- Email: [email protected]
Effect of long-range order on the cyclic deformation behaviour of Ni3Fe single crystals
Acta mater. 48 (2000) 3401±3408 www.elsevier.com/locate/actamat
EFFECT OF LONG-RANGE ORDER ON THE CYCLIC DEFORMATION BEHAVIOUR OF Ni3Fe SINGLE CRYSTALS H. Y. YASUDA, D. FURUTA and Y. UMAKOSHI{ Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka, 565-0871, Japan (Received 24 January 2000; received in revised form 8 May 2000; accepted 8 May 2000) AbstractÐEect of ordering on cyclic deformation in disordered and ordered Ni3Fe single crystals was investigated focusing on stress±strain response and deformation substructure. The cyclic hardening depended strongly on the long range order. The maximum stress in the disordered crystals increased gradually with increasing number of cycles and then reached a saturation, while ordered ones exhibited cyclic softening after an initial strong cyclic hardening. The cyclic hardening at an early stage of fatigue in ordered crystals may be due to APB tubes and debris which were produced by the intersection between primary and secondary slips. Coarse slip bands were observed in fatigued ordered Ni3Fe single crystals. In the bands, three-dimensional dislocation structure was formed accompanied by a decrease in the degree of order, which was responsible for the cyclic softening. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Intermetallic; Fatigue; Dislocations; Transmission electron microscopy
1. INTRODUCTION
Cyclic deformation and fatigue property of ordered alloys are important factors for industrial application. The earliest studies on fatigue behaviour of ordered alloys were carried out by Boettner et al. [1]. The long-range degree of order in some ordered alloys signi®cantly aected the high-cycle fatigue life, notch sensitivity and environmental embrittlement under cyclic loading [2, 3]. The ease of crossslip of superdislocations was closely related to the fatigue property [1]. Recently, much attention has been paid to understanding the cyclic deformation behaviour of intermetallics such as Ni3Al [4, 5, 32± 34], NiAl [6, 7], TiAl [8] and Ti3Al [9, 10] focusing on cyclic hardening, fatigue life and deformation substructure. Ni3Fe is a Kurnakov-type intermetallic compound with the L12 structure and order±disorder transition occurring at 5038C. Since disordered and ordered states are realized at room temperature by an appropriate heat treatment [11], the eect of the long-range ordering on the cyclic deformation behaviour of Ni3Fe can be examined. Moreover, both disordered and ordered Ni3Fe are ferromagnetic
{ To whom all correspondence should be addressed.
and, in particular, the disordered Ni3Fe is known as a superior soft magnetic material called Permalloy. Not only plastic behaviour, but also the magnetic properties of Ni3Fe depend on the degree of order [12±16, 35]. The magnetic properties are known to be in¯uenced by lattice defects such as dislocations and planar faults in magnetic materials [14±17]. Dislocation arrangements and planar faults in deformed ferromagnetic materials were observed using magnetic techniques. In this paper, we report on the cyclic deformation behaviour of disordered and ordered Ni3Fe single crystals focusing on the cyclic hardening, the surface topography, and the deformation substructure. Observations of dislocations and planar faults using a magnetic technique are described in a separate paper [18]. 2. EXPERIMENTAL PROCEDURE
Master ingots of Ni±25.0 at.%Fe alloy were prepared by melting high-purity Ni and Fe in a plasma arc furnace. Single crystals were grown from the ingots by a ¯oating zone method at a rate of 10 mm/h under a high-purity Ar gas ¯ow. The crystals were homogenized at 1473 K for 24 h, and their orientation was determined using X-ray Laue back diraction technique. Fatigue specimens
1359-6454/00/$20.00 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 1 4 3 - 9
3402
YASUDA et al.: CYCLIC DEFORMATION
whose loading axis is [149] were cut from the crystals by spark machining. The front and side sur-faces of the specimens were (941) and (121), respectively. Schmid factors for the primary (111)[101] and the secondary (111)[101] slip systems were 0.5 and 0.467, respectively. The specimens were annealed at 735 K for 1 103 h, then slowly cooled at 10 K/h for chemical ordering and the degree of long-range order was about 0.9 after heat treatment [11]. The disordered state was obtained by quenching from 873 K in ice-brine. Plastic strain-controlled fatigue tests in a tension/ compression mode, all beginning in tension, were done at room temperature in air using a servo hydraulic system (Shimadzu servo pulsar EHFED5-10L type) with a clip-on extensometer for strain measurement. Plastic strain amplitude (Dep) was controlled in the range between 20.25 and 20.45%. All the tests were conducted at a constant strain rate of 3 10ÿ4/s. Slip traces were examined using an optical microscope equipped with Nomarski interference contrast. The surface topography on the front surface of fatigued specimens was examined by an atomic force microscope (AFM) (Shimadzu SPM-9500), and the fracture surface by a scanning electron microscope (SEM). Thin foils for transmission electron microscopic (TEM) observation were cut parallel and perpendicular to the primary (111) slip plane by spark machining and polished using the twin jet technique. Deformation substructure was observed by a Hitachi H-800 electron microscope operated at 200 kV. 3. RESULTS
3.1. Cyclic stress±strain response Figure 1 shows cyclic hardening curves of disordered and ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%: The maximum stress is represented as the average of the absolute values of maximum tensile and compressive stresses in each stress±strain hysteresis loop, since no signi®cant
Fig. 1. Cyclic hardening curves of disordered and ordered Ni3Fe single crystals fatigued at Dep 20:25%:
Fig. 2. Variation in the maximum stress of disordered Ni3Fe single crystals cyclically deformed at various plastic strain amplitudes as a function of cumulative plastic strain.
asymmetry between tensile and compressive stresses was observed in the loop of Ni3Fe single crystals. The cyclic hardening depends strongly on the longrange order. In the disordered crystal, the maximum stress gradually increases with increasing number of cycles (N ) and is saturated at about 2 103 cycles. In contrast, the ordered crystal shows strong cyclic hardening up to N = 1 102 and then the maximum stress gradually decreases after peaking at about 300 MPa. A rapid drop in the maximum stress occurs in the ®nal stage of fatigue where cracks were initiated at the surface of the specimen. Similar cyclic hardening/softening behaviour was also seen in Ni3Al polycrystals [19]. Figures 2 and 3 show variation in maximum stress with cumulative plastic strain of disordered and ordered Ni3Fe single crystals cyclically deformed at various Dep, respectively. In Fig. 2 similar cyclic hardening curves are seen at various Dep and the saturation stresses reach the same value of about 150 MPa regardless of dierent Dep. This may correspond to the stress in the plateau region of typical cyclic stress±strain curves of Cu single crystals [20, 21]. In contrast, the peculiar cyclic hardening/softening behaviour is always recognized in the ordered
Fig. 3. Variation in the maximum stress of ordered Ni3Fe single crystals cyclically deformed at various plastic strain amplitudes as a function of cumulative plastic strain.
