Materials Science and Engineering A239–240 (1997) 169–173

Mechanical strength of the binary compound Ni3Al

B. Matterstock *, J.L. Martin, J. Bonneville, T. Kruml, P. SpatigDepartement de Physique, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland

Abstract

The mechanical properties of binary Ni76.6Al23.4 single crystalline specimens have been studied in compression tests over a widerange of temperatures (293–1100 K). The resolved proof stress (t0.2%) and the corresponding work-hardening rate (u0.2%) havebeen measured as a function of temperature. A technique of repeated stress relaxations has been used to investigate the variationin the density of mobile dislocations that occurs during such transient tests. These experiments have been complemented withstrain-rate changes for characterising the strain-rate sensitivity of the flow stress. The high hardening rates and their variation withtemperature seem to correlate well with the values of the mobile dislocation exhaustion rates. The strain rate sensitivity of thestress exhibits negative values between 293 and 600 K, below the stress anomaly domain. These mechanical parameters arediscussed in terms of the available dislocation mechanisms. © 1997 Elsevier Science S.A.

Keywords: Ordered intermetallics; Strain rate sensitivity; Portevin–Le Chatelier effect; Strength anomaly

1. Introduction

During the last 10 years, a considerable effort hasbeen devoted to the characterisation and understandingof the strength anomaly of Ni3Al compounds [1]. Oneof our contributions to this effort has been the mea-surement and interpretation of the mechanical proper-ties, using conventional compression tests, but alsodeveloping appropriate transient tests (successive relax-ations [2] and repeated creep tests [3]) in an attempt toseparate the respective contributions to the strain-rateof the mobile dislocation density on the one hand andtheir velocities on the other hand. These methods weresuccessfully applied to single crystals of Ni3 (Al, 1 at.%Ta), Ni3 (Al, 3 at.% Hf) [4,5]. The results suggested thatalloying effects are quite complex, i.e. not only influ-ence the stress level at a given temperature, but alsoalter the deformation mechanisms as indicated by thevariation of the activation volume with temperature.Therefore, the investigation of the mechanical proper-ties of the binary compound has been undertaken withspecial emphasis on off stoichiometry effects. Thepresent report deals with single crystals of compositionNi76.6Al23.4, i.e. on the Nickel rich side of the stoichio-metric compound.

2. Experimental details

Two sets of binary Ni3Al specimens of two differentorigins but having similar compositions (76.6% Ni and23.4% Al) have been used. One set of specimens wasprepared from a single crystalline bar that has beenkindly supplied by Professor D.P. Pope at the Univer-sity of Pennsylvania. In this case, the specimens wereelectro-discharge machined with a [1( 23] compressionaxis, yielding a Schmid factor of 0.46 for the primary[1( 01](111) octahedral slip system. Shear stresses t andshear strains g are resolved on the primary octahedralglide system. The second set of specimens was preparedfrom a columnar grained rod kindly provided by Pro-fessor S. Hanada at the Tohoku University, Sendai.The growth direction of the columnar grains corre-sponds to a �011� crystallographic orientation and thegrains are slightly misoriented, the misorientations areless than 5° from each other about this direction, sothat the rod can be considered as a highly polygonisedsingle crystal. The deformation axes were chosen per-pendicular to the �011� average growth direction andwere approximately parallel to a [2( 33] orientationwhich, in this case, corresponds to a maximum Schmidfactor of about 0.37 for octahedral slip. For all thespecimens, the gauge length was 6.5 mm with a gaugecross section of 3.2 mm×3.2 mm.* Corresponding author.

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The mechanical tests were performed in compressionunder helium atmosphere in a Schenck RMC 100 test-ing machine, at an engineering strain-rate of 5×10−5

s−1. The specimens were deformed at various tempera-tures that range from 295 to 1100 K. Each deformationtest has been performed on a virgin specimen. Indeed, itwas shown that the well accepted procedure whichconsists in performing successive deformation tests atdifferent temperatures on the same sample, yields sig-nificant errors [4]. The strain-rate sensitivity of the flowstress (S) has been measured by using both stressrelaxation tests and strain-rate changes. In the latterexperiment, the nominal strain-rate was abruptly in-creased, respectively decreased, by a factor of 11. Atechnique of repeated stress relaxations has been usedfor measuring the variation in the density of mobiledislocations [6] that takes place during relaxation. Thepoint of the latter technique is to measure the trueactivation area of the dislocation velocity which isconsidered as thermally activated. The structuralchanges which are included in the apparent activationarea, which is measured in single relaxation tests orstrain rate jump experiments, are therefore isolated.They can be characterised by the plastic hardening rateand the change of mobile dislocation density during thetransient. These two parameters can be determined,with a satisfactory accuracy, by comparing the valuesof the true and apparent activation areas, respectively.Detailed information about this experimental procedurehas already been given elsewhere [2].

