Dynamic strain ageing and crack propagation in the 2091 AlLi alloy

Scripta METALLURGICA et MATERIALIA

Vol.

29, pp. 1379-1384, 1993 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

DYNAMIC STRAIN A G E I N G AND CRACK P R O P A G A T I O N IN T H E 2091 AI-Li ALLOY D. Delafosse*, G. Lapasset** and L.P. Kubin* * LEM, CNRS-ONERA, 29 Av. de la Division Leclerc, BP 72, 92322 Chfitillon Cedex, France ** ONERA (OM), 29 Av. de la Division Leclerc, BP 72, 92322 Chgtillon Cedex, France

(Received June 16, 1993) Introduction Dynamic strain ageing (DSA) phenomena, which stem from the dynamic interaction of mobile dislocations and diffusing solute atoms, are observed in many dilute alloys at intermediate temperatures. In uniaxial deformation, DSA may lead to a negative strain rate sensitivity (SRS) of the flow stress and to the occurrence of jerky flow, alias the Portevin-Le Chfitelier (PLC) effect [1]. The load serrations associated with the PLC effect are related to the propagation of deformation bands and the formation of visible surface markings at the surface of the deformed samples [2]. Another detrimental influence of the PLC effect on material properties is a characteristic loss of ductility [ 1, 3], as a local increase of the strain rate is no longer counteracted by the stabilizing effect of the SRS. In some aluminium alloys, this ductility through has unambiguously been related to the behaviour of the SRS in DSA conditions [4, 5]. In AI-Li based alloys, characteristic manifestations of jerky flow have also been observed [6-8] and a recent study has conclusively shown that the occurrence of jerky flow is promoted by the presence of shearable 8' (AI3Li) precipitates [9]. As little is known to date about the influence of DSA and the PLC effect on fracture-related properties, the aim of the present study is to investigate ductile fracture in a commercial AI-Li alloy, the 2091 alloy. In the standart treatment conditions used all through this work, this alloy exhibits serrated yielding in uniaxial deformation, associated with a depletion of the SRS (cf. figure 2, below). In the same alloy, Gomiero et al. [6] reported a loss of toughness with increasing heat treatment times and attributed it to an enhancement of PLC instabilities. However, it is also recognized [ 10] that grain boundary decohesion is favoured by an increase of the ageing time. Indeed, the detrimental effect of intergranular fracture is so important that it may well mask any other phenomenon influencing the toughness. The present study is, therefore, focussed on properties related to one single heat treatment, for which fracture is purely intragranular. In what follows, experimental results are reported and discussed on the influence of strain rate and temperature on both uniaxial deformation and the tearing behaviour of the 2091 alloy. Evidence is found for an anomalous behaviour of the tearing resistance in the temperature domain where DSA occurs. Further, heterogeneous deformation is shown to take place in such conditions in the plastic zone ahead of the crack tip region. Material and Experimental Conditions The 2091 AI-Li alloy is a low density, damage tolerant alloy which has been under intense development in the past few years for weight critical applications in the aircraft industry. The material was supplied by CRVP6chiney in the form of a 1.6 mm thick sheet in an as cold-rolled condition. The alloy was solution heat-treated 20 minutes at 527°C, then water-quenched to room temperature and immediately stretched by 2.5 %. After a 24 h. holding time at room temperature, it was further aged 45 minutes at 150°C. The thermal treatments were performed in salt or oil bathes, temperature being controlled within an accuracy of-+- 1°. The chemical composition of the alloy is (in wt.%): Li: 2.0, Cu: 2.0, Mg: 1.3, Zr: 0.10, balance AI. Fe, Si and Ti contents are less than 0.05 %. The microstructure is almost totally recrystallized with a grain size of about 25 p.m. Metallographic examination revealed the presence of coarse particles of two different phases, a Fe-rich phase and a quaternary AI-Li-Cu-Mg phase very similar to the so-called R phase [6]. Gomiero et al. [6] have shown that after a slightly underaged temper at 150°C, the microstructure contains 8' precipitates and GPB (Guinier-PrestonBagaryatskii) zones. From TEM examinations of our alloy, the mean radius of the 8' particles was estimated to

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about 12 nm. These examinations showed no evidence for any S' (A12CuMg), as well as intergranular, precipitation. Flat specimens for uniaxial deformation tests were machined with the load axis parallel to the longtransverse direction. The sheet thickness was further reduced from 1.6 to 1.4 mm by milling. Tension tests were carried out in the temperature range from -70 to +100°C on a Schenck electromechanical machine. The imposed cross-head velocity was 0.1 mm per minute and the corresponding nominal strain rate 10-4 s-1. The strain rate sensitivity was measured at 2% strain through upwards strain rate changes by a factor of 5. The resulting stress change was estimated from the stable portions of the stress-strain curve, carefully avoiding the transient regime. The tearing experiments were performed on deep notched tear test specimens (cf. figure 1), ensuring quasistatic ductile crack propagation in Mode I and in plane stress conditions. Kobayashi et al. [11] have assessed this geometry by Finite Element Analysis as a satisfactory alternative to conventional pre-cracked CT specimens for the R curve testing of Aluminium alloys, provided the reduced notch depth is above 0.3 and the notch root radius below 50 gtm. Crack extension was monitored by means of 'lost wire gages'. These gages are made of 10 parallel conducting wires, spaced 0.25 mm apart, which break as the crack propagates. They present the advantage of requiring no preliminary calibration, since a 0.25 mm increment in crack advance is evidenced by a clear step in the recording of the gage electrical resistance. The gages are bonded to the specimen surface with a high elongation epoxy resin. Preliminary tests showed that problems of delamination are encountered when the gages are placed just on top of the machined notch. Our gages are, thus, bonded 0.5 mm away from the initial notch root, ensuring a crack extension monitoring within the range of 0.5 to 3 mm, but forbidding a reliable JIc measurement. The tear tests were, then, performed at constant cross-head velocity. Three velocities were used, 0.2, 0.75 and 2 mm/min, at test temperatures ranging from -70 to +60°C. The load, point load displacement and gage electrical resistance were numerically recorded during the test with a sampling rate between 1 and 10 Hz, depending on the expected duration of the test. The tearing resistance of the material and its dependence on temperature and applied crack mouth opening rate were estimated as follows. From the area A under the load vs. point load displacement curve the value of the J integral is obtained with help of the relation: J = 2A/[B(W - a)]

(1)

where B is the specimen thickness, W the ligament length and a the crack length. The tearing modulus Tm is estimated within the crack extension range of 1 to 2 mm through Tm = OJ/Oa and it was checked that the J vs. Aa curves are linear between 0.5 and 3 mm of crack extension. Results

Uniaxial Deformation Tests Serrated yielding was observed in the temperature range from -50 to + 50°C. Figure 2 shows the temperature dependence of two characteristic features, the SRS and the critical strain for the onset of unstable plastic flow. The SRS undergoes a pronounced, negative, minimum around 0°C and serrations are recorded when the SRS drops beyond a critical value of about -0.8 MPa. Such slightly negative critical values of the SRS, as well as the occurrence of a minimum in the temperature dependence are now well explained by current models of the PLC effect [1, 12]. The temperature dependence of the critical strain exhibits a distinct minimum which seems characteristic of most Aluminium alloys exhibiting jerky flow [13]. The fact that the two minima coincide in figure 2 is probably accidental since the SRS is strain-dependent (measurements were performed here at a fixed strain of 2%). These results are sufficient to unambiguously identify the PLC effect as being responsible for jerky flow in this alloy. Tearin~ Tests The results of this set of experiments are summarized in figure 3. For each velocity, a clear maximum in tearing resistance occurs at intermediate temperatures. Bearing in mind that each cross-head velocity is related to a given crack extension rate and to a finite range of strain rates in the near tip area, one can notice from the position of the three maxima that they exhibit a negative strain rate sensitivity: an increase in the applied strain rate shifts the maximum tearing resistance towards higher temperatures. This is to be related to the behaviour of the flow stress in uniaxial tension whose strain rate sensitivity is also negative during jerky flow. DSA effects are maximum when the mobility of solute atoms is comparable to the velocity of mobile dislocations. Both phenomena are thermally activated, but only the latter depends on strain rate. As a consequence, if the same tearing or tensile test

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is performed at a higher strain rate, the temperature dependence of DSA and related phenomena (e.g., the tearing resistance) should be shifted towards higher temperatures as is, indeed, observed. This dependence can be set in form of an Arrhenius law: AG = kT ln(g0/g)

