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LLNL-CONF-400006
Isothermal Martensitic andPressure-Induced (delta) to (alpha)'Phase Transformations in a Pu-Ga Alloy
A. J. Schwartz, M. A. Wall, D. L. Farber, K. T.Moore, K. J. M. Blobaum
December 19, 2007
Plasticity 2008Kona, HI, United StatesJanuary 3, 2008 through January 8, 2008
Disclaimer
This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.
A.J. Schwartz, M.A. Wall, D.L. Farber,K.T. Moore, and K.J.M. Blobaum
LLNL-CONF-400006
Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551This work performed under the auspices of the U.S. Department of Energy byLawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
Presentation Title
Date
Isothermal Martensitic and Pressure-Induced δ to α′ Phase Transformations in a Pu-Ga Alloy
Plasticity 2008January 3, 2008
Kona, Hawaii
Lawrence Livermore National Laboratory
2LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Understanding the phase transformations remainsas one of the significant Pu metallurgical challenges
Equilibrium phase diagram 5 allotropic phase transformations Effects of alloying on phase stability and properties Phase transformations and phase stability
• The δ → α′ isothermal martensitic transformation—Mechanism or mechanisms—Double-C curve kinetics
• The δ → α′ transformation under pressure— Pu-Al— Pu-Ga— Amorphous phase?— Characterization of the recovered sample
3LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
For decades, the “West” accepted that the δ phase wasthermodynamically stable at ambient conditions
Hecker and Timofeeva, LA Science (2000)
US Diagram Russian Diagram
The δ-phase retained to room temperature is metastableTimofeeva (2003) estimated 10,000 years to decompose
Chebotarev, Plutonium and Other Actinides 1975 (1975)Adler, Met Trans (1991)Timofeeva, Aging Studies and Lifetime Extension of Materials (2003)
Ellinger, Land, and Struebing, J. Nuc. Mat. (1964)
Equilibrium phase diagram
4LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
δ (323°C - 468°C)f.c. cubic ( )ρ = 15.9 g/cm3
!
Fm3m
ε (486°C - 640°C)b.c. cubic ( )ρ = 16.5 g/cm3
!
Im3m
δ′ (468°C - 486°C)b.c. tetragonal (I4/mmm)ρ = 16.0 g/cm3
γ (214°C - 323°C)f.c. orthorhombic (Fddd)ρ = 17.1 g/cm3
α (Low temperature - 126°C)monoclinic (P21/m)ρ = 19.8 g/cm3
β (126°C - 214°C)base.c. monoclinic (C2/m)ρ = 17.8 g/cm3
Liquid (640°C +)ρ = 16.5 g/cm3
0 200 400 600 800—2
0
2
4
6
8
Pure Pu L
β
γ
δε
δ′
α
Temperature (°C)
Leng
th c
hang
e (%
)
Hecker, LA Science (2000)
Plutonium undergoes five solid-solid allotropic phasetransformations between the ground state and the liquid
Allotropic phase transformations
10% 3.5% 7%
–0.5%
–3%
5LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Alloying plutonium with Ga retains the fcc δ-phase,reduces volume change, and improves ductility
A few atomic percent Ga make plutonium easier to cast and to shape
Strain (%)0 10 20 30 40
20
40
60
00
100
200
300
400
500
Aluminumδ-Pu
Cast Iron
α-Pu
Stress (KSI)
Stre
ss (M
Pa)
Dilatometry of unalloyed and alloyed Pu Tensile properties of selected materials
0 200 400 600 800—2
0
2
4
6
8
Pure Pu L
Pu 3 at.% Ga
β
γ
δε
δ′
αL
δεδ
εL
Pu 4.5 at.% Ga
Temperature (°C)
Leng
th c
hang
e (%
)
Hecker, LA Science (2000)
Hecker and Stevens, LA Science (2000)
Effect of alloying
6LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Retention of the δ-phase is dependent on the composition,cooling rate, and homogenization treatment
Ga-rich core
Ga-poor edge
Intergranular α Pu
Mitchell et al. Met Trans (2001)
Mitchell et al. Met Trans. (2001)
Mitchell et al. Met Trans. (2001)
Pu - 1 wt.% (3.3 at.%) Ga
Oudot, Ph.D. Thesis (2005)
Before homogenizationAfter homogenization
Ga distribution
Effect of alloying
7LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Upon cooling to sub-ambient temperatures, δ transformsto α′ via an isothermal martensitic transformation
Adler et al. Met Trans. (1988)
Hecker et al. Prog Mat Sci. (2004)
Similar to δ-phase at roomtemperature, α′ is also metastable
-200
-150
-100
-50
0
50
100
150
200
0 0.5 1 1.5 2 2.5 3
Ms (C)Rs (C)
Tem
per
atu
re (
C)
Ga concentration (at.%)
Massalski & Schwartz, JALCOM (2007)
Low-temperature δ → α′ martensitic transformation
8LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
The α′ particles that form from the isothermal martensitictransformation appear as lathes in optical microscopy
