Correlation of the microstructure and the tensile deformation of incology MA956

Journal of

ELSEVIER Journal of Materials Processing Technology 53 (1995) 121-130

Materials Processing Technology

Correlation of the Microstructure and the Tensile Deformation of Incoloy MA956 B.Dubiel a, W.Osuch a, M.Wrobel a, P.JEnnis b, A.Czyrska-Filemonowicza aUniversity of Mining and Metallurgy (AGH), Faculty of Metallurgy and Material Science, AI. Mickiewicza 30, 30-059 Krakow, Poland bResearch Centre Jtilich GmbH (KFA), Institute for Materials in Energy Systems, D-52425 Jialich, Germany

The microstructures of INCOLOY MA956 in the as-received condition and after hot tensile testing have been investigated by optical and transmission electron microscopy. The results of the structural analyses were correlated with the tensile properties in the temperature range 20-950°C. Significant changes in the deformation behaviour and the dislocation structures were observed as the test temperature was increased. Up to 400°C, strain hardening was observed and dislocation densities were correspondingly low. An attractive dislocationparticle interaction was seen, dislocations remaining in contact with the oxide dispersoid particles on the departure side of the particles.

1. I N T R O D U C T I O N INCOLOY alloy MA956 is an oxide dispersion strengthened (ODS) alloy produced by the mechanical alloying process. The microstructure consists of dispersion of very fine yttrium oxide particles in a ferritic FeCrAI matrix. The high AI content of the matrix (5%) leads to the formation of the tightly adherent, slow growing alumina scale during exposure at high temperatures, which provides excellent resistance to aggresive gaseous enviroments [1]. The oxide particles are much less susceptible to coarsening than precipitates [2-4] and remain as discrete particles at temperatures near the melting point of the matrix. ODS materials possess sufficient strength to be considered for applications at temperatures around ll00oc. The strengthening and failure mechanisms in ODS alloys are therefore of considerable interest and in the present work, the microstructures of as-received material and of tensile test specimens have been investigated and correlated with the strength and ductility properties.

2. EXPERIMENTAL PROCEDURE MA956 bar (20 mm thick) was supplied by Inco Alloys International in the hot extruded and recrystallized condition (1330oc/1 h). The nominal composition of the alloy investigated is as follows (in wt %): Fe-20Cr-4.5AI- 0.5Ti-0.5Y203 . For mechanical testing specimens were cut parallel to the extrusion axis of the bar. The gauge length of the specimens tested was 25 mm and the gauge diameter was 6 mm. The tensile Elsevier Science S.A. 0924-0136(95)01968-K

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tests were performed in air at macroscopically constant deformation rates. Temperatures and strain rates were as follows: 20, 400, 600, 800, 900 and 950oc, at a strain rate of 10-3s -l, and 10-2, 10 -3, 10-4 and 10-5s-1 at 950 ° C. The load-extension curves were continuously plotted during tests. Microstructural examinations were performed using optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of thin foils and extraction double replicas [5]. For the tensile specimens the systematic examinations of the microstructure were performed on the thin foils prepared both parallel and perpendicular to the deformation axis from different places of the deformed zone. Replicas were used for quantitative measurement of dispersoid size. The determination of chemical composition of dispersoids was performed by use X-ray diffraction analysis of extracted isolates (XRD). The determination of the crystallographic orientation of the as-received bar was performed using three different methods: X-ray Laue method, electron channelling pattern technique using SEM and selected area diffraction patterns of TEM thin foils.

3. R E S U L T S AND DISCUSSION 3.1. Microstructure and texture of as-received material

Figure 1 shows the macrostructure of the bar tested.

Fig. I. Macrostructure of INCOLOY MA956 bar: cross (a) and longitudinal (b) sections

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The highly serrated grain boundaries enabled a high degree of interlocking between adjacent grains. The grain size was in the range of some millimeters in the plane perpendicular to extrusion axis of the rod. The grains were highly elongated in direction of the axis, some grains extending over the 300 mm length of the bar examined. The grain aspect ratio (i.e. length/diameter) was 100:1, about one order of magnitude higher than the specified minimum [6]. Accumulations of small grains elongated in the extrusion direction, with grain diameters about 0.03 mm were sporadically present. The analysis performed by electron and by X-ray diffraction method on over 70 grains showed a crystallographic texture. The crystallographic directions <111> and <001> were approximately parallel to the extrusion direction of the material. Strong textures of MA956 and similar MA957 alloy have been also reported [7,8]; the extruded and annealed MA957 had the orientation scattered among <001>, <113> and <111> poles. The microstructure consisted of oxide dispersoids as well as pure alumina and titanium carbonitrides of up to few pm diameter in a ferritic matrix (Fig.2a). The analyses of extracted particles showed that the most of the of yttrium- aluminium oxides were tetragonal Y3A15OI2. The presence of this type of oxides was reported by Cama and Hughes [4]. Quantitative analysis using the TEM micrographs showed three classes of size distributions of the Y-AI oxides in the size range 5-300 nm. The size distribution of the finest dispersoids (Fig.2b) had mean size of 9 nm. The results were in agreement with those obtained by TEM and small angle neutron scattering technique [3,9].

