# Presentation: 085542

## Basic research needsmaterials under extreme conditions

**Provide a basic understanding of materials response to extreme conditions (irradiation) environments**
**dpa production, “freely migrating defects,” defect evolution, alloy stability, mechanical properties, etc.**
**Enable materials design (new concepts, new materials)**
**Fundamental challenge: What makes a material radiation resistant?**
**Development of predictive computational models**
**multiscale models**
**critical experiments**
**validation**
**Interplay between experiment and modeling**
**models choose and help analyze experiments while critical and validation experiments should make modeling practical**

- Bob Averback
- University of Illinois at Urbana-Champaign

## Materials designexplore new materials and concepts

**an example:**
**Nanostructures: it has long been recognized that a high density of sinks would suppress radiation damage:**
**Encouraging work:  ****“12YWT” and 14YWT”.** Hoelzer et al. (ORNL)
- 		**(mechanically alloyed ODS ferritic alloys)**
_**critical questions**_
**Are nanostructured alloys stable under irradiation?**
**High temperature?**
**What determines this?**

## Self-organization in Driven Systems

**Bellon and Averback (1995)**
**Ball milling**

**Enrique and Bellon (2000)**
**Radiation effects**

**Largest length scale ****~ ********R**

**Competition between ballistic and diffusive mixing:**

**Steady states**

**Increasing ******

## Experiments using MeV Kr ions

**Cu – 15%Co**

**Cu-15%Ag**

**Ag-15%Co**

**Experiments using MeV Kr ions**

**Krasnochtchekov,Chee,Averback and Bellon (2005)**

**Notice that dynamic phase boundaries can be shifted by choice of solute**
**Can alloys be engineered (tailored) in this way?**
**Are the MA/ODS ferritic alloys an example?**

_**Need to know**_
**Mixing of oxide dispersoids in metals, diffusive properties? Structure of “nanoprecipitates”**
**Effects of interstitials on precipitation in metals**

## Glasses

**Good corrosion resistance, encapsulation of wastes**
- Interesting property:
- 	Radiation induced viscosity seems the same in all glasses 	measured, and nearly independent of temperature

**Similar ******** although different density, thermal diffusivity, T****g****, median recoil energy ...**

**Snoeks E. et.al. , J App. Phys. 78 4723**

| Sample | T(1/2) [keV] | E [MeV] | Ion | H 
(pa dpa)-1 |
| --- | --- | --- | --- | --- |
| Ni78B14Si8 | 0.4 | 7 | Protons | 610-9 |
| ZrCuAl | 20 | 800 | Kr | 310-9 |
| SiO2 | 30 | 0.27-4 | Xe,Ne,Er | 2.810-9 |
| Si | 40 | 2.0 | Xe | 110-9 |
| MD simulation (Mayr, Averback et al PRL 2003) |  |  |  |  |
| CuTi | 100-10 keV |  | Self-ion | 3 x 10-9 |
| CuTi | Defect injection |  |  | 3 x 10-9 |

**Viscosity scales inversely with nuclear energy loss (proportional to number of defects)**

**Are there other new materials of possible interest?**

## Need for a Generic Radiation Damage Code

**For example**

**Materials, operating conditions, choice of fuels **_**all to be made**_

**From Zinkle?**

## What should we hope to get from a radiation damage code

**It is unreasonable to think that any code will provide the microstructural evolution of a complex alloy under prolonged irradiation starting from scratch****.**
**More reasonably, the code should extrapolate the microstrucutural evolution of an alloy from a wide set of data to different primary recoil spectra, fluxes, times, and temperature.**
**Requires multiscale modeling**
**Requires critical experiments ****{input: concepts and data}**
**Requires validation: ****{output}**

## What role do experiments play in modern radiation damage

**1987 ****Fusion workshop San Diego: ****with Roy Benedek, Tomas Diaz de la Rubia, Wayne King**

**experiment**

**theory and simulation**

**Need for critical experiments**

**2006**

**Many unexplained experiments**

## An example: A quantitative phase field model for irradiated materialsBadillo, Krasnochtchekov, Bellon, Averback: UIUCBulatov and Gilmer: LLNL