YASUDA et al.: CYCLIC DEFORMATION
Fig. 4. Variation in the area fraction of the coarse slip bands in ordereded Ni3Fe single crystals fatigued at Dep 20:25% with number of cycles.
crystals fatigued at various Dep in Fig. 3. The peak of maximum stress appears at the same cumulative plastic strain of 1 102% at various Dep and specimens fatigued at dierent Dep fractured at the same cumulative strain as shown in the ®gure. 3.2. Slip traces, surface topography and fracture behaviour Slip traces and surface topography of disordered and ordered Ni3Fe single crystals cyclically deformed at De 20:25% to various number of cycles were examined on the primary (111) plane. Although the data are not shown here, in the disordered crystals ®ne and fairly homogeneously distrib-
3403
uted slip traces were observed and the density increased with an increase in the number of cycles. In contrast, the coarse slip bands were observed along the primary (111) plane in the ordered crystals even at an early stage of fatigue. Fine slip traces with very weak contrast were also observed between these bands; the width and number of the bands increased as the number of cycles increased. Figure 4 shows variation in the area fraction of the slip bands in the ordered crystals with the number of cycles. The fraction of the bands increases as the number of cycles rises, reaching about 77% at N = 1 104. When shear deformation occurred in a localized region and formed a slip band, the slip band induced a sharp surface step. Figures 5 and 6 show the frequency of surface steps measured by AFM in disordered and ordered Ni3Fe single crystals fatigued at Dep 20:25%, respectively. In disordered crystals, a high frequency of ®ne steps was observed at an early stage of fatigue. As the number of cycles rose, the frequency of steps with higher height increased, but the height of the steps was almost below 0.5 mm even after N = 1 104. The distribution of the frequency showed no remarkable change after N = 1 104. The variation in height and frequency of surface steps in the ordered crystals was quite dierent from that in the disordered crystals. At N = 10 there was a high frequency of ®ne steps similar to that in the disordered crystals. However, surface steps more than 0.5 mm in height corresponding to the coarse slip bands appeared at N = 1 102. As the number of cycles increased, slip bands concentrated in a localized region, resulting
Fig. 5. Distribution of the height of surface steps in disordered Ni3Fe single crystals cyclically deformed at Dep 20:25% to various number of cycles (N); (a) N = 10, (b) N = 1 102, (c) N = 1 103, (d) N = 1 104.
3404
YASUDA et al.: CYCLIC DEFORMATION
Fig. 6. Distribution of the height of surface steps in ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%; (a) N = 10, (b) N = 1 102, (c) N = 1 103, (d) N = 1 104.
in the development of high steps whose high frequency showed wide distribution. Steps more than 1 mm were formed at and above N = 1 103 as shown in Fig. 6. These steps result in the crack initiation in ordered Ni3Fe crystals shown in Fig. 7. Figure 8 shows scanning electron fractographs of disordered and ordered Ni3Fe single crystals cyclically deformed at Dep 20:45%: In both specimens the stage I cracks were initiated parallel to the primary (111) plane and stage II cracks propagated perpendicular to the stress axis showing striation. It is known that the crack growth rate is accelerated by chemical ordering and the notch sensitivity becomes high in the ordered state. However, the fractograph in the ordered state [Fig. 8(b)] shows no brittle ¯at fracture surface.
disordered Ni3Fe, which can hardly be suppressed by quenching, may induce the planar dislocation arrangement in the disordered crystals as proposed by Gerold et al. [23]. Figure 10 shows deformation substructures in the (111) foils of ordered Ni3Fe single crystals cyclically deformed at De 20:25%: At N = 1 102 screw dislocations and many pieces of debris with b=[101] are observed in Fig. 10(a). Two types of edge and 608 debris are con®rmed. Dislocation bundles with b=[101] are arranged along [011] direction inclined at 608 to their Burgers vector at N = 1 103 and 1 104 as shown in Fig. 10(b) and (c). The density of the bundles increased with the number of
3.3. Deformation substructure in fatigued Ni3Fe single crystals Figure 9 shows deformation substructures in disordered Ni3Fe single crystals cyclically deformed to N 1 103 at De 20:25%: Well-developed dislocation bundles perpendicular to the Burgers vector of b=[101] are observed in the (111) foil in the disordered crystal, as shown in Fig. 9(a). These bundles were often observed in fatigued Cu single crystals [20, 21]. Planar arrangement of dislocations was observed in a (121) foil, and the ladder or vein structure seen in fatigued Cu single crystals was not formed in this crystal. The morphology of the dislocation arrangement in the (121) foil is similar to that in austenitic stainless steel cyclically deformed at small strain amplitudes [22], as shown in Fig. 9(b). Formation of the short-range order in
Fig. 7. A SEM image of the fatigue cracks at a slip band in ordered Ni3Fe single crystal cyclically deformed at Dep 20:25% to N = 1 104.
