Morgan tms ods 2015 02-29 v4.4 dist

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Structure and Thermokinetics of Y-Ti-O Precipitates in Nanostructured

Ferritic AlloysDane Morgan

University of Wisconsin, Madison

Leland Barnard Knolls Atomic Power Laboratory

Nicholas Cunningham, G.R. OdetteUniversity of California, Santa Barbara

Samrat Choudhury, Blas UberuagaLos Alamos National Laboratory

March 18, 2015TMS

Orlando, Florida

The Idea Behind Nanostructured Ferritic Alloys

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Steel (Fe, C, W, …)

Oxide (Y2O3, TiO2, …)

Mix+Consolidate (Mechanical ball

milling, HIP)

Steel with fine grains, high density of nanoscale (1-3nm) stable precipitates• Enhances mechanical properties• Enhances radiation resistance

• Called Nanostructured Ferritic Alloys (NFAs) or Oxide Dispersion Strengthened (ODS) Alloys

• Of interest for applications in next generation nuclear reactors which include high temperature, high radiation dose conditions

• Practical and fundamental science issues related to nature and evolution of nanoscale precipitates

Outline

• Introduction to Nanostructured Ferritic Alloys

• Precipitate “bulk” structure [1]

• Precipitate interfacial structure [2]

• Thermal Aging [3]

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[1] L. Barnard, G. R. Odette, I. Szlufarska, and D. Morgan, An ab initio study of Ti-Y-O nanocluster energetics in nanostructured ferritic alloys, Acta Materialia 60, p. 935-947 (2012).

[2] S. Choudhury, D. Morgan, and B. P. Uberuaga, Massive Interfacial Reconstruction at Misfit Dislocations in Metal/Oxide Interfaces, Scientific Reports 4, p. 8 (2014)

[3] L. Barnard, N. Cunningham, G. R. Odette, I. Szlufarska, and D. Morgan, Thermodynamic and kinetic modeling of oxide precipitation in nanostructured ferritic alloys, To be published in Acta Materialia (2015).

Outline





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Nanostructured Ferritic Alloy Mechanical Properties

• Excellent tensile, creep, fatigue strength

• Good fracture toughness

• Stable to high temperatures

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G.R. Odette, et al., Annu Rev Mater Res ‘08; G.R. Odette, JOM ‘14

Nanostructured Ferritic Alloy Mechanical Properties

• Excellent tensile, creep, fatigue strength

• Good fracture toughness

• Stable to high temperatures

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Klueh, et al., JNM, ‘02

800°C, 138 MPa

Nanostructured Ferritic Alloy Radiation Resistance

High sink strength reduces

• He bubble/Void, loop growth

• Radiation embrittlement

• Swelling7

G.R. Odette, JOM ‘14

Thin lines – unirradiatedThick lines - irradiated

Nanostructured Ferritic Alloy Radiation Resistance

High sink strength reduces

• He bubble/Void, loop growth

• Radiation embrittlement

• Swelling8

G.R. Odette, JOM ‘14

Open Questions about Nanostructured Ferritic Alloys

• What alloying elements and heat treatments are needed for optimum nanocluster density/size distribution?

• What is the thermal and radiation stability of nanoclusters?

• What is the matrix-nanocluster interface structure and it segregation tendencies (e.g. He trapping)?

• What are the nanocluster-dislocation interactions and their effects on mechanical properties?

A detailed, atomistic-level understanding of the Y-Ti-O precipitates and their energetics is a crucial step toward addressing all of these concerns.

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Todays Key Questions

• What “bulk” structures of oxide precipitates form in Fe at ~1nm – coherent vs. incoherent?

• What interfacial structures occur at the oxide-metal interface?

• What controls the thermal stability of the precipitates?

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Outline


• Precipitate “bulk’ structure [1]



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Y2TiO5+Y2Ti2O7

Y2O3

Y2Ti2O7+TiO2

Y2O3+Y2TiO5

FeO↔Fe+1/2O2

Cr2O3↔2Cr+3/2O2

TiO2↔Ti+O2

The Y-Ti-O Phase Diagram

The Nature of the Nanoprecipitates

• Typical values: Number density=1023-1024/m3, Volume fraction=0.5-1%, Diameter=1.5-3.0nm

• Explored with SANS/SAXS, Atom Probe, TEM, Ab Initio Tools

• Generally pyrochlore Y2Ti2O7

(227) but significant uncertainty due to conditions and interpretation challenges (Y2TiO5, rocksalt,

amorphous) 13

TEM showing lattice spacings of Y2Ti2O7

J. Ribis, R. de Carlan , Acta Mat, ‘12

Fe–14Cr–1W–0.3Ti–0.3Y2O3 wt.%






amorphous) 14

A. Hirata, Nat Mat, ‘11

14YWT (Fe-14Cr-3W-0.4Ti-0.25-Y2O3 wt.%)

