atom probe tomography of ods steels containing hf 2 nd oddisseus workshop. 22 nd march, 2015 maria a...

Atom Probe Tomography of ODS steels containing Hf

2nd ODDISSEUS Workshop. 22nd March, 2015

Maria A Auger

Postdoctoral Research AssistantDepartment of Materials, University of Oxford

Outline• Motivation to include Hf in ODS steels• Manufacturing process• Characterisation: - Powder samples

- Consolidated samples• Atom Probe Tomography• Conclusions


Motivation to include Hf in ODS steels

Addition of elements from the IVB group to ODS steels has been extensively studied.

Ti and Zr addition produces grain refinement and dispersoid size reduction in ODS steels.Not many references can be found about Hf addition.


Manufacturing process

Powder metallurgy route

Fe-14Cr pre-alloyed powder

Hf powder

Mechanical alloying (MA)

Spark plasma sintering (SPS)

Hf + Y2O3 powder

Fe-14Cr-0.22Hf (wt. %)

Fe-14Cr-0.25Y2O3-0.22Hf (wt. %)

Nominal compositions

Chromium steel vessels and ballsBalls to powder ratio 10:1Rotation speed: 150 rpmMilling time: 60 hMilling atmosphere: high purity Ar

Temperature: 1150oCPressure: 50 MPaTime: 5 min

++

Fe-14Cr-Hf

Fe-14Cr-Y-Hf


Powder characterisation

SEM / EDS

Fe-14Cr-Hf

Fe-14Cr-Y-Hf

Most of the powder grains are plate like shaped ( 400 μm in size)

Round profiles are more frequent; irregular shapes are also found.

Bimodal size distribution is found.

The smaller powder grains (tenths of μm in size) show irregular shapes.

The larger powder grains (up to 300 μm in size) are plate like shaped.


SEM / EDS Fe-14Cr-Hf

X

X Spectrum 1

Spectrum 1

Nominal composition is found for Cr and Hf

Si contamination is present



Spectrum 1

SEM / EDS Fe-14Cr-Y-Hf

Nominal composition is found for Cr and Hf.

Y is not detected.

Si contamination is present (also C and Al)

X

X Spectrum 1



XRD

Fe-14Cr-Y-Hf

Fe-14Cr-Hf

bcc phase

bcc phase



Nanoindentation measurements

Fe-14Cr-Y-HfFe-14Cr-Hf

Powder sample embedded in a phenolic resin and grinded and polished



Nanoindentation measurements Fe-14Cr-Hf

29 measurements were averaged (1 to 9 nanoindentations per grain)

Average hardness = 7.87 ± 0.43 GPa Average modulus = 134 ± 33 GPa



Nanoindentation measurements Fe-14Cr-Y-Hf



1st set



Nanoindentation measurements Fe-14Cr-Y-Hf



2nd set (new sample holder)



Fe-14Cr-Y-Hf hardness value is 10% higher than the one for Fe-14Cr-Hf

Fe-14Cr-Y-Hf modulus value is 30% higher than the one for Fe-14Cr-Hf

SEM of nanoindentation marks Fe-14Cr-Hf

Some marks are asimetrical (indenter tip effect? Sample effect?)2nd ODDISSEUS Workshop. 22nd March, 2015


SEM of nanoindentation marks Fe-14Cr-Hf

Some powder grains detached from the holder…



SEM of nanoindentation marks Fe-14Cr-Y-Hf

Difficult to distinguish between marks and grain defects2nd ODDISSEUS Workshop. 22nd March, 2015



SPSed material characterisation

Fe-14Cr-Y-Hf



Fe-14Cr-Hf

SEM

SEM



Fe-14Cr-Hf

Fe-14Cr-Y-Hf

SEM



Fe-14Cr-Hf Fe-14Cr-Y-Hf

Matrix average composition

Nominal Cr percentage is confirmedSi impurities are present in the matrix

XRD



Fe-14Cr-Hf

Fe-14Cr-Y-Hf

bcc phase + fcc phase + unidentified phase (Fe-Si compound?)