YASUDA et al.: CYCLIC DEFORMATION
3405
Fig. 8. Fracture surfaces of disordered (a) and ordered (b) Ni3Fe single crystals cyclically deformed at Dep 20:25%:
cycles. In the bundles, a large number of pieces of 608 debris extend along the [011] direction, as shown in Fig. 11. Primary dislocations are piled-up at the debris and form bundles along [011] resulting in cyclic hardening in the ordered Ni3Fe single crystals at an early stage. TEM images in the (121) foils of ordered Ni3Fe single crystals fatigued at Dep 20:25% are shown in Fig. 12. At N = 1 102, primary [101] dislo cations show planar con®guration with g~ 202 in Fig. 12(a), while these dislocations are out of contrast and sharp bands composed of secondary [101] dislocations are observed with g~ 111 in Fig. 12(b). The width of the bands increased with increasing number of cycles and irregular cells of three-dimensional dislocation arrangement were formed in the bands at N = 1 103, although the planar dislocation arrangement still remained between the bands [Fig. 12(c)]. The formation of the cell structure was accompanied by dislocation rearrangement, which may require the cross slip of dislocations. Further development of the irregular cell was observed at N = 1 104 [Fig. 12(d)]. The
formation of the coarse bands in the ordered Ni3Fe may correspond to the cyclic softening after N = 1 102. 4. DISCUSSION
The cyclic deformation behaviour of Ni3Fe single crystals depended strongly on the long-range order: typical cyclic hardening was observed in the disordered state similar to usual metals, while cyclic softening occurred after initial strong cyclic hardening in the ordered state. In ordered Ni3Fe, a h110i superlattice dislocation can be dissociated into four 1/6h112i Shockley superpartials bound by an anti-phase boundary (APB) and two complex stacking faults [24±26]. They spread widely on the {111} plane, but no dissociation on the cube cross-slip plane occurred at room temperature. Since extension or constriction of the dislocation core occurs due to the edge component of their Shockley partials in an elementary process for the cross-slip onto the cube plane, depending on the applied stress mode such as ten-
Fig. 9. TEM images taken from the (111) (a) and (121) (b) foils of disordered Ni3Fe single crystals ---cyclically deformed at Dep 20:25% to N = 1 103. (a) Incident beam B 1 [211], g = 111, b=[101], e is perpendicular to b; (b) B 1 [121], g = 202.
3406
YASUDA et al.: CYCLIC DEFORMATION
Fig. 10. Deformation substructure taken from the (111) foils of ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%: Burgers vector parallel to [101], and 608 debris represented by b and 60 in the ®gure, respectively. B 1 [101] and g=111; (a) N = 1 102, (b) N = 1 103, (c) N = 1 104.
sion and compression, asymmetry of yield stress in tension and compression appears in the L12-type compounds such as Ni3Al[4] and Ni3Ge[27]. However, since no signi®cant asymmetry of plastic behaviour in tension and compression was observed in the ordered Ni3Fe single crystals at room temperature, the tendency of dislocations to cross-slip onto the cube plane may be very low. Moreover, low APB energy in ordered Ni3Fe induced a planar dislocation arrangement in a monotonic deformation. Under the cyclic loading, however, dislocations in the coarse slip bands formed a threedimensional structure. In the ordered Ni3Fe single crystals, rapid cyclic hardening was observed at an early stage of fatigue, and in this alloy a well-developed ordered domain structure was formed during heat treatment. The ordered domain was cut by dislocations during their to-and-fro motion resulting in a decrease in
Fig. 11. A high magni®cation image of the dislocation bundle formed in ordered Ni3Fe single crystal cyclically deformed at Dep 20:25% to N = 1 103. B 1 [101], g=111.
the domain size. However, the eect of change in the domain size on the shear stress is too small to explain the strong cyclic hardening [26]. At the strong cyclic hardening stage, dislocation bundles and debris along the [011] direction, which is the intersection between primary (111) and secondary (111) slip planes, were inclined at 608 to b=[101]. The intersection and reaction between the primary and secondary dislocations may produce the 608 bundles and the 608 debris. When the primary superlattice dislocations dissociated into unit dislocations bound by APB intersected by secondary dislocations, APB tubes may be produced, as proposed by Vidoz and Brown [28]. However, APB tubes were rarely observed by TEM observation in fatigued Ni3Fe, since the scattering factors of Ni and Fe are very similar. If such tubes are formed during cyclic deformation, they may interrupt the motion of dislocations resulting in strong work hardening. Moreover, the 608 debris as well as edge debris was frequently observed in the ordered crystals. The 608 debris behaved similarly to the faulted superdipoles seen in cyclically deformed Ni3Al±B single crystals. It may also interrupt the motion of primary dislocations and contribute to the cyclic hardening in ordered Ni3Fe single crystals. In addition, Lomer±Cottrell locking due to the reaction between primary and secondary dislocations may contribute to the cyclic hardening. When the cyclic softening occurred after N = 1 102, slip bands with more than 0.5 mm height were developed. A large number of dislocations due to the to-and-fro motion and APBs may be produced in these bands, resulting in the destruction of the ordered structure and decreasing the degree of order in the localized region. The decrease in the long-range order during cyclic deformation was con®rmed in Ni3Al polycrystals by the X-ray diffraction method [19]. Evidence of the decrease in the degree of order in Ni3Fe during cyclic deformation was obtained by magnetic measurement as described in a separate paper [18]. Figure 13 shows a high magni®cation image of
YASUDA et al.: CYCLIC DEFORMATION
3407
Fig. 12. Deformation substructure in the (121) foils of ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%; (a) N = 1 102, B 1 [121], g = 202, (b) N = 1 102, B 1 [121], g = 111, (c) N = 1 3 4 10 , B 1 [121], g = 111, and (d) N = 1 10 , B 1 [121], g = 111.
the coarse slip bands in the fatigued ordered Ni3Fe crystal. Secondary dislocations with a scalloped shape are also observed in the bands. If these dislocations interact with some lattice defects such as
Fig. 13. A high magni®cation image of the coarse slip bands formed in ordered Ni3Fe single crystal cyclically deformed at Dep 20:25% to N = 1 103; B 1 [121], g = 111.
