22.08.2015   Comprehensive nuclear materials

Nanoscale Oxide Particle Control

4.08.2.1 Dissociation and Precipitation

The fine distribution of Y2O3 particles, which is essential to improving the high temperature strength of ODS steels, is attained by the dissociation of oxide particles during MA processing.2 The thermodynam­ically stale Y2O3 particles are forcedly decomposed into the ferritic steel matrix during the MA process. Subsequent annealing induces oxide particles to pre­cipitate finely at elevated temperature of around 1000 °C. The co-addition of Ti during MA proces­sing promotes the decomposition of Y2O3 and then the precipitation of Y-Ti complex oxide particles through an annealing heat treatment.3,4 A field emis­sion ion micro-probe (FIM) analysis confirmed that this type of complex oxide is constituted of several nanometer-sized Y-Ti-O compounds.5-7

The precipitation process of the decomposed Y2O3 was investigated by means of a small angle neutron scattering (SANS) experiment.8 The neutron-scattering cross-section (dS/dO) versus scattering vector (q2) plots for the milled U 14YWT(Fe-14Cr-0.4Ti-3W — 0.25Y2O3) are shown in Figure 1(a). They indicate that the hot isostatic pressing (HIP) of U14YWT at 850 °C leads to the precipitation of a high number density of nanoclusters, as designated by Odette. Figure 1(b) shows the effects of HIP (filled symbols) and powder annealing (open symbols) at tempera­tures of 700, 850, 1000, and 1150 °C. The increase in magnitude and decrease in slope of the dS/dO versus q2 curves indicate that the radius of nanoclus­ters decreases and their number density increases with decreasing temperature at HIP and powder annealing. Annealing at 700 °C produces the highest scattering and lowest sloping, which indicates that the smallest-sized nanoclusters precipitate with the high­est number density at lower temperatures. In terms of an X-ray diffraction experiment using Super Photon

ring-8 eV (Spring-8) constructed in Japan, Kim eta/, recently reported that nanoclusters could be in a noncrystalline state and can be transformed to nano­crystalline oxide particles at around 10000C9

4.08.2.2 Structure and Coherency

With regard to ODS steels without Ti, high resolu­tion (HR) TEM investigations were performed by Klimiankou to investigate the structure of Y2O3.10 The crystallographic lattice of the metal matrix cor­responds to a-Fe with a bcc structure and a lattice constant of a0 = 0.287 nm.11 The Y2O3 has a crystal­line bcc structure with a 1.06 nm lattice constant.11 Figure 2 shows an HRTEM image taken from an Y2O3 particle that is surrounded by the matrix (M) lattice. This image was taken from the grain, oriented with [1 1 1]M zone axis to the electron beam. A fast Fourier transformation (FFT) of the image shows the matrix lattice as a hexagonal pattern with diffraction spots of the {110} type and dM(110) = 0.203 nm dis­tance. In the FFT image, the Y2O3 (YO) lattice is rectangular, with diffraction spots of the {2 2 2} type and a corresponding atomic planes distance of dYO(2 2 2) = 0.306 nm. The angle of 70.50 between diffraction spots of the {2 2 2}YO type marked in Figure 2(a) confirms that the Y2O3 particle is oriented with the [110] zone axis, and consequently [1 1 0]YO// [1 1 1]M. The orientation correlation of both lattices is (1 I I)YO//(1 T 0)M. Therefore, the following Kurdjumov-Sachs orientation relationship12 is satisfied:

(111 )yo//(110)m ;

[110] YO//[111]m : I1]

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Figure 1 Results of a SANS experiments for as-mechanically alloyed powders and after HIP and annealing in U14YWT (Fe-14Cr-0.4Ti-3W-0.25Y2O3). (a) As-MA, 8500C HIP and annealing and (b) HIP (filled) and annealing (annealing) at 1150,1000, 850, and 7000C. Reproduced from Alinger, M. J.; Odette, G. R.; Hoelzer, D. T. J. Nucl. Mater. 2004,382, 329-333.

 

_ (011)

101)(2^^>.(1T0) _ (2TT)

(222) _

(T10)<^j22?)(101) , (011)

 

3.06A

 

70.5

 

3.06 A

 

(a)

 

(b)

 

Figure 2 HRTEM micrograph of the Y2O3 particle with surrounding matrix (a) and FFT image of micrograph (b). The diffraction spots from Y2O3 particle of {2 2 2} type form the rectangle, whereas diffraction spots from the matrix of {1 1 0} type form the hexagon at the [1 1 0] zone axis and [1 1 1] of matrix. Reproduced from Kliniankou, M.; Lindau, R.; Moslang, A. J. Nucl. Mater. 2004, 329-333, 347-351.

 
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Figure 3 EFTEM images of Y2Ti2O7 particles. Reproduced from Kliniankou, M.; Lindau, R.; Moslang, A. J. Nucl. Mater. 2004, 329-333, 347-351.

data are equal to the following data calculated from the Y2Ti2O7 structure: d2 2 2 = 0.29 nm and d0 0 4 = 0.25, and an angle between the (0 0 4) and (2 2 2) atomic planes of 54.7°. The analysis of EFTEM results defini­tively shows that Y-Ti-O particles have a Y2Ti2O7 composition.

These findings suggest that nano-oxide particles precipitate from the ferritic matrix, maintaining crys­talline coherency or partial-coherency with a ferritic matrix. In general, the nucleation and growth of pre­cipitates proceeds, as both interfacial and strain ener­gies become minimal. In the case of ODS steels, interfacial coherency could be maintained between thermodynamically stable nanoparticle precipitates and the ferritic matrix in order to decrease the free energy in the system from the extremely high energy state induced by MA. Elucidation of the details of the nanoscale precipitation is important not only as basic materials science research but also as the develop­ment of high-strength engineering materials.