Vickers hardness



Fe-14Cr-Hf

HV = 2.31 ± 0.13 GPa

Fe-14Cr-Y-Hf

HV = 2.34 ± 0.18 GPa


Atom Probe Tomography



Parameters:

Temperature: 50K

Laser energy: 0.3-0.4 nJ

Repetition rate: 200 kHz

Sample preparation:

Electropolishing: 25% perchloric acid, 75% acetic acid (V = 15 – 7 V DC)

Micropolishing: 2% perchloric acid, 98% 2-butoxyethanol (V = 7 – 4 V DC)

Instrument used:

CAMECA LEAPTM 3000HR (Dep. Materials-Atom Probe Group)

Data analysis:

CAMECA IVASTM software

Origin

Corel (Draw/Photopaint)

APT tip(final radius < 100 nm)



Fe-14Cr-Y-Hf



Fe-14Cr-Y-Hf

20 nm

CrFe Si

Hf-OY-OY



Fe-14Cr-Y-Hf Cluster analysis – Cluster shape

Best-fit ellipsoid approximation 3 characteristic lenghts



Fe-14Cr-Y-Hf

𝑂𝑏𝑙𝑎𝑡𝑒𝑛𝑒𝑠𝑠=𝑆𝑚𝑎𝑙𝑙𝑒𝑠𝑡 h𝑐 𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐 h𝑙𝑒𝑛𝑔𝑡𝑀𝑖𝑑𝑑𝑙𝑒 h𝑐 𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐 h𝑙𝑒𝑛𝑔𝑡

𝐴𝑠𝑝𝑒𝑐𝑡𝑟𝑎𝑡𝑖𝑜=𝑀𝑖𝑑𝑑𝑙𝑒 h𝑐 𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐 h𝑙𝑒𝑛𝑔𝑡𝐿𝑎𝑟𝑔𝑒𝑠𝑡 h𝑐 𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐 h𝑙𝑒𝑛𝑔𝑡

The cluster morphology can be defined in terms of the Oblateness and Aspect ratio:

Cluster analysis – Cluster shape



Fe-14Cr-Y-Hf

83% of the clusters are spherical

9% are disc shaped

7% are rod shaped

Cluster analysis – Cluster shape



Fe-14Cr-Y-Hf Cluster analysis – Cluster size

The centre-of-mass (CM) of each cluster can be calculated as:

�⃗�𝐶𝑀=∑𝑖=1

𝑁𝐶

𝑚𝑖 ∙𝑟 𝑖

∑𝑖=1

𝑁𝐶

𝑚𝑖

mass of each ion in the cluster coordinates of each ion in the cluster Number of ions in the cluster

It is straigthforward to calculate the radius of gyration:

𝑅𝑔=√∑𝑖=1𝑁𝐶

(�⃗�𝐶𝑀−𝑟 𝑖 )2

∑𝑖=1

𝑁 𝐶

𝑚𝑖

The Guinier radius relates to as:

𝑅𝐺=√ 53 ∙𝑅𝑔



Fe-14Cr-Y-Hf Cluster analysis – Cluster size

Mean value = 2.9 ± 0.7 nm



Fe-14Cr-Y-Hf Cluster analysis – Cluster composition

Variable stoichiometry in clusters < 4 nm in size showClusters > 4 nm in size show almost constant composition of ~ Y2HfO3

Conclusions

• Fe-14Cr-0.22Hf (wt. %) and Fe-14Cr-0.25Y2O3-0.22Hf (wt. %) were successfully

produced by mechanical alloying and consolidated by SPS

• The nominal composition is achieved in both alloys (Si impurities are present)

• bcc crystal structure for powder samples and bcc+fcc+unknown phase for consolidated samples

• Similar hardness values are obtained in the consolidated material

• Fe-14Cr-0.25Y2O3-0.22Hf (wt. %) shows a nanometric distribution of Y-Hf-O

clusters, mainly spherical and 3 nm in size



Hongtao Zhang (Material Processing)

Chris Jones (Nanoindentation)

Yina Huang (TEM)

Paul A J Bagot (APT)

Michael P Moody (APT)

Steve G Roberts (MFFP)

Patrick S Grant (Advanced Processing Laboratory)

Acknowledgements

atom probe tomography of ods steels containing hf 2 nd oddisseus workshop. 22 nd march, 2015 maria a...

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