(2)

where AG is an activation energy, k is Boltzmann's constant, T the absolute temperature, g the applied strain rate and g0 a pre-exponential factor. Figure 4 shows a plot of I/T max vs. lnV, where T max is the temperature of the maximum tearing resistance and V the cross-head velocity. Assuming that the cross-head velocity is proportional to some average strain rate in the plastic zone, the slope of the straight line of figure 4 yields an activation energy of 0.5 eV. This value is comparable to the one determined from uniaxial tests in substitutional aluminum alloys exhibiting jerky flow around room temperature, typically 0.6 eV [13]. Jerky Flow in the Plastic Zone ahead the Crack Tip The question remains of whether the observed increase in tearing resistance is only due to a change in the stress-strain relationship with the occurrence of DSA or to the onset of jerky flow in the crack tip plastic zone. Although the temperature and estimated plastic strain rates in the near tip region are comparable to the conditions for the onset of jerky flow in tension, no detectable sudden load drops were recorded during the tear tests. The possible formation of PLC bands was thus investigated through their effect on the surface roughness. Specimens were mirror polished down to a 0.25 ~tm surface roughness and then torn in conditions yielding the maximum tearing resistance. In such conditions, optical observations of the surface reveal striped patterns ahead of the crack which seem characteristic of the development of PLC bands parallel to the crack paths (cf. figure 5). A further investigation by Nomarski interferometry in the region where the bands are the most clearly defined (cf. figure 5.c) yields an average band spacing of 150 I.tm (i.e. 6 times the grain size), somewhat smaller than what is usually observed in tension for this alloy [9]. This difference may be ascribed to the difference in strain rates between these two types of tests or to differences in boundary conditions for the region of the specimen undergoing unstable flow. The fact that the band contrast is optimum in conditions of normal lighting is at variance with what is observed in uniaxial deformation. In the latter case bands are better evidenced at glancing angle [2]. This difference indicates that the bands observed in the plastic zone correspond to a significant surface roughness, i.e. to a shear component predominantly perpendicular to the surface of the specimens. Indeed, as can be checked from figure 5.b, polishing scratches are not sheared by the bands.

Discussion and Concluding Remarks So far, DSA and PLC instabilities have been characterized in tension and an anomaly in tearing resistance has been evidenced, the temperature and strain rate dependence of this anomaly being closely related to DSA. A better understanding of the mechanisms involved in this anomaly is necessary to predict the fracture behaviour of other classes of alloys exhibiting DSA in tension. To date, however, the influence of DSA and PLC on fracturerelated properties, like the fracture toughness, has not been systematically investigated. From a theoretical point of view, the incorporation of rate effects and of strain rate softening phenomena have recently been considered within a simplified frame by Br6chet and Louchet [14,15]. This model, based on a crack stability analysis, relies on both assumptions of small scale yielding and crack tip blunting being the dominant effects of plasticity. It predicts that, in certain conditions, the fracture toughness can be enhanced by unstable plastic flow. One can, however, hardly apply this model to the problem of plane stress propagation where the influence of crack tip blunting is significantly reduced [ 16]. Experimentally, Srinivas et al. [17] have recently shown the beneficial effect of DSA on JIc in an Armco iron whose hardening coefficient is substantially increased by PLC instabilities. This is not the case in the 2091 alloy where tension tests showed that the SRS and, to some extent, the ductility are the only mechanical properties affected by DSA. The loss of ductility would imply a reduction in tearing resistance if crack propagation were controlled by some critical strain ahead of the tip. On the other hand, the reduction in flow stress consecutive to a negative SRS would cause, in the frame of the Lin and Thomson [18] analysis for the propagation of shielded cracks, an increase in the plastic part of the crack opening displacement. A rough numerical estimate of this effect for the 2091 alloy yields an alteration of the flow stress in the range 1-2%, somewhat insufficient to account for the 10 to 15% increase of tearing modulus measured in this study. In this respect, a continuous variation of the mechanical properties with DSA seems to fail in explaining the amplitude of the observed variation in crack

