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• 1. [Article] Long-Term Stability of 14YT-4Sc Alloy at High Temperature

  14YT alloy (Fe-14Cr-0.25wt.%Y2O3-0.4wt.%Ti) with 4 at.% Sc addition was previously reported to exhibit a nanoscale microstructure and high strength when annealed at temperatures up to 1000 °C (0.65Tm) ...

  Citation × Citation Citation Abstract Copy Title: Long-Term Stability of 14YT-4Sc Alloy at High Temperature Author: Li, Lulu, Xu, Weizong, Saber, Mostafa, Zhu, Yuntian, Koch, Carl C., Scattergood, Ronald O. Year: 2015 14YT alloy (Fe-14Cr-0.25wt.%Y2O3-0.4wt.%Ti) with 4 at.% Sc addition was previously reported to exhibit a nanoscale microstructure and high strength when annealed at temperatures up to 1000 °C (0.65Tm) for 1 hour. Here we report that the microstructure and mechanical behavior of 14YT-4Sc alloy after long-term annealing for up to 60 hours at 1000 °C. FIB analysis shows abnormal grain growth with annealing time, while a large fraction of the matrix still consists of nanoscale grains. TEM images reveal a slight growth of nano grains, with estimated grain growth exponent, n, to be 0.29. Sc-Ti-Y-O enriched nano oxide particles (nm) were observed in EDS mapping of the 14YT-4Sc-60h sample. The nano-structure retention at the high temperature of 1000 °C for 60 hours is attributed to these complex nano oxides. Title: Long-Term Stability of 14YT-4Sc Alloy at High Temperature Author: Li, Lulu, Xu, Weizong, Saber, Mostafa, Zhu, Yuntian, Koch, Carl C., Scattergood, Ronald O. Year: 2015 14YT alloy (Fe-14Cr-0.25wt.%Y2O3-0.4wt.%Ti) with 4 at.% Sc addition was previously reported to exhibit a nanoscale microstructure and high strength when annealed at temperatures up to 1000 °C (0.65Tm) for 1 hour. Here we report that the microstructure and mechanical behavior of 14YT-4Sc alloy after long-term annealing for up to 60 hours at 1000 °C. FIB analysis shows abnormal grain growth with annealing time, while a large fraction of the matrix still consists of nanoscale grains. TEM images reveal a slight growth of nano grains, with estimated grain growth exponent, n, to be 0.29. Sc-Ti-Y-O enriched nano oxide particles (nm) were observed in EDS mapping of the 14YT-4Sc-60h sample. The nano-structure retention at the high temperature of 1000 °C for 60 hours is attributed to these complex nano oxides. Close

• 2. [Article] Influence of Scandium Addition on the High-Temperature Grain Size Stabilization of Oxide-Dispersion-Strengthened (ODS) Ferritic Alloy

  The influence of 1–4 at% Sc addition on the thermal stability of mechanically alloyed ODS ferritic alloy was studied in this work. Sc addition was found to significantly stabilize grain size and microhardness ...

  Citation × Citation Citation Abstract Copy Title: Influence of Scandium Addition on the High-Temperature Grain Size Stabilization of Oxide-Dispersion-Strengthened (ODS) Ferritic Alloy Author: Li, Lulu, Xu, Weizong, Saber, Mostafa, Zhu, Yuntian, Koch, Carl C., Scattergood, Ronald O. Year: 2015 The influence of 1–4 at% Sc addition on the thermal stability of mechanically alloyed ODS ferritic alloy was studied in this work. Sc addition was found to significantly stabilize grain size and microhardness at high temperatures. Grain sizes of samples with 1 and 4 at% Sc was found maintained in the nanoscale range at temperatures up to 1000 °C with hardness maintained at 5.6 and 6.7 GPa, respectively. The detailed microstructure was also investigated from EDS elemental mapping, where nanofeatures [ScTiO] were observed, while nanosized [YTiO] particles were rarely seen. This is probably due to the concentration difference between Sc and Y, leading to the formation of [ScTiO] favoring that of [YTiO]. Precipitation was considered as the major source for the observed high temperature stabilization. In addition, 14YT–Sc alloys without large second phases such as Ti-oxide can exhibit better performance compared to conventional ODS materials. Title: Influence of Scandium Addition on the High-Temperature Grain Size Stabilization of Oxide-Dispersion-Strengthened (ODS) Ferritic Alloy Author: Li, Lulu, Xu, Weizong, Saber, Mostafa, Zhu, Yuntian, Koch, Carl C., Scattergood, Ronald O. Year: 2015 The influence of 1–4 at% Sc addition on the thermal stability of mechanically alloyed ODS ferritic alloy was studied in this work. Sc addition was found to significantly stabilize grain size and microhardness at high temperatures. Grain sizes of samples with 1 and 4 at% Sc was found maintained in the nanoscale range at temperatures up to 1000 °C with hardness maintained at 5.6 and 6.7 GPa, respectively. The detailed microstructure was also investigated from EDS elemental mapping, where nanofeatures [ScTiO] were observed, while nanosized [YTiO] particles were rarely seen. This is probably due to the concentration difference between Sc and Y, leading to the formation of [ScTiO] favoring that of [YTiO]. Precipitation was considered as the major source for the observed high temperature stabilization. In addition, 14YT–Sc alloys without large second phases such as Ti-oxide can exhibit better performance compared to conventional ODS materials. Close

• 3. [Article] Effect of Oxygen Content on Thermal Stability of Grain Size for Nanocrystalline Fe10Cr and Fe14Cr4Hf Alloy Powders

  Low oxygen content powders of high purity elemental Fe, Cr and Hf were produced in a glove box by mechanically filing the solid materials. Fe10Cr and Fe14Cr4Hf nanocrystalline alloy powders were processed ...

