Materials Science on the Atomic Scale with the 3-D Atom Probe: Experiments and ModellingGeorge SmithDepartment of Materials Oxford University

Acknowledgements Alfred Cerezo, Professor, Oxford University Paul Warren, Gang Sha, Rohan Setna, Andrew Morley, Paul Bagot, Terry Godfrey, Oxford University Kazuhiro Seto JFE Steel Shoichi Hirosawa Tokyo Institute of Technology David Larson Seagate Technology, USA (now at Imago Scientific Instruments) Funding: EPSRC, The Royal Society, Rolls-Royce, Oxford Nanoscience

Declaration of Interest In 1987, together with family and university colleagues, I set up a small company, Kindbrisk Ltd (subsequently renamed Oxford nanoScience) Oxford nanoScience became part of the Polaron Group of companies, and is now owned by Imago Scientific Instruments I and my family and colleagues are shareholders in Polaron, which in turn is a stakeholder in Imago.

Outline of Talk Knowledge requirements for materials design: advantages of combining experimental data with modelling studies Seeing atomic structure with the field ion microscope Studying 3-D atomic composition: atom probe analysis Modelling atomic processes by Kinetic Monte Carlo Case studies: examples from Cu, Al and Fe-based alloys Extension to thin films, catalysts and semiconductors

Field Ion Microscope Image of Pt

Field ion microscope Specimen in form of needle, 100nm end radius Voltage applied to specimen generates high field Gas ionised at apex generates image on screenChannel plate and phosphor screen

High voltage (d.c.)

Field ionised gas atoms

Needle-shaped specimen (cooled) Vacuum chamber

FIM image formation+Polarised gas atom Gas ion

Thermal accommodation Ionisation Adsorption

Critical surface

Specimen

Field ionisation Strong electric field distorts electron potential of gas atom and allows ionisation by electron tunneling Ionisation can only occur when electron level in atom lies above the Fermi level in the metal This defines minimum or critical distance, xc within which ionisation cannot occurGas atom

I e Metal

xc

Ball model of FIM Specimen

Grain Boundary in Tungsten

Field evaporation Electric field distorts the potential curve for ion on metal surface At very high field, ionic and atomic curves cross Thermal activation can produce ionisation - even at low temperatures Evaporation field depends on sublimation energy (thus melting temperature)V(x)

I

n

n

Ion (no field) x

Atom neFx Ion (in field)

Atom probe microanalysis Single atoms field evaporated from specimen and identified by timeofflight mass spectrometry. Local composition measured directly by counting atoms Spatial resolution is approx. 2nm laterally 0.2nm in depth.High voltage (d.c. + pulse) FIM image screen Single-ion sensitive detector

Needle-shaped specimen (cooled)

Field evaporated ions

Vacuum chamber

Identifying single atoms Flight time t of ions removed from specimen is measured over flight length d with (sub-)nanosecond resolution. By equating potential energy for ion at specimen and kinetic energy after field evaporation, can calculate massto-charge ratio:Kinetic energy1

mv2 = neV 2

Potential energy

m 2 = V + (2t) n

2 2 m 1 t = 2eV = 2 eV v d n

The 3-dimensional atom probe Single atoms removed from specimen and identified Position sensing gives original position to sub-nm Continued removal gives 3-D atomic-scale mapHigh voltage (d.c. + pulse) Positionsensitive detector

Specimen (cooled)

Field evaporated ions

Flight time signal

3DAP Data Example: Stainless SteelMass Resolution: FW1%M ~ 200 All peaks resolved Cu2+ and Mo3+ species separated by 0.17 amuLog scale 24.5 to 33.5 amu

Duplex Stainless Steel Linear Scale

Duplex Stainless Steel Linear Scale

3D atomic scale tomography(x,y)

At any point, 3DAP gives analysis of the surface elemental distribution Field evaporation leads to atomic layer slicing through the material Data allows reconstruction of original 3D distribution of elements

z=0 z=1 z=2 z=3 z=4 z=5

3-dimensional atom probe (3DAP) 3DAP allows mapping of element distributions with atomic-scale resolution and in 3-dimensions Local compositions can be measured directly Example: Mg and Si distributions from 6000 series Al alloy aged for 16hours at 180C Atom map clearly shows (100) oriented precipitatesMg Si

5nmData courtesy S. Pilcher, Oxford University

High resolution wide angle 3DAP study of Cu precipitation processHigh Ni steel aged for 50,000 hours at 330C Fe Cu

10nm

(only 10% of Fe atoms from analysis shown)

3-D Atomic Scale Computer Simulation of Alloy Systems Accurate simulation of subtle solute effects requires model which includes vacancy exchange and vacancy-solute interactions Large number of parameters required, but these are determined from independent sources: like atom-atom interactions (Eii) from cohesive energies, corrected for different lattice parameters unlike interactions (Eij) from phase diagram information vacancy-solute interactions (Eiv) from vacancy formation energies in pure materials

Kinetic Monte Carlo model Alloy represented by atoms on fixed lattice: no strain Thermodynamics given by nearest neighbour interactions Diffusion modelled statistically Atomic diffusion steps modelled by vacancy migration Migration probability depends on energy change Realistic timescales calculated using residence time algorithm

