Abstract
The Fe-Ni-Al-Cr (FBB8) ferritic alloy contains B2-ordered NiAl precipitates which, upon minor addition of Ti to the alloy, display L21-ordered Ni2TiAl sub-precipitates; this improves creep resistance, consistent with an increase in coherency strains from the hierarchical NiAl/Ni2TiAl precipitates. Here, we study the effect of a small addition of Hf (0.5 wt%) to FBB8-1.5wt.%Ti alloy on precipitate structure and creep properties. The main microstructural changes with Hf addition are relatively minor: (i) decrease of mean radius of Ni2TiAl/NiAl hierarchical precipitates from 84 ± 14 to 78 ± 13 nm, (ii) increase of volume fraction of these precipitates from 18 ± 2 to 19 ± 2%, (iii) increase of their number density from 9 ± 0.3 × 10−19 to 11 ± 0.3 × 10−19 m−3, and (iv) decrease of precipitate edge-to-edge distance from 187 ± 57 to 160 ± 48 nm. The larger volume fraction, higher number density and smaller edge-to-edge distance of B2/L21 precipitates in the FBB8-1.5Ti-0.5Hf alloys, are all favorable to higher creep resistance for a strengthening mechanism based on precipitate climb bypass. However, the threshold stress for creep at 700 °C decreases upon Hf addition to FBB8-1.5Ti, from 156 to 122 MPa. The lower creep resistance is explained by a decrease of the lattice misfit between the Hf-enriched B2-precipitates and the bcc-matrix, and by a decrease of the volume fraction of L21-Ni2TiAl sub-precipitates within B2-NiAl precipitates from 16 ± 4% to 10 ± 3% without and with Hf, respectively.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 126-135 |
| Number of pages | 10 |
| Journal | Acta Materialia |
| Volume | 153 |
| DOIs | |
| State | Published - Jul 2018 |
Funding
This research was supported financially by the US Department of Energy (DoE) , Office of Fossil Energy, under Grant DE-FE0005868 (Dr. V. Cedro, monitor). The authors gratefully acknowledge Prof. P.K. Liaw, Mr. Z. Sun and Mr. G. Song (University of Tennessee) for providing the alloys and helpful discussions. Dr. Baik thanks Drs. Anthony De Luca and Dinc Erdeniz for assistance of creep experiment and useful discussion. Dr. Baik also thanks Prof. David N. Seidman and Prof. Dieter Isheim for supporting the APT experiment. APT was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The local-electrode atom-probe tomograph at NUCAPT was acquired and upgraded with equipment grants from the MRI program of the National Science Foundation (grant number DMR-0420532 ) and the DURIP program of the Office of Naval Research (grant numbers N00014–0400798 , N00014–0610539 , N00014–0910781 , N00014-1712870 ). NUCAPT is a Research Facility at the Materials Research Center of Northwestern University and received support through the National Science Foundation's MRSEC program (grant number NSF DMR-1720139 ) and from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource ( NSF ECCS-1542205 ). Additional instrumentation at NUCAPT was supported by the Initiative for Sustainability and Energy at Northwestern (ISEN) . This research was supported financially by the US Department of Energy (DoE), Office of Fossil Energy, under Grant DE-FE0005868 (Dr. V. Cedro, monitor). The authors gratefully acknowledge Prof. P.K. Liaw, Mr. Z. Sun and Mr. G. Song (University of Tennessee) for providing the alloys and helpful discussions. Dr. Baik thanks Drs. Anthony De Luca and Dinc Erdeniz for assistance of creep experiment and useful discussion. Dr. Baik also thanks Prof. David N. Seidman and Prof. Dieter Isheim for supporting the APT experiment. APT was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The local-electrode atom-probe tomograph at NUCAPT was acquired and upgraded with equipment grants from the MRI program of the National Science Foundation (grant number DMR-0420532) and the DURIP program of the Office of Naval Research (grant numbers N00014–0400798, N00014–0610539, N00014–0910781, N00014-1712870). NUCAPT is a Research Facility at the Materials Research Center of Northwestern University and received support through the National Science Foundation's MRSEC program (grant number NSF DMR-1720139) and from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205). Additional instrumentation at NUCAPT was supported by the Initiative for Sustainability and Energy at Northwestern (ISEN).
Keywords
- B2/L2 hierarchical precipitate
- Creep
- Ferritic steel
- Hafnium effect
- Precipitate-strengthening
- Superalloy
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys