscope (DTEM), which combines pulsed laser systems and pump-probe techniques with a conventional TEM. The DTEM thereby enables studies of transformations that are (1) far too fast to be captured by conventional bulk techniques, and (2) difficult to study with current ultrafast electron diffraction (UED) instruments (which typically require an accumulation of multiple shots for each diffraction pattern). Martensitic transformations in nanocrystalline materials meet both criteria, with their rapid nucleation, characteristic interface velocities ~1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of ~1010 K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long intense electron pulse. Diffraction patterns show an almost complete transition to the β phase within 500 ns. Post-mortem analysis (after the sample is allowed to cool) shows a reversion to the α phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a "massive" short-range diffusion mechanism. INTRODUCTION Martensitic transformations have been studied for over a century, and current understanding of these rapid transformations is still limited to phenomenological models based on postmortem observations of crystallographic relationships between the parent and product phases [1]. These limitations stem from the inability to directly observe the transformation due to the inherent rapid growth rates and interface velocities that can be on the order of 103 m/s. In spite of this, some kinetic models have been developed for alloy systems that exhibit time-dependent growth (isothermal) and have slow nucleation and growth rates [1]. The development of more sophisticated models that extend to other kinetic modes of the martensitic transformation, such as athermal modes, necessitates a technique that has both high spatial and high temporal resolution. One such instrument has been constructed at Lawrence Livermore National Laboratory (LLNL), the dynamic transmission electron microscope or DTEM. This provides a means to observe the dynamics of rapid material processes such as the martensitic transformation. The LLNL DTEM is a next-generation version of
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