APB and debris, the degree of long-range order may be destroyed after their motion. Moreover, the to-and-fro motion of dislocations in the localized region under cyclic loading may accelerate the disordering. Such wiping out of defects by secondary dislocations was observed in irradiated metals during deformation [29]. The disordering on the slip plane was also reported in stoichiometric Ni3Al [30, 31]. As the degree of order decreases, the individual unit dislocations can be activated without creating APB and the dislocation rearrangement occurs by their easy cross-slip. The slip disordering facilitates the motion of primary dislocations in the slip bands and therefore the cyclic softening occurs in the later stage of fatigue in ordered Ni3Fe. However, cyclic softening following initial hardening was not seen in Ni3Al±B [4, 32, 33] and Ni3Ge [27] single crystals cyclically deformed at constant total strain amplitude. The initial strong cyclic hardening in those crystals may hinder the eect of cyclic softening at the further stage. The number and area fraction of slip bands increased as plastic strain amplitude grew. However, the distribution of the step heights of the slip bands was almost independent of these amplitudes, ranging from 20.25 to 20.45% at the same cumulative plastic strain. Therefore, the peak stress and the corresponding cumulative plastic strain was
3408
YASUDA et al.: CYCLIC DEFORMATION
approximately constant at the amplitudes in this study, although there was a small deviation in the peak and the cumulative plastic strain depending on the plastic strain amplitude. 5. CONCLUSIONS
The cyclic deformation and deformation substructure in disordered and ordered Ni3Fe single crystals were investigated and the following conclusions were reached. The cyclic hardening of Ni3Fe depended strongly on the degree of long-range order. In the disordered state the maximum stress gradually increased, showing cyclic hardening, and reached a saturated stress around N = 2 103 at De 20:25%, while strong initial cyclic hardening was observed in the ordered state, showing a peak at a stress of about 300 MPa at N = 1 102 following by cyclic softening. Variation in the maximum stress with the cumulative plastic strain showed no Dep dependence. The initial strong hardening in the ordered state is due to the formation of APB tubes and debris created by the interaction and reaction between the primary and secondary slips. Formation of the coarse slip bands more than 0.5 mm in height in the ordered state corresponds to the cyclic softening. Concentrated shear deformation at the slip bands during fatigue induces the slip disordering and the disordering facilitates the motion of primary dislocations under cyclic loading resulting in cyclic softening. AcknowledgementsÐThis work was supported by a Grantin-Aid for Scienti®c Research and Development from the Ministry of Education, Science, Sports and Culture of Japan. REFERENCES 1. Boettner, R. C., Stolo, N. S. and Davies, R. G., Trans. Metall. Soc. AIME, 1966, 236, 131. 2. Stolo, N. S., Fuchs, G. E., Kuruvilla, A. K. and Choe, S. J., High temperature ordered intermetallic alloys. In II MRS Symposium Proceedings, Vol. 81, 1987, p. 247. 3. Stolo, N. S., ISIJ Inter., 1997, 37, 1197. 4. Hsiung, L. M. and Stolo, N. S., Acta metall. mater., 1990, 38, 1191. 5. Bonda, N. R., Pope, D. P. and Laird, C., Acta metall., 1987, 35, 2371.
6. Busso, E. P. and McClintock, F. A., Acta metall. mater., 1994, 42, 3263. 7. Kallingal, C. G., Smith, T. R., Stolo, N. S. and Rajan, K., Acta metall. mater., 1994, 42, 3731. 8. Umakoshi, Y., Yasuda, H. Y. and Nakano, T., Intermetallics, 1996, 4, S65. 9. Nakano, T., Ogawa, B., Koizumi, Y. and Umakoshi, Y., Acta mater., 1998, 46, 4311. 10. Koizumi, Y., Nakano, T. and Umakoshi, Y., Acta mater., 1998, 46, 4743. 11. Morris, D. G., Brown, G. T., Piller, R. C. and Smallman, R. E., Acta metall., 1976, 24, 21. 12. Victoria, M. and Vidoz, A. E., Phys. Stat. Sol. (a), 1968, 28, 131. 13. Wee, D.-M. and Suzuki, T., Trans. Jpn. Inst. Metals, 1981, 22, 163. 14. Dahl, O., Z. Metallk., 1936, 28, 133. 15. Chikazumi, S., Suzuki, K. and Iwata, H., J. Phys. Soc. Jpn., 1957, 12, 1259. 16. Takahashi, S., Phys. Stat. Sol. (a), 1977, 42, 201. 17. KronmuÈller, H., Inter. J. Nondestructive Testing, 1972, 3, 315. 18. Yasuda, H. Y., Furuta, D., Sasaki, A. and Umakoshi, Y., J. appl. Phys., submitted. 19. Webb, G. and Antolovich, S. D., Metall. Trans. A, 1994, 25, 2469. 20. Mughrabi, H., Mater. Sci. Engng, 1978, 33, 207. 21. Neumann, P., Physical Metallurgy, 3rd edn, R. W. Cahn and P. Haasen, 1983, p. 1554. 22. Mughrabi, H., ISIJ Inter., 1997, 37, 1154. 23. Gerold, V. and Karnthaler, H. P., Acta metall., 1989, 37, 2177. 24. Korner, A. and Karnthaler, H. P., Phil. Mag. A., 1985, 52, 29. 25. Korner, A., Karnthaler, H. P. and Hitzenberger, C., Phil. Mag. A, 1987, 56, 73. 26. Takasugi, T., Eguchi, T., Yoshida, M. and Izumi, O., J. Jpn. Inst. Metals, 1989, 53, 34. 27. Pak, H. R., Hsiung, L.-M. and Kato, M., High temperature ordered intermetallic alloys. In MRS Symposium Proceedings, Vol. 39, 1985, p. 239. 28. Vidoz, A. E. and Brown, L. M., Phil. Mag., 1962, 7, 1167. 29. Wechsler, M. S., in The Inhomogeneity of Plastic Deformation. ASM, Ohio, 1971, p. 19. 30. Yoo, M. H., Horton, J. A. and Liu, C., Acta metall., 1988, 36, 2935. 31. Horton, J. A., Baker, I. and Yoo, M. H., Phil. Mag. A, 1991, 63, 319. 32. Hsiung, L. M. and Stolo, N. S., Acta metall. mater., 1992, 40, 2993. 33. Hsiung, L. M. and Stolo, N. S., Acta metall. mater., 1994, 42, 1457. 34. Bonda, N. R., Pope, D. P. and Laird, C., Acta metall., 1987, 35, 2385. 35. Takahashi, S., Phys. Stat. Sol. (a), 1977, 42, 529.
