On the temperature dependence of the critical resolvedshear stress of the c0-strengthened superalloy NIMONIC PE16

Angelika Vennemann 1, Eckhard Langmaack, Eckhard Nembach *

Institut f€uur Materialphysik der Universit€aat M€uunster, Wilhelm-Klemm-Strasse 10, 48149 M€uunster, Germany

Received 18 December 2001; accepted 11 February 2002

Abstract

The temperature dependence of c0-strengthening of the commercial nickel-base superalloy NIMONIC PE16 has been

investigated by measuring the critical resolved shear stress in the temperature range 373–1173 K and observing the

resulting slip line patterns. � 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Materials; Superalloy; Properties and phenomena; Aging; Mechanical properties; High temperature; Shear bands; Yield

phenomena

1. Introduction

The high strength of nickel-base superalloysderives from fine precipitates of the L12-long rangeordered c0-phase [1–3]. Up to the temperatureTmax P 1000 K, the critical resolved shear stress(CRSS) st of c0-particle strengthened superalloysis virtually independent of the deformation tem-perature Tdeform. Above Tmax, stðTdeformÞ decreasesprecipitously. In order to contribute to the un-derstanding of this temperature dependence ofstrength, the following investigations have beencarried out for the commercial c0-strengthenednickel-base superalloy NIMONIC PE16 [2] (com-position in at.%: Ni 41.3, Fe 34.4, Cr 17.5, Al 2.6,

Mo 2.0, Ti 1.5, Co 0.5, C 0.25, Si 0.11): (1) com-pression testing single crystals of near [�1111]-ori-entation in the temperature range 373–1173 K anddetermining the CRSS and (2) observation of sliplines in the same temperature range. Tmax of NI-MONIC PE16 is around 1050 K.

2. Experiments

Since single phase Ll2-ordered c0-intermetallicsare known to slip on {0 0 1}-planes at elevatedtemperatures [3,4], the principal aim of the presentslip line study was to look for f001g-slip traces.Therefore the nominal orientation of the cylin-drical single crystals was chosen to be [�1111]. Forthis orientation the maximum Schmid factor forf001g-glide is close to twice the maximum onefor f111g-glide. Two opposite sides of the crys-tals were machined flat for the slip line observa-tions. The nominal Miller indices of these planeswere f112g. In fact, both observation planes were

Scripta Materialia 46 (2002) 723–728

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*Corresponding author. Tel.: +49-251-8333570; fax: +49-

251-8338346.

E-mail address: nembach@nwz.uni-muenster.de (E. Nem-

bach).1 Present address: Stiftung Institut f€uur Werkstofftechnik

Badgasteiner Strasse 3, 28359 Bremen, Germany.

1359-6462/02/$ - see front matter � 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

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orientated such that they were parallel to thespecimen axis, but deviated by up to 10� fromf112g. They were chosen such that the traces off001g- and f111g-slip planes included as large aspossible angles and that the relevant Burgers vec-tors had a strong component parallel to h112i.

2.1. Specimen preparation

The preparation schedule of the single crystalspecimens was as follows: (i) growth of cylindricalcrystals of 4 mm diameter, (ii) spark machiningof two opposite f112g-planes, (iii) mechanicallypolishing them, finally with 0.25 lm diamondspray, (iv) recovery treatment: 24 h at 979 K (thistreatment was meant to definitely avoid recrystal-lization of parts of the f112g-planes), (v) ho-mogenization treatment: 4 h at 1473 K, (vi) waterquench. Part of the single crystal specimens havebeen studied in this state. They will be referredto as ‘‘only homogenized, not aged’’. Others havebeen subjected to aging treatments: (vii) c0-nucle-ation treatment: 2 h at 979 K, (viii) Ostwald rip-ening treatment [2,5]: either 29 or 137 or 192 h atTage ¼ 1029 K. After these latter treatments at Tagethe c0-volume fraction f was 0.089 and the averageradii r of the c0-particles were: 10.1 nm (under-aged), 18.0 nm (peak-aged), and 20.4 nm (peak-aged). The respective states of aging [2,5] areindicated. The actual orientation of each specimenwas determined; it deviated by up to 6� from [�1111].Just prior to the compression tests the f112g-

planes were repolished with 0.25 lm diamondspray.

