Final Report 02-ERD-033: Rapid Resolidification of Metals using Dynamic Compression

Abstract

The purpose of this project is to develop a greater understanding of the kinetics involved during a liquid-solid phase transition occurring at high pressure and temperature. Kinetic limitations are known to play a large role in the dynamics of solidification at low temperatures, determining, e.g., whether a material crystallizes upon freezing or becomes an amorphous solid. The role of kinetics is not at all understood in transitions at high temperature when extreme pressures are involved. In order to investigate time scales during a dynamic compression experiment we needed to create an ability to alter the length of time spent by the sample in the transition region. Traditionally, the extreme high-pressure phase diagram is studied through a few static and dynamic techniques: static compression involving diamond anvil cells (DAC) [1], shock compression [2, 3], and quasi-isentropic compression [4, 5, 6, 7, 8, 9, 10]. Static DAC experiments explore equilibrium material properties along an isotherm or an isobar [1]. Dynamic material properties can be explored with shock compression [2, 3], probing single states on the Hugoniot, or with quasi-isentropic compression [4, 5, 6, 7, 8, 9, 10]. In the case of shocks, pressures variation typically occurs on a sub-nanosecond time scale or faster [11]. Previous quasi-isentropic techniques have yielded pressure ramps on the 10-100 nanosecond time-scale for samples that are several hundred microns thick [4, 5, 6, 7]. In order to understand kinetic effects at high temperatures and high pressures, we need to span a large dynamic range (strain rates, relaxation times, etc.) as well as control the thermodynamic path that the material experiences. Compression rates, for instance, need to bridge those of static experiments (seconds to hours) and those of the Z-accelerator (10{sup 6} s{sup -1}) [4] or even laser ablation techniques (10{sup 6} s{sup -1} to 10{sup 8} s{sup -1}) [7]. Here, we present a new technique that both extends the compression time to several microseconds and makes accessible states beyond the principal Hugoniot and isentrope. The strain rate in these quasi-isentropic compression experiments vary from 10{sup 4} - 10{sup 6} s{sup -1}, effectively bridging the gap between static compression and previous quasi-isentropic compression techniques [4, 7]. The primary deliverable associated with this LDRD-ER is the creation a new experimental capability for the lab: the ability to control pressure and temperature loading rates in a dynamic compression experiment by using functionally graded impactors in the light gas gun facility. The new capability will enable dynamic experiments exploring a broader area of pressure and temperature phase space, ultimately enabling further experiments on the kinetics of phase transitions at high temperature and pressure. Using our unique arbitrary-density graded impactors, scientists can now investigate various aspects of the solidification phase transition including (a) time scale, (b) loading rate dependence and (c) sample size effects.