Phase transformation analysis in titanium at nanosecond time resolution

We aim to develop a fundamental understanding of materials dynamics (from {micro}s to ns) in systems where the required combination of spatial and temporal resolution can only be reached by the dynamic transmission electron microscope (DTEM). In this regime, the DTEM is capable of studying complex transient phenomena with several orders of magnitude time resolution advantage over any existing in-situ TEM. Using the unique in situ capabilities and the nanosecond time resolution of the DTEM, we seek to study complex transient phenomena associated with rapid processes in materials, such as active sites on nanoscale catalysts and the atomic level mechanisms and microstructural features for nucleation and growth associated with phase transformations in materials, specifically in martensite formation and crystallization reactions from the amorphous phase. We also will study the transient phase evolution in rapid solid-state reactions, such as those occurring in reactive multilayer foils (RMLF). Program Impact: The LLNL DTEM possesses unique capabilities for capturing time resolved images and diffraction patterns of rapidly evolving materials microstructure under strongly driven conditions. No other instrument in the world can capture images with <10 nm spatial resolution of interesting irreversible materials processes such as phase transformations, plasticity, or morphology changes with 15 ns time resolution. The development of this innovative capability requires the continuing collaboration of laser scientists, electron microscopists, and materials scientists experienced in time resolved observations of materials that exist with particularly relevant backgrounds at LLNL. The research team has made observations of materials processes that are possible by no other method, such as the rapid crystallization of thin film NiTi that identified a change in mechanism at high heating rates as compared to isothermal anneals through changes in nucleation and growth rates of the crystalline phase. The project is designed to reveal these fundamental processes and mechanisms in rapid microstructure evolution that form the foundation of understanding that is an integral part of the DOE-BES mission.

The impact phase transformation of body-centered-cubic iron is one of the hotspots in current research. Many studies demonstrated that when iron is impacted along the [100] direction, body-centered-cubic phase will transform into hexagonal close-packed phase; while when it is impacted along the [101] direction, a certain amount of face-centered-cubic phase will also appear besides hexagonal close-packed phase. The transformation from body-centered-cubic to hexagonal close-packed phase has been clarified, however, the transformation from body-centered-cubic to face-centered-cubic phase still needs further exploring. In the present work, molecular dynamics simulation is used to study the phase transformation of body-centered-cubic iron impacted along the [101] direction. The results show that the body-centered-cubic phase will transform into a close-packed structure including hexagonal close-packed phase and face-centered-cubic phase). The formation mechanism of face-centered-cubic phase is as follow. In the loading process, single crystal iron suddenly contracts along the [101] and &lt;inline-formula&gt;&lt;tex-math id="Z-20200630101515-1"&gt;\begin{document}$ [\bar101] $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; directions, and expands along the [010] direction, leading to the transformation from body-centered-cubic phase to face-centered-cubic phase. The formation mechanism of hexagonal close-packed phase can be divided into two stages: first, (101) plane is compressed into close-packed plane, then hexagonal close-packed phase is obtained by the relative sliding of adjacent close-packed planes. To further investigate the formation mechanism of the close packed structure, the effect of stress state on the phase transformation of body-centered-cubic iron is further studied. Under one-dimensional (along the [101] direction) or two-dimensional loading (along [101] and &lt;inline-formula&gt;&lt;tex-math id="Z-20200630101515-2"&gt;\begin{document}$ [\bar101] $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101515-2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; directions), the body-centered-cubic iron transforms into face-centered-cubic iron. In the loading process the lattice constants along the three dimensions are monitored. When the transformation from body-centered-cubic phase to face-centered-cubic phase finishes, the ratio of lattice constants along three directions is 1∶1∶1.31 under one-dimensional loading; while the ratio of lattice constants is 1∶1∶1 under two-dimensional loading. Obviously, the body-centered-cubic phase transforms into distorted face-centered-cubic phase under one-dimensional loading. Under two-dimensional (along the [101] and [010] direction) and three-dimensional loading (along the [101], [010] and &lt;inline-formula&gt;&lt;tex-math id="Z-20200630101616-1"&gt;\begin{document}$ [\bar101] $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101616-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191877_Z-20200630101616-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; direction), the body-centered-cubic phase transforms into hexagonal close-packed phase. Gibbs free energy value for each of BCC, HCP and FCC phase is calculated. The calculation results show that the BCC phase is stable under low pressure, while the HCP and FCC phase are stable under high pressure. Finally, based on Gibbs free energy and the effect of stress state on the phase transformation, the phase transformation mechanism of body-centered-cubic iron under [101] impaction is investigated, and a reasonable explanation for the phase transformation is obtained.

