Session N4: MS: Metals Phase Transitions II

Phase Transformation of Nitinol Shape Memory Alloy under Dynamic Uniaxial Strain Compression

The stress-induced martensitic transformation of Nitinol, a near equi-atomic NiTi based shape memory alloy (SMA), has been studied extensively, but primarily under uniaxial stress loading. The transformation stress under such loading is quite low (0.5-0.8 GPa), and appears to depend on the loading rate. In this study, various types of plate impact experiments over the loading rate range of $10^{5}$-$10^{7}$$s^{-1}$ were designed and conducted to a peak stress of 6 GPa to capture the phase transition of Nitinol under dynamic uniaxial strain loading. Particle velocity profiles and wave velocities were measured using laser interferometry under shock and ramp compression. Additionally, sound speed measurements were made at various peak stresses. All of these experiments indicate that under dynamic uniaxial strain loading, the martensitic transformation of Nitinol occurs at $\sim$ 2 GPa. Further research is ongoing to gain insights on the role of stress states and loading rates on the stress-induced transformation of Nitinol, and to develop an understanding of the transformation behavior under different loading conditions.   

Studying the dynamic phase transitions in tin with impact experiments on pre-heated samples

The dynamic $\beta $-$\gamma $ transition in high-purity tin (Sn) has been studied in a series of VISAR-instrumented plate impact experiments in which pre-heated $\beta $-Sn samples of different, 0.5 to 5 mm, thickness were shock-transformed into $\gamma $-phase. The initial sample temperature was varied from room temperature up to 505 K (melting point). The experiments allowed pressure-temperature mapping of the upper boundary of the existence of tin $\beta $-phase, while varying the sample thickness gave access to information about the transformation kinetics. The resulting $\beta $-$\gamma $ boundary is compared with that obtained in static and dynamic experiments. Comparisons between experimental results and hydrodynamic calculations are made.   

Constitutive modelling of phase transition in iron under sweeping detonation wave loading

The well-known alpha-epsilon phase transition in iron has a very accurately determined phase transition pressure value of 13 GPa from numerous plane impact tests. However, a more recent experimental study, using a sample loaded by a sweeping detonation wave, has measured a decreased transition value to that observed in the plane tests. The present analysis is developed in order to study if martensitic character of the phase transition is a possible mechanism of the transition in the sliding wave loading conditions. A two-phase model is employed that is complemented with the phase-transition kinetic taking into account the shear stress factor. A set of constitutive equations describing the rate sensitive strength response and the phase transition kinetic complements the conservation laws of the model. The model implemented in CTH is used for simulation of the experiments. Results of modelling of a sliding wave loading experiment demonstrate that the observed phase transition occurring at a reduced transition pressure can be described with the martensitic mechanism.   

Investigation of tantalum room temperature isothermal compression to multi-megabar pressures using two-stage diamond anvils

Ta has a bcc structure at room $P$-$T$ and based on previous studies this structure has been proposed to be stable up to $P$\textgreater 100 GPa and temperatures up to the melt. Extending $P$-$V$ data to higher $P$ and obtaining accurate isothermal data is vital for further development of SESAME-EOS tables and next generation modeling of EOS and strength of Ta and other metals. In this study, we performed multiple compression studies of Ta. We primarily employed the new two-stage toroidal diamond anvil concept to achieve $P\approx $270 GPa ($V$/$V_{0}=$0.60) using rhenium as a pressure standard, while performing \textit{in situ} micro x-ray diffraction measurements at the High Pressure Collaborative Access Team (HPCAT) facility at APS. Concurrently, we have performed multiple hydrostatic and non-hydrostatic measurements of Ta using traditional single-stage anvils. Comparing data collected under both conditions provides better insight into the effects of non-hydrostatic stress distribution on behavior of Ta.   

X-Ray Diffraction of Platinum at High Pressures

Laser-driven experiments at the University of Rochester's Laboratory for Laser Energetics were performed to test theoretical predictions of a solid--solid phase transformation in platinum at high pressures and temperatures.\footnote{ L. Burakovsky\textit{ et al.}, J. Phys.: Conf. Ser. \textbf{500}, 162001 (2014).} Platinum is of interest because it is often used as a calibration standard in high-pressure experiments. Powder x-ray diffraction\footnote{ J. R. Rygg \textit{et al.}, Rev. Sci. Instrum. \textbf{83}, 113904 (2012).} was used to measure the crystal structure of platinum shocked, then ramp compressed to pressures up to 300 GPa. These experiments serve as additional measurements of shock-ramped, shocked, and shock-released platinum completed at Sandia National Laboratories and the National Ignition Facility. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority.