Modeling in support of the development of steels for high-temperature applications

Oxide dispersion strengthened alloys are known for their better chemical and mechanical

properties affected by composition and microstructure, respectively. Oxide Dispersion

Strengthened (ODS) steels are potential candidates for high temperature applications such as

future fusion and nuclear reactor, chemical reactors, ultra supercritical steam turbines, etc in the

form of structural members. The addition of nano-size ODS particles enhances the high

temperature creep strength of these steels. In addition such steels require good oxidation

resistance, reduced activation when exposed to neutron irradiation, and long-term microstructure

stability at high temperatures as those expected during service life of component [1, 2, 3].

Radiation damage in materials is quantified by the displacement per atom (DPA). DPA

represents how many times an atom is displaced from its original atomic lattice position to a new

lattice position as a result of given radiation flux. Candidate materials intended for fusion

structural materials and plasma facing materials should consist of following elements for low or

reduced activation property of composition to meet low level waste criteria: Cr, Ti, V, Fe, W, Si,

and C. High temperature stability of microstructure is decided by the amount of recrystallization

occurring which in turn is affected by the amount of applied strain, particle volume fraction, size

distribution, and spacing between them. High temperature corrosion resistance is enhanced by

presence of Cr in steel; however, its content needs to be optimized in order to balance the

demerit of aging embrittlement while maintaining strength at high temperature [4]. Excess

oxygen (except for oxygen concentration in Y2O3) resulting from processing is considered

harmful for microstructure and mechanical properties of steel and hence needs to be controlled at

the initial stages of processing.

Dispersion of fine ODS particles cause zener drag and renders recrystallization sluggish.

Nano ODS particles of Y2O3 or Ti2Y2O7 are known to trap irradiation induced point defects and

diffusing atoms like Cr, Nb, Mo, V; at the particle-matrix interface, thereby suppressing aging

embrittlement [2]. Bimodal distribution of Y-containing ODS particles is present in reduced

activated ferritic steels (RAF) ODS steel wherein Ti is also found at the particles. Y2O3

dispersoid shows coarsening above 600 oC, however addition of Ti refines the size of

dispersoids, increases the number density and no significant dispersoid coarsening for Ti2Y2O7

particles was observed up to 1000 oC [3]. Hence steels with Y-Ti-O type nano particles exhibit

higher elevated temperature strength and improved creep and irradiation resistance. When Y/Ti

ratio is less than 1, the dispersoids are confirmed to be Ti2Y2O7. These particles are formed

homogeneously, in the grain interior as well as at the grain boundaries [5].

A minimum of 4 hrs milling of the atomized steel and Y2O3 powders is performed to get

optimum properties [3]. In RAF-ODS steel bimodal grain distribution is observed which may be

attributed to the hot isostatic pressing (HIP) processing parameters and needs to be studied

further [5]. With change in hot extrusion temperature from 1150 oC to 1200

oC, degradation in

creep and tensile strength was observed and needs to be investigated further [6].

A strong coherency between oxide lattice and matrix results in effective strengthening

due to oxide dispersion. In case of considerable differences in their lattice parameters, smaller

particles (< 5 nm) ensures better coherency with the matrix, resulting in superior high and low

temperature creep resistance with increased strength. S. Ohtsuka et. al. [6] has suggested that in a

9Cr ferritic-martensitic ODS steel, finer and denser particle precipitation is observed in presence

of elongated residual ferrite in the structure after annealing treatment. The fraction of elongated

ferrite grains is controlled through titanium, which in excess combines with carbon, and

resultantly increases the ferrite concentration. According to the theory on precipitate coarsening

[7, 8] oxide particle growth is controlled by the interfacial energy between an oxide particle and

matrix. If the lattice misfit between ferrite phase and oxide particles is smaller than that between

austenite phase and oxide particles, the enhanced and finer precipitation of oxide dispersion in

elongated residual ferrite can be interpreted in light of the above precipitation theory.

Oka et. al. [9] have observed an orientation relationship between the ODS particles and

matrix similar as in case of precipitates; oxide // matrix for Cr-NiODS (fcc structure)

and oxide // matrix for 12CrODS (bcc structure). AP-FIM analysis of ODS particles

showed inner and outer shell like structure with larger Ti-enriched region compared to Y-

enriched region, indicating Ti enrichment occurred at the outer region of the particles during

fabrication process at high temperature, as Ti was not contained in original Yttria powder. Under

electron irradiation particles less than 10 nm dissolved at lower temperature, whereas in case of

larger particles some amount of sputtering was observed on the surface of the particle including

the interface. Stability of ODS particles is commented to be dependent upon the composition and

purity (amount of Ex. O) of oxide particles. In lab scale studies irradiation environment such as

vacuum condition also plays a role in accurately determining irradiation damage in material.

