New research finds lead toughens up under extremes

NIF Target Chamber (Download Image)

Artist’s rendering of the interior of the National Ignition Facility’s (NIF) Target Chamber. In these experiments, 160 of NIF’s 192 beams converge at the center of the chamber, driving the sample to pressures several million times the pressure of the atmosphere, similar to those found at the center of the Earth.

In a new paper published as an “Editors’ Suggestion” in Physical Review Letters, a team of researchers from Lawrence Livermore National Laboratory has demonstrated that lead — a metal so soft that it is difficult to machine at ambient conditions — responds similarly to other much stronger metals when rapidly compressed at high pressure. The paper also was highlighted by a “Viewpoint” in Physics.

“This is the first paper to directly probe the strength of a heavy metal at Earth-core pressures following a phase transformation,” said Andy Krygier, LLNL physicist and lead author. “The pressure and the phase transformation turned soft lead very hard. This hardening had been predicted, but actually seeing that take place was quite remarkable and exciting for our team.”

“It’s clear now — metallurgy isn’t the same at high pressure, and we have been fortunate enough to get a first look at what’s different at this extreme regime,” added Rob Rudd, LLNL materials strength theorist and co-author.

This research is part of a series of high-pressure materials science campaigns led by co-author Hye-Sook Park at LLNL’s National Ignition Facility (NIF), aiming to reveal material properties at extreme conditions. Materials exhibit a wide range of exotic behaviors at high pressure, and NIF — the world’s largest and most energetic laser — provides the capability to drive experimental samples to pressures several million times the pressure of the atmosphere, similar to those found at the center of the Earth.

To infer the high-pressure material strength, the team’s experiments exploited a phenomenon called the Rayleigh-Taylor (RT) instability, which occurs when material is accelerated across different densities. They employed precisely formed rippled samples that seeded RT instability, causing ripple growth in a controlled manner during the high-pressure conditions. The RT growth of surface perturbation occurs due to material density variations at the interface. However, this growth can be changed by the material flow stress. A strong material will produce less growth than a weak one, measured using an X-ray diagnostic available at NIF. By combining results from multiple experiments, the researchers compared the evolution of the RT growth in time to predictions from simulations testing various strength models.

“Material strength at high pressure and strain-rate is notoriously difficult to model as it is influenced by effects that occur over many different scales, all the way from the smallest scales that take into account how atoms bond to each other, to the explicitly macroscopic scale,” Krygier said. “We have experimentally validated a new strength model in the high pressure, high strain-rate regime with careful measurements using techniques developed over many years. This work truly demonstrates the ability of LLNL to solve hard and important problems by harnessing the unique capabilities and experts across so many different domains.”

“This endeavor took multiple years to develop,” Park said, highlighting several innovations that made the lead experiments possible, including multi-layer reservoirs designed to keep the lead solid as it was compressed to extremely high pressure; functionality that enabled direct comparison of the strength of pure and alloy samples; in-situ radiographic standards that significantly reduced the error bars; and important advancements in target fabrication.

“We rely heavily on the expertise and capabilities of the LLNL target fabrication team,” Krygier said. “Among the many small miracles that take place in order to perform this work, our experiments require very thin, large hohlraums, careful design and characterization of the low-density foams and precisely formed ripples. The team has learned how to regularly do all of this including, crucially, production of high-quality ripples in a range of materials with vastly different properties and with a range of amplitudes and wavelengths.”

In addition to the targets, the interpretation and understanding of the lead strength in this unique regime required developing a new strength model created by co-author Rudd. This theory is fed to the sophisticated simulation tools that predict experimental observables using LLNL’s high performance computing capability. This integrated effort made it possible to make meaningful interpretations of the experimental results.    

Looking ahead, the team will continue research in this area by investigating lead alloys, as well as compressing to higher pressures. Results from future experiments will guide development of new strength models for materials under extreme conditions.

Co-authors include Philip Powell, James McNaney, Channing Huntington, Shon Prisbrey, Bruce Remington, Rob Rudd, Damian Swift, Christopher Wehrenberg, Tom Arsenlis and Hye-Sook Park from LLNL, and Peter Graham, Edward Gumbrell, Matt Hill, Andy Comley and Steve Rothman from the United Kingdom’s Atomic Weapons Establishment.