The influence of long range order on fatigue crack initiation in an FeCoV intermetallic compound

Scripta METALLURGICA et MATERIALIA

Vol. 26, pp. 331-336, 1992 Printed in the U.S.A.

Pergamon Press plc All rights reserved

THE INFLUENCE OF LONG RANGE ORDER ON FATIGUE CRACK INITIATION IN AN FeCo-V INTERMETALLIC COMPOUND N.S. Stoloff, S.J. Choe and K. Rajan Rensselaer Polytechnic Institute Materials Engineering Department Troy, New York 12180-3590 (Received October 3, 1991) (Revised November 15, 1991)

Introduction The formation of persistent slip bands, extrusions and intrusions at crystal surfaces has been well documented in cyclically deformed cubic metals such as copper [1,2], nickel [3,4], and iron [3]. A variety of dislocation substructures associated with changes in surface topography also have been reported, one of the most common being a ladder structure developed in copper and nickel single crystals [3]. In the case of intermetallic compounds, persistent slip bands, extrusions and intrusions have been associated with crack initiation in Ni3AI+B crystals oriented for single slip [5]. In this alloy a ladder structure was not observed. Previously, we have reported that transgranular crack initiation along slip bands occurs in polycrystals of the B2 intermetallic FeCo-2%V, both in the ordered and disordered condition whether cycling is carried out in air or in (Sxl0 - S P a ) vacuum [6]. The purpose of this paper is to give a detailed account of both surface and sub-surface damage accumulation in polycrystalline FeCo-2%V and to relate the observations to those made on another ordered intermetallic, Ni3AI. Experimental Procedures Warm worked rods of FeCo-2%V were obtained from a commercial source. The alloy was annealed at 1000°C, and water quenched to produce the disordered condition. Ordering was accomplished by slow cooling to 25°C. The critical temperature, To, for long range order is 720°C. Strain controlled experiments, in fully reversed tension-compression, (R=-I) were carried out on FeCo-V with a cylindrical specimen. All cycling was carried out under total strain control at a frequency of 0.33Hz. Cycling was interrupted periodically to examine the specimen surfaces by optical examination of replicas as well as by SEM. Although most tests were carried out in air, a few were conducted in 2 . 7 x l O - s P a vacuum. Exoerimental Results i).

Low Cycle Fatigue Lives Cycling under strain control in air resulted in improved fatigue lives when samples were tested in the ordered condition, see Fig. i. Cycling in vacuum produced a small improvement in fatigue lives in both the ordered and disordered conditions.

331 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc

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Cyclic Hardening and Softening A distinct difference in cyclic hardening was noted between ordered and disordered FeCo-V. Hardening was more rapid in the ordered condition, see Fig. 2. Also, the maximum stress increased with increasing strain amplitude. 3).

Surface Observations Intense slip bands were observed in disordered FeCo-V, cycled with ~ T = I . 0 6 % , ~ p = 0 . 2 3 % after about 300 cycles, see Fig. 3a). The slip bands continued to develop up to 5000 cycles, at which time distinct cracks could be observed, Fig. 3a). Cracks continued to grow and link up to 8000 cycles, Fig. 3b). Clear evidence that extrusions had formed along persistent slip bands is shown in Fig. 4. Note that cracks propagate along the extrusions. Fully ordered FeCo-V also developed persistent slip bands by about the first 300 cycles. Extrusions were clearly visible within the first 600 cycles, see Fig. 5. A crack was found along one of the extrusions elsewhere on this specimen. 4).

Dislocation substructures Transmission electron microscopy revealed that disordered material contained only unit dislocations, while ordered specimens contained superlattice dislocations. In disordered samples, bundles of dislocations lined up in some regions along <200>, see Fig. 6a); at other places dislocations seemed to be distributed randomly. Dislocation densities were low in ordered samples, and the dislocations were distributed more homogeneously, see Fig. 6b). 5).

