Kinetics of solid-solid phase transitions in metals using proton radiography (u)
- Los Alamos National Laboratory
When a compressed material changes phase it doesn't do so instantly. Instead it transitions through a mixed phase as it transforms to the end state phase for a given pressure, volume and temperature. Common phase diagrams show the phase boundaries as sharp lines when compression has been slowly applied and held for an infinite amount of time. When the compression is applied with high strain rate, however, the phase boundaries are no longer crisp as the kinetic effects of the crystal reorientation delay the transitions, resulting in regions of mixed phase. This opens up the possibility that some degree of metastability exists for such transition in dynamic compression. The compression path can go past the equilibrium phase boundary and the transition happen from a metastable state because of the very short timescale of the compression process. Molecular dynamics (MD) simulations recently have been used to examine shock-induced phase transitions in single crystal materials illustrating an orientation dependence of the transition stress, mechanisms, kinetics, and Hugoniot response. For example, the [100] orientation of iron had a simulated transition stress higher than the experimentally determined polycrystalline value of 13 GPa by 2 GPa. Previously, dynamic experiments on iron have observed a non-zero transition time and width in the solid-solid {alpha}-{var_epsilon} phase transition. Using Proton Radiography at the Los Alamos Neutron Science Center, we have performed plate impact experiments on iron to further study the {alpha}-{var_epsilon} phase transition which occurs at 13 GPa. A 40mm bore powder gun was coupled to a proton radiography beam line and imaging system and synchronized to the impact of the projectile on the target sample with the proton beam pattern. A typical experimental configuration for the iron study, as shown below in 3 color-enhanced radiographs, is a 40mm diameter aluminum sabot impacting a 40mm diameter sample of polycrystalline ARMCO iron. The iron is backed by a sapphire optical window for velocimetry measurements. The aluminum flyer on the left of the iron is barely visible for visual display purposes.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC52-06NA25396
- OSTI ID:
- 1049963
- Report Number(s):
- LA-UR-11-00569; LA-UR-11-569; TRN: US201218%%385
- Resource Relation:
- Conference: 31st International Workshop on Physics of High Energy Density in Matter ; January 30, 2011 ; Hirschegg, Austris
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ALUMINIUM
COMPRESSION
CONFIGURATION
ENERGY DENSITY
IMAGES
IRON
KINETICS
METASTABLE STATES
MONOCRYSTALS
NEUTRONS
ORIENTATION
PHASE DIAGRAMS
PHYSICS
PROJECTILES
PROTON BEAMS
PROTON RADIOGRAPHY
SAPPHIRE
STRAIN RATE
TARGETS