Boll, Torben

Pint, Bruce A.

Stiller, Krystyna

Unocic, Kinga A.

KITopen

www.kit.eduKIT – The Research University in the Helmholtz AssociationTransport along grain boundaries through alumina investigated by atom probe tomographyTorben Boll*, Kinga A. Unocic, Bruce A. Pint, Krystyna StillerAtom Probe Tomography:Zr at a grain boudary in Al2O3,Each dot represents one atom, Al and O atoms are not displayed20 nmTEM of typical oxide on NiAl2• Protective Al2O3 coating on NiAl-alloy• O (and all other elements) in α-alumina diffuse mostly via grain boundaries (GBs)• Minor outward diffusion of metal• Decoration of GBs will influence the diffusion and thus oxidation• Apparently grows inwardsJOJAl, JMMaterial Ni at.% Al at.% Zr ppma Hf ppma N ppma C ppma Sxx ppma O ppma B ppma Cr ppma Zr-doped 49.95 49.99 520 0 0 0 3 48 30 0 Hf-doped 49.83 50.07 0 480 30 36 0 43 0 100  3[1] V. K. Tolpygo and D. R. Clarke, Mater. High Temp., vol. 20, no. 3, pp. 261–271, 2003[2] J. a. Nychka and D. R. Clarke,  Oxid. Met., vol. 63, no. 5–6, pp. 325–352, 2005Outward diffusion: Exp. ideaHf 10h exposureafterbeforeTEM of mech. pol. Hf sample • No Ga contamination• GB enriched with Hf and some Ni5400 nmPtAuAl2O3 GBOHfGaNiAlTEM of Zr sample6• Zr enriched at the GBHow to calculate the flux• Calculate the flux– Number of diffused Al-atoms 𝑁𝐺𝐵𝐴𝑙– Exposure time Δ𝑡 (10h)• Calculate number of atoms– Volume of ridge 𝑉𝐴𝑙=𝐴𝐴𝑙 𝐿𝐺𝐵– Length of GB 𝐿𝐺𝐵 not height!– Cross section area of ridge 𝐴𝐴𝑙– Volume of Al2O3 unit cell: Vu=2.54 10-22 cm³– Number of Al atoms per unit cell: 127𝐽𝐴𝑙=𝑁𝐺𝐵𝐴𝑙𝐿𝐺𝐵Δ𝑡𝑁𝐺𝐵𝐴𝑙 =12 𝑉𝐴𝑙𝑉𝑢𝐽𝐴𝑙=12 𝐴𝐴𝑙𝑉𝑢GBoxidemetalridge𝐴𝐴𝑙hoxideFlux of Al through GBs at 1100°• Mech. polishing enhances ridge growth• Zr allows higher outward flux than Hf8100 10000.1110J [nm-1s-1]Zr mech.Hf mech. Hf-Xe Hf-Gathickness [nm]Should follow Fick’s 1. law (assuming hoxide is constant)Flux of Al through GBs at 1100°• Mech. polishing enhances ridge growth• Zr allows higher outward flux than Hf• Inward flux six orders of magnitude larger9100 10000.111010010001000010000010000001E7J [nm-1s-1]Zr mech.Hf mech. Hf-Xe Hf-Gathickness [nm]Should follow Fick’s 1. law (assuming hoxide is constant)APT of Hf sample10Au,PtAl2O3• Protective Au,Ptcoating• GB with Hf, Ni• Ni enriched at surface• Gibbsian excess Γ(Number of additional atoms per area in GB):NiHf AlO40 nmHf:  0.5 nm-2Ni:  2.6 nm-2Hf:  0.35 nm-2Ni:  0.59 nm-2APT of Zr sample11• Protective Ag on top of ridge-GB• No Ni found• ΓZr: 2.5 nm-2Zr20 nm At the ridgeOutward flux of Ni, Hf, Zr• Outward flux into ridge GB• Gibbsian excess• Additional GB length (ridge height from SEM)• JX= ΓX hridge12→ amount of diffused material100 10001E-41E-30.010.1110J [nm-1s-1] Zr mech Hf-mech Hf-Xe Hf-Ga Ni (APT) Hf (APT) Zr (APT)thickness (nm)GBoxidemetalridgeOutward flux of Ni, Hf, Cr• Outward flux into ridge GB• Gibbsian excess• Additional GB length (ridge height from SEM)• JX= ΓX hridge13→ amount of diffused material100 10001E-41E-30.010.111010010001000010000010000001E7J [nm-1s-1] Zr mech Hf-mech Hf-Xe Hf-Ga Ni (APT) Hf (APT) Zr (APT)thickness (nm)JO>>JAl>>JHf,Ni,ZrConclusions• Outward Diffusion of Al along Al2O3 GBs is observed by STEM Mechanical polishing introduces defects that promote diffusion– Hf reduces Al-outward diffusion stronger than Zr• Zr is enriched at GBs • Hf is enriched at GBs• Ni is found at the GB and at the topof the ridge in the Hf sample• JO~106 nm-1s-1>>JAl~1 nm-1s-1>>JHf,Ni,Zr ~10-3 nm-1s-114→ Outward diffusion of Zr, Hf→ Outward diffusion of Ni15Thank you for your attentionAcknowledgementsFor help with 2nd exposure and sputter coating:Patrik AlnegrenYou also want APT results: knmf.kit.edu, or contact meKNMF grants APT time to suitable projects 

Transport along grain boundaries through alumina investigated by atom probe tomography

https://publikationen.bibliothek.kit.edu/1000069806/4163057

Transport along grain boundaries through alumina investigated by atom probe tomography

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