EFFECT OF LONG-RANGE ORDER ON THE CYCLIC DEFORMATION BEHAVIOUR OF Ni3Fe SINGLE CRYSTALS H. Y. YASUDA, D. FURUTA and Y. UMAKOSHI{ Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka, 565-0871, Japan (Received 24 January 2000; received in revised form 8 May 2000; accepted 8 May 2000) AbstractÐEect of ordering on cyclic deformation in disordered and ordered Ni3Fe single crystals was investigated focusing on stress±strain response and deformation substructure. The cyclic hardening depended strongly on the long range order. The maximum stress in the disordered crystals increased gradually with increasing number of cycles and then reached a saturation, while ordered ones exhibited cyclic softening after an initial strong cyclic hardening. The cyclic hardening at an early stage of fatigue in ordered crystals may be due to APB tubes and debris which were produced by the intersection between primary and secondary slips. Coarse slip bands were observed in fatigued ordered Ni3Fe single crystals. In the bands, three-dimensional dislocation structure was formed accompanied by a decrease in the degree of order, which was responsible for the cyclic softening. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Intermetallic; Fatigue; Dislocations; Transmission electron microscopy
1. INTRODUCTION
Cyclic deformation and fatigue property of ordered alloys are important factors for industrial application. The earliest studies on fatigue behaviour of ordered alloys were carried out by Boettner et al. [1]. The long-range degree of order in some ordered alloys signi®cantly aected the high-cycle fatigue life, notch sensitivity and environmental embrittlement under cyclic loading [2, 3]. The ease of crossslip of superdislocations was closely related to the fatigue property [1]. Recently, much attention has been paid to understanding the cyclic deformation behaviour of intermetallics such as Ni3Al [4, 5, 32± 34], NiAl [6, 7], TiAl [8] and Ti3Al [9, 10] focusing on cyclic hardening, fatigue life and deformation substructure. Ni3Fe is a Kurnakov-type intermetallic compound with the L12 structure and order±disorder transition occurring at 5038C. Since disordered and ordered states are realized at room temperature by an appropriate heat treatment [11], the eect of the long-range ordering on the cyclic deformation behaviour of Ni3Fe can be examined. Moreover, both disordered and ordered Ni3Fe are ferromagnetic
{ To whom all correspondence should be addressed.
and, in particular, the disordered Ni3Fe is known as a superior soft magnetic material called Permalloy. Not only plastic behaviour, but also the magnetic properties of Ni3Fe depend on the degree of order [12±16, 35]. The magnetic properties are known to be in¯uenced by lattice defects such as dislocations and planar faults in magnetic materials [14±17]. Dislocation arrangements and planar faults in deformed ferromagnetic materials were observed using magnetic techniques. In this paper, we report on the cyclic deformation behaviour of disordered and ordered Ni3Fe single crystals focusing on the cyclic hardening, the surface topography, and the deformation substructure. Observations of dislocations and planar faults using a magnetic technique are described in a separate paper [18]. 2. EXPERIMENTAL PROCEDURE
Master ingots of Ni±25.0 at.%Fe alloy were prepared by melting high-purity Ni and Fe in a plasma arc furnace. Single crystals were grown from the ingots by a ¯oating zone method at a rate of 10 mm/h under a high-purity Ar gas ¯ow. The crystals were homogenized at 1473 K for 24 h, and their orientation was determined using X-ray Laue back diraction technique. Fatigue specimens
1359-6454/00/$20.00 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 1 4 3 - 9
3402
YASUDA et al.: CYCLIC DEFORMATION
whose loading axis is [149] were cut from the crystals by spark machining. The front and side sur-faces of the specimens were (941) and (121), respectively. Schmid factors for the primary (111)[101] and the secondary (111)[101] slip systems were 0.5 and 0.467, respectively. The specimens were annealed at 735 K for 1 103 h, then slowly cooled at 10 K/h for chemical ordering and the degree of long-range order was about 0.9 after heat treatment [11]. The disordered state was obtained by quenching from 873 K in ice-brine. Plastic strain-controlled fatigue tests in a tension/ compression mode, all beginning in tension, were done at room temperature in air using a servo hydraulic system (Shimadzu servo pulsar EHFED5-10L type) with a clip-on extensometer for strain measurement. Plastic strain amplitude (Dep) was controlled in the range between 20.25 and 20.45%. All the tests were conducted at a constant strain rate of 3 10ÿ4/s. Slip traces were examined using an optical microscope equipped with Nomarski interference contrast. The surface topography on the front surface of fatigued specimens was examined by an atomic force microscope (AFM) (Shimadzu SPM-9500), and the fracture surface by a scanning electron microscope (SEM). Thin foils for transmission electron microscopic (TEM) observation were cut parallel and perpendicular to the primary (111) slip plane by spark machining and polished using the twin jet technique. Deformation substructure was observed by a Hitachi H-800 electron microscope operated at 200 kV. 3. RESULTS
3.1. Cyclic stress±strain response Figure 1 shows cyclic hardening curves of disordered and ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%: The maximum stress is represented as the average of the absolute values of maximum tensile and compressive stresses in each stress±strain hysteresis loop, since no signi®cant
Fig. 1. Cyclic hardening curves of disordered and ordered Ni3Fe single crystals fatigued at Dep 20:25%:
Fig. 2. Variation in the maximum stress of disordered Ni3Fe single crystals cyclically deformed at various plastic strain amplitudes as a function of cumulative plastic strain.
asymmetry between tensile and compressive stresses was observed in the loop of Ni3Fe single crystals. The cyclic hardening depends strongly on the longrange order. In the disordered crystal, the maximum stress gradually increases with increasing number of cycles (N ) and is saturated at about 2 103 cycles. In contrast, the ordered crystal shows strong cyclic hardening up to N = 1 102 and then the maximum stress gradually decreases after peaking at about 300 MPa. A rapid drop in the maximum stress occurs in the ®nal stage of fatigue where cracks were initiated at the surface of the specimen. Similar cyclic hardening/softening behaviour was also seen in Ni3Al polycrystals [19]. Figures 2 and 3 show variation in maximum stress with cumulative plastic strain of disordered and ordered Ni3Fe single crystals cyclically deformed at various Dep, respectively. In Fig. 2 similar cyclic hardening curves are seen at various Dep and the saturation stresses reach the same value of about 150 MPa regardless of dierent Dep. This may correspond to the stress in the plateau region of typical cyclic stress±strain curves of Cu single crystals [20, 21]. In contrast, the peculiar cyclic hardening/softening behaviour is always recognized in the ordered
Fig. 3. Variation in the maximum stress of ordered Ni3Fe single crystals cyclically deformed at various plastic strain amplitudes as a function of cumulative plastic strain.
