The effect of high temperature exposure on the structure and oxidation behaviour of mechanically alloyed ferritic ODS alloys

Journal of

ELSEVIER Journal of Materials Processing Technology 53 (1995) 93--100

Materials Processing Technology

The Effect of High Temperature Exposure on the Structure and Oxidation Beha~a~ur of Mechanically Alloyed Ferritie ODS Alloys A. Czyrska-Filemonowicz*, D. Clemens, WJ. 0uadakkers Institute for Materials in Energy Systems (IWE 1), Research Centre Jiilich, 52425 Jiilich,

Germany

* Faculty of Metallurgy and Material Science, University of Mining and Metallurgy, Cracow, Poland

As a part of a broader study of FeCrAl-base ODS alloys, the effect of yttria content on the oxidation behaviour of PM 2000-type alloys at 1200"C has been investigated. The growth rate of the alumina surface scale increases with yttria content in the alloys. SEM studies of oxide fracture surfaces and TEM analyses of the bulk material and of the oxide mierostrueture revealed, that the decreased oxidation resistance of high yttria containing alloys is caused by finer oxide grains resulting in an increased number of oxygen diffusion paths. After longer times formation of voids and mierocracks at the grain boundaries in the oxide scales of the high-yttria alloys leads to a further increase in growth rate because of additional oxygen transport. The microstructural changes caused by high temperature exposure were correlated with the differences in oxidation resistance of the alloys investigated. 1. I N T R O D U C T I O N Due to excellent corrosion resistance and good mechanical strength at high temperatures, oxide dispersion strengthened (ODS) alloys are promising materials for high temperature components operating in aggressive environments [1]. The ODS alloys produced by mechanical alloying (MA) derive their specific properties from a dispersion of very fine oxide particles (preferentially yttria) in a metallic matrix. The oxide dispersoids, which are much more stable than precipitates such as carbides or intermetallic phases in conventional superalloys, act as obstacles for dislocations and therefore improve the creep behaviour at high temperatures. The high aluminium content in the ma~ix (5%) leads to the formation of a slowly growing, alumina scale during exposure at high temperatures which provides excellent oxidation resistance. The presence of yttria (around 0.5% in commerical alloys) is necessary to improve the oxide scale adherence. However, previous studies on MA 956 type materials have shown that the oxide growth rate is affected by the exact alloy yttria content, an increase of the yttria content (from 0.17 to 0.7%) led to an increase in oxide growth rate [2, 3]. In the present investigations the mierostrueture of PM 2000 type ferritie ODS alloys with different yttria contents has been studied after high temperature oxidation. Scanning- and transmission electron microscopy (SEM, TEM) were used in order to correlate the alloy yttria content, the mierostructure of the oxide scale and the microstructure of the bulk materials with the oxidation behaviour. Elsevier Science S.A. S S D I 0924-0136(95)01965-H

94

A. Czyrska-Filemonowicz et al. / Journal of Materials Processing Technology 53 (1995) 93-100

2. EXPERIMENTAL

Two mechanically alloyed (MA) ODS materials were investigated: PM 2000 (Fe, 20Cr, 5.5A1, 0.3Ti, 0.5Y203 in wt-%) and PM 2002 (composition as alloy PM 2000 but additionally containing 0.25% metallic yttrium, which was completely oxidized to yttria during the MA process). The alloys were supplied as recrystallized bars by Plansee Metall AG, Reutte, Aus(ria. Oxidation studies were carded out at 1200"C with flat specimens of size 20xl0mm and 2mm thickness with an 800 grit surface finish. Oxide growth rates and spalling behaviour were investigated by gravimetrical analysis after intermediate cooling at regular time intervalls during the long time exposures up to 3000h in air. The sa'uctural investigations were performed by optical metallography, SEM and TEM. Oxide scale microstructure was studied by SEM (fracture surfaces) and by TEM (thin foils prepared from the scale close to the oxide surface). The bulk material was examined by TEM using thin foils and extraction double-replicas. Replicas were used for EDS analysis of the dispersoid composition and for measurements of dispersoid size distribution. TEM specimen preparation is described in refs. [2,4]. TEM investigations were carded out using a JEM 200 CX equipped with an energy dispersive X-ray spectrometer (EDS) and JEM 2010 URP electron microscopes. Quantitative microstructural analysis of TEM micrographs was carded out using an interactive image analysis system (IBAS) for dispersoid size and scale grain size determination. 3. RESULTS AND DISCUSSION

Fig. 1 shows weight changes as a function of time during cyclic oxidation of PM 2000 type materials with d~fferent yttria contents at 1200°C. The gravimetrical analyses conf!rm the results obtained for other ferritic ODS alloys, namely MA 956 ~pe materials 1.~]: alh)y PM 2002, which contained the higher yttria content, showed higfier oxide growth rates.

E o d, E

j

..j

..."

(u o~ cO cc-

o.,

...... -I u

9 ~

o

1

~ we;oh', l o s s d'~e .o

/

!

oxide scale sDa::;ng [

5oo

|

1ooo

15oo

Time ,/ n

Fig. 1

Effect of ym-ia content on oxidation at 1200°C in air of PM 2000 type alloys.