Real space STEM showing NaCl structures






amorphous) 15

G.R. Odette and D.T. Hoelzer, JOM ’10G.R. Odette, JOM ‘14

Atom Probe: Ti/Y≈1.5-4, O/(Ti+Y)<1Y2Ti2O7: Ti/Y=1, O/(Ti+Y)=7/4>1

MA957 (Fe–14Cr–0.3Mo–1Ti–0.3Y–0.2O–0.03C wt.%)Ti+Y >3% isocomposition contours

Atomistic models of coherent structures show unusual chemistry – off stoichiometry, high vacancy stability






amorphous) 16

Posselt, et al. MSMSE ‘14


Why so much uncertainty?

• Complex heterogeneous non-equilibrium system with many possible behaviors (e.g., multiple phases can be present, coherent vs. incoherent)

• Systems may be quite different: stoichiometry, mixing, consolidation differences

• Data interpretation challenging (e.g. atom probe stoichiometry)

• Sampling different precipitates (e.g., with TEM)

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Need to guidance from Y-Ti-O precipitate structure-stability relationships

Density Functional Theory Calculation of Y-Ti-O Clustering Energetics

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• How do we search for stable clusters, considering• Structure• Coherence• Stoichiometry

• Different approaches:• Clusters based around strongly bound O-Vac pairs [1].• Clusters that minimize interaction energies [2].• Clusters that match bulk oxide stoichiometry [3].• All assume clusters restricted to the Fe lattice.

• Here, we will investigate including some clusters not restricted to the Fe lattice.

[1] C.L. Fu, M. Krcmar, G. S. Painter, and X. Q. Chen, Physical Review Letters 99 (2007).[2] Y. Jiang, J. R. Smith, and G. R. Odette, Physical Review B 79 (2009); A. Gopejenko, Y. Zhukovskii, P. Vladimirov, E. Kotomin, A. Moslang, and X. Q. Chen, Journal of Nuclear Materials 406 (2010); M Posselt, D Murali, and B K Panigrahi, MSMSE 22 (2014).[3] C. Hin, B. D. Wirth, and J. B. Neaton, Physical review B 80 (2009).

Cluster Searching Methods

• On-lattice clusters:• Clusters restricted to the bcc Fe lattice

• Structure matched clusters:• Clusters guided by the structure of known bulk oxides (e.g,

rutile TiO2 and bixbyite Y2O3).

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Methods: On Lattice Clusters

= Fe or Ti/Y

= O

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• Metal atoms restricted to bcc Fe lattice

• O atoms in interstitial stites

[1] C.L. Fu, M. Krcmar, G. S. Painter, and X. Q. Chen, Physical Review Letters 99 (2007).

[2] Y. Jiang, J. R. Smith, and G. R. Odette, Physical Review B 79 (2009); A. Gopejenko, Y. Zhukovskii, P. Vladimirov, E. Kotomin, A. Moslang, and X. Q. Chen, Journal of Nuclear Materials 406 (2010); M Posselt, D Murali, and B K Panigrahi, MSMSE 22 (2014)

[3] C. Hin, B. D. Wirth, and J. B. Neaton, Physical review B 80 (2009).

Methods: Structure Matched Clusters

• Some Ti, Y atoms mapped onto Fe lattice sites

• O atoms placed relative to Ti, Y atoms according to oxide structure.

• Fe atoms impinging closely upon Ti,Y,O atoms removed.

• Ti-O/Y-O matched to rutile TiO2 / bixbyite Y2O3 21

+z

Methods: Formation Energy Calculation

• Reference states:• Pure Fe.• Isolated Ti, Y on Fe substitutional site.• Isolated O on octahedral Fe interstitial site.

• Calculations performed using Density Functional Theory (VASP, PAW, GGA) according to methods developed in [1].

[1] Y. Jiang, J. R. Smith, and G. R. Odette, Physical Review B 79 (2009).

x +y-=

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Ti-O Cluster Formation Energies

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24


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• Given a fixed number of Ti atoms but allowing any number of O atoms, what sort of Ti-O cluster will be most stable?

• Predicated on relative diffusivities:• At 1150 oC:• Fe: 1.1E-20 m2/sec• Y: 1.5E-23 m2/sec• Ti: 1.7E-20 m2/sec• O: 1.0E-14 m2/sec


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HypostoichiometricM Terminated

StoichiometricMixed Termination

HypertoichiometricO Termination


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HypostoichiometricTi Terminated



Increasing O

Y-O Cluster Formation Energies

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HypostoichiometricTi Terminated



Increasing O

Y-Ti-O Clusters

• To assess whether these trends continue in the full Y-Ti-O system, we will perform a much smaller suite of calculations on Y-Ti-O on-lattice and structure matched clusters.