YASUDA et al.: CYCLIC DEFORMATION
Fig. 4. Variation in the area fraction of the coarse slip bands in ordereded Ni3Fe single crystals fatigued at Dep 20:25% with number of cycles.
crystals fatigued at various Dep in Fig. 3. The peak of maximum stress appears at the same cumulative plastic strain of 1 102% at various Dep and specimens fatigued at dierent Dep fractured at the same cumulative strain as shown in the ®gure. 3.2. Slip traces, surface topography and fracture behaviour Slip traces and surface topography of disordered and ordered Ni3Fe single crystals cyclically deformed at De 20:25% to various number of cycles were examined on the primary (111) plane. Although the data are not shown here, in the disordered crystals ®ne and fairly homogeneously distrib-
3403
uted slip traces were observed and the density increased with an increase in the number of cycles. In contrast, the coarse slip bands were observed along the primary (111) plane in the ordered crystals even at an early stage of fatigue. Fine slip traces with very weak contrast were also observed between these bands; the width and number of the bands increased as the number of cycles increased. Figure 4 shows variation in the area fraction of the slip bands in the ordered crystals with the number of cycles. The fraction of the bands increases as the number of cycles rises, reaching about 77% at N = 1 104. When shear deformation occurred in a localized region and formed a slip band, the slip band induced a sharp surface step. Figures 5 and 6 show the frequency of surface steps measured by AFM in disordered and ordered Ni3Fe single crystals fatigued at Dep 20:25%, respectively. In disordered crystals, a high frequency of ®ne steps was observed at an early stage of fatigue. As the number of cycles rose, the frequency of steps with higher height increased, but the height of the steps was almost below 0.5 mm even after N = 1 104. The distribution of the frequency showed no remarkable change after N = 1 104. The variation in height and frequency of surface steps in the ordered crystals was quite dierent from that in the disordered crystals. At N = 10 there was a high frequency of ®ne steps similar to that in the disordered crystals. However, surface steps more than 0.5 mm in height corresponding to the coarse slip bands appeared at N = 1 102. As the number of cycles increased, slip bands concentrated in a localized region, resulting
Fig. 5. Distribution of the height of surface steps in disordered Ni3Fe single crystals cyclically deformed at Dep 20:25% to various number of cycles (N); (a) N = 10, (b) N = 1 102, (c) N = 1 103, (d) N = 1 104.
3404
YASUDA et al.: CYCLIC DEFORMATION
Fig. 6. Distribution of the height of surface steps in ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%; (a) N = 10, (b) N = 1 102, (c) N = 1 103, (d) N = 1 104.
in the development of high steps whose high frequency showed wide distribution. Steps more than 1 mm were formed at and above N = 1 103 as shown in Fig. 6. These steps result in the crack initiation in ordered Ni3Fe crystals shown in Fig. 7. Figure 8 shows scanning electron fractographs of disordered and ordered Ni3Fe single crystals cyclically deformed at Dep 20:45%: In both specimens the stage I cracks were initiated parallel to the primary (111) plane and stage II cracks propagated perpendicular to the stress axis showing striation. It is known that the crack growth rate is accelerated by chemical ordering and the notch sensitivity becomes high in the ordered state. However, the fractograph in the ordered state [Fig. 8(b)] shows no brittle ¯at fracture surface.
disordered Ni3Fe, which can hardly be suppressed by quenching, may induce the planar dislocation arrangement in the disordered crystals as proposed by Gerold et al. [23]. Figure 10 shows deformation substructures in the (111) foils of ordered Ni3Fe single crystals cyclically deformed at De 20:25%: At N = 1 102 screw dislocations and many pieces of debris with b=[101] are observed in Fig. 10(a). Two types of edge and 608 debris are con®rmed. Dislocation bundles with b=[101] are arranged along [011] direction inclined at 608 to their Burgers vector at N = 1 103 and 1 104 as shown in Fig. 10(b) and (c). The density of the bundles increased with the number of
3.3. Deformation substructure in fatigued Ni3Fe single crystals Figure 9 shows deformation substructures in disordered Ni3Fe single crystals cyclically deformed to N 1 103 at De 20:25%: Well-developed dislocation bundles perpendicular to the Burgers vector of b=[101] are observed in the (111) foil in the disordered crystal, as shown in Fig. 9(a). These bundles were often observed in fatigued Cu single crystals [20, 21]. Planar arrangement of dislocations was observed in a (121) foil, and the ladder or vein structure seen in fatigued Cu single crystals was not formed in this crystal. The morphology of the dislocation arrangement in the (121) foil is similar to that in austenitic stainless steel cyclically deformed at small strain amplitudes [22], as shown in Fig. 9(b). Formation of the short-range order in
Fig. 7. A SEM image of the fatigue cracks at a slip band in ordered Ni3Fe single crystal cyclically deformed at Dep 20:25% to N = 1 104.
