Modelling of Processing Steps of New Generation ODS Alloys
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INTRODUCTION AND MOTIVATION
AT temperatures above 1000 C, oxide dispersion strengthened (ODS) alloys exhibit better microstructural stability than the established Ni base superalloys. The outstanding creep properties of the latter result from a high volume fraction of ordered c¢ precipitates.[1] At temperatures above 1000 C, however, these precipitates are not stable enough against coarsening and morphological changes (rafting),[2,3] and the creep strength of the alloys drops accordingly. Oxides are sufficiently stable at temperatures above 1000 C and yttrium-based oxides exhibit exceptional resistance against coarsening up to 1200 C.[4,5] Moreover, the oxides are effective obstacles against dislocation motion as explained in the model proposed by Ro¨sler and Arzt.[6] Recent ODS alloys are strengthened by about 0.5 vol pct yttrium-based oxides with typical sizes 5 to 10 nm. They are produced by mechanical alloying of the powders of the matrix forming metals and of yttria. Until recently, it was generally accepted that the yttria particles are fragmented during mechanical alloying to achieve sizes in the nm range at the end. New findings, however, indicate that the yttria particles totally dissolve in the matrix during mechanical alloying. The oxygen released from the dissolved yttria is trapped at the abundantly available dislocation cores. Thus up to about 1 wt pct oxygen can be
J. SVOBODA and V. HORNI´K are with the Institute of Physics of Materials, Academy of Science of the Czech Republic, Zˇizˇkova 22, 616 62 Brno, Czech Republic. Contact e-mail: [email protected] H. RIEDEL is with the Fraunhofer-Institut fu¨r Werkstoffmechanik IWM, Wo¨hlerstra&z.crepsve 11, 79108 Freiburg, Germany. Manuscript submitted June 17, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
accommodated, if the dislocation density in the powder reaches 1017 m2.[7] Such a high amount of dissolved oxygen in the mechanically alloyed powder of the Fe-Al-O, Fe-Al-O-Y, Fe-Al-Cr-O-Y or Fe-Al-CrMo-O-Y systems is also verified experimentally by two of the present authors (JS and VH). This finding provides the basis for a new generation of ODS alloys with an oxide volume fraction by one order of magnitude higher than in classical ODS alloys. Necessary prerequisites for excellent creep properties of ODS alloys at temperatures above 1000 C are coarse grains and a homogeneous dispersion of nano-oxides in the grains. Such a microstructure can be achieved by consolidation of the mechanically alloyed powders using hot rolling, which leads to an ultra-fine-grained microstructure, and by subsequent annealing leading to formation of a coarse-grained microstructure.[5] To understand and predict the microstructural evolution during processing, a model for recrystallization and grain growth is needed. Doherty et al.[8] summarize the material science knowledge of the 1990s on recrystallization. Most of the modelling work at that time was based on the classical Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach,[9] which describes the recrystallization process by an equ
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