Session Q05: Mechanisms of Solid-Solid Phase Transformation

Study of β-γ phase transition of tin single cristal under dynamic compression by X-ray diffraction

We investigate the β-γ phase transition of tin single crystal under shock loading by X-ray diffraction and velocimetry. The observation of the γ phase appearance is needed to validate the mechanism and the pressure of the transition. This last one is established between 7 and 11.5 GPa in static and dynamic experiments. Primary X-ray diffraction combined with plate impact experiments were carried out on both CEA Gramat, using a gold X-Pinch X-ray source, and the APS synchrotron on a unique single crystal orientation. A new configuration of the experiment has been elaborated to emphasize the γ-phase from three orientations (110), (100) and (001) of the β-phase single crystal. For this new configuration, a molybdenum source with X-ray emission more penetrating than gold (17.4 keV over 9.7 keV), was selected at CEA Gramat to bring out more information. Additional experiments were carried out for each orientation on both CEA Gramat and APS synchrotron facilities to confirm the hypothesis.   

Mechanochemical Influence on the Graphite to Diamond Phase Transformation via Controlled Out of Plane Strain

Mechanical strain in covalent bonds is well-known to accelerate chemical reactions and alter reaction pathways. Previous results have shown that rotational and bending strains can result in the lowering of the total energy needed to induce reaction. However, it is not entirely clear how this effect influences and couples with other condensed phase effects that alter reaction kinetics and energetics. Hence, we utilize steered molecular dynamics to impose out-of-plane strains throughout a graphite lattice at a variety of levels of strain. These strained systems are then compressed at high strain rates to form diamond phases. We assess the level of compression and compressive work necessary to induce phase transformation for various out of plane strains. This results in a consistent lowering of the necessary work/compression needed to begin diamond formation. However, beyond a certain level of out of plane strain, it also increases defect nucleation and forms disordered regions, which leads to slowed transformation rates.   

Shock Induced Phase Transitions in a High Entropy Rare Earth Sesquioxide, (La0.2Y0.2Ce0.2Pr0.2Sm0.2)2O3

This presentation will discuss the role of high strain rate and pressure from mechanical shock on the induced phase transitions in the high entropy oxide (HEO), (La_0.2Y_0.2Ce_0.2Pr_0.2Sm_0.2)₂O₃. The facilities at the Dynamic Compression Sector at Argonne National Laboratory enable in situ phase and crystal structure analysis by X-ray diffraction under high strain rate (up to 7 km/s impact velocities) and pressures using a powder gun flyer to impact the HEO target. Analysis of the diffraction data will be used to track crystallographic modification induced by this mechanical shock and its role in the observed phase transitions. In this work, crystallographic phase transitions and amorphization under dynamic (ns) conditions of the above HEO are shown and compared with existing quasi-static compression phase transition measurements.

 

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.   

X-ray free electron laser observation of ultrafast lattice behaviour under femtosecond laser-driven shock compression in iron

Understanding the natures of shock compression of condensed matter has been an important subject over the past century. A femtosecond laser emerged as a new shock-driver approximately 20 years ago. Femtosecond laser-driven shock wave generates unique microstructures inside materials unlike conventional shock waves. Therefore, properties of this shock wave may differ from conventional ones, however, the lattice behaviours under the femtosecond laser-driven shock compression have never been clarified. Here we report the ultrafast lattice behaviours in iron shocked by a direct irradiation of a femtosecond laser pulse diagnosed using X-ray free electron laser diffraction. We found that the initial compression state caused by the femtosecond laser-driven shock wave is the same as that caused by conventional shock waves. We also found that the temporal deviation of peaks of stress and strain waves for the first time experimentally, which was predicted theoretically. Furthermore, the existence of plastic wave peak between the stress and strain wave peaks is a new finding that has not been predicted even theoretically. Our findings will open up new avenues for the development of novel materials that combine strength and toughness in a trade-off relationship.   

Competing pathways during a shock-induced phase transition in Zr

Shock compression can cause materials to undergo structural transformations and form new phases of matter with novel material properties. Determining the phase transition mechanism using the orientation relations (ORs) between ambient and high-pressure phases have been typically gleaned from examinations of recovered samples. But the results of such efforts are often complicated by issues such as post-shock annealing and wave interactions. The ductile alpha to brittle omega transition in group IV metals (Zr, Ti) has been studied extensively. Despite decades of study, the pathway by which the high-pressure omega phase is formed remains controversial with several ORs reported in literature. Here we present in-situ x-ray diffraction measurements which provide a snapshot of the active alpha to omega ORs during nanosecond shock-compression of a single crystal [0001] Zr sample. Our data shows that the transformation proceeds by three concurrent and distinct mechanisms which compete for dominance. Our results indicate the alpha to omega transformation is more complex than previously understood, with shear stress playing a critical role in determining the favorability of a particular mechanism. The insights gathered in our study point to a new intricate picture of how materials transform under extreme conditions and take steps to reconcile the discordant results of previous experimental and theoretical investigations.   

High-pressure crystal structure investigation of the cage compound Fe1+δGa3

The pristine compound FeGa3 crystalizes in a tetragonal P42/mnm crystal symmetry, where Fe atoms surrounded by two non-equivalent Ga sites feature a cage-type structure. It has been shown in single crystal FeGa3 that an intrinsic energy gap ~ 0.4 eV develops, establishing a semiconducting and diamagnetic ground state [1]. However, at low temperatures, additional in-gap states related to the presence of intrinsic disorder induces a small intrinsic energy gap Δe, followed by putative weak ferromagnetism (below Tm) [2], which open a debate about the role of disorder in FeGa3 ground state. 

In order to investigate the role of disorder, we synthesize FeGa3 with a delicate inclusion of Fe-antisite disorder, named as Fe1+δGa3 (δ ~ 0.16). Our initial electrical transport and magnetization studies under pressure (up to 2 GPa) reveals a non-canonical phase diagram for semiconducting electronic correlated material: Δe decreases while Tm increases with pressure. These findings call for X-ray diffraction (XRD) experiments under high-pressure to demystify whether this unusual phase diagram is due either to Anderson localization or to structural phase transition. 

In this work, we present XRD results of Fe1+δGa3 up to 38 GPa and at 294 K, performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. Preliminary XRD data analysis distinguishes two different pressure ranges separated by critical pressure Pc  ~ 18 GPa. In particular at Pc, the average volume grain size reduces ~ 30%, the microstrain increases ~ 50% drastically, accompanied by strong broadening in the XRD pattern. We infer that the system undergoes a structural transition from crystalline to amorphous phase around Pc.

Our findings are of extremely relevance to understand an intriguing metalization reported for the pristine single crystal FeGa3 at very high pressure [3].