YASUDA et al.: CYCLIC DEFORMATION
3405
Fig. 8. Fracture surfaces of disordered (a) and ordered (b) Ni3Fe single crystals cyclically deformed at Dep 20:25%:
cycles. In the bundles, a large number of pieces of 608 debris extend along the [011] direction, as shown in Fig. 11. Primary dislocations are piled-up at the debris and form bundles along [011] resulting in cyclic hardening in the ordered Ni3Fe single crystals at an early stage. TEM images in the (121) foils of ordered Ni3Fe single crystals fatigued at Dep 20:25% are shown in Fig. 12. At N = 1 102, primary [101] dislo cations show planar con®guration with g~ 202 in Fig. 12(a), while these dislocations are out of contrast and sharp bands composed of secondary [101] dislocations are observed with g~ 111 in Fig. 12(b). The width of the bands increased with increasing number of cycles and irregular cells of three-dimensional dislocation arrangement were formed in the bands at N = 1 103, although the planar dislocation arrangement still remained between the bands [Fig. 12(c)]. The formation of the cell structure was accompanied by dislocation rearrangement, which may require the cross slip of dislocations. Further development of the irregular cell was observed at N = 1 104 [Fig. 12(d)]. The
formation of the coarse bands in the ordered Ni3Fe may correspond to the cyclic softening after N = 1 102. 4. DISCUSSION
The cyclic deformation behaviour of Ni3Fe single crystals depended strongly on the long-range order: typical cyclic hardening was observed in the disordered state similar to usual metals, while cyclic softening occurred after initial strong cyclic hardening in the ordered state. In ordered Ni3Fe, a h110i superlattice dislocation can be dissociated into four 1/6h112i Shockley superpartials bound by an anti-phase boundary (APB) and two complex stacking faults [24±26]. They spread widely on the {111} plane, but no dissociation on the cube cross-slip plane occurred at room temperature. Since extension or constriction of the dislocation core occurs due to the edge component of their Shockley partials in an elementary process for the cross-slip onto the cube plane, depending on the applied stress mode such as ten-
Fig. 9. TEM images taken from the (111) (a) and (121) (b) foils of disordered Ni3Fe single crystals ---cyclically deformed at Dep 20:25% to N = 1 103. (a) Incident beam B 1 [211], g = 111, b=[101], e is perpendicular to b; (b) B 1 [121], g = 202.
3406
YASUDA et al.: CYCLIC DEFORMATION
Fig. 10. Deformation substructure taken from the (111) foils of ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%: Burgers vector parallel to [101], and 608 debris represented by b and 60 in the ®gure, respectively. B 1 [101] and g=111; (a) N = 1 102, (b) N = 1 103, (c) N = 1 104.
sion and compression, asymmetry of yield stress in tension and compression appears in the L12-type compounds such as Ni3Al[4] and Ni3Ge[27]. However, since no signi®cant asymmetry of plastic behaviour in tension and compression was observed in the ordered Ni3Fe single crystals at room temperature, the tendency of dislocations to cross-slip onto the cube plane may be very low. Moreover, low APB energy in ordered Ni3Fe induced a planar dislocation arrangement in a monotonic deformation. Under the cyclic loading, however, dislocations in the coarse slip bands formed a threedimensional structure. In the ordered Ni3Fe single crystals, rapid cyclic hardening was observed at an early stage of fatigue, and in this alloy a well-developed ordered domain structure was formed during heat treatment. The ordered domain was cut by dislocations during their to-and-fro motion resulting in a decrease in
Fig. 11. A high magni®cation image of the dislocation bundle formed in ordered Ni3Fe single crystal cyclically deformed at Dep 20:25% to N = 1 103. B 1 [101], g=111.
the domain size. However, the eect of change in the domain size on the shear stress is too small to explain the strong cyclic hardening [26]. At the strong cyclic hardening stage, dislocation bundles and debris along the [011] direction, which is the intersection between primary (111) and secondary (111) slip planes, were inclined at 608 to b=[101]. The intersection and reaction between the primary and secondary dislocations may produce the 608 bundles and the 608 debris. When the primary superlattice dislocations dissociated into unit dislocations bound by APB intersected by secondary dislocations, APB tubes may be produced, as proposed by Vidoz and Brown [28]. However, APB tubes were rarely observed by TEM observation in fatigued Ni3Fe, since the scattering factors of Ni and Fe are very similar. If such tubes are formed during cyclic deformation, they may interrupt the motion of dislocations resulting in strong work hardening. Moreover, the 608 debris as well as edge debris was frequently observed in the ordered crystals. The 608 debris behaved similarly to the faulted superdipoles seen in cyclically deformed Ni3Al±B single crystals. It may also interrupt the motion of primary dislocations and contribute to the cyclic hardening in ordered Ni3Fe single crystals. In addition, Lomer±Cottrell locking due to the reaction between primary and secondary dislocations may contribute to the cyclic hardening. When the cyclic softening occurred after N = 1 102, slip bands with more than 0.5 mm height were developed. A large number of dislocations due to the to-and-fro motion and APBs may be produced in these bands, resulting in the destruction of the ordered structure and decreasing the degree of order in the localized region. The decrease in the long-range order during cyclic deformation was con®rmed in Ni3Al polycrystals by the X-ray diffraction method [19]. Evidence of the decrease in the degree of order in Ni3Fe during cyclic deformation was obtained by magnetic measurement as described in a separate paper [18]. Figure 13 shows a high magni®cation image of
YASUDA et al.: CYCLIC DEFORMATION
3407
Fig. 12. Deformation substructure in the (121) foils of ordered Ni3Fe single crystals cyclically deformed at Dep 20:25%; (a) N = 1 102, B 1 [121], g = 202, (b) N = 1 102, B 1 [121], g = 111, (c) N = 1 3 4 10 , B 1 [121], g = 111, and (d) N = 1 10 , B 1 [121], g = 111.
the coarse slip bands in the fatigued ordered Ni3Fe crystal. Secondary dislocations with a scalloped shape are also observed in the bands. If these dislocations interact with some lattice defects such as
Fig. 13. A high magni®cation image of the coarse slip bands formed in ordered Ni3Fe single crystal cyclically deformed at Dep 20:25% to N = 1 103; B 1 [121], g = 111.