2.2. Compression tests

The single crystal specimens were compressiontested at temperatures Tdeform between 373 and 1173K. The technical strain rate was approximately1:5 104/s. In order to reach Tdeform fast, the fur-nace was allowed to overshoot slightly. Specimensdeformed at or above 1073 K were above Tage ¼1029 K for up to 17 min. This time includes heatingup to Tdeform, deforming, and cooling down. Oxi-dation of the specimens was prevented by a pro-tective argon atmosphere and by shielding them bya split titanium tube.The CRSS st was calculated from the load L

needed to achieve 0.002 plastic technical strain epl.The compression tests were stopped at epl � 0:02.The highest Schmid factor of all f111gh110i-slipsystems, i.e. that of the primary system, has beeninserted. The results for the CRSS st are presentedin Table 1. The column r ¼ 0 nm refers to the onlyhomogenized, not aged crystals. For all st-dataquoted new, not previously deformed specimenshave been used. Most st-data have been derivedfrom one specimen; there were, however, two ex-ceptions: (i) most of the st-data quoted for r ¼ 20:4nm represent averages over two specimens and (ii)at 1173 K four only homogenized, not aged spec-imens have been deformed. In Table 1, estimatederror limits are given; their major part stems from

Table 1

CRSS st of single crystals versus the deformation temperature Tdeform (K)

st (MPa)

Tdeform r ¼ 0:0 nm r ¼ 10:1 nm r ¼ 18:0 nm r ¼ 20:4 nm

373 + 60� 6 0 179� 19 + 187� 20 + 184� 19723 + 199� 20873 m 45� 5 162� 17 + 192� 20 + 180� 13923 + 185� 14973 m 46� 5 0 183� 19 0 210� 21 0 210� 161023 + 185� 20 – 194� 151073 m 99� 11 156� 16 158� 17 160� 121123 m 96� 10 y 141� 14 115� 8 117� 81173 0 66� 6

The column with the heading r ¼ 0 nm refers to homogenized, not aged specimens. + (work hardening), 0 (no work hardening),

(work softening), m (serrated flow) indicate the mode of slip, y (yield point).

724 A. Vennemann et al. / Scripta Materialia 46 (2002) 723–728

the Schmid factor, which is very sensitive to theexact orientation of the specimen. In view of theserather large error limits the agreement between thepresent st-data and those published for NIMONICPE16-single crystals with middle orientations [2] isjudged to be satisfactory.In many cases there was no work hardening,

often there was even work softening (¼ ‘‘nega-tive’’ work hardening). This is indicated in Table 1;there the sign of oL=oepl is given. The quoted signapplies to the entire range 0 < epl6 �0.02. Be-tween 873 and 1123 K, all of the only homogenizedcrystals showed serrated flow. This too is indicatedin Table 1. Except for the underaged specimenswith r ¼ 10:1 nm, oL=oepl of c0-strengthened speci-mens decreases systematically as Tdeform is raised.In case of serrated flow, st has been derived fromthe peaks of the curve LðeplÞ.

2.3. Observation of slip lines

Since the direction of the final mechanical0.25 lm-diamond spray polish was parallel to theaxes of the specimens, i.e. to the [�1111]-direction,scratches caused by the polish could easily bedistinguished from slip lines. Cross slip was veryfrequent. Examples of slip line patterns are pre-sented in Fig. 1. With one exception only traces off111g-glide planes have been found. This excep-tion are the only homogenized, not aged singlecrystals deformed at 1173 K: 29% of their in-spected f112g-planes showed traces of (1 0 0)-and/or (0 0 1)-slip planes. Most only homogenizedspecimens meant to be deformed at 1173 K werechosen such that the expected angles betweenf111g- and f001g-slip traces were not less than14�. Fig. 1(b) shows (1 0 0)-slip traces observed onan only homogenized specimen. It cannot be def-initely decided whether the approximately hori-zontal slip traces are due to (0 0 1)-or ð1�111Þ-glide.High resolution scanning electron microscopy re-vealed no details of the slip traces beyond the in-formation gained by optical microscopy.A measure of the probability to see traces of the

various f111g-glide planes in the optical micro-scope is the ratio n=g listed in Table 2: g is the totalnumber of f112g-planes inspected and n is thenumber of inspected f112g-planes in which at

least five traces of the respective slip plane systemhave been seen. For Tdeform > 373 K, this proba-bility often differed from specimen to specimen de-formed at the same temperature; sometimes it waseven different for the two opposite f112g-obser-vation planes of the same specimen. Due to thesymmetry of the specimen, the definitions of (1 1 1)-and (�11�111)-slip traces are arbitrary; both includeangles of about �20� with the specimen axis. Thatslip plane was given the indices (1 1 1) for whichmore traces of activated slip planes were seen. Sincefor the chosen nominal specimen orientation theSchmid factor of (�1111)-planes vanishes, no entries

Fig. 1. Optical micrographs showing slip traces on nominal

ð1�112Þ-planes. The directions of the various slip traces are in-dicated, the vertical arrow marks the nominal [�1111]-specimenaxis. The different orientations of the ð1�111Þ-traces in (a) and(b) reflect the differences of the actual orientations of the

two specimens. (a) Aged specimen, f ¼ 0:089, r ¼ 10:1 nm,

Tdeform ¼ 1023 K. The actual observation plane deviates by 2�from (1 �11 2). (b) Only homogenized, not aged specimen,

Tdeform ¼ 1173 K. The actual observation plane deviates by 1�from ð1�112Þ.

A. Vennemann et al. / Scripta Materialia 46 (2002) 723–728 725

are listed for these planes in Table 2. It transpiresfrom Table 2, that there is no systematic variationof the ratio n=g with Tdeform.For c0-strengthened specimens deformed at and

above Tmax, three distinctly different slip line pat-terns have been found by optical microscopy: (i)few, rather widely spaced (�0.1 mm) sharp sliplines, (ii) diffuse, homogeneously distributed sliplines, (iii) no slip lines at all. The observations ofthese patterns were, however, not well reproduc-ible; even on the two f112g-planes of a specimendifferent patterns have been found.

3. Discussion

3.1. Homogenized specimens

The high temperature CRSS st of those speci-mens which have only been homogenized, but notaged, has a maximum at Tdeform � 1100 K. Thereason is probably that during heating up to high

temperatures Tdeform and during the deformation,small c0-particles form [6]. Above the peak-tem-perature of st, they start to dissolve and stðTdeformÞdecreases again. After deformation at 1173 K,which is about 40 K above the c0-solvus tempera-ture, f001g-slip traces were frequently observed.Since st (1173 K) is 78% higher than st (973 K), it isconcluded that at 1173 K the specimens containedc0-precipitates. At 373 K, there was work harden-ing; between 873 and 1123 K the flow was serrated.

3.2. c0-strengthened specimens

Up to Tdeform ¼ Tmax � 1050 K, the CRSS stvaries only slightly; in fact there is a weakly pro-nounced maximum below Tmax. The work hard-ening rate varies monotonically with increasingTdeform (Table 1): from work hardening to worksoftening. There is, however, one exception: thespecimens with the smallest c0-precipitates (r ¼10:1 nm) show no systematic variation of the work

Table 2

Results of the slip line observations. g ¼ number of inspected f112g-planes, n ¼ number of f112g-planes in which at least five tracesof the respective glide plane have been seen

r (nm) Tdeform (K) g n=g

ð111Þ ð�11�111Þ ð1�111Þ f001g10.1 373 1 1.00 0.00 0.00 0.00

873 2 1.00 1.00 1.00 0.00

973 2 1.00 1.00 1.00 0.00

1023 2 1.00 1.00 1.00 0.00

1123 1 1.00 1.00 0.00 0.00

18.0 873 2 1.00 1.00 0.50 0.00

973 2 1.00 0.50 0.50 0.00

1073 2 1.00 0.00 0.50 0.00

1123 1 1.00 1.00 1.00 0.00

20.4 373 2 1.00 1.00 1.00 0.00

723 2 1.00 1.00 0.00 0.00

873 3 1.00 1.00 0.33 0.00

923 4 1.00 1.00 0.50 0.00

973 3 1.00 1.00 0.33 0.00

1073 1 1.00 0.00 0.00 0.00

1123 2 1.00 0.00 0.00 0.00

0.0 373 2 1.00 1.00 0.00 0.00

873 2 1.00 1.00 1.00 0.00

973 2 1.00 0.50 0.00 0.00

1073 1 1.00 1.00 0.00 0.00

1123 2 1.00 1.00 1.00 0.00

1173 7 0.86 0.29 0.29 0.29

726 A. Vennemann et al. / Scripta Materialia 46 (2002) 723–728

hardening rate with Tdeform; their work hardeningrate alternates between positive, zero, and nega-tive. The monotonic variation of the work harden-ing rate with Tdeform shown by the other specimensindicates some monotonic temperature variationof the relevant dislocation mechanisms.Concerning the question which mechanisms

cause the drastic decrease of stðTdeform > TmaxÞ ofc0-strengthened specimens, two groups of mecha-nisms must be distinguished.

1. The c0-strengthened single crystal specimenshad been aged at Tage ¼ 1029 K. This is about20 K below Tmax. Above about 950 K the ther-modynamic equilibrium c0-volume fraction fequdecreases markedly if the aging temperature israised [2]. The c0-solvus temperature is around1133 K. Since st increases with the actual c0-vol-ume fraction fact, the mentioned decrease of fequwill cause a corresponding decrease of st. For atleast two reasons, it is difficult to measure factaccurately: (i) in general, the error limits of c0-volume fractions determined by transmissionelectron microscopy are about 20% [5,7] and(ii) for Tdeform > Tage, the c0-volume fractionchanges during heating the specimen to Tdeform,deforming it, and cooling it. For Tdeform > 950K, fact probably exceeds fequ. On the basis ofthe temperature dependence of fequ [2], it is esti-mated that an appreciable part if not all of thedrastic decrease of stðTdeform > TmaxÞ is causedby the dissolution of c0-particles.

2. The monotonic decrease of the work hardeningrate as Tdeform is raised suggests that some other,so far unidentified thermally activated disloca-tion process operates in the c0-strengthenedspecimens. This point will be further investi-gated by deforming the same specimen at differ-ent temperatures and observing the ensuingeffects on st.

Miner et al. [8] reported cube slip for the c0-strengthened nickel-base superalloy Ren�ee N4. Thec0-volume fraction was 0.65 and the size of the c0-particles was 0.25 lm. Bettge and €OOsterle [9] in-vestigated slip in the c0-strengthened nickel-basesuperalloy SC16. The c0-particle dispersion wasbimodal: c0-cuboids (450 nm edge length, 0.35

volume fraction) and c0-spheres (diameter 80 nm,0.05 volume fraction). In the optical microscopespecimens deformed at 923 and 1023 K seemed toindicate cube slip. Transmission electron micros-copy revealed, however, that there was no cubeslip: since dislocations could not enter the c0-cu-boids, there was multiple cross slip on f111g-planes in the c-matrix channels between thec0-cuboids. These channels were about 200 nmwide and parallel to f001g. This zigzag glide onf111g-planes simulated glide on f001g-planes.Bettge and €OOsterle’s results shed some doubt onMiner et al.’s conclusions. The small size and thelow volume fraction of the present c0-particles,which precipitated during heating up and de-forming the only homogenized, not aged NI-MONIC PE16 single crystals, preclude theoperation of a mechanism similar to the one ob-served by Bettge and €OOsterle.

4. Conclusions

1. c0-strengthened specimens: The drastic decreaseof stðTdeformÞ above Tmax � 1050 K is at least inpart caused by the dissolution of c0-particles.But there may also be other thermally activatedprocesses which contribute to the high tempera-ture decrease of st.

2. c0-strengthened specimens: With the exceptionof the specimens with r ¼ 10:1 nm, the workhardening rate decreases as Tdeform is raised.

3. Homogenized, un-aged specimens: f001g-sliptraces are displayed at 1173 K.

Acknowledgements

Prof. R. Reichelt, University of M€uunster, isthanked for performing the high resolution scan-ning electron microscopy. Financial support bythe Deutsche Forschungsgemeinschaft is gratefullyacknowledged.

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