A. Czyrska-Filemonowicz et al. / Journal of Materials Processing Technology 53 (1995) 93-100

95

Detailed analysis of the data revealed, that the scale growth kinetics of the alloy with an yttria content of 0.5%, obeys a rate law Am = k.t n where ~ m is the weight change, t the exposure time and k the oxidation rate constant. The value of n is approximately 0.35. The alloy with the highest ytWia content however, shows an n which is near to 0.5; the n even increases after exposure times of more than around 500h. Figs. 2-4 shows the microstructure of the bulk material of the alloys investigated.

Fig. 2

The microstructure of the Alloy PM 2000 exposed for 1000 h at 1200°C. a) Fine Y-AI oxides and Ti (C,N) in a ferritic matrix. b) Yttria denuded zone along the grain boundary. c) HREM image of the dispersoid and the matrix.

96

A. Czyrska-Filemonowicz et al. / Journal of Materials Processing Technology 53 (1995) 93-100

Tile microstructure consists mostly of mixed Y-A1 oxide dispersoids and some larger particles of pure A1203 and titanium carbonitrides Ti(C, N) in a ferritic matrix (Fig. 2a). An interesting observation was an yttria denuded zone along some high-angle grain boundaries (Fig. 2b), what could be related to enhanced diffusion processes along these grain boundaries. Fig. 2c shows high resolution electron microscopy (HREM) image of the finest dispersoid. Statistical measurements of dispersoid size (Figs. 3,4) revealed that the dominant small dispersoids, which influence the creep resistance, were in the size range 3 to about 60 nm. The mean diameters d of these dispersoids were measured as d -- 9.2nm for alloy PM 2000 and as d = 18.0rim for alloy PM 2002. They were identified as yttrium aluminium lnonoclinic (YAM) 2Y2OyA1203 or yttrium aluminium tetragonal (YAT) 3Y2Oy5AI20 3. Previous studies on the stability of dispersoids in PM 2000 tyj?e ODS alloys showed that dispersoids coarsened during heat treatment above about 1150°C [5,6]. (b) 1.40~

....

-

/ 1.00-

i

O180 " ~ 0.60-! z

tl ~'£-~ ~

I !

01201.... J ~ 0,00 1E+00

Fig. 3

1

2

4

6 1E+01 2 P o r t i c l e size / n m

4

6

1E402

a) Fine dispersoids extracted from the Alloy PM 2000 exposed for ] 000h at 1200°C b) Size distribution of the dispersoids (TEM-IBAS)

(a) ~:~

:

(b) 10

8-

o z

6-

4-

2-

i:

............

~ ~:

1E+O0 ;

Fig. 4

I

.

2

.

.4

.

.

.

.

6 1E+01 P a r t i c l e size /

.

2 nm

4

6

a) Fine dispersoids extracted from Alloy PM 2002 exposed for 100011 at ] 200°C b) Size distribution of the dispersoids (TEM-IBAS)

1E+02

A. Czyrska-Filemonowicz et aL / Journal of Materials Processing Technology 53 (1995) 93-100

97

A typical example of the effect of ytlria content on oxide microslructure is shown in Figure 5. Fracture surfaces of oxide scales formed during 1000h oxidation at 1200"C showed on the alloy with low yttria (I'M 2000), small grains at the scale/gas interface and columnar grains which were enlarged in direction of the scale/alloy interface. The scale on the high yttria alloy PM 2002 does not explicitly show this increasing grain size of the oxide in direction of the interface with the alloy. A second striking difference is, that the high yttria alloy exhibits a large number of cavities (microcracks) at the oxide grain boundaries. These effects were observed in all studied cases (exposures at 1100 and 1200"C for times of 5 - 1000h) for the PM 2000 and also for the MA 956 type ODS materials [2, 7].

Fig. 5 Alloy PM 2000 (a) and PM 2002 (b) exposed for 1000h at 1200"C. Fracture surfaces of oxide scales (SEM). In the previous studies of oxidation behaviour of ferritic MA 956 type materials [7], the growth mechanisms of the oxide scales were studied by two-stage oxidation. The oxygen isotope profiles in the alumina scales showed for both alloys the 1°O to be enriched near the scale/alloy interface, indicating that the scales are growing by oxygen grain boundary diffusion. Apparently a variation of the yttria content in the studied range does not fundamentally affect the scale growth mechanisms, what is in agreement with previous studies on MA 956 type materials [7]. This is confirmed by the scale structure revealed by SEM (Fig. 5) which shows that the grain size increases in growth disection.

98

.4. Czyrska-Filemonowicz et al. / Journal of Materials Processing Technology 53 (1995) 93-100

TEM investigations of thin foils prepared from the oxide scale allowed to reveal details of the scale morphology formed on the alloys investigated (Figs. 6-8).

Fig. 6:

Alloy PM 2002 exposed for 1000h at 1200"C. Morphology of the alumina scale (TEM mierograph of scale thin foil).

