Abstract
Two Z-phase strengthened 12% Cr steels were investigated: they are similar in composition, however one steel contains Nb and the other contains Ta. Z-phase precipitates (CrMN, M = Nb or Ta) provide precipitation hardening for creep resistance at 650 °C in these steels. Experimental data based on the transmission electron microscopy investigation of the size evolution of Z-phase precipitates during isothermal ageing at 650 °C show that the Ta-based Z-phase benefits from a five times smaller coarsening constant compared to the Nb-based Z-phase. Theoretical calculations show that this is attributed to the smaller diffusivity of Ta compared to Nb in the steel matrix. Besides, comparing the size of the Ta-based Z-phase precipitates in the gauge and head portion of a crept specimen, it is shown that Z-phase coarsens faster under stress.
| Original language | English |
|---|---|
| Pages (from-to) | 247-254 |
| Number of pages | 8 |
| Journal | Materialia |
| Volume | 4 |
| DOIs | |
| State | Published - Dec 2018 |
| Externally published | Yes |
Funding
Financial support from the Swedish Energy Agency, KME (contract number: 710 ), the Research Foundation of VGB in Germany (contract number: 397 ), and the EU FP7 Z-ULTRA (contract number: 309916 ) is gratefully acknowledged. FL is partly funded via LIGHTer, a program financed within Strategic Innovation Areas, a mutual venture between VINNOVA, the Swedish Energy Agency, and Formas. Torsten-Ulf Kern of Siemens AG is also gratefully thanked for fruitful discussions.
Keywords
- Coarsening rate
- Creep
- Diffusivity
- Electron microscopy
- Precipitate