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Fig.2. Oxide dispersoids and Ti(C,N) particles in the ferritic matrix (a), (thin foil from cross section) and size distribution of the finest dispersoids (b)

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Because of the large grain size, the structural examinations performed on the thin foils prepared both from the cross and longitudinal sections of the bar did not often show the grain boundaries. Fig.3a shows the grain boundary observed on the thin foil prepared from the longitudinal section of the specimen.

Fig3. Microstructure of the as-received MA956: pinning of the grain boundary and particle denuded zone along the grain boundary (a) and particle denuded band inside a large grain (b), (thin foil from logitudinal section) The pinning of the grain boundary by coarse particles and particle denuded band along the grain boundary is seen. Dispersoid depletion in the form of bands was also observed inside a large grains, as shown in Fig3b. The observations of the thin foils prepared both from transverse and longitudinal sections of the bar indicated the presence of finer grains in the size range of some micrometres incorporated within the large grains. These fine grains probably result from incomplete recrystallization. 3.2. Tensile tests

Stress-strain curves determined at deformation rate 10-3s -1 are presented in Fig.4. At room temperature and temperatures between 600 and 900oc, a pronounced yield point was observed while at 400°C no yielding was seen. At room temperature and 400°C, pronounced strain hardening was found in the stress-strain curves, the stress continuously decreasing until fracture, indicating localization of deformation at low strains. For almost the whole of the plastic range of high temperature deformation, the slope of stress-strain curves was not especially sensitive to the test temperature. The influence of the strain rate on the tensile properties of MA956 is shown in Fig5. A well defined yield point was only observed for a strain rate 10-2s - 1 The maximum stress and the ductility decreased with decreasing strain rate. As the strain rate decreased from l0 -2 to 10-3s -1, the highest drop in the maximum stresses

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was exhibited. Further reduction of the strain rate to 10-4-10-5s "1 dramatically reduced the fracture ductility.

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Fig.4. Stress-strain curves for MA956; strain rate 10-3s-1

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olo5

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o515

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Fig.5. Stress-strain curves for MA956 tested at 950°C and strain rates from 10-2 to 10-5s-1

3.3. Microstructure of tensile tested specimens The etching technique revealed slip lines and shear bands for specimens deformed at temperatures of 600°C and below (Fig.6a).

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b

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distance from the fracture surface(ram) Fig6. Microstructure (a) and microhardness (b) of the fracture zone of specimen tested at 600oc

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The density o f shear bands increased with deformation temperature. Increasing the test temperature led to increased necking of the specimen. For the specimen tested at 800oc, although the necking was considerable, no deformation marks were observed on the etched cross section. The microhardness of particular grains at the different distance from fracture surface was measured for the specimens deformed at 600oc and at 800°C. For the specimen deformed at 600°C, the microhardness decreased with the distance from fracture surface (Fig.6b). In contrast, no essential difference in microhardness inside and outside the neck zone was found for the sample deformed at 800°C. TEM investigations o f deformed specimens were aimed at determining the dislocation configurations within the grains and interactions o f dislocations with dispersoid particles. The microstructures of the specimens deformed at 20 and 400°C exhibited relatively high dislocation densities. The dislocations had tendency to form structures similar to cell structures (Fig.7). The grains were orientated with the <111> direction in the tensile axis.

Fig7. Microstructure o f specimen tested at 2 0 o c High dislocation density region (thin foil from prepared from the cross section ca 3mm from fracture face) The observations o f the structure of the specimen tested at 600°C on the thin foils prepared from the longitudinal section indicated the inhomogenity in the dislocation structure in different places o f the tensile specimen. An increased dislocation density was seen near the fracture zone (Fig.8a). In the regions located far from the fracture zone a low dislocation density was observed (FigSb).

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Fig.8. Microstructure of specimen tested at 600°C, showing a high dislocation density, ca 5 mm from fracture face (a) and a low dislocation density, ca 10 mm from fracture face (b), thin roils from longitudinal section

Fig.9. Microstmcture of specimen tested at 600°C showing non-uniformity of particles distribution correlated with heterogenous dislocation distribution (a) and subgrain boundary (b).Thin foil from cross section ca 12 mm from fracture face

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On the thin foil prepared from the cross section ca 12 mm from fracture, non unifbrm distributions o f dispersoids have been observed (Fig.9a). Areas with fewer dispersoid particles exhibited low dislocation density. This type o f the microstructure could result from higher probability o f the dislocation recovery in the area of lower dispersoid density. For the sample prepared from cross section, the presence ofsubgrain boundaries was also detected (Fig.gb). In the specimens deformed at 800°C and 900°C no differences in the dislocation arrangements in the different parts of the deformed zone were observed. The dislocation density was low and dislocations were elongated in the directions close to <111> as shown in Figure 10.

