Session B2: CM.2 Phase Transitions: Zirconium and Other Single-Component Systems

The Influence of Peak Shock Stress on the High Pressure Phase Transformation in Zirconium

At high pressures zirconium is known to undergo a phase transformation from the hexagonal close packed (HCP) alpha phase to the simple hexagonal omega phase. Under conditions of shock loading, the high-pressure omega phase is retained upon release. However, the hysteresis in this transformation is not well represented by equilibrium phase diagrams and the multi-phase plasticity likely under shock conditions is not well understood. For these reasons, the influence of peak shock stress and temperature on the retention of omega phase in Zr has been explored. In-situ VISAR and PDV measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to quantify the volume fraction of retained omega phase, characterize the morphology of the shocked alpha and omega phases, and qualitatively understand the kinetics of this transformation. In turn, soft recovered specimens with varying volume fractions of retained omega phase have been utilized to understand the contribution of omega and alpha phases respectively to strength in Zr.   

Time-Resolved X-ray Diffraction and Electrical Resistance Measurements of Structural Phase Transitions in Zirconium

We have designed a portable pressure controller module to tune compression rates and maximum pressures attainable in a standard gas-membrane diamond anvil cell (DAC). During preliminary experiments, performed on zirconium (Zr) metal sample, pressure jumps of 80 GPa or higher were systematically obtained in less than 0.2s (400GPa/s). In-situ x-ray diffraction and electrical resistance measurements were performed simultaneously during this rapid pressure increase to provide the first time resolved data on $\alpha $ $\to \quad \omega \quad \to \quad \beta $ structural evolution in Zr at high pressures. Direct control of compression rates and peak pressures, which can be held for prolonged time, allows for investigation of structural evolution and kinetics of structural phase transitions of materials under previously unexplored compression rate-pressure conditions that bridge traditional static and shock/dynamic experimental platforms.   

Sound Speed Measurements in Zirconium using the Front Surface Impact Technique

We have performed a series of experiments impacting zirconium samples of varying purity level directly onto lithium fluoride (LiF) windows to determine both the Hugoniot and sound speed as a function of stress up to 70 GPa. This front surface impact (FSI) geometry is useful for determining sound speed in shock-compression experiments because wave interactions are mostly eliminated and multiple sample thicknesses are not needed in each experiment. The experimental results show a kink in the sound speed above 30 GPa, which is where we expect to see the transition from the $\omega$ (hex-3) to $\beta$ (bcc) phase. A rarefaction shock also forms in the release wave in experiments conducted above 30 GPa giving further evidence that this phase transition is being observed. In this presentation, I will present the details of the technique used to obtain and analyze the data and a summary of the results.   

The influence of high temperature shock compression conditions on transformation degree of silicon nitride to cubic phase

In the present work a research of silicon nitride transformation to high pressure cubic phase was performed in plane recovery ampoules using high temperature shock compression method (HTSC). The samples containing different concentrations of silicon nitride powder and high compressible temperature raising inert additive (KBr, KCl) were subjected to shock loading at 36 and 50 GPa. Similar experiments were performed with commonly used copper powder additive and without any additives for comparison. It was shown that increase of additive concentration leads to increase in transformation degree of silicon nitride to cubic phase. At 36 GPa transformation degree for KBr additive ($\le $35{\%}) is higher than for KCl additive ($\le $22{\%}) and much higher than that in experiments with copper powder additive ($\le $9{\%}). At 50 GPa transformation degree reaches 96{\%} (KBr additive). In these experiments cubic phase was obtained in a nanocrystalline form. Cubic phase was not found in recovered silicon nitride after loading of samples which contained no additives.   

Structural phase transition in bismuth under shock compression measured via nanosecond time-resolved X-ray diffraction

Structural phase transition in bismuth under laser-shock compression up to 11 GPa has been studied via nanosecond time-resolved X-ray diffraction. The nanosecond time-resolved single-shot X-ray diffraction was performed using a laser-pump and X-ray probe technique with a 100-ps X-ray pulse from the synchrotron radiation facility (Photon Factory Advanced Ring, KEK). The sample was a polycrystalline bismuth foil with the thickness of 20 micrometers. The target assembly has a plasma-confined scheme and been irradiated by a 8-ns laser pulse. The results show that the shocked bismuth undergoes a series of structural transformations involving four solid structures: the Bi-I, Bi-II, Bi-III, and Bi-V phases. The transformation form the Bi-I phase to the Bi-V phase occurs within 4 ns under shock compression at maximum pressure of approximately 11 GPa, showing no transient phases with available experimental conditions. Successive phase transformations from the high-pressure Bi-V phase to the Bi-I phase via the Bi-III and the Bi-II phases during shock release within 30 ns have also been unambiguously resolved.   

A multiphase equation of state for Be based on the modified mean-field potential approach

We present a first-principles scheme to study the multiphase equation of state (EOS) for Be, based on our recently developed modified mean-field potential (MMFP) approach. We first calculate the EOS for Be of hcp and bcc structure, and then compute the melting curve of Be by using MMFP approach. At last, based on the EOS of bcc Be and melting curve and considing the effcet of the melting entropy, we obtain the EOS of liquid Be. Based on the results, we obtain the multiphase EOS of Be. The calcutlated Hugoiot of solid and liquid phase as well as the melting curve are in agreement with available experimental data.