  Citation × Citation Citation Abstract Copy Title: Effect of Oxygen Content on Thermal Stability of Grain Size for Nanocrystalline Fe10Cr and Fe14Cr4Hf Alloy Powders Author: Roodposhti, Peiman Shahbeigi, Saber, Mostafa, Koch, Carl, Scattergood, Ronald Year: 2017 Low oxygen content powders of high purity elemental Fe, Cr and Hf were produced in a glove box by mechanically filing the solid materials. Fe10Cr and Fe14Cr4Hf nanocrystalline alloy powders were processed using these elemental powders in conjunction with SPEX ball milling. The grain-size stability of the nanocrystalline alloy powders was investigated for selected annealing temperatures. High temperature stabilization can be achieved by Zener pinning (kinetic mechanism) or segregation of Hf to grain boundaries (thermodynamic mechanism). Solute drag mechanisms can be effective at lower annealing temperatures. Recent regular solution models developed by the authors predict that Hf would facilitate thermodynamic grain-size stabilization in Fe14Cr4Hf alloys at high temperatures. However Hf-base reactions such as intermetallic phase or oxide formation can favor kinetic stabilization and this can dominate over a contribution from thermodynamic stabilization. In contrast, grain-size stabilization in Fe10Cr alloy would be a result of solute drag by the Cr solutes at lower temperatures. The results from previous investigations on Fe10Cr and Fe14Cr4Hf nanocrystalline alloys were (unknowingly at the time) influenced by the fact that the commercially available high purity elemental powders used for SPEX ball mill processing contained significant amounts of oxygen impurity. The results obtained in this investigation using identical processing methods with low oxygen content powders for synthesizing the alloys provide further insight into their stabilization mechanisms. Title: Effect of Oxygen Content on Thermal Stability of Grain Size for Nanocrystalline Fe10Cr and Fe14Cr4Hf Alloy Powders Author: Roodposhti, Peiman Shahbeigi, Saber, Mostafa, Koch, Carl, Scattergood, Ronald Year: 2017 Low oxygen content powders of high purity elemental Fe, Cr and Hf were produced in a glove box by mechanically filing the solid materials. Fe10Cr and Fe14Cr4Hf nanocrystalline alloy powders were processed using these elemental powders in conjunction with SPEX ball milling. The grain-size stability of the nanocrystalline alloy powders was investigated for selected annealing temperatures. High temperature stabilization can be achieved by Zener pinning (kinetic mechanism) or segregation of Hf to grain boundaries (thermodynamic mechanism). Solute drag mechanisms can be effective at lower annealing temperatures. Recent regular solution models developed by the authors predict that Hf would facilitate thermodynamic grain-size stabilization in Fe14Cr4Hf alloys at high temperatures. However Hf-base reactions such as intermetallic phase or oxide formation can favor kinetic stabilization and this can dominate over a contribution from thermodynamic stabilization. In contrast, grain-size stabilization in Fe10Cr alloy would be a result of solute drag by the Cr solutes at lower temperatures. The results from previous investigations on Fe10Cr and Fe14Cr4Hf nanocrystalline alloys were (unknowingly at the time) influenced by the fact that the commercially available high purity elemental powders used for SPEX ball mill processing contained significant amounts of oxygen impurity. The results obtained in this investigation using identical processing methods with low oxygen content powders for synthesizing the alloys provide further insight into their stabilization mechanisms. Close

• 4. [Article] Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

  The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys ...

  Citation × Citation Citation Abstract Copy Title: Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition Author: Xu, Weizong, Li, Lulu, Saber, Mostafa, Koch, Carl C., Zhu, Yuntian, Scattergood, Ronald O. Year: 2015 The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys with 1, 2, and 4 at. pct Hf addition at 1173 K (900 °C). Microstructural characterizations using aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy reveal high density of HfO2 nanoparticles with sizes of ~4 nm dispersed throughout the ferritic matrix. This indicates that kinetic stabilization by HfO2 nanoparticle pinning is primarily responsible for the observed high thermal stability. In addition, some Hf and Cr segregation on grain boundaries is observed in the Fe-14Cr-4Hf, suggesting the existence of thermodynamic stabilization at high Hf content. Second-phase precipitations such as hafnium carbide, M23C6, and Fe-Cr-Hf intermetallic phase are also found in the Fe-14Cr-4Hf, but their large sizes and inter-spacing suggest that their contribution to stabilization is minimal. Title: Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition Author: Xu, Weizong, Li, Lulu, Saber, Mostafa, Koch, Carl C., Zhu, Yuntian, Scattergood, Ronald O. Year: 2015 The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys with 1, 2, and 4 at. pct Hf addition at 1173 K (900 °C). Microstructural characterizations using aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy reveal high density of HfO2 nanoparticles with sizes of ~4 nm dispersed throughout the ferritic matrix. This indicates that kinetic stabilization by HfO2 nanoparticle pinning is primarily responsible for the observed high thermal stability. In addition, some Hf and Cr segregation on grain boundaries is observed in the Fe-14Cr-4Hf, suggesting the existence of thermodynamic stabilization at high Hf content. Second-phase precipitations such as hafnium carbide, M23C6, and Fe-Cr-Hf intermetallic phase are also found in the Fe-14Cr-4Hf, but their large sizes and inter-spacing suggest that their contribution to stabilization is minimal. Close