The δ → α′ isothermal martensitic transformation goes to ~ 25% completion
Untransformed(δ phase)
Partiallytransformed
(δ + α' phases)
Pu - 2.0 at.% Ga440°C/12 hours -120°C/10 hours
α' particle
Low-temperature δ → α′ martensitic transformation
9LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
α′ → δ reversion has been shown to occur via a burstmartensitic mode
Blobaum et al. Met Trans (2005)Blobaum et al. Met Trans (2005)
Dilatometry traces through the α′ → δreversion exhibit stepsThe derivative (dL/dt) reveals periodicspikes
Differential scanning calorimetry of theα′ → δ reversion shows periodic spikes
δ
Pu - 2.0 at.% Ga
The δ → α′ isothermal martensitic transformation requires nucleation of anew phase, the reverse α′ → δ transformation does not
Low-temperature δ → α′ martensitic transformation
10LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
The amount of the δ → α′ transformation is dependenton details of the thermal cycling
Blobaum et al. Acta Mat (2006)
Conditioning times of ~6 hours arerequired for reproducible amounts oftransformation
αm embryos may be forming as aprecursor to the δ → α + Pu3Ga
These αm embryos initiate α′ onsubsequent cooling
The amount of transformation inPu - 1.8 at.% Ga alloys decreaseswith each thermal cycle
Hecker et al. Prog Mat Sci. (2004)
Pu - 1.8 at.% Ga Pu - 2.0 at.% Ga
Low-temperature δ → α′ martensitic transformation
11LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Orme et al. experimentally determined the kineticsof the δ → α′ isothermal martensitic transformation
Pu - 0.6 at.% Ga Pu - 0.7 at.% Ga
Pu - 1.4 at.% Ga Pu - 1.9 at.% Ga
TTT diagrams of Pu-1.4 & 1.9 at.% Gaalloys show two separate knees
This behavior implies two distinct,thermally activated mechanisms
must exist for this transformation
Orme et al. Pu & Other Actinides (1975)
Low-temperature δ → α′ martensitic transformation
12LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Recent DSC work by Oudot et al. confirms the double-Cbehavior and reveals interesting precursor phenomena
Oudot et al. JALCOM (2007)
We still do not understand the origin of the double-C behavior !
The reversion peak integral (amount ofδ → α′ reversion) reveals two maximaafter 18-hours holds
DSC scans on cooling to isothermalhold temperature reveal three peaks
Oudot et al. JALCOM (2007)
Low-temperature δ → α′ martensitic transformation
13LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
The crystallography of the low-temperature δ → α′transformation has been characterized with TEM
153K, 10hours
δ matrixα’ particles
The orientationrelationship betweenα' and δ is:(111)δ || (020)α'
[-110]δ || [100]α'
α′ particles consist of 2variants rotated 60° around<020>α′
TEM shows (205)α' twinningas a lattice invariantdeformation mode
The α'− δ interface iscomposed of a terrace andledge structure that isfaceted on 111δ
Moore et al. Met. Trans (2007)
Moore et al. Met. Trans (2007)
Zocco et al. Acta Met. (1990)
Low-temperature δ → α′ martensitic transformation
14LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
The large volume difference between δ and α′ isaccommodated by dislocation formation and migration
Increased dislocationdensity at tip of α′ particle~ 1.7 x 1011 /cm2
Moore et al. Met. Trans (2007)
δ
α'
Moore et al. Met Trans. (2007)
Elastic-plastic FEManalysis reveals regionsof compression andtension during reversion
The dislocation density increases in the vicinity of α′ particles
Background dislocationdensity ~ 2.2 x 1010 /cm2
Low-temperature δ → α′ martensitic transformation
15LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
The δ → α′ transformation can also be induced bypressure
Hecker, MRS Bulletin (2001)
Hecker et al. Prog Mat Sci. (2004)
Densities of homogenizedPu-Ga alloys after isostaticpressing to 1 GPa
Pu - 2 at.% Al alloystransform first to β′ then to α′under pressure
Pressure-induced δ → α′ martensitic transformation
16LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
The δ → α′ transformation and reversion characteristicsare a strong function of composition
Under pressure, Pu - Ga alloys transform directly to α′ and undergo either adirect (α′ → δ) or indirect (α′ → β + δ → γ + δ → δ) reversion
Reversion characteristics are similar to those in thermally-inducedtransformations
Hecker et al. Prog Mat Sci. (2004)
Hecker et al. Prog Mat Sci. (2004)
Why do Pu-Al alloys transform through β′whereas Pu-Ga alloys transform directly to α′?
