Mechanical Characterization of a Nano-ODS Steel Prepared by Low-Energy Mechanical Alloying
Abstract
:1. Introduction
2. Materials and Methods
3. Results
- (i)
- Original powders and ODS precursor;
- (ii)
- Hot extruded material;
- (iii)
- eat treated ODS steel.
3.1. Original Powders and ODS Precursor
3.2. Hot Extruded Material
3.3. Heat Treated ODS Steel
4. Discussion
- (i)
- In the case of examined ODS steel the solid solution strengthening mainly depends on the amounts of Cr and W and may be evaluated as [32]:
- (ii)
- The Bailey-Hirsch stress is expressed by the relationship:
- (iii)
- The Hall-Petch stress (σHP) vs. temperature T is given by:
- (iv)
- ν = 0.3 is the Poisson’s ratio, L = 63 nm and d = 18 nm are the mean spacing between particles and particle size, respectively, determined from TEM observations (average values of 20 micrographs).
- (v)
- The Arzt-Rősler-Wilkinson stress (σARW), connected to the process of dislocation climb over hard particles, can be written according to the equation proposed by Reppich [37] as:
5. Conclusions
- (i)
- The method permits treatment of large amounts of powder in the order of kilograms.
- (ii)
- The material has equiaxed grains (mean size of 400 nm) and a dislocation density of 4 × 1012 m−2. Two types of second phase particles were observed: (i) round or irregular-shaped Y-Ti-oxides with different contents of titanium; (ii) cubic or rectangular fine precipitates of few nanometers. Therefore, low-energy MA decomposes Y2O3 into Y and O atoms, which are then bonded with Ti and precipitate to form extremely fine Y-Ti-O complex oxides during HE. Nano-oxides are stable up to 800 °C and guarantee the stability of microstructure and mechanical properties.
- (iii)
- With respect to the conventional preparation route fine equiaxed grains are observed instead of a bimodal grain size distribution. This microstructure involves better mechanical properties up to 500 °C, however, the precipitate distribution in the matrix is not so homogeneous as that obtained from high-energy MA, thus at higher temperature, where the Petch-Hall contribution breakdowns and the role played by precipitate in strengthening is dominant, YS and UTS remarkably decrease and are a little lower than those of conventional ODS steels.
- (iv)
- ODS steel heat treated above 1000 °C exhibits defect recovery and partial grain growth that can be also ascribed to the not homogeneous original distribution of Y2O3 particles.
- (v)
- In conclusion the examined method seems promising but a longer time of MA seems necessary for achieving a more homogeneous distribution of nano-oxides and consequently further enhancing mechanical properties at high temperature.
Author Contributions
Conflicts of Interest
References
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Sample | Temperature (°C) | Time (min) | Hardness HV |
---|---|---|---|
HE | No | No | 447 ± 5 |
HE + 1050 °C/30 min | 1050 | 30 | 315 ± 6 |
HE + 1100 °C/30 min | 1100 | 30 | 327 ± 10 |
HE + 1150 °C/30 min | 1150 | 30 | 305 ± 4 |
Intensities of the Most Strong Reflections | HE | A (HE + 1050 °C/30 min) | B (HE + 1100 °C/30 min) | C (HE + 1150 °C/30 min) |
---|---|---|---|---|
I110 | 100 | 100 | 100 | 100 |
I200 | 37 | 41 | 36 | 49 |
I211 | 43 | 61 | 50 | 59 |
Strengthening Contributions to Yield Stress (MPa) | Temperature (°C) | ||||
---|---|---|---|---|---|
25 | 400 | 500 | 600 | 700 | |
σSS | 100 | 100 | 100 | 100 | 100 |
σBH | 28 | 24 | 23 | 21 | 16 |
σ0 | 13 | 13 | 13 | 13 | 13 |
σD | 285 | 265 | 259 | - | - |
σK | 657 | 571 | 543 | - | - |
σARW | - | - | 133 | 122 | 94 |
σCC | - | - | 285 | 16 | 2 |
σSS + σBH + σ0 + σD + σK (ODS steel) | 1083 | 973 | 938 | - | - |
σSS + σBH + σ0 + σCC + σARW (ODS steel) | - | - | 554 | 272 | 225 |
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Sanctis, M.D.; Fava, A.; Lovicu, G.; Montanari, R.; Richetta, M.; Testani, C.; Varone, A. Mechanical Characterization of a Nano-ODS Steel Prepared by Low-Energy Mechanical Alloying. Metals 2017, 7, 283. https://doi.org/10.3390/met7080283
Sanctis MD, Fava A, Lovicu G, Montanari R, Richetta M, Testani C, Varone A. Mechanical Characterization of a Nano-ODS Steel Prepared by Low-Energy Mechanical Alloying. Metals. 2017; 7(8):283. https://doi.org/10.3390/met7080283
Chicago/Turabian StyleSanctis, Massimo De, Alessandra Fava, Gianfranco Lovicu, Roberto Montanari, Maria Richetta, Claudio Testani, and Alessandra Varone. 2017. "Mechanical Characterization of a Nano-ODS Steel Prepared by Low-Energy Mechanical Alloying" Metals 7, no. 8: 283. https://doi.org/10.3390/met7080283