Cerium: The western perspective

Transcription

1 Cerium: The western perspective F. J. Cherne (DE-9) Los Alamos National Laboratory Collaborators Include: P. A. Rigg, B. J. Jensen, W. W. Anderson, and D. B. Hayes (DE-9, LANL)

2 Introduction A grand challenge in dynamic materials research is to obtain validated multi-phase Equations-of-State (EOS) for metals Cerium is a good candidate for this challenge because of its complex dynamic response Because of it s low pressure large volume phase change we expect that cerium should melt at a very low stress This needs to be experimentally verified If true, cerium will be an ideal metal in which to study the dynamic melting process

3 Cerium the complex material: What we know and don t know. Four solid phases known at zero pressure, with at least three more phases at high pressure. Only pure element with a solid-solid critical point Complicated f-electron behavior. γ-phase has negative dk/dp as it approaches the low pressure phase transition (γ α). Phase diagram uncertain above 3 GPa. Slope (and sign) of α-α phase boundary uncertain. Little is known about the spallation behavior of pure cerium metal. Anomalous melting at low pressure though dynamic melt boundary uncertain LANL data (Marsh compendium) suggests ~120 kbar Data from Zhernokletov, et. al. suggests that the melt transition is at pressures greater than 200 kbar Objective of current work is to use Photon doppler velocimetry (PDV) / Velocity Interferometer for Any Reflector (V ISAR) to obtain spallation characteristics, Hugoniot measurements, and C L (Lagrangian sound speed at pressure) determination Zhao & Holzapfel; J. Alloys and Compounds (1997)

4 Samples of cerium used for this work We had two sources of our cerium metal Commercially pure % from Alfa-Aesar no additional heat treatment performed (2 samples) Pure % with special processing to obtain fine grain material (7 samples) For most of the spallation experiments the nominal thickness was 2 mm with one exception being 5 mm. Cerium oxidizes readily in air and thus for VISAR measurements the cerium had to be polished just before the shot. Cerium is a pyrophoric material which makes it difficult to recover for post shot analysis.

5 First results from Ce showing large volume phase transition Isentropic (Z) Shock (Gas Gun) Ce (82μm) Drive 0.6 Ce (2mm) 0.20 Particle Velocity (mm/μs) Very 0.05 Very large large volume volume change change phase phase transition transition mm thick sample

6 Cerium data is now routinely being obtained Ce spall experiments have been done below and above the phase transition overdriven stress Reverse impact experiments are beginning; may provide more details on how phase change happens Δu p shows reduced strength on release for higher stress Particle Velocity (mm/μs) Brittle Fracture? Ductile Fracture? Rarefaction shock shows Isostructural phase change Is reversible!

7 Δu p indicates that processing may play a role in the spallation behavior of cerium Blue dots are commercially available % pure cerium Red squares are specially processed % pure cerium in order to make fine grained samples Ductile? Brittle?

8 Experimental configuration for sound speed measurements in cerium Multi-slug experiments performed using PDV and VISAR to obtain shock velocity, wave profiles including release, and sound speeds at pressure 4 cerium samples used per target backed by LiF windows 8 PDV probes used to measure shock velocity and wave profiles 4 VISAR probes used Experimental Configuration (only 2 samples shown) PDV probes VISAR probe LiF window VISAR probe PDV probes Copper impactor Ce Copper base plate Copper base plate, Ce, and LiF Completed Target in prep. room

9 VISAR Data on cerium Four VISAR probes were used to obtain the wave profile (ns resolution) at the cerium/lif interface Time between shock arrival and release for multiple sample thicknesses (overtake method) provides a measure of the sound speed at pressure Sound speed analysis underway Calculations using a 1-D code underway PDV and VISAR (white trace) comparison VISAR profiles for Exp 2 (Vp = 0.92 km/s) VISAR profiles for Exp 3 (Vp = km/s) VISAR profiles for Exp 4 (Vp = km/s) Particle Velocity (km/s) Thin sample After Overtake Thick Sample Particle Velocity (km/s) Near overtake Particle Velocity (km/s)

10 Overtake method used to determine sound speeds Sound speeds are more sensitive to phase transitions VISAR profiles for 4 different Ce thicknesses Exp #4 (Vp = km/s) Overtake method utilizes multiple sample thicknesses to monitor release wave position with respect to shock wave Plot of sample thickness versus peak state duration provides the overtake thickness Particle Velocity (km/s) Δt Monte-Carlo error analysis used to calculate uncertainty in overtake distance Ce thickness vs. time difference (error bars not shown) Sound speed (C L ) is related to the overtake distance (analysis in progress) Ce Thickness (mm) Δt (μs)

11 Us-Up data obtained for cerium (using PDV) Us-Up data obtained from PDV and VISAR shock velocity measurements Additional data obtained from LANL and Russian experiments (recent) Data shows two linear trends with a kink at approximately 131 kbar indicating the melt transition Monte-Carlo error analysis reveals a melt transition stress of 131 ± 2 kbar Melt transition is higher than that estimated from LANL compendium (120 Kbar) and lower than the Russian estimate (> 200 Kbar) Us-Up data for shocked cerium (LANL and Russian) Us (km/s) Melt Transition Melt Transition Sound speed data (analysis in progress) (error bars not shown) Up (km/s) Long. Stress Histogram generated at Us-Up intersection point

12 A preliminary multi-phase EOS for Ce has been constructed K T0 n C V ργ P 0 V 0 T E 0 S 0 (dyn/cm 2 ) (erg/gk) (g/cm 3 ) (dyn/cm 2 ) (cm 2 /g) (K) (erg/g) (erg/gk) γ K T Polynomial* e e6 0 α 1.641e e e e e5 α 4.766e e e e e5 This EOS has γ, α, and a third phase. Includes anomalous compressibility in the γ phase Does a good job Assume form of the Helmholtz Potential 1 n KV T o V V o F ( TV, ) = F S ( T T ) + P ( V V ) + o o o o o 1 ( n 1) 1 n ( n 1 ) V o V o C ( T T) ( ) V V V T To + γ + V o o log + 1 T Second derivatives are expressed To completely define as EOS V n o K = T K T O V γ = V C = v constant constant = 3R Initial state PVT,,, E, S o o o o o -and- Parameters: K, n, C, γ T o V V

13 Summary New dynamic data is being obtained on cerium metal, and is of unprecedented quality using both PDV and VISAR to obtain accurate shock velocities and wave profiles Data obtained for cerium better constrains Hugoniot - dynamic melt transition occurs at 131 kbar Photo (right) shows multi-slug cerium experiment in progress on the LANL largebore gun The data obtained has been used to constrain a new multi-phase EOS yet more work is needed to include other phases, using the high stress hugoniot and sound speed data currently being obtained. Recovery experiments involving cerium should be performed to examine the spallation mechanisms LANL Large-Bore Powder Gun at TA (cerium experiment) Gun Breech Projectile impacting 55 gallon water drum