Mechanism of Proton Irradiation-Induced Creep of

Ultra-Fine Grain Graphite ZXF-5Q

A.A. Campbell & G.S. Was

INGSM-15

September 18, 2014

Research Supported by: US DOE under NERI Contract # FC07-06ID14732 INL under Contract # DE-AC07-

05ID14517

Outline

• Objective• Experimental Methodology• Results• Discussion• Conclusions

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Objective

• Determine the mechanism of proton irradiation-induced creep for an ultra-fine grain graphite

• This work is published in:A.A. Campbell & G.S. Was, “Proton Irradiation-Induced Creep of Ultra-Fine Grain Graphite”, Carbon, 77 (2014) 993-1010.

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Experimental

• POCO grade ZXF-5Q– Particle size < 1 µm– Pore size < 0.3 µm– Density 1.78g/cm3

(80% theoretical density)– Tensile Strength 79MPa– Anisotropy < 1.03 BAF– Young’s Modulus 14.5 GPa– Compressive Strength 175MPa– Thermal Conductivity 70 W/m/K– Green Pet Coke Filler, milled to size, isostatically molded*

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POCO Grade ZXF-5Q Data sheet.

Experimental Methodology

• Irradiation creep experiments – utilize novel system designed to perform proton irradiation-induced creep experiments

• Post-irradiation Analysis– Crystal parameters – Analyze X-Ray Diffraction spectra

with Williamson-Hall methodology

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Irradiation Chamber

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Campbell and Was, Journal of Nuclear Materials, 433 (2013) 86-94.

Irradiation Creep Experimental Conditions

• Applied tensile stress (1000ºC, 1.15x10-6dpa/s)– 5 MPa, 10 MPa, 20 MPa, 40 MPa

• Dose Rate (700ºC, 20MPa)– 2.95x10-7dpa/s to 5.51x10-7dpa/s

• Temperature (20MPa, variable dose rate)– 700ºC, 900ºC, 1000ºC, 1100ºC, 1200ºC

• Two samples used for each experiment, one with stress and one without stress– Residual stress from EDM machining resulted in curvature of the

unstressed sample

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Results

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Irradiation Creep Example Data

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Dose Rate & Temperature Control

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Applied Stress Dependence (1000ºC, 1.15x10-6dpa/s)

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Stress Dependence Comparison

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AGOT H-337 and AXF-8QBGI from Gray, Carbon, 11, (1973) 183 SM1-24 from Oku et al., JNM, 152, (1988) 225IG-110 from Oku et al., JNM, 172, (1990) 77

Dose Rate Dependence(700ºC, 20MPa)

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Dose Rate Comparison

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Veringa from Veringa and Blackstone, Carbon, 14, (1976) 279. SM1-24 from Oku et al., JNM, 152, (1988) 225IG-110 from Oku et al., JNM, 172, (1990) 77

Accumulated Dose Dependence(1000ºC, 1.15x10-6dpa/s, 20MPa)

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Accumulated Dose Comparison

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Neutron Data for H-451 from:Burchell, T.D., JNM, 381, (2008) 46.

Temperature Dependence (20MPa)

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Temperature 700ºC 700ºC 900ºC 1000ºC 1000ºC 1100ºC 1200ºC

Dose Rate 2.95x10-7 4.39x10-7 8.10x10-7 1.06x10-6 1.17x10-6 1.56x10-6 2.04x10-6

𝑘=𝑒(− 𝐸𝑎

𝑘𝐵𝑇 )𝐸𝑎 0.3𝑒𝑉

Temperature Comparison

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Veringa and Dragon from Veringa and Blackstone, Carbon, 14, (1976) 279 Burchell, T.D., JNM, 381, (2008) 46Gray et al., Carbon, 5, (1967) 173 Kelly and Burchell, Carbon, 32, (1994) 119Mitchell et al., Nuc Energy, 41, (2002) 63 SM1-24 from Oku et al., JNM, 152, (1988) 225IG-110 from Oku et al., JNM, 172, (1990) 77 Perks PGA from Perks and Simmons, Carbon, 1, (1964) 441Perks AGOT H-337 and AXF-8QBGI from Perks and Simmons, Carbon, 4, (1966) 85

Temperature Comparison

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Veringa and Dragon from Veringa and Blackstone, Carbon, 14, (1976) 279 SM1-24 from Oku et al., JNM, 152, (1988) 225IG-110 from Oku et al., JNM, 172, (1990) 77

Difference Between Proton and Neutron Results

• Last year I presented these differences• Showed work from Russia that found that the neutron to

gamma flux ratio has significant effect on turn-around dose [1]• Showed work from China that showed exposure to γ-rays at

room temperature increased graphitization [2,3]• I presented a hypothesis that γ-rays are annealing damage as it

is being caused by neutrons in-reactor– Effectively reducing the number of defects available to assist with

driving creep– For example, 900°C proton irradiation wouldn’t experience turn-

around until 21dpa

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[1] Nikolaenko et al., Atomic Energy, 87, (1999) 480.[2] Li, B. et al., Carbon, 60, (2013) 186.[3] Xu, Z. et al., Materials Letters, 63, (2009) 1814.