Vacancy exchange: parametersE Ea E

x

Activation barrier given by: EEa = 0 +

Single vacancy used in model (conc. 10-6). Model time is scaled to take into account real vacancy concentration:Cveq t = model Cv MCSf b

0 = Qidiffusion where:= Qidiffusion

2 Hvf

[Hv (Fe) Eiv ]

Nucleation in copper-cobalt Cu-1at%Co solutionised and aged at 723K (T300K)Data courtesy of R. Setna, University of Oxford30 minutes 1 hour 24 hours

5nm

Model: nucleation in Cu-1at.%Co Simulation of ageing in Cu-1at.%Co at 723K600 MCS 1200 MCS 29000 MCS

Hardness evolution of 7050 Al alloy200

Ageing at 121C190

180

170

160

150 1 10 100 1000 10000

Ageing time (min

Alloy composition: 6.29 wt% Zn, 2.22 wt% Mg, 2.28 wt% Cu, 0.11 wt% Zr, 0.09 wt% Fe and 0.05 wt% Si

7050 Al alloy: 3DAP observationsAQ 30 min

60 min

240 min

1440 min Zn Mg Cu

5 nm

Simulation of 7050 Al alloyDevelopment of precipitates during simulated ageing at 121C 25 mins 45 mins 810 mins 1500 mins C D D C D N D A B B A C A

B Zn Mg Cu

Hardness for Cu-containing steel250 240 230 220 210 200 190 180 170 160 1501x10 -1

Vickers hardness / HV

1.5%Ni-containing alloy 0.3%Ni-containing alloy

Before ageing

10

102

103

104

105

Ageing time at 365C / hrs

Fe-0.5at.%Cu-Ni-1.5at.%Mn-0.75at%Si Higher Ni alloy shows a more pronounced hardening

Analysis of steel aged at 365CBefore ageing 1.5 at.% Ni 100hrs 1000hrs 3000hrs 10000hrs

0.3 at.% Ni

10nm

Higher density of Cu clusters are formed in higher Ni alloy

Analysis of steel aged at 365C A shell of Ni, Mn and Si formed around Cu clusters after ageing at 365C for 3000 hours Other solutes segregate weakly, or not at all

Cu

Ni

Mn

Si

5nm

Interface segregation in high-Ni steelAged for 18620 hours at 330 C20.0 18.0 16.0 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0 -0.3 0 0.3 0.6

Aged for 100 hours at 405 C20.0 18.0 16.0 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0 -0.3 0 0.3 0.6

Matrix Ppt.

Si Mn Ni

Matrix Ppt.

Si Mn Ni

Distance from interface (nm

Distance from interface (nm

Simulation of steel aged at 365CBefore ageing ~370hrs ~1860hrs ~9240hrs ~36720hrs

0.3 at.%Ni

1.5 at.%Ni

5nm

Simulated microstructures are a good match with those observed in 3DAP analysis of steels

Origin of interface segregationNi Ni Ni Cu Fe Cu Fe Cu Fe

Eint=5(CuCu+FeCu+2FeNi) = 5 (-247.3) kJ/mol

Eint=5(CuCu+CuNi+FeNi+FeFe) Eint= 5(2CuNi+FeCu+FeFe) =5(-254.6) kJ/mol =5(-252.3) kJ/mol

Energy parameters from model can be used to compare internal energies of three different atom configurations Lowest energy configuration is for Ni at ppt. interface. Parameters for Mn and Si show similar tendency

3DAP Analysis of Multilayer StackCo Cu Ni

growth direction

5 nm Whole analysis (800k atoms)

2 nm Selected volume 1694 nm

MD Simulation of Deposition vs. 3DAP Data3D Atom Probe Data C o C u Ni Fe Molecular Dynamics Model

2 nm

Modelling results from collaboration with X. Zhou and H. Wadley (Univ. of Virginia)

Pulsed Laser Atom Probe

Unannealed TMR structureCap FM layer Barrier FM layer

Seed

2nmSlice 4nm thick

Analysis of TMR structure (similar to that used in hard disks and MRAM) with laser pulsed 3DAP Laser pulsing makes analysis of these complex structures more routine, especially in the case where insulating layers are present Note atomic planes in lower FM layer

Boron Dopant Atoms in Silicon(specimen courtesy of D. Larson)

PLAP Spectrum from GaInAsP

Mass spectrum from GaNN2+

69Ga+

71Ga+

N+

69Ga+ 71Ga+

Field Evaporated Pt-Rh Surface

Pt-Rh after exposure to NO

Pt-Rh (111) surface exposed to 10 mbar NO at 573K

Pt-Rh (001) surface exposed to 10 mbar NO at 573 K

Molecular Adsorption Sites: NO on Pt(012), 292K

Molecular NO adsorption at step edges

Summary 3DAP is powerful technique for studying the atomic-scale structure and composition of materials 3D experiments can be used to benchmark computer models of materials behaviour 3-D atomic scale characterisation of processing of metallic multilayer structures is now achieved Study of semiconductors and catalysts possible Foundation for atomic scale materials design but still a huge amount more work to do!

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