The competition among multiple solid phases determines the final microstructures of a material. Such competition can originate at the very beginning of the solidification process. We report the results of molecular dynamics simulation of the phase competition between the hexagonal close-packed (hcp), face-centered cubic (fcc), and body-centered cubic (bcc) phases during the solidification of pure Tb. We found that the liquid supercooled below the hcp melting temperature has both bcc and hcp/fcc nuclei, but only the bcc nuclei grow such that the liquid always solidifies into the bcc phase, even at temperatures where the hcp phase is more stable. The hcp phase can only form in the last liquid droplet or at the bcc grain boundaries. Depending on the bcc grain orientations, the hcp phase jammed between the bcc grains either completely disappears or slowly grows via a solid-state massive transformation mechanism. Once the hcp phase becomes large enough, the stresses associated with its appearance can trigger a martensitic transformation. Yet, not the entire bcc phase is consumed by the martensitic transformation and the remaining bcc phase is transformed into the hcp phase via the solid-state massive transformation mechanism. Finally, if the supercooling is too large, the nucleation becomes almost barrier free and the liquid solidifies into a structure consisting of ultra-fine hcp and bcc grains after which the bcc phase quickly disappears.

The case for a body-centered cubic phase (α′) for iron at inner core conditions

We performe first-principles density functional theory calculations to investigate the stability and mechanical properties of various HfHx (0 ≤ x ≤ 1) phases. For pure Hf phases, the calculated results show that the HCP and FCC phases are mechanically stable, while the BCC phase is unstable at 0 K. Also, as for various HfHx phases, we find that H location and concentration could have a significant effect on their stability and mechanical properties. When 0 ≤ x ≤ 0.25, the HCP phases with H at (tetrahedral) T sites are energetically most stable among various phases. The FCC and BCC phases with H at T sites turn to be relatively more favorable than the HCP phase when H concentration is higher than 0.25. Furthermore, our calculated results indicate that the H solution in Hf can largely affect their mechanical properties such as the bulk moduli (B) and shear moduli (G).

When a short, high peak-power laser pulse is focused on a material surface, vigorous material removal called ablation occurs. The ablation process is one of the major processes for laser microfabrication. We have observed a dynamics of infrared laser ablation process of copper, zinc and aluminum by a stroboscopic imaging technique with a few nanoseconds time resolution and found a growth of jet-like plasma during the laser pulse duration.Output of Q-switched Nd:YAG lasers passed through a doubling crystal contains the fundamental and doubled radiation; the former was used as ablation pulse and the latter as illumination light. Metal targets were irradiated in air, argon or helium environment. Image was photographed by a gated CCD camera with a band pass filter at 532 nm from a tilted direction while the ablation laser hit perpendicular to the surface and the illuminating light hit at 60 degrees to the normal. Interval between ablating and illuminating laser pulses was varied by optical delay line.When the laser fluence is high, a jet-like plasma column growing toward the incoming laser beam direction is observed. Its growth speed during the laser pulse duration is of 100 km/s, which is significantly faster than shock wave propagation under atmospheric pressure. The speed slows down to about 10 km/s when the pulse is terminated. The growth speed dependence on laser fluence and the effects of metal properties and gas environment are examined.A possible mechanism of the jet formation would be laser-induced breakdown of environment gas triggered by electrons ejected from the sample surface at very beginning of the laser pulse.When a short, high peak-power laser pulse is focused on a material surface, vigorous material removal called ablation occurs. The ablation process is one of the major processes for laser microfabrication. We have observed a dynamics of infrared laser ablation process of copper, zinc and aluminum by a stroboscopic imaging technique with a few nanoseconds time resolution and found a growth of jet-like plasma during the laser pulse duration.Output of Q-switched Nd:YAG lasers passed through a doubling crystal contains the fundamental and doubled radiation; the former was used as ablation pulse and the latter as illumination light. Metal targets were irradiated in air, argon or helium environment. Image was photographed by a gated CCD camera with a band pass filter at 532 nm from a tilted direction while the ablation laser hit perpendicular to the surface and the illuminating light hit at 60 degrees to the normal. Interval between ablating and illuminating laser pulses was varied by optical delay line.When the...

