Scripta h4atedia, Vol. 37, No. 8, pp. 1215-1220,1997 Elsmier Science Ltd

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MICROSTRUCTURE OF INCOLOY MA956 AFTER LOW AND HIGH TEMPERATURE DEFORMATION

EL Dubiel’, M. Whbel’, P.J. Ermis2, A. Czyrska-Filemonowicz’ ‘University of Mining and Metallurgy (AGH), Faculty of Metalhqy and Materials Science,

Al. Mickiewicza 30,30-059 Krakbw, Poland ‘Research Centre J&h, Institute for Materials in Energy Systems 1,

D-52425 JUlich, Germany

(Received March 6, 1997) (Accepted May 2, 1997)

Introduction

INCOLOY alloy MA956 is an Fe20Cr5A10.5Y203 alloy produced by the mechanical alloying process which provides a fine dispersion of the yttrium oxide phase in the ferritic Fe-Cr-Al matrix. The alloy exhibits outstanding strength and corrosion resistance at temperatures even above 1000°C. Calcula- tions based on the difference in the elastic moduli of the oxide particles (dispersoids) and the ferritic matrix (1, 2) led to the conclusion that the repulsive dislocation-dispersoid interaction can be the source of the strengthening due to Orowan bowing of dislocations between particles. However, after high temperature deformation an attractive interaction between dislocations and hard incoherent oxide particles has been observed (3, 4).The aim of the present investigations was to reveal the nature of interaction between dispersoids and dislocations developed in INCOLOY MA956 during plastic de- formation performed at systematically varied temperatures up to 950°C. An intention of such investi- gations was improving our understanding of the deformation mechanisms in ferritic ODS alloys.

Experimental Procedure

INCOLOY alloy MA956 was supplied by INCO Alloys International as hot extruded and recrystal- lized (133OWl h) bars of 20 and 30 mm diameter. The composition of the alloy investigated was as follows (in wl: %): 0.02 C, 0.09 Si, 19.8 Cr, 0.13 Ni, 0.03 Co, 0.31 Ti, 4.6 Al, 0.03 N, 0.52 YZ03, bal. Fe. Because tbe 30 mm diameter bar was particularly coarse grained, it was possible to cut single crystal specimens of 6 mm diameter by spark machining. The resulting rods of 150 mm length were polished, etched and examined for grain boundaries to ensure that they were really single crystals. X-ray diffiction analysis was used for selection of the single crystals with orientation of the longitu- dinal axis very close to the crystallographic direction ~11 l>.

Specimens for tensile testing were prepared Tom the bars; gauge lengths of the polycrystalline and the single crystalline specimens were 25 mm and 15 mm and the gauge diameters were 6 mm and 3 mm, respectively. After testing in tension the specimens were furnace cooled to about lOO”C, due to

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TABLE 1

Details of Tensile Tests Carried Out

experimental limitations. Because such slow cooling can influence the microstructure, compression tests (which enable rapid cooling of the deformed specimens), were carried out on single crystal cylin- drical specimens (6 mm diameter, 9 mm height). The friction between the specimen and the compres- sion platens was reduced by a high temperature lubricant. After testing, the specimens were quickly cooled down in liquid nitrogen. Details of the mechanical tests carried out are presented in Table 1.

Structural analysis was performed using optical metallography and transmission electron micros- copy (TEM). The TEM investigations were carried out using JEM 1OOC and JEM 2010 ABE micro- scopes. Thin foils were prepared by double-jet electropolishing in 10% solution of the perchloric acid in glacial acetic acid at about 10°C and 50 V.

Results and Discussion

The microstructure of INCOLOY MA956 in the as received condition (Figure 1) consisted of mixed Y-AI dispersoids mostly tetragonal Y3AlsO12 (YA’I’) and some larger particles of pure alumina and titanium carbon&ides in a ferritic matrix with extremely low dislocation density. The mean diameter

Figure 1. TEM micrograph of INCOLOY MA956, as received (extruded and recrystallized 1 h/1330°C, air-cooled).

Vol. 37, No. 8 INCOLOY MA956 1217

___

I MA956 single ctystol specimens

0 0.65 o.io 0.i5 0

strain 10

Figure 2. Effect of temperatme on stress-strain curves of INCOLOY MA956 single crystals; strain rate 10%“.

of the fine dispersoids YAT was measured as 9 nm [5]. Figure 2 shows typical tensile curves of single crystal specimens. In the temperature range 20-400°C the tensile curves were typical and revealed strain hardening. At temperatures above 600°C an early onset of necking was observed. The results of tensile tests are discussed in [5,6].

In Figure 3 the normalized maximum tensile and compressive stress (o,dG) versus temperature is presented. The shape of the stress/strain curves showed that the maximum stress was reached at rather low strains; the maximum stress may therefore be taken as an indication of dislocation/particle interac- tion. Two temperature regions of plastic deformation can be distinguished. It suggests that in the tem- perature range 400-600°C a change of dislocation-dispersoid interaction occurred, thermally activated processes beginning to play an important role in plastic deformation of INCOLOY MA956.

