Atom Probe Tomography of ODS steels containing Hf 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Maria A Auger Postdoctoral Research Assistant Department of Materials, University of Oxford

Outline Motivation to include Hf in ODS steels Manufacturing process Characterisation:- Powder samples - Consolidated samples Atom Probe Tomography Conclusions 2 nd ODDISSEUS Workshop. 22 nd March, 2015

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. 2 nd ODDISSEUS Workshop. 22 nd March, 2015

Manufacturing process Powder metallurgy route Fe-14Cr pre-alloyed powder Hf powder Mechanical alloying (MA) Spark plasma sintering (SPS) Hf + Y 2 O 3 powder Fe-14Cr-0.22Hf (wt. %) Fe-14Cr-0.25Y 2 O Hf (wt. %) Nominal compositions Chromium steel vessels and balls Balls to powder ratio 10:1 Rotation speed: 150 rpm Milling time: 60 h Milling atmosphere: high purity Ar Temperature: 1150 o C Pressure: 50 MPa Time: 5 min + + Fe-14Cr-Hf Fe-14Cr-Y-Hf 2 nd ODDISSEUS Workshop. 22 nd March, 2015

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. 2 nd ODDISSEUS Workshop. 22 nd March, 2015

SEM / EDS Fe-14Cr-Hf X X  Spectrum 1 Spectrum 1 Nominal composition is found for Cr and Hf Si contamination is present 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

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 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

XRD Fe-14Cr-Y-Hf Fe-14Cr-Hf bcc phase 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

Nanoindentation measurements Fe-14Cr-Y-HfFe-14Cr-Hf Powder sample embedded in a phenolic resin and grinded and polished 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

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 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

Nanoindentation measurements Fe-14Cr-Y-Hf Average hardness = 8.73 ± 0.48 GPa Average modulus = 176 ± 30 GPa 29 measurements were averaged (1 to 4 nanoindentations per grain) 1 st set 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

Nanoindentation measurements Fe-14Cr-Y-Hf Average hardness = 8.50 ± 0.57 GPa Average modulus = 165 ± 30 GPa 27 measurements were averaged (1 to 4 nanoindentations per grain) 2 nd set (new sample holder) 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation 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?) 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

SEM of nanoindentation marks Fe-14Cr-Hf Some powder grains detached from the holder… 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

SEM of nanoindentation marks Fe-14Cr-Y-Hf Difficult to distinguish between marks and grain defects 2 nd ODDISSEUS Workshop. 22 nd March, 2015 Powder characterisation

2 nd ODDISSEUS Workshop. 22 nd March, 2015 SPSed material characterisation

Fe-14Cr-Y-Hf 2 nd ODDISSEUS Workshop. 22 nd March, 2015 SPSed material characterisation Fe-14Cr-Hf SEM

2 nd ODDISSEUS Workshop. 22 nd March, 2015 SPSed material characterisation Fe-14Cr-Hf Fe-14Cr-Y-Hf

SEM 2 nd ODDISSEUS Workshop. 22 nd March, 2015 SPSed material characterisation Fe-14Cr-HfFe-14Cr-Y-Hf Matrix average composition Nominal Cr percentage is confirmed Si impurities are present in the matrix

XRD 2 nd ODDISSEUS Workshop. 22 nd March, 2015 SPSed material characterisation Fe-14Cr-Hf Fe-14Cr-Y-Hf bcc phase + fcc phase + unidentified phase (Fe-Si compound?)

Vickers hardness 2 nd ODDISSEUS Workshop. 22 nd March, 2015 SPSed material characterisation Fe-14Cr-Hf HV = 2.31 ± 0.13 GPa Fe-14Cr-Y-Hf HV = 2.34 ± 0.18 GPa

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Parameters: Temperature: 50K Laser energy: 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 LEAP TM 3000HR (Dep. Materials-Atom Probe Group) Data analysis: CAMECA IVAS TM software Origin Corel (Draw/Photopaint) APT tip (final radius < 100 nm)

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf 20 nm CrFeSi Hf-O Y-O Y

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf Cluster analysis – Cluster shape Best-fit ellipsoid approximation  3 characteristic lenghts

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf The cluster morphology can be defined in terms of the Oblateness and Aspect ratio: Cluster analysis – Cluster shape

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf 83% of the clusters are spherical 9% are disc shaped 7% are rod shaped Cluster analysis – Cluster shape

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf Cluster analysis – Cluster size The centre-of-mass (CM) of each cluster can be calculated as: It is straigthforward to calculate the radius of gyration:

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf Cluster analysis – Cluster size Mean value = 2.9 ± 0.7 nm

2 nd ODDISSEUS Workshop. 22 nd March, 2015 Atom Probe Tomography Fe-14Cr-Y-Hf Cluster analysis – Cluster composition Variable stoichiometry in clusters < 4 nm in size show Clusters > 4 nm in size show almost constant composition of ~ Y 2 HfO 3

Conclusions Fe-14Cr-0.22Hf (wt. %) and Fe-14Cr-0.25Y 2 O Hf (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-0.25Y 2 O Hf (wt. %) shows a nanometric distribution of Y-Hf-O clusters, mainly spherical and  3 nm in size 2 nd ODDISSEUS Workshop. 22 nd March, 2015

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