**Current PFMs for solid-solid transformations, provide no information on the absolute time scale and length scale of the simulations, ****they are not predictive****.**
**This limitation is a consequence of the phenomenological approach used to derive the free energy of the alloy and the mobility of the various field variables.**
**New approach employs a**** ****microscopic model with a clear physical input for atom-atom interactions and for atom jumps****. **
**The relevant variables and quantities of the PFM are then obtained by coarse graining**

## Schematic of approach

**Mean field**

**Vacancy loop**

**Interstitial loop or cluster**

**Requirements:**
**Primary state of damage**
**Freely migrating defects**
**Defect interactions (including He)**
**Diffusion matrix in concentrated alloy**
**Elastic strain fields**
**Dislocations (overlay DD code)**
**Methods:**
**Atomistic modeling for local events**
**or**
**Experimental input at mesoscale**

_**Critical issues:**_
**Can this framework be implemented?**
**Can it work for complex alloys?**

**50 nm**

## Defect production-effects of recoil spectrum 

**Do we have any direct information on defect clustering in cascades?**
**Not much**** **

**RSA and TDR (1998)**

**Malerba (2006)**
**(Bacon, Stoller, etc.)**

- Iron

-  **It seems pretty good, and we now understand why – it is an insensitive property**
** ****what about PKA effects on clustering? **
**Is MD presently sufficient?**
**Potentials**
**Electron phonon coupling**
**Unresolved since Flynn and Averback (1988); Caro and Victoria (1989)**

## But we have the means

**But we have the means**

**Pure Ni**

**Ni-1% Si**

**Advances since 1989:**** **
**can fully simulate DXS from any defect cluster**
**	****Since difficult to obtain bulk single crystals**
**Grazing incidence at synchrotrons**
**Micro x-ray beams – just need large grains**
**Diffuse electron scattering in TEM see local regions**
**Kirk and Jenkins Phil Mag A (2005)**
**For chemistry: ****	****AP-FIM “LEAP”, analytical TEM, SANS and SAXS**

**Size distributions of defect clusters****after neutron irradiation Ehrhart and Averback (1989)**

_**Critical issues**_
**Need for user centers:**
**Methods, sample fabrication**

## Surface effectsan example of interplay between experiment and simulation

**Surface effects****an example of interplay between experiment and simulation**

**Nordlund,Averback..**
**1999 (Nature)**

**Stoller 2002 (JNM)**

**50 keV Cu -> Cu**

**10 keV in Fe**

**1970’s Beautiful FIM results of Seidman et al in W are mostly surface effects – tie experiment to simulation**

## Migration of clusters

- Simulations:  cluster glide in one dimensions: Osetsky and Bacon
- Experiments:

## Freely migrating defects

**Old Argonne experiments: Rehn, Okamoto,Averback, and Wollenberger Berlin group required several dpa: unclear what they probe. ****Need more specific experiments: an example**

**Probability for    escape depends on concentration of    and specimen thickness: gives concentration of “traps”**** **

**Results from Cu****3****Au**
**(highly perfect single crystals)**

## Validation of modelsneed for specific information

## Relaxation to steady states of order

_**Sequence**_
**System responds to applied flux with response time t****1**
**System responds to an increment of flux ******** with response time, t****2**
**System returns to steady state with response time t****1**
**System returns to equilibrium state of order with response time t****o****.**
**	**

***Since ****************** is proportional to D = D******v******c******v***
**obtain**
******o****/********(********) = c(********)/c****o**

**Analysis of curves yields:**
**   ****= 1 x 10****-8**
**All other values of c****v**** follow**
**Wei, Flynn and Averback PRL 1999**

## Freely migrating defects

_**Experiment**_
**Quench in antisite defects**
**Irradiate with He ions**
**Measure dS/df**
**Irradiate with Ar **
**(to produce cascades)**
**Measure dS/d******

**Notice: dS/d******** scales with (1-S)**

## Freely migrating defects: Temperature dependence

## Critical basic research areas

**Develop experimental methods to provide information on the meso scale**
**Freely migrating and clustered defects associated with cascades**
**Atomic mixing and diffusion across interfaces of dissimilar materials (unlikely to come from simulation in the near future)**
**Examples: DXS, AP-FIM, **
**Develop methods to input experimental facts into simulations**
**EAM is a good guide**
** **
**Understand phase stability in nanostructure and other inherently metastable materials**
**Evaluate the role of electron-phonon coupling in MD simulations**
**Find solution if it is important**