Fractography Fracture in both ordered and disordered material was by transgranular cleavage, see Figs. 8a) and b) respectively. Cracks initiated near or at the surface, consistent with the observation of cracks at extrusion-intrusion pairs, see Fig. 3. Samples fractured in vacuum showed the same features as those in air. Discussion The observation of increased life in disordered FeCo-V during strain controlled cycling is in marked contrast to earlier results under stress controlled cycling [7]. In that work, long range order extended lives significantly. This apparent discrepancy is readily explained in terms of the observed cyclic hardening behavior shown in Fig. 2. Cyclic hardening is so rapid under strain control that a critical fracture stress level is reached sooner than in the case of disordered material. In stress control, on the other hand, cyclic hardening in the ordered condition causes the plastic strain applied per cycle to rapidly decrease as the flow stress increases. Accordingly, cycling of ordered material results in the delay of both crack initiation and final fracture. The TEM and SEM results show that the formation of persistent slip bands, extrusions and intrusions is not fundamentally affected by the preponderance of superlattice dislocations in the ordered material. In fact, persistent slip bands were noted at 300 cycles in both ordered and disordered samples. Although the dislocation substructures shown in Fig. 6 are fundamentally different than those observed in cubic metals such as copper, the appearance of surface damage is very similar. The observation that extrusions form readily in ordered material deforming by superlattice dislocation motion is consistent with our findings for NI3AI+B single crystals [5]. In the earlier work it was shown that single crystals oriented for single slip developed persistent slip bands very early in a strain controlled fatigue test and that a characteristic intrusion-extruslon morphology accompanied saturation of the cyclic flow stress. Similar observations recently have been made on binary Ni3A1 crystals oriented for multiple slip, in stress control, see Fig. 7 [8]. In Ni3AI, also, there was no evidence of either the ladder or cell structure typical of many cubic metals. It is known that long

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range order ~Dd short range order (in quenched samples) suppress cell formation in FeCo-V tested under monotonic conditions [9]. Essmann et al [2] have suggested, based on work on fcc metals and alloys, that in planar slip materials random irreversible slip of large dislocation groups resulted in pronounced surface roughness, leading to slip band cracking. In wavy slip materials, on the other hand, damage was suggested to result mainly from plastic strain localization in PSBs. Since disordered FeCo-V displays wavy slip and ordered FeCo-V deforms by homogeneous planar slip, Mughrabi's analysis suggests that PSBs should be more likely to develop in the disordered material. In fact, comparison of Figs. 4 and 5 shows that extrusions formed early in both conditions, but the intrusion/extrusion morphology was more clearly developed in disordered material. Therefore, our results on FeCo-V as well as the previously cited results on Ni3AI crystals suggest that the distinction between planar and wavy slip is not as important in PSB-extrusion/intrusion development of intermetallic compounds. Acknowledqements This research was supported by the National Science Foundation under Grants No. DMR84-09593 and DMR 89-11975. The authors are grateful to Mr. D. Wenman for assistance with the experimental program. References 1.

2. 3. 4. 5.

6.

7. 8.

9.

D. Kuhlmann-Wilsdorf and C. Laird, Mat. Sci. and Eng., 27, 137 (1977). U. Essmann, U. Gosele and H. Mugrahbi, Phil. Mag., 44, 405 (1981). H. Mughrabi, F. Ackermann and K. Herty in Fatigue Mechanisms, ASTM STP 675, Philadelphia, PA, 1979, p. 69. T.L. Grobstein, S. Sivashankavan, G. Welsch, N. Panigrahi, J.D. McGervey and J.W. Blue, Mat. Sci. and Eng., A138, 191 (1991). L.M. Hsiung and N.S. Stoloff, Acta Metall., 38, 119, (1990). N.S. Stoloff, G.E. Fuchs, A.K. Kuruvilla and S.J. Choe, in High Temperature Ordered Intermetallic Alloys II, MRS Symposia Proceedings, v. 81, ed. by N.S. Stoloff, C.C. Koch, C.T. Liu and O. Izumi, Pittsburgh, PA, 1987, p. 247. R.C. Boettner, N.S. Stoloff and R.G. Davies, Trans. AIME, 236 131 (1966). D. Wenman, K. Rajan and N.S. Stoloff, unpublished. K.R. Jordan and N.S. Stoloff, Trans. AIME, ~45, 2027 (1969). 1700

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Fig. 3. Development of cracks along persistent slip bands in disordered FeCo-V ~fT=l.06%, A~p=0.23% a)5000 cycles b) 8000 cycles.

Fig. 4. Extrusions along persistent slip bands in disordered FeCo-V, air, AfT=I.06%, 8000 cycles.

Fig. 5. Extrusions in ordered FeCo-V, air, A~T=I.03%, 600 cycles.

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a) disordered Fig. 6. Dislocation substructures ~T=I%,N=I9,770.

b) ordered in FeCo-V,

tested in air,

Fig. 7. Surface damage and crack in Ni3AI single crystal tested in air at room temperature; stress control, a m a x = 7 3 0 MPa T M

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SEM fractographs ~T=0.7%.

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b) disordered of FeCo-V,

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