Statistical measurements of scale grain sizes close to the sta'faces performed on thin foils using TEM and IBAS has been determined in both specimens as 1.2 to 1.4 Ixm. On the grain boundaries large yta-ium containing particles were observed (Figs. 6,7), which also were found by other investigators [8]. The yttria particles precipitated on the grain boundaries were much larger in comparison with the dispersoids in the bulk material (Fig. 7). EDS analysis showed that they contained aluminium or titanium [9].

Fig. 7:

The Y-A1 oxides in the alumina scale (a) and in the bulk material (b) of Alloy 2002 exposed for 1000h at 1200"C

A. Czyrska-Filemonowicz et aL / Journal of Materials Processing Technology 53 (1995) 93-100

99

TEM analysis revealed other structural defects formed within the alumina scale: dislocations and voids which were formed homogeneously or on dislocations within the grains. The latter were found with the image slightly out of focus and under kinematical diffraction conditions. In underfocused conditions, as presented in Fig. 8, dark Fresnel fringes which surrounded the voids, often faceted, were observed.

F~g.8

Alloy PM 2002 exposed for 1000h at 1200"C. Structural defects within the alumina scale; voids formed on the grain boundaries (a) as well as on dislocations and homogeneously within the alumina scale (b) (TEM micrograph of scale thin foil).

100

A. Czyrska-Filemonowicz et al. / Journal of Materials Processing Technology 53 (1995) 93-100

4. SUMMARY To study the effect of high temperature exposure on the structure and oxidation behaviour of ferritic ODS alloys, specimens of two alloys PM 2000 and PM 2002 with different yttria content were oxidized at 1200"C up to 3000h. The microstructure of bulk materials consisted mostly of fine yttrium alumim'um oxide dispersoids, in the size range 3 to about 60nm in a ferritic matrix. In spite of their high thermodynamic stability, the dispersoids coarsen and yttrium is transported towards the alumina surface scale. The transport of yttrium from the alloy into the scale requires a relatively easy solubility of the yttria dispersion in the alloy, despite their high thermodynamic stability. Strong indications for desolving of the yttria particles were found by formation of dispersion depleted zone along the alloy grain boundaries. Yttrium diffuses along the grain boundaries of the alumina scale where it forms Y-A1 and Y-Ti oxide precipitates. The oxide growth rate of these alloys at temperatures up to 1200"C increases with increasing yttria content. The high growth rate of the scale can be explained by the observed scale microstructure. High yttria content in the bulk alloy does not fundamentally change the growth mechanisms in the scale; in both studied alloys the alumina scales appeared to grow by oxygen grain boundary diffusion. The high yttria content in the alloy seems to enhance void formation in the alumina scale, thereby decreasing its protective properties. The scale of alloy PM 2002 exhibited voids, often faceted, and microcracking of the oxide grain boundaries after longer oxidation times. As the yttria contents above 0.5wt-% lead to inferior of oxidation resistance, an yttria content which is lower than the amount in commercial ferritic ODS alloys, like PM 2000 (< 0.5 %), is favourable in respect to oxidation resistance.

Acknowledgements The authors appreciate the support of Mr. Baumanns for carrying out the oxidation experiments and Mrs. D. ESer for assistance in TEM specimen preparation. Dr Wallura and Dr R. Ravelle-Chapuis 0EOL) are gratefully acknowledged for carrying out the SEM studies and HREM investigations.

REFERENCES 1. MJ. Bennett, M.R. Houlton; Proc. Conf. High Temperature Materials for Power Engineering Litge, Vol. B (24-27 September 1990) Kluwer Academic Publisher, Dordrecht, the Netherlands, p. 227 2. A. Czyrska-Filemonowicz, R. Versaci, D. Clemens, W.I. Quadakkers; 2nd Int. Conf. Microscopy of Oxidation, 29-31 March, 1993, Cambridge, UK, S.B. Newcomb, MJ. Bennett (eds), The Institute of Materials, London, p. 288 3. WJ. Quadakkers, K. Schmidt, H, Griibmeier, E. Wallura, Materials at High Temperatures 10 (1992) 23 4. A. Czyrska-Filemonowicz, K. Spiradek, S. Gorczyca; Praktische Metallographie 22 (1991) 217 5. P. Krautwasser, A. Czyrska-Filemonowicz, M. Widera, F. Carsughi; Materials Science and Engineering A 177 (1994) 199 6. P. Krantwasser, M. Widera, D. F_~r, B.D. Wirth; Proc. 13th Plansee Seminar, 2A.-28.May 1993, Reutte, Austria, H. Bildstein, R. Eck (eds.) 7. D. Clemens, K. Bongartz, W. Speier, RJ.. Hussey, WJ. Quadakkers; Fresenius J. Analytical Chemistry, 346 (1993) 318 8. K. Przybylski, A.J. Garratt-Reed, B.A. Pint, E.P. Katz, G.J. Yurek;(1987) J Electrochem Society 134 (1987) 3207 9. R. Versaci, D. Clemens, R. Hussey, W.I. Quadakkers; Solid State Ionics, 59 (1993) 235