Recovery is the dominant softening mechanism in these particles containing steels [3, 5]

whereas recrystallized grains are formed in the vicinity of the prior grain boundaries or coarse

precipitate particles due to particle stimulated nucleation (PSN). However, no significant

dispersoid coarsening for Ti2Y2O7 particles was observed up to 1000 oC and softening due to

grain coarsening is ruled out. During annealing, recrystallization occurs preferentially in grains

belonging to the -fiber texture component on account of their higher stored energy.

Electron backscattered diffraction (EBSD) has been successfully used to characterize the

grain boundaries up to 2o misorientation, however in order to study the nature of

particle/boundary interaction resulting in recovery/recrystallization, TEM is essential. EBSD has

also been used to study the microtexture evolution in differently strained components [5].

As can be seen, very fine oxide particles contribute strongly to the strength and creep-

rupture properties but are dissolved easily during electron irradiation. Stability of oxide particles

under irradiation conditions is reported to be dependent on its composition and purity. Numerous

sources reported refinement and dense formation of ODS particles in presence of Ti and is also

found to be dependent on the matrix phase and its orientation. High temperature service

conditions cause temperature-dependent softening phenomenon in material reducing its high

temperature strength as well as aging embrittlement causing premature failure. Recovery is the

most dominant while limited texture-dependent recrystallization is also observed. HRTEM

studies of the particle-matrix interface under irradiative and high temperature conditions forms

the major part of the project to study the softening mechanism, their kinetics, aging

embrittlement, and defect-sink phenomenon at particle interface. Hence for future development

of ODS steels, oxide particle composition, size, density and volume fraction needs to be

finalized. This configuration should be such to balance the strength and stability of

microstructure against irradiation damage, at intended high temperatures. For the purpose,

categorical effect of thermo-mechanical processing of ODS steel on development of oxide

particle composition, dispersion as well as textural changes in the matrix, needs to be simulated

physically and/or mathematically for next stage of development.

References

1. H. R. Z. Sandim, R. A. Renzetti, A. F. Padilha, A. Moslang, R. Lindau, and D. Raabe.

Annealing behavior of RAFM ODS-Eurofer steel. Fusion Science and Technology, Vol. 61,

2012, pp. 136.

2. H. R. Z. Sandim, R. A. Renzetti, A. F. Padilha, D. Raabe , M. Klimenkov, R. Lindau and A.

Moslang. Annealing behavior of ferritic-martensitic 9% Cr-ODS-Eurofer steel. Materials

Science and Engineering A, A 527, 2010, pp. 3602.

3. G. Sundararajan, R. Vijay, and A. V. Reddy. Development of 9Cr ferritic-martensitic and

18Cr ferritic oxide dispersion strengthened steels. Current Science, Vol. 105, No. 8, 2013,

pp. 1100.

4. S. Li, Z. Zhou, M. Wang, H. Hu, L. Zou, G. Zhang, and L. Zhang. Microstructure and

mechanical properties of 16 Cr-ODS ferritic steel for advanced nuclear energy system.

Journal of Physics: Conference Series, DOI: 10.1088/1742-6596/419/1/012036.

5. Ch. Ch. Eiselt, M. Klimenkov, R. Lindau, A. Moslang, H. R. Z. Sandim, A. F. Padilha, and

D. Raabe. High resolution transmission electron microscopy and electron backscatter

diffraction in nanoscaled ferritic and ferritic-martensitic oxide dispersion strengthened-steels.

Journal of Nuclear Materials, 385, 2009, pp. 231.

6. S. Ohtsuka, S. Ukai, M. Fujiwara, T. Kaito, and T. Narita. Improvement of creep strength of

9CrODS martensitic steel by controlling excess oxygen and titanium concentrations.

Materials Transactions, Vol. 46, No. 3, 2005, pp. 487.

7. D. A. Porter and K. E. Easterling. Phase transformations in metals and alloys, Stonely

Thrones Publishers Ltd., Cheltenham, 2000, pp. 314.

8. J. W. Martin and R. D. Doherty. Stability of microstructure in metallic systems, Cambridge

University Press, Cambridge, 1976, pp. 173.

9. K. Oka, S. Ohnuki, S. Yamashita, N. Akasaka, S. Ohtsuka, and H. Tanigawa. Structure of

nano-size oxides in ODS steels and its stability under electron irradiation. Materials

Transactions, Vol. 48, No. 10, 2007, pp. 2563.