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propagation energy. Therefore, it seems that the occurence of heterogenous flow has to be taken into account explicitly. Indeed, experimental evidence has been found for heterogeneous flow in the plastic zone associated with the PLC effect. As stresses and strain rates are heterogeneous and far from uniaxial in the plastic zone, a simulation of this effect with help of conventional Finite Element codes seems difficult, all the more as the latter do not usually incorporate strain rate effects and negative SRSs. Further, experimental analysis is needed to explain the strikingly regular pattern of the deformation bands in a non uniaxial state of stress and strain, and to check the direction of the dominating shear component responsible for the observed surface roughness. The beneficial influence of DSA on the tearing resistance of this alloy takes place in a narrow temperature range which is compatible with that of an aircraft structure in service conditions. Besides, one can see from figure 3 that the decreasing branch on the right hand side of each maximum is relatively steep. This has important practical consequences for the R curve testing of the alloy. A slight variation in the test temperature, applied machine displacement rate or specimen geometry can lead to a 10 to 15% scatter in the experimental data. It follows that utmost care should be taken in defining the experimental conditions for the R curve testing of alloys exhibiting DSA. Further, infrared thermography experiments have been carried out during tearing tests on the 2091 alloy [19], showing that dissipation in the crack tip plastic zone could induce temperature increases of 5-10 ° with the cross-head velocities used in the present study. Such temperature rises are sufficient to affect substantially the tearing resistance in the domain where it exhibits a strong temperature dependence. This experimental study is a first approach to the problem of the influence of strain rate effects on crack propagation. The examination of a wider range of experimental conditions (e.g., strain rate and temperature as well as microstructure through different heat treatments) is currently under way. A similar study concerning crack initiation by Jlc measurements will follow, the aim being the understanding of the interactions between strain rate sensitivity, jerky flow and crack stability in ductile alloys exhibiting DSA.

References 1. 2. 3. 4. 5. 6. 7.

L.P. Kubin and Y. Estrin, J. Phys. III, 1,929 (1991). K. Chihab, Y. Estrin, L.P. Kubin and J. Vergnol, Scripta metall., 21,203 (1987). A.K. Ghosh, Metall. Trans., 8A, 1221 (1977). E. Bouchaud, L.P. Kubin and H. Octor, Metall. Trans., 22A, 1021 (1991). L.P. Kubin, A. Styczynski and Y. Estrin, Scripta metall., 26, 1423 (1992). P. Gomiero, Y. Br6chet, F. Louchet, A. Tourabi and B. Wack, Acta metall., 40, 863 (1992). C. Damerval, G. Lapasset and L,P. Kubin, Aluminum-Lithium Alloys, Proc. 5 th Int. Aluminium-Lithium Conf., Williamsburg, Virginia, T.H. Sanders Jr. and E.A. Starke Jr. (Eds), Vol. 2, p. 569, Materials and Component Engineering Publications Ltd., 1989. 8. J.C. Huang and G.T. Gray, Scripta metall., 24, 2707 (1990). 9. S. Kumar and H.B. McShane, Scripta metall., 28, 1149 (1993). 10. W.G.J. 't Hart, L. Schra, M.C. McDarmaid and M. Peters, Technical Report, NLR TP 90172 U, National Aerospace Laboratory, Amsterdam, 1990. 11. T. Kobayashi, M. Niinomi and Y. Takabayashi, in Advances in Fracture Research, Proc. 7 th Int. Conf. on Fracture (ICF7), Houston, Texas, K. Salama, K. Ravi-Chandar, D.M.R. Taplin and P., Rama Rao (Eds.), Vol. 1, p. 779, Pergamon Press (Oxford), 1989. 12. P.G. McCormick, Acta metall., 36, 3061 (1988). 13. L.P. Kubin and Y. Estrin, Acta metall., 38, 697 (1990). 14. Y. Br6chet and F. Louchet, Acta metall., 41, 3, 783 (1993). 15. F. Louchet and Y. Br6chet, Acta metall., 41, 3, 793 (1993). 16. Q.S. Nguyen and M. Rahimian, J. M6ca. Appl., 5, 1,95 (1981). 17. M. Srinivas, G. Malakondaiah and P. Rama Rao, Acta metall., 41, 1301 (1993). 18. I.H. Lin and R. Thomson, Acta metall., 34, 2, 187 (1986). 19. D. Delafosse and L.P. Kubin, in Dislocations 93 : Microstructures and Physical Properties, Transtech Publications, in Press.

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FIG. 5.b Optical Micrograph of the specimen surface near the upper boundary of the plastic zone. Each horizontal band is seen to contain about 6 grains. The thin, nearly vertical, lines are polishing scratches which do not seem to be sheared by the bands.

FIG.5.a Optical observation of the plastic zone around the crack tip with a CCD camera under intense normal lighting (inverse video). The specimen was torn at -20°C with a cross-head velocity of 0.2 mm/min, up to a crack extension of 3 mm. The band spacing is constant all over the plastic zone. The dashed rectangles (b) and (c) correspond to the areas shown in figs. 5.b and c, respectively.

FIG. 5.c Low magnification micrograph obtained in Nomarski contrast near the notch tip of the specimen.

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