3. Experimental results

3.1. [1( 23] oriented specimens

The resolved proof stress measured at a 0.2% offsetplastic strain (t0.2%) is shown in Fig. 1 as a function of

Fig. 2. Work-hardening coefficient as a function of temperature forthe [1( 23] single crystals.

the deformation temperature. t0.2% is approximatelyconstant from room temperature up to 425 K and thensharply increases up to a temperature of about 1000 K,referred to as the peak temperature (Tp,t) in the follow-ing, above which it finally decreases. The general aspectof t0.2% with temperature is in agreement with previ-ously reported results on Ni3Al single crystals havingboth identical orientation and composition [7].

The related work-hardening rate (u0.2%) reported inFig. 2 exhibits a temperature dependence which issimilar to that of t0.2%. However, u0.2% peaks at atemperature of about 850 K (Tp,u) which is below Tp,t.This anomalous dependence of u with temperature hasalready been reported by various authors for the binarycompound as well as for alloyed compounds. It hasalso been shown that the variation of u with tempera-ture is dependent on the deformation procedure, that is:u exhibits two peaks as a function of temperature whenmeasured on a specimen that is successively deformedat several increasing temperatures, while one peak onlyis present when virgin specimens are used for eachtemperature [4].

The repeated stress relaxation technique allows forthe measurement of a structural parameter (V) whichaccounts for the microstructural changes that takeplace during relaxations. The latter include both thevariation in the density of mobile dislocations (rm) andof the internal stress (ti) [6]. Since V results from thevariations in both ti and rm, it is not possible todetermine each of these parameters separately withoutfurther hypothesis. However, the plastic deformationbeing always very small during a stress relaxation test,it seems reasonable to assume that ti does not vary toomuch during relaxation. Then, the value of V allows usto determine the variation of rm during relaxation. Fig.3 shows, in the anomaly domain, the values of u0.2%

together with that of d0.2%=Drm/rmo(rmo is the mobileFig. 1. t0.2% as a function of temperature for the [1( 23] single crystals.

B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173 171

Fig. 3. Work-hardening coefficient and dislocation exhaustion rate atgp=0.2%. [1( 23] single crystals.

Table 1Comparison of the mechanical parameters of three different Ni3Alcompounds [4,9]

Ni3 (Al, Ta)Ni3Al Ni3 (Al, Hf)

650 KTp,t 800 K1000 K230 MPat0.2% max 400 MPa 190 MPa

600 K 750 K850 KTp,u

u0.2% max 5300 MPa5400 MPa 7800 MPa

were also performed on the samples. According to theobserved transient, two temperature domains could bedefined below the peak temperature (see Fig. 4): for300BTB600 K (domain I), the stress jump, followinga strain rate increment, includes a positive transient Dttr

followed by a stress decrease, measured through Dtss

which exhibits negative values. Stress instabilities arealso visible along the deformation curves in this temper-ature range. In domains II (700 K5T5900 K) and III(900 K5T51100 K), below and above the peak tem-perature respectively, the transients are ‘normal’ ones,with a larger amplitude in domain III. A detaileddescription of these transients can be found in [8].

4. Discussion

A comparison of the mechanical properties of singlecrystals of the binary compound (present study) andsingle crystals containing Ta and Hf additions respec-tively [4,5], yields the following remarks:� For each of the three compounds, a peak tempera-

ture for t0.2% can be defined which is higher than thepeak temperature for u0.2%.

� The respective values for the peak temperatures andmaximum values of t0.2% and u0.2% are listed in Table1 for three different compounds.

dislocation density at the beginning of the relaxation)as a function of t0.2%. The d0.2% values are calculatedfor a decrease in strain-rate of one order of magnitudeduring relaxation. A fair correlation is found in thevariation of d0.2% and u0.2% with t0.2%. They bothrapidly increase in the first part of the anomaly domain,indicating that an increase in the exhaustion rate resultsin an increase of the work-hardening rate and vice-versa. They peak at approximately the same stress(temperature) level, before the stress peak, above whichd0.2% drastically decreases while the decrease of u0.2% ismuch more moderate.

3.2. [2( 33] oriented specimens

As a whole, the plots of t0.2% and u0.2%, respectively,as a function of temperature, exhibit the same trends asabove (see Figs. 2 and 3). Strain-rate jump experiments

Fig. 4. The temperature domains of the proof stress and the associated transients in strain-rate jump experiments. [2( 33] single crystals.

B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173172

Table 1 shows that the effects of alloying are quitecomplex. It affects the various peak temperatures, aswell as the maximum stress and the maximum work-hardening rate. This is corroborated by activation vol-ume measurements. The plots of the latter parameter asa function of temperature exhibit different trends, de-pending on the presence of a given alloying element[4,5]. This means that different deformation mecha-nisms operate in the various types of compounds.

The most significant results of stress relaxation exper-iments reported here, concern the values of the exhaus-tion rate of mobile dislocations. The same experimentshave also yielded values of the true activation areas [9].Their variations with temperature are as follows. As thetemperature increases in domain I, this parameter in-creases from 1200b2 to 1800b2, it decreases in regime IIfrom 1900b2 to 900b2, then exhibits low values(5100b2) in regime III. The relatively large values ofthe activation area, in domains I and II, clearly indicatethat thermal activation is not too effective for disloca-tion motion in the corresponding temperature ranges.However, the same experiments provide values for theexhaustion rate of mobile dislocations at least duringthe relaxations.

Looking at the plots of Fig. 3, it is remarkable thatthe stress at which the work-hardening rate u0.2% ismaximum, coincides with a maximum dislocation ex-haustion rate. This can be interpreted by consideringthat dislocation exhaustion takes place in the crystal bystorage without (or with restricted) annihilation. Thistype of correlation also supports the method that weuse to determine the dislocation exhaustion rate, andthe related assumptions for interpretation. It also indi-cates that the exhaustion phenomena, which take placeduring plastic deformation at the onset of the transientexperiment, are quite comparable with those of thestress–strain curve just before the relaxation. In addi-tion, the values of exhaustion rates which are measuredhere between 50 and 70% are quite high, correspondingto the high work-hardening rates that these compoundsexhibit, as compared with pure metals. This suggeststhat the present Ni3Al compounds exhibit specific dislo-cation storage mechanisms which affect both the flowstress and the work-hardening coefficient.

From the data of strain rate jump experiments, therate sensitivity can be measured as a function of tem-perature. The latter parameter is defined as:

S= (Dt/D ln g; ) (1)

From early investigations of the mechanical proper-ties of Ni3Al compounds, this parameter has beenmeasured by some authors [10]. The stress transientfollowing an increase of strain rate is usually reportedto look like the schematics of Fig. 5. Dttr is positive (ornegligible). Consequently, Str, defined by relation Eq.(1) above, is positive (or negligible) [10,11]. However,

the present data show that Sss (though of small magni-tude) is negative in the present experiments in tempera-ture domain I, where t0.2% is constant or slightlyincreases with temperature. This behaviour which isdifferent from the previously reported ones, may beattributed to different alloying elements (Ti in [10] andHf+B in [11]).

It is well known that the two effects observed here, intemperature domain I, i.e. serrations on the stress–strain curve and a negative strain-rate sensitivity, canbe attributed to the Portevin–Le Chatelier effect. Thisis clearly evidenced in copper solid solutions as anexample [12]. In addition, for the latter alloys, ananomalous increase of the flow stress is also observed,due to the Portevin–Le Chatelier effect. However, wedo not think that this effect is responsible for thestrength anomaly of the present Ni3Al samples, for thefollowing reasons:� the anomaly is observed in the transition domain

(I+II) and domain II in Fig. 4 in which S is positiveand no serrations are observed;

� the present increase in flow stress is large as com-pared with that of the Cu solid solutions.The Portevin–Le Chatelier effect is connected with

the mechanism of dynamic strain ageing, in whichmoving dislocations interact with diffusing point de-fects, impurities or solute atoms. In the present state ofour investigations, the diffusing species is not identifiedyet. However, these results emphasise the role of pointdefects or impurities with regard to the flow stress ofNi3Al compounds.

Finally, a comparison of the mechanical properties ofthe two sets of specimens investigated in this studyshows that at a given temperature in the strengthanomaly domain t0.2% is lower in the [1( 23] than in the[2( 33] orientation (see Figs. 1 and 4) but peaks at ahigher temperature in the former orientation. Thesefeatures about the stress orientation dependence of t0.2%

are in fair agreement with results previously reportedfor Ni3(Al,Ta) single crystals [13].

Fig. 5. Schematic stress transient associated with a strain-rate jump.

B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173 173

5. Conclusion

The mechanical parameters of the binary compoundNi76.6Al23.4 have been determined and a fair agreementis found between the temperature variations of thework-hardening coefficient and the mobile dislocationexhaustion rate. In the low temperature domain, whichcorresponds to the onset of the strength anomaly, thePortevin–Le Chatelier effect is evidenced. This effectalone is not likely to account for the large amplitude ofthe strength anomaly of this compound.

Acknowledgements

The financial support of Fonds National Suisse isgratefully acknowledged.

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