• We will restrict our search to clusters with Y:Ti ratio of 1:1, matching the pyrochlore oxide Y2Ti2O7.

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Ti-Y-O Cluster Formation Energies

HypostoichiometricM Terminated



Increasing O

[1] Y. Jiang, J. R. Smith, and G. R. Odette, Physical Review B 79 (2009).[2] D. Murali et al. Journal of Nuclear Materials 113 (2010). 30

• Again, most stable clusters are structure-matched, hyperstoichiometric

Conclusion - Clusters that Resemble Bulk Oxide are Most Stable

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Bulk oxide Embedded Cluster

Ti-O(Rutile TiO2)

Y-O(Bixbyite Y2O3)

Ti-Y-O(Pyrochlore Y2Ti2O7)

Outline





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Atomic Structure of the Y2O3/Fe Interface

{010}FeAl|| {011}YO, <100>YO|| <001>FeAl

Inks

on e

t al.

MR

S P

roc

,199

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Relaxed Structure of the bi-layer of metal and oxide

Iron Yttrium Oxygen

Fe

Y2O3

Orientation Relationship between Y2O3/Fe

Misfit dislocation at the interface results in excessive

Fe/O ratio

Local structure of misfit dislocation in metal/oxide is a

f (strain, chemistry)

Fe bcc {010} planeY2O3 {011} plane

Restoring Chemical Balance at Dislocation (Fe/O > 1)

Taking out Y

Interfacial Fe Vacancy

Taking out Fe

Interfacial Y Vacancy

Inserting Oxygen

Oxygen in Interfacial Fe layer

Iron

Yttrium

Oxygen

Interstitial Oxygen

Reducing Conditions

Oxidizing Conditions

Change in Energy of the System with Point DefectsFe Vacancies

Most of the vacancies/oxygen interstitials enter at the dislocation

Interstitial Oxygen

Change in Energy of the System with Point DefectsFe Vacancies Interstitial Oxygen + Fe Vacancies

Under More Reducing Conditions: Fe vacancies

Under More Oxidizing Conditions (~Cr/Cr2O3): Interstitial Oxygen + Fe Vacancies

Conclusions - Fe/Y2O3 Interfaces are Highly Defected

• Fe/Y2O3 semi-coherent interface shows highly defected structure

• Undefected Fe/O=1.5, Equilibrium Fe/O~0.5 (~50% Fe vac, ~50% extra O interstitials at PO2=Cr/Cr2O3)

• Will impact interface segregation, stability.

Outline





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Thermal Aging Nanostructured Ferritic Alloy

• Long-term stability of nanoprecipitates at elevated temperature (potentially under irradiation) is critical for sustained performance.

• Thermal aging experiments show excellent stability.

• Goal is to model these experiments to develop molecular scale understanding of mechanisms controlling stability of nanoprecipitates.

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Experimental Thermal Aging Data from Odette Group (UCSB)MA957 (Fe–14Cr–0.3Mo–1Ti–0.3Y–0.2O–0.03C wt.%)

40M. Alinger, PhD Thesis, University of California Santa Barbara, 2004. N. Cunningham, et al, Mat Sci & Eng A (2014)N. Cunningham, et al., Fusion Materials Report June 30, 2012, DOE/ER-0313/52

-4 -3 -2 -1 0 1 2 3 4 5 61

2

3

4

51223K Cunningham1273K Cunningham1423K Alinger1473K Alinger1523K Alinger1573K Alinger

LOG Aging Time (hr)

Mea

n R

adiu

s (n

m)

Fits to classical coarsening models suggest pipe diffusion

Chemical rate theory/mass action kinetics

Method – Cluster Dynamics (CD)

• Cluster growth/shrink rates determined from diffusion coefficients, thermodynamics, and interfacial energy.

• Solve coupled ODEs to obtain the number of clusters at each size. Generalized for standard and pipe diffusion.

Time evolution

V. Slezov, Kinetics of First-Order Phase Transitions, 1st ed., Wiley-VCH, 2009.

Parameterizing Cluster Dynamics Model

• Fe-Y-Ti-O Thermodynamics– Y-Ti-O Bulk + Impurity (CALPHAD)

– Interfacial (Fitting)

– PO2 (Fitting)

– Y–dislocation binding (ab initio)

• Fe-Y-Ti-O Kinetics– Bulk impurity diffusion (experiments, ab initio (Y in

Fe))

– Dislocation impurity diffusion (empirical correlation)

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Parameterizing Cluster Dynamics Model

• Fe-Y-Ti-O Thermodynamics– Y-Ti-O Bulk + Impurity (CALPHAD)

– Interfacial (Fitting)

– PO2 (Fitting)

– Y–dislocation binding (ab initio)

• Fe-Y-Ti-O Kinetics– Bulk impurity diffusion (experiments, ab initio (Y in

Fe))

– Dislocation impurity diffusion (empirical correlation)

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Parameterizing Cluster Dynamics Model: Interfacial Energy

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TiAx 00

Simple model to get one fitting parameter s0. Set by bare s(TiO2)-s(Y2O3)

0.00 0.33 0.67 1.000.0

1.0

2.0

3.0 Y2O3 Surface EnergyTiO2 Surface EnergyTiO2/liquid Fe Interface EnergyPipe Diffusion Model Best FitStandard Model Best Fit

Ti fraction of metal atoms in oxide

Inte

rfac

ial

En

ergy

(J/

m2)

Close agreement with bare and liquid Fe interfacial energies validates approach

Parameterizing Cluster Dynamics Model: PO2

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PO2 fit to give best agreement to coarsening data

1200 1300 1400 1500 1600-30

-25

-20

-15

-10Pipe Diffusion Best Fit

Standard Model Best Fit

Cr/Cr2O3 Equillibrium

Ti/TiO2 Equilibrium

Temperature (K)

LO

G P

O2

• Close agreement with Cr/Cr2O3 equilibrium validates approach

• Suggests no exception PO2 in NFA steels

Parameterizing Cluster Dynamics Model: Y–dislocation binding (ab initio)

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Calculate dislocation binding energy for multiple elements

• Good agreement with experiment, elasticity for C, N, O

• Y exceptionally stable – drives Y solubility for pipe diffusion!

C N O Y

-3

-2

-1

0

Elasticity TheoryAb InitioExperiment

Bind

ing

Ener

gy (e

V)

Cluster Dynamics Modeling of Thermal Aging

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-4 -3 -2 -1 0 1 2 3 4 5 61

2

3

4

5 1223K Cun-ningham1273K Cun-ningham1423K Alinger1473K Alinger1523K Alinger1573K Alinger

LOG Aging Time (hr)

Mea

n R

adiu

s (n

m)

1000 1050 1100 1150 1200 1250 1300 1350 14000.0

0.5

1.0

1.5

2.0

2.550 years80 years

Temperature (K)

Ch

ange

in m

ean

rad

ius

(nm

)

Predictions of Coarsening Over Reactor Lifetimes

Excellent stability up to over 1,100K

Conclusions – Successful Y-Ti-O Nanocluster Coarsening

• Confirms results of reduced order fitting from Odette et al that process is pipe diffusion

• Predicts long term stability of >100 years at >1,100K.

• Suggests PO2 may be controlled by Cr/Cr2O3 in Nanostructured Ferritic Alloys with Cr

• Provides useful molecular scale parameters (interfacial energies, Y diffusivity, …) for models of processing and thermal/irradiation stability

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-4 -2 0 2 4 61

2

3

4

5

1223K Cunningham 1273K Cunningham

1423K Alinger 1473K Alinger

1523K Alinger 1573K Alinger

Pipe Model Standard Model

LOG Aging Time (hr)

Mea

n R

adiu

s (n

m)

Summary Conclusions on Y-Ti-O Precipitates in Nanostructured Ferritic Alloys

• Nanoprecipitates are bulk-like structures down to very small sizes – remaining on bcc lattice is higher in energy

• Larger particle semi-coherent interfaces create complex defect structure to maintain Fe/O balance

• Molecular understanding of coarsening is available– Confirms pipe diffusion– Shows exceptional stability (>100 years at

>1100K)– Foundation for composition, processing,

irradiation modeling50

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http://matmodel.engr.wisc.edu/

COMPUTATIONAL MATERIALS GROUP

Faculty* Izabela Szlufarska * Dane Morgan

Postdocs* Guangfu Luo * Georgios Bokas* Henry Wu * Jia-Hong Ke* Mahmood Mamivand * Min Yu* Wei Xie * Yueh-Lin Lee

Graduate Students

* Amy Kaczmarowski * Ao Li* Austin Way * Benjamin Afflerbach* Cheng Liu * Chaiyapat Tangpatjaroen* Franklin Hobbs * Hao Jiang * Huibin Ke * Hyunseok Ko* James Gilbert * Jie Feng* Kai Huang * Kumaresh Murugan* Lei Zhao * Mehrdad Arjmand* Ryan Jacobs * Shenzen Xu* Tam Mayeshiba * Xing Wang

* Yipeng Cao * Zhewen Song

* Zhizhang Shen

Acknowledgements

U.S. DEPARTMENT OF ENERGYRickover Fellowship Program In Nuclear Engineering

DMR MMN (110564)

10-888

Computing time provided by NSF TG-DMR110074 and NSF TG-DMR090023, NSF grant number OCI-1053575

Funding/Resources Acknowledgements

Thank You for Your Attention

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