Transformation induced softening and plasticity in high entropy alloys

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 ∼1010K/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.

Revealing the phase-transformation path in a FeCoNiSnx eutectic high entropy alloy system by crystallographic orientation relationships

Phase transformations in the Mo - Ru alloy system were investigated at high pressures and high temperatures in a laser-heated diamond anvil cell. The room temperature compression of Mo - 20 at% Ru alloy to 67 GPa revealed that the BCC phase is stable to the highest pressure. Laser heating at high pressures performed without an insulating medium resulted in the formation of phase. However, controlled laser heating to 2200 K at 20 GPa with KCl insulating medium showed that the BCC phase transforms into a mixture of BCC and sigma phases. The present phase-stability data are discussed within the context of the rigid-band model and electron transfer under high pressures in transition metal systems.

The effect of silicon impurities on the phase diagram of iron and possible implications for the Earth's core structure

Multiple electron scattering and the nonintuitive nature of image formation with coherent radiation complicate the interpretation of conventional transmission electron microscopy images. Precession of the illuminating beam in transmission electron microscopy (TEM) can lead to more robust and interpretable images with some penalty to image contrast, a technique known as dynamic hollow-cone illumination TEM. We demonstrate direct and robust imaging of light and heavy atoms in a crystalline environment with this technique. This method is similar to the annular bright-field technique in scanning transmission electron microscopy, via the principle of reciprocity. Dynamic hollow-cone illumination TEM is challenging in practice due to sensitivity to the misalignment of the precession axis, microscope objective aperture, and crystal zone axis.

Comparative study of the expansion dynamics of laser-driven plasma and shock wave in in-air and underwater ablation regimes

For fusion to be realized as a safe, sustainable source of power, new structural materials need to be developed which can withstand high temperatures and the unique fusion radiation environment. An attractive aspect of fusion is that no long-lived radioactive wastes will be produced, but to achieve this structural materials must comprise reduced activation elements. Compositionally complex alloys (CCAs) (also called high entropy alloys, HEAs) are promising candidates for use in extreme environments, including fusion, but few reported to date have low activation. To address these material challenges, we have produced novel, reduced activation, HEAs by arc-melting, and investigated their thermal stability, and radiation damage resistance using 5 MeV Au2+ ion implantation. Whilst the alloys were designed to form single phase BCC, using room temperature and non-ambient in situ X-ray diffraction we have revealed the thermodynamically stable structure of these alloys is in fact a sigma phase. We propose that a BCC phase is formed in these alloys, but at high temperatures (>1000°C). A BCC phase was also formed during heavy ion implantation, which we propose to be due to the rapid heating and cooling that occurs during the thermal spike, effectively freezing in the BCC phase produced by an implantation induced phase transformation. The BCC phase was found to have high hardness and a degree of ductility, making these new alloys attractive in the development of reduced activation HEAs for nuclear applications.