Scripta Metallurgica e t Materialia, Vol. 31, No. 11, pp. 1513-1518, 1994 Copyright © 1994 Elsevier ScienceLtd Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00
BEHAVIOR OF MONOCRYSTALLINE
ALLOY AT LOW TEMPERATURES
T.Y. Z h a n g
State Key L a b o r a t o r y F o r Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Shenyang 110015, P.R. China
(Received March 15, 1994 (Revised July 8, 1994) Introduction Investigations[I-3] have indicated that at low temperatures a l u m i n u m - lithium alloys display improved toughness and an improved strength-toughness relationship. The yield strength, ultimate tensile strength, elongation and the fracture toughness increase with decreasing temperatures. Several mechanisms have been proposed to explain this most striking feature. Webster claimed that low melting point impurities, such as sodium and potassium, are responsible for the improvement of mechanical properties in A1-Li alloys at low temperatures. H o w e v e r , Venkateswara Rao et al. indicated that the increased delamination at low temperatures can increase the degree of i n - p l a n e crack deflection, resulting in toughening of the alloys. On the basis of their own results, Xu and coworker pointed out that the improvement of tensile and fatigue properties at liquid nitrogen temperatures is also presumably attributable to the delamination.Therefore, the mechanisms responsible for the variation in mechanical properties with temperature are not currently w e l l - u n d e r s t o o d . In order to elucidate the real situation, single crystals of a binary a l u m i n u m - l i t h i u m alloy were adopted in the present study. This paper is devoted to the description of the behavior of the l o a d - d i s p l a c e m e n t curves and ~hc associated slip traces on the sample surfaces.
Expcrimental details Single crystals of a A1-2.2%Li(in wt.) alloy were grown under an argon atmosphere by using the Bridgman technique. The tensile specimens with a gauge length of 12 mm and a cross section of 4 x 4 mm were solutionized at 510"C for 20 minutes and quenched into water. The specimens were then aged at either 175"C for 48 hrs or 220"C for 48 h to develop peakaged or overaged conditions which were designated as PAS or OAS. The specimens were carefully electropolished before tensile deformation and subsequently observed with a scanning electron microscope(SEM).
Vol. 31, No. 11
The tensile tests were performed under displacement control on a Schcnck machine at temperatures ranging between 296K and 90K. The nominal strain rate was about 1.5 x 10-3s-~. The load versus displacement curves were recorded during tensile tests.
Experimental results 1. Tensile deformation behavior of p e a k - and overaged monocrystalline specimens In figure 1 is shown the recorded load versus displacement curves of peakaged monocrystalline specimens, the tensile axis of which is close to the  crystallographic orientation. It is very interesting to note from Fig. 1 that unlike the polycrystallinc A1-Li alloys, the monocrystalline A1-Li alloy studied in the present work displays a decrease in the yield strength and an increase in the ultimate tensile strength as the temperature decreases from room temperature. The ductility in terms of the elongation remains almost unchanged with decreasing the temperature. The tensile deformation of A1-Li single crystals is characterized by discontinuous (serrated) yielding or the so-called Portevin-Le Chatelier (P-L) effect. As can be seen from Fig.l, this effect depends strongly on the temperature. It becomes weaker with decreasing the temperature and almost disappears at 90K. Figure 2 illustrates the l o a d / d i s p l a c e m e n t curves of pcakagcd specimcns with a tensile axis close to IT13]. Similar to the single crystals close to  (Fig.l), a decrease in temperature gives rise to a decrease in yield strength. However, both ultimate tensile strength and ductility significantly increase as the temperature reduces. As compared to the near-J013] orientation crystals, the single crystals with n e a r - I l l 3 ] orientation show higher tensile strength and hardening rate, but weaker discontinuous yielding or P - L effect. This result implies that the mechanical properties of the monocrystallinc A1-Li alloy are strongly orientation-dependent. In
figure 3 are shown
the l o a d / d i s p l a c e m e n t
curves of ovcraged single crystals with various
crystallographic orientations at different temperatures. Thc tensile axes of the specimens OAS3, OAS4 and OAS5 or OAS6 arc close to the ,  and  orientations, respectively. Several intcresting points arc worthy to be mentioned. First, as compared to the crystal OAS3 which is close to the  crystallographic orientation, the  crystal (OAS4) shows a much higher yield strength and ultimate tensile strength, but the same level oF ductility. Second, by comparison betwccn crystals OAS5 and OAS6 with the same orientation, it is seen that the decrease in temperature leads to drops in both yield strength and ultimate tensile strength, but the same ductility. Third, ovcraged crystals display less intensive P - L effects as compared to the peakaged crystals at the same temperatures(see Fig.l). The crystal deformed at 140K shows no P - L effect. 2. Investigations of surface slip traces by SEM After stretched to failure, the slip features on the specimen surfaces were observed using scanning electron microscopy. Some typical micrographs for peakaged and overaged specimens were selected in figures 4 and 5, respectively. Generally speaking, all specimen surfaces arc characterized by straight and uniformly distributed
Vol. 31, No. 11
slip traces, indicating good ductility of the specimens. However, by comparison of Fig.4(a) with Fig.4(b), the slip traces at low temperature are shorter, less intensive and more irregular. The influence of crystallographic orientation on the slip behavior can be seen by comparing Fig.4(a) with Fig.4(c). It is clear that the orientation close to - side in the standard triangle gives rise to a less uniform slip pattern. Multiple slip traces were commonly seen when the overaged crystals were deformed (Fig.5). Again, the decrease in temperature developed more irregular slip traces.
Discussion 1. Variation of mechanical properties with temperature It is considercd typical for polycrystalline AI-Li alloys that the yield strength and ultimate tensile strngth increase as temperature decreases from ambient temperature. However, as reported in the present work, this is not the case for monocrystallinc A1-Li alloys. For example, the yield strength of the peakaged and overaged specimens with the same orientation decreases with decreasing temperature. This is similar to the results reported by Miura et al. for AI-Li single crystals in compression tests. It is generally accepted that with decrease in temperature the yield strength is increased because the thermally activated slip of dislocation is involved in the yielding process. Since 61 is the main precipitates in the aged A1-Li alloy and has a structure of intermetallic compound Ni3A1, the anomalous temperature dependence of the yield strength observed in the present study for aged monoerystalline A1-Li alloy can be reasonably explained by the mechanism proposed for Ni3AI[7 ]. 2. Orientation dependence of mechanical behavior As indicated above, the specimen close to  displays a stronger strain hardening response and higher strength as compared to the specimen close to . This orientation dependence can be explained by the dislocation interaction and its end products. For  which is at the - side in the standard triangle, the dislocation interaction will produce the Lomcr-Cottrell lock. However, for  orientation which is at the - side in the standard triangle, there is no any strong dislocation interaction other than the formation of jogs. On the basis of this argument, the obscrved experimental results can be easily understood. This is also supported by the investigation of surface slip traces (sec Fig.4). 3. Discontinuous yielding ( P - L effect) in monocrystalline A1-Li alloy The intensity of the discontinuous (serrated) yielding ( P - L effect) observed in monocrystalline A1-Li alloy depends on temperature, crystallographic orientation and the aging conditions. Dynamic strain ageing and the shear of 8' preciptates are two mechanisms which have been proposed for the explanation of the P - L effect in A1-Li alloys. As indicated by the present investigation, the peak aged specimens show a more pronounccd P - L cffect compared to the over aged specimens and particularly, the P - L effect of the near-t013] crystals