Or do they?
Pressure-induced δ → α′ martensitic transformation
17LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Diamond anvil cell experiments on a Pu - 2 at.% Gaalloy reveal δ → γ′ → α′ transformation sequence
Faure et al. MRS Proceedings (2006)
Faure et al. MRS Proceedings (2006)
Does the time dependence of the α′ volumesuggest Ga hopping to site 8? Sadigh and Wolfer, PRB (2005)
Pu - 2 at.% Ga, Pressurized to 1.76 GPa and held
Pure Pu, 1.76 GPa
Pu - 2 at.% Ga, to 1.76 GPa at ~0.1 GPa/day
Faure et al. MRS Proceedings (2006)
In the DAC, Pu - 2 at. Ga transformsthrough the sequence δ → γ′ → α′
Pressure-induced δ → α′ martensitic transformation
18LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Upon cooling, Harbur reported that a 0.68 at.% Ga alloyhas a density intermediate between δ and α phases
Harbur proposes that the δ phase transforms to α′ + amorphous phase– on cooling low solute alloys– under pressure
Pressure-induced δ → α′ martensitic transformation
Harbur, JALCOM (2007)
After compressing to 1 GPaAlloy % α′ % δ % amorphous1.0 at.% Ga 87 0 131.7 at.% Ga 66 0 342.5 at.% Ga 68 12 20
Harbur, JALCOM (2007)
19LLNL-CONF-400006 AJS: Plasticity 2008
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We are coupling low pressure recovery experiments withTEM to elucidate the mechanism and morphology
2.3 mm diameter specimens are slowly compressed to 1 GPa inthe large volume moissanite anvil cell
Pressure-induced δ → α′ martensitic transformation
20LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National LaboratoryUncompressed alloy, as received microstructure
Optical microscopy of the uncompressed alloy revealsevidence of Pu6Fe and Ga segregation
Pu6Fe
Ga segregation
Pressure-induced δ → α′ martensitic transformation
21LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Optical microscopy images of reference alloy after hydrostatic compression
Optical microscopy of the compressed specimen revealsα′ and former δ phase grain boundaries
Former δ phasegrain boundaries
α′
Optical microscopy does not have the resolution todifferentiate between phases
Pressure-induced δ → α′ martensitic transformation
22LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
X-ray diffraction of the compressed sample revealspeaks from α′ and δ
Pressure-induced δ → α′ martensitic transformation
Our X-ray diffraction does not indicate the presence of anamorphous phase
23LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Preliminary TEM reveals fine-grained α′ and smallamounts of δ – no evidence of an amorphous phase
Pressure-induced δ → α′ transformationAverage α′ grain size ~ 100s nmImplies nucleation dominated mechanism
Low-temperature-induced δ → α′ isothermal martensitic transformationAverage α′ particle size ~ 1000s x 10,000s nmImplies nucleation limited mechanism (strain)
Pressure-induced δ → α′ martensitic transformation
24LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Preliminary TEM reveals fine-grained α′ and smallamounts of δ – no evidence of an amorphous phase
α′
δ
δ
α′
δ phase is observed dispersed between the α′ grainsHigh dislocation densityNo apparent orientation relationship (yet)
Pressure-induced δ → α′ martensitic transformation
25LLNL-CONF-400006 AJS: Plasticity 2008
Lawrence Livermore National Laboratory
Summary Low temperature isothermal δ → α′ transformation
• Nucleation limited• Lath-shaped particles• Intermediate phases possible
Pressure-induced δ → α′ transformation• Nucleation dominated• Very fine grain size• No evidence of the amorphous phase• Intermediate phases likely
β′-phase (C2/m)
Isothermal δ → α′
Pressure-induced δ → α′
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