Creep Mechanism Comparison

• Experimental dependencies to compare with mechanisms:– Linear with Stress– Linear with Dose Rate– Arrhenius with Temperature

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Basal Pinning-Unpinning

• High density of lightly pinned dislocation• Irradiation produces and destroys pinning points

• From definition of mechanism creep rate should be:– Linear with stress– Not effected by dose rate– Increase with temperature

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Stress-Induced Preferred Absorption (SIPA)

• Preferential absorption of defects at dislocations, strain occurs as dislocations climb

– Linearly dependent on stress

– Dose rate dependence arises in Ci and Cv

– Temperature dependence arises in DiCi and DvCv

– Should not be effected by accumulated dose if dislocation density does not change

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Glide with SIPA enhanced climb (PAG)

• Additive to SIPA, but strain arises from dislocation glide rather than climb

– Squared stress dependence

– Dose rate dependence arises in Ci (linear)

– Temperature dependence arises in DiCi (Arrhenius)

– Should not be effected by accumulated dose if dislocation density does not change

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Climb and Glide from Dislocation Bias

• Similar to PAG but interstitials are absorbed at dislocations and vacancies are absorbed at voids

– Squared stress dependence

– Dose rate dependence arises in Ci (linear)

– Temperature dependence arises in DiCi (Arrhenius)

– Should not be effected by accumulated dose if dislocation density does not change

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Creep Rate Comparison

Experimental Creep Rate Dependencies 

Basal Pinning-

UnpinningSIPA PAG CGDB

Linear with Stress Agree Agree Disagree DisagreeLinear with Dose Rate Disagree Agree Agree Agree

Arrhenius with Temperature Agree Agree Agree AgreeConstant with Dose Disagree Agree Agree Agree

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Mechanism Creep Rate Dependencies

• SIPA – only mechanism that had significant agreement of experimentally-determined and mechanism-predicted creep rate dependencies

• If creep is driven by a mechanism dependent on defects for creep to occur (climb driven) then the effect of applied stress should be observed in the microstructure changes

Dose and Stress Affects on Crystallite Dimensions

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Dose and Stress Affects on Lattice Parameters

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Sources of Lattice Spacing Changes

• C-spacing – single interstitials, interstitial clusters, interstitial loops– Single interstitials not stable at these temperatures– Loops observed only cause increase around the loop edge [1]– Primary source must have [1]:

• Stable configuration, low diffusibility, no tendency to grow, subject to radiation annealing• Six atom hexagonal clusters – where density is dependent on dose rate and temperature

• Interatomic spacing – Poisson’s ratio effect, vacancies– Poisson’s does not account for all the contraction– Single and di-vacancies cause rearrangement of covalent bonds, but above 500ºC

vacancies are mobile– Vacancy lines form at high dose (low temperature) and with onset of irradiation at

high temperature• Average number of vacancies in a line increases with temperature• Atoms around uncollapsed lines will have rearranged covalent bonds [2]• Concentration of uncollapsed lines will saturate with dose, and saturation density should

decrease for higher temperatures [3]

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[1] Reynolds and Thrower, Philosophical Magazine, 12, (1965) 573-593. [2] Kelly et al., Journal of Nuclear Materials, 20, (1966) 195-209. [3] Henson et al., Carbon, 6, (1968) 789-806.

Sources of Crystal Parameter Changes

• C-spacing variation decreases with increasing dose– Seems counter intuitive– In immediate vicinity of

cluster, increase is greater than average [1]

– Distribution of clusters is fairly uniform [1]

• Crystallite size – measure of size of regions in graphite with perfect structure

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[1] Bacon and Warren, Acta Crystallographica, 9, (1956) 1029-1035.

Stress Effects on Crystal Parameters

• Samples only received a dose of 0.25dpa– Below dose to reach the saturation values

• In neutron irradiations, the lattice parameter change of a crept sample is less than uncrept samples [1]– Smaller interstitial concentrations due to stress-enhanced

recombination or stress-enhanced interstitial mobility– Concentration remains constant but cluster density

decreases due to clusters being swept together by gliding dislocations

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[1] Richards and Kellett, Journal of Nuclear Materials, 25, (1968) 45-57.

Discussion

• Arrhenius temperature dependence gives an irradiation creep activation energy of ~0.3eV– Same order of magnitude of interstitial migration energy

• Results from microstructure analysis show the crept samples had less lattice and crystallite change than the uncrept sample– Suggests lower interstitial concentration in the crept samples due to

interstitials driving creep

• Best-agreement between experimental results and creep mechanisms suggests a mechanism similar to SIPA is the controlling mechanism

• Results from Karthik showed positive climb of partial basal dislocations– Climb occurs when interstitials are absorbed at the edge of the defect

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Radiation Damage in Graphite

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Karthik et al., Journal of Nuclear Materials, 412 (2011) 321–326"Carbon Materials for Advanced Technologies", ed. Timothy D. Burchell, 1999.

Is there a Glide Path for Dislocations?

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Images courtesy of Helen Freeman – work to be published

Proposed Mechanism

• Stress-Induced Climb of Basal Plane Dislocations– Dislocation climb driven mechanism, rather than glide

driven (agreement with linear stress dependence)– ~0.3eV activation energy suggests interstitial migration is

the rate-limiting mechanism– In-situ TEM from Karthik observed basal plane climb– Same behavior as SIPA, but ignore term due to vacancy

absorption at dislocations

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Conclusions

• Irradiation-induced creep rate dependencies found to be:– Linear dependence on applied stress and dose rate– Arrhenius dependence on irradiation temperature (approximately

linear in this temperature range)– No dependence on accumulated dose

• Creep rate dependencies on experimental conditions mostly agree with the dependencies observed for neutron irradiation creep of graphite

• Proposed mechanism is modified SIPA to only depend on interstitial absorption at dislocations

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Mechanism of Proton Irradiation-Induced Creep of

Ultra-Fine Grain Graphite ZXF-5Q

A.A. Campbell & G.S. Was

INGSM-15

September 18, 2014

Research Supported by: US DOE under NERI Contract # FC07-06ID14732 INL under Contract # DE-AC07-

05ID14517