Microstructure Evolution in ODS Alloys with a High-Volume Fraction of Nano Oxides

Abstract

Existing oxide dispersion strengthened (ODS) alloys are, besides Ni-based superalloy single crystals, the most creep-resistant materials. The creep resistance of the ODS alloys may, moreover, be significantly improved thanks to increasing the volume fraction of the nano oxides by more than one order of magnitude so that the oxides play a decisive role in strengthening. The present experimental study deals with two systems of such a high-volume fraction of nano oxides, namely, the Fe-11Al-1O and Fe-17Cr-7Al-4Y 2 O 3 systems prepared by mechanical alloying and hot rolling leading to a rather stable fine-grained microstructure. This microstructure undergoes static recrystallization at high temperatures. The kinetics of static recrystallization and coarsening of nano oxides in recrystallized grains is determined for both systems. The difference in kinetics of coarsening of Al-based and Y-based oxides in the Fe-11Al-1O and Fe-17Cr-7Al-4Y 2 O 3 systems is expressive and predetermines the Fe-17Cr-7Al-4Y 2 O 3 system or similar ones to become the new leading system among creep-and oxidation-resistant materials for applications up to 1200 • C. fraction of even finer sizes of oxides. Then, however, one cannot expect the resistivity of oxides against coarsening at very high temperatures. ODS alloys are produced in two steps: nano-composite powder from the matrix of the respective chemical composition and Y 2 O 3 is produced by mechanical alloying (MA), which is then hot consolidated in the second step. The recent commercially produced ODS alloys are represented by MA956 or MA957 [4], PM 2000 or PM 2010 [5], ODM alloys [6], 1DK or 1DS [7]. The non-commercially produced, experimental and advanced versions of ODS alloys are ODS Eurofer [8], 12YWT [9], 14YWT [10] and 9YWT [11]. New research on 14YWT [12] showed that mechanical properties of ODS alloys can significantly be improved via optimized processing consisting of a controlled thermo-mechanical treatment and in lowering content of N and C introduced during MA. Both steps led to a significant increase in fracture toughness attributed to improved cohesion of grain boundaries. However, processing of 14YWT is still extremely demanding [12].The excellent creep properties of the ODS alloys are explained by an attractive interaction of dislocations with oxides modeled by Rösler and Arzt [13], who predicted a threshold stress for creep decreasing with coarsening of oxides. A more detailed discrete dislocation model treating the collective interaction of precipitates with dislocations during creep has been published very recently [14]. A model for coarsening in multicomponent systems [15] shows that coarsening kinetics is proportional to the product of the solubility and the diffusion coefficient of O in the matrix. The solubility of O is determined by the chemical stability of the oxide (yttria is reported as the chemically most stable oxide) and a significant resistance against coarsening was confirmed by Krautwasser et al. [16] for yttria particles. The creep properties of ODS a...

The Influence of Aluminum Content on Oxidation Resistance of New-Generation ODS Alloy at 1200 °C

“…Excellent creep strength of the ODS alloys is associated with an attractive interaction between dislocations and nano-oxides as it is successfully modelled by Rösler and Arzt [19], who also predict a threshold stress for creep and its significant drop with coarsening of the nano-oxides. The coarsening kinetics of particles in multicomponent systems is modelled by Fisher et al [20] and experimentally studied e.g., by Bartkova et al [21] or by Svoboda et al [22] indicating a very high 2 of 13 resistance of Y-nano-oxides against coarsening in ODS alloys. The benefits of high creep resistance of ODS alloys at very high temperatures are conditioned by a sufficiently good oxidation resistance.…”

“…The research effort of the authors resulted in development of a mechanically alloyed powder with a very high content of O, allowing production of an ODS alloy (called new-generation ODS alloy by the authors) of rather high nano-oxide volume fraction of about 5%, which exceeds the volume fraction in classical ODS alloys by one order of magnitude [4,6]. The processing of the new-generation ODS alloy consists of three steps [22]: (1) MA to produce chemically homogeneous powders, (2) hot consolidation of the powders by intensive plastic deformation, e.g., by rolling, providing a compact ultra-fine grained alloy due to an intensive dynamic recrystallization and (3) heat treatment to provoke secondary recrystallization leading to rather coarse-grained microstructure conditioning an excellent creep resistance. A complex optimization of the chemical composition of the new-generation ODS alloy performed recently by the authors of the Institute of Physics of Materials leads to conclusion that pure Y-nano-oxides as strengthening particles are much better with respect to coarsening than other nano-oxides [21,22], as well as in terms of the cohesive strength of grain boundaries being influenced by wetting of the nano-oxides (unpublished results).…”

“…The processing of the new-generation ODS alloy consists of three steps [22]: (1) MA to produce chemically homogeneous powders, (2) hot consolidation of the powders by intensive plastic deformation, e.g., by rolling, providing a compact ultra-fine grained alloy due to an intensive dynamic recrystallization and (3) heat treatment to provoke secondary recrystallization leading to rather coarse-grained microstructure conditioning an excellent creep resistance. A complex optimization of the chemical composition of the new-generation ODS alloy performed recently by the authors of the Institute of Physics of Materials leads to conclusion that pure Y-nano-oxides as strengthening particles are much better with respect to coarsening than other nano-oxides [21,22], as well as in terms of the cohesive strength of grain boundaries being influenced by wetting of the nano-oxides (unpublished results). Moreover, the influence of Mo, Ta, La, Ti, Co and Ni alloying on the creep resistance is evaluated as negative or neutral and Cr alloying as slightly positive (unpublished results).…”

High Temperature and Corrosion Properties of A Newly Developed Fe-Al-O Based OPH Alloy

“…However, it was confirmed that they grow during annealing at 1200 • C. As the nano-oxides are very stable against coarsening up to temperatures of 1000 • C, and their number density is kept very high, the Zener pinning effect is also active at these temperatures. The Zener pinning effect decreases due to the coarsening of nano-oxides at temperatures over 1200 • C, and secondary recrystallization takes place [25].…”

“…As the nano-oxides are very stable against coarsening up to temperatures of 1000 °C, and their number density is kept very high, the Zener pinning effect is also active at these temperatures. The Zener pinning effect decreases due to the coarsening of nano-oxides at temperatures over 1200 °C, and secondary recrystallization takes place [25]. It is very difficult to measure the grain size with automatic software and therefore the evaluation is done by in-house software according to the American Society for Testing and Materials (ASTM) E112-13, "Standard Test Methods for Determining Average Grain Size".…”