APB and debris, the degree of long-range order may be destroyed after their motion. Moreover, the to-and-fro motion of dislocations in the localized region under cyclic loading may accelerate the disordering. Such wiping out of defects by secondary dislocations was observed in irradiated metals during deformation [29]. The disordering on the slip plane was also reported in stoichiometric Ni3Al [30, 31]. As the degree of order decreases, the individual unit dislocations can be activated without creating APB and the dislocation rearrangement occurs by their easy cross-slip. The slip disordering facilitates the motion of primary dislocations in the slip bands and therefore the cyclic softening occurs in the later stage of fatigue in ordered Ni3Fe. However, cyclic softening following initial hardening was not seen in Ni3Al±B [4, 32, 33] and Ni3Ge [27] single crystals cyclically deformed at constant total strain amplitude. The initial strong cyclic hardening in those crystals may hinder the eect of cyclic softening at the further stage. The number and area fraction of slip bands increased as plastic strain amplitude grew. However, the distribution of the step heights of the slip bands was almost independent of these amplitudes, ranging from 20.25 to 20.45% at the same cumulative plastic strain. Therefore, the peak stress and the corresponding cumulative plastic strain was
3408
YASUDA et al.: CYCLIC DEFORMATION
approximately constant at the amplitudes in this study, although there was a small deviation in the peak and the cumulative plastic strain depending on the plastic strain amplitude. 5. CONCLUSIONS
The cyclic deformation and deformation substructure in disordered and ordered Ni3Fe single crystals were investigated and the following conclusions were reached. The cyclic hardening of Ni3Fe depended strongly on the degree of long-range order. In the disordered state the maximum stress gradually increased, showing cyclic hardening, and reached a saturated stress around N = 2 103 at De 20:25%, while strong initial cyclic hardening was observed in the ordered state, showing a peak at a stress of about 300 MPa at N = 1 102 following by cyclic softening. Variation in the maximum stress with the cumulative plastic strain showed no Dep dependence. The initial strong hardening in the ordered state is due to the formation of APB tubes and debris created by the interaction and reaction between the primary and secondary slips. Formation of the coarse slip bands more than 0.5 mm in height in the ordered state corresponds to the cyclic softening. Concentrated shear deformation at the slip bands during fatigue induces the slip disordering and the disordering facilitates the motion of primary dislocations under cyclic loading resulting in cyclic softening. AcknowledgementsÐThis work was supported by a Grantin-Aid for Scienti®c Research and Development from the Ministry of Education, Science, Sports and Culture of Japan. REFERENCES 1. Boettner, R. C., Stolo, N. S. and Davies, R. G., Trans. Metall. Soc. AIME, 1966, 236, 131. 2. Stolo, N. S., Fuchs, G. E., Kuruvilla, A. K. and Choe, S. J., High temperature ordered intermetallic alloys. In II MRS Symposium Proceedings, Vol. 81, 1987, p. 247. 3. Stolo, N. S., ISIJ Inter., 1997, 37, 1197. 4. Hsiung, L. M. and Stolo, N. S., Acta metall. mater., 1990, 38, 1191. 5. Bonda, N. R., Pope, D. P. and Laird, C., Acta metall., 1987, 35, 2371.
6. Busso, E. P. and McClintock, F. A., Acta metall. mater., 1994, 42, 3263. 7. Kallingal, C. G., Smith, T. R., Stolo, N. S. and Rajan, K., Acta metall. mater., 1994, 42, 3731. 8. Umakoshi, Y., Yasuda, H. Y. and Nakano, T., Intermetallics, 1996, 4, S65. 9. Nakano, T., Ogawa, B., Koizumi, Y. and Umakoshi, Y., Acta mater., 1998, 46, 4311. 10. Koizumi, Y., Nakano, T. and Umakoshi, Y., Acta mater., 1998, 46, 4743. 11. Morris, D. G., Brown, G. T., Piller, R. C. and Smallman, R. E., Acta metall., 1976, 24, 21. 12. Victoria, M. and Vidoz, A. E., Phys. Stat. Sol. (a), 1968, 28, 131. 13. Wee, D.-M. and Suzuki, T., Trans. Jpn. Inst. Metals, 1981, 22, 163. 14. Dahl, O., Z. Metallk., 1936, 28, 133. 15. Chikazumi, S., Suzuki, K. and Iwata, H., J. Phys. Soc. Jpn., 1957, 12, 1259. 16. Takahashi, S., Phys. Stat. Sol. (a), 1977, 42, 201. 17. KronmuÈller, H., Inter. J. Nondestructive Testing, 1972, 3, 315. 18. Yasuda, H. Y., Furuta, D., Sasaki, A. and Umakoshi, Y., J. appl. Phys., submitted. 19. Webb, G. and Antolovich, S. D., Metall. Trans. A, 1994, 25, 2469. 20. Mughrabi, H., Mater. Sci. Engng, 1978, 33, 207. 21. Neumann, P., Physical Metallurgy, 3rd edn, R. W. Cahn and P. Haasen, 1983, p. 1554. 22. Mughrabi, H., ISIJ Inter., 1997, 37, 1154. 23. Gerold, V. and Karnthaler, H. P., Acta metall., 1989, 37, 2177. 24. Korner, A. and Karnthaler, H. P., Phil. Mag. A., 1985, 52, 29. 25. Korner, A., Karnthaler, H. P. and Hitzenberger, C., Phil. Mag. A, 1987, 56, 73. 26. Takasugi, T., Eguchi, T., Yoshida, M. and Izumi, O., J. Jpn. Inst. Metals, 1989, 53, 34. 27. Pak, H. R., Hsiung, L.-M. and Kato, M., High temperature ordered intermetallic alloys. In MRS Symposium Proceedings, Vol. 39, 1985, p. 239. 28. Vidoz, A. E. and Brown, L. M., Phil. Mag., 1962, 7, 1167. 29. Wechsler, M. S., in The Inhomogeneity of Plastic Deformation. ASM, Ohio, 1971, p. 19. 30. Yoo, M. H., Horton, J. A. and Liu, C., Acta metall., 1988, 36, 2935. 31. Horton, J. A., Baker, I. and Yoo, M. H., Phil. Mag. A, 1991, 63, 319. 32. Hsiung, L. M. and Stolo, N. S., Acta metall. mater., 1992, 40, 2993. 33. Hsiung, L. M. and Stolo, N. S., Acta metall. mater., 1994, 42, 1457. 34. Bonda, N. R., Pope, D. P. and Laird, C., Acta metall., 1987, 35, 2385. 35. Takahashi, S., Phys. Stat. Sol. (a), 1977, 42, 529.