Fig. 10. Microstructure o f specimen tested at 800°C; thin foil from cross section ca 12 mm fi'om fracture face The dislocation density was low enough to allow the the interaction o f single dislocations with the dispersoid to be investigated. Very often the dislocations were pinned at the departure side o f particles, as shown in Fig. 11. This type o f the structure gave the impression of an attractive dislocation-particle interaction as was postulated in the literature for ODS alloys [10-15]. Such dislocation configurations suggesting an attractive interaction between dislocations and dispersoids appear to be characteristic for the structure of 1NCOLOY MA956 after high temperature deformation.

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Fig.ll. Microstructure of specimen tested at 800°C, showing dislocation in the vicinity of dispersoids in bright (a) and dark (b) field images (longitudinal section, ca 6 mm from fracture face)

4. SUMMARY 1. The extruded bar exhibited a crystallographic texture, the <111> and <001> crystal directions lying parallel to the extrusion axis. 2. The microstructure of the 1NCOLOY alloy MA956 in as-received condition consisted of large grained ferritic matrix and mixed Y-AI dispersoids (mainly tetragonal Y3A15012) 3. In tensile tests, INCOLOY MA956 indicated a strong tendency to locallization of the plastic deformation in shear bands. At test temperatures above 400oc this led to early onset of necking. 4. The microstructure of the specimens deformed at temperatures up to 600°C exhibited heterogenous dislocation arrangement with a tendency to cell formation. A high density of dislocations was found near the fracture face. In the regions located far from the fracture zone, a low dislocation density was observed. 5. Specimens deformed at 800 and 900oc exhibited very low dislocation density, with large dislocation-free areas. High curvature of dislocation lines in the vicinity of dispersoids gave the impression of an attractive interaction between dislocations and dispersoid particles.

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ACKNOWLEDGMENTS The authors appreciate to support of Polish Committee of the Scientific Research (KBN), grant nr 3087101. The cooperation in this study of A.Zielifiska-Lipiec, J.St~pifiski (AGH) and D.Schwarze, V.Gutzeit (KFA) is gratefuly acknowledged. The authors are also grateful to S.Gorczyca, A.Korbel (AGH) and H. Schuster (KFA) for helpful discussions.

REFERENCES

1. A.Czyrska-Filemonowicz, D.Clemens, W.J.Quadakkers, paper submitted to this Conference 2. B.Dubiel, M.Wrobel, W.Osuch, A.Zielifiska-Lipiec, P.J.Ennis, D.Schwarze, A.CzyrskaFilemonowicz, Proc. of VIII Conference on Electron Microscopy of Solid State, WroclawSzklarska Por~ba, April 20-23, 1993, Poland, S.Gorczyca et al (eds), (1993) 255 3. P.Krautwasser, A.Czyrska-Filemonowicz, M.Widera, F.Carsughi, Materials Science and Engineering, A 177 (1994) 199 4. H.Cama, T.A.Hughes; Proc. of VIII Conference on Electron Microscopy of Solid State, Wroc~aw-Szklarska Por~ba, April 20-23, 1993, Poland, S.Gorczyca et al (eds), (1993) 276 5. A.Czyrska-Filemonowicz, K.Spiradek, S.Gorczyca, Prakt. Metallographie, 22 (1991) 217 6. INCOLOY alloy MA956, brochure from lncoMAP 7. ELEvens, J.W.Martin; Materials Science Forum, 94-96 (1992) 643 8. A.Alamo, HRegle, G.Pons, J.L.Bechade; Materials Science Forum, 88- 90 (1992) 183 9. A.Czyrska-Filemonowicz, P.Krautwasser, M.Widera; ibid, 276 10. A.Czyrska-Filemonowicz, P.J.Ennis, B.Dubiel, M.Wr6bel; Scripta Met.et Mat. (in print) 11. D.J.Srolovitz, R.A.Petkovic-Luton, M.J.Luton; Acta Metallurgica, Vol.31 (1983) 2151 12. D.J.Srolovitz, M.J.Luton, R.A.Petkovic-Luton, D.MBarnet, W.D.Nix: ibid, Vol.32 (1984) 1079 13. E.Arzt, D.S.Wilkinson; ibid, Vol.34 (1986) 1893 14. E.Arzt, J.ROsler; ibid, Vol.36 (1988) 1053 15. J.ROsler, E.Arzt; ibid, Vol.38 (1990)671