To cover different regions of plastic deformation the specimens compression tested at 20,400 and 8OO’C were selected for microstructural investigations. In the microstructural analyses attention was focused on the examination of a dislocation substructure, especially dislocation configurations in the vicinity of dispersoids. This allows investigation of the nature of dislocation-dispersoid interactions,

0. I8 I

00 compression tests

c 0 i I I 0 200 400 So0 800 l&O

test temperature, ‘C

Figure 3. Normalized maximum tensile and compressive strengths as a function of test temperature.

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Figure 4. Microstructure of the specimen compression tested at 4OOT, showing dislocation loops around dispersoids; ‘EM micrographs, (a) bright field and (b) dark field images.

which limit a mobility of dislocations and therefore influence the strength of the material. There are at least two possibilities for the interaction with non-shearable dispersoids.

The first is connected with a repulsive dislocation-dispersoid interaction. According to the Orowan mechanism [7], this leads to strain hardening as was observed on the tensile curves. The characteristic microstructural feature for this mechanism are dislocation loops around particles. In the specimens tensile tested at 20 and 400°C and compression tested at 400°C such features were observed. Figures

Figure 5. Microstructure of the specimen compression tested at 4OO“C, showing shear bands in the localized region of plastic deformation.

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Figure 6. Dislocalrions in the vicinity of dispersoids in the microstructure of INCOLOY MA956 compression tested at SOO’C; TEM bright (a) and dark (b) field images.

4 a, b show the dislocation-dispersoid configurations in the microstructure of a specimen compression tested at 400°C. TEM bright and dark field images revealed the contrast of dislocations in the vicinity of dispersoids, which suggests Orowan bowing of dislocations between particles. The specimens de- formed at 400°C were characterized by strong localization of plastic deformation with the appearance of thin shear bands (Figure 5). The dislocation density p was high and exhibited significant differences between plastic deformation localization area (p = 3.5. 10” m-‘) and the remaining part of tested specimen (p = 4.6. x lOI m”).

The second type of mechanism is based on the local climb over particles and an attractive disloca- tion-particle interaction (S-11). In the specimens that were tensile and compression tested at SOOT, the dislocation density was very low (p = 1.4. x 10” m”) and the dislocations were often pinned at the departure side of particles, as shown in Figures 6 a, b. The curvatures of dislocation lines in the matrix and in the vicinity of dispersoids had the same sign. This type of the structure gave the impression of an attractive interaction between dislocations and dispersoids. The same configurations of dislocation and dispersoids were also observed in the structure after creep deformation at 950°C and 1050°C (4). It is not expected that such a dislocation-dispersoid interaction can produce a significant strain hardening, hence in specimens tensile tested at 800°C and above an early onset of necking was observed. It can, therefore, be concluded that the local climb mechanism and/or an attractive interaction between the dislocation and the yttrium-aluminium oxides postulated by R&ler and Arzt (8, 9) can be applied for high temperature deformation of the ferritic ODS alloy INCOLOY MA956.

Summary

The ferritic Fe-&Al alloy INCOLOY MA956 strengthened by oxide dispersoids is considered for applications al temperatures around 1000°C. The oxide particles act as obstacles for moving disloca- tions. The results of tensile and compression tests showed that there are two different mechanisms of strengthening depending on the deformation temperature of the alloy. The microstructural investiga-

1220 INCOLOY MA956 Vol. 37, No. 8

tions revealed the different dislocation substructure, especially dislocation-dispersoid interaction in the specimens deformed in low and high temperatures. At temperatures in the range 20-4OO”C, dislocation loops around the dispersoid particles were observed, indicating an Growan type of strengthening ef- fect. During high temperature tensile and compression deformation, the shape of dislocations pinned at the departure side of particles indicated an attractive dislocation-particle interaction as described by the Rosier-Arzt model of dispersion strengthening.

Acknowledgments

The valuable contributions of Z. Kedzierski, W. Osuch and A. Zielinska-Lipiec (AGH) as well as H. Schuster (Research Centre JUlich) are gratefully acknowledged. Most of the electron microscopy was carried out using a JEM20 10 ARP instrument funded by the Foundation for Polish Science.

1. 2.

3. 4. 5.

A. Kelly and R.B. Nicholson, Prog. Mat. Sci., .lO (1963) 5 1. L.M. Brown and R.K. Ham in A. Kelly, R.B. Nicholson, Strengthening Methods in Crystals, Elsevier Publishing Com- pany, Amsterdam (197 1) 9. J. Preston, B. Wilshire and E.A. Little, Scripta Met. et Mat. 25 (1991) 183. A. Czyrska-Filemonowicz, M. Wrbbel, B. Dubiel and P.J. Emus, ibid, 32 (1994) 33 1. B. Dubiel, W. Osuch, M. Wr6be1, P.J. Ennis and A. Czyrska-Filemonowicz, Journal of Materials Processing Technology, 53 (1995) 121.

6. M. Wr&el, D. Schwarze, B. Dubiel, P.J. Ermis, A. Czyrska-Filemonowicz, Archives of Metallurgy 40 (1995) 447. 7. E. Orowan, Proc. Symposium on Internal Stresses in Metals and Alloys, The Institute of Metals, 1948,45 1. 8. E. Arzt and J. ROsler, Acta Met. 36 (1988) 1053. 9. J. Rosier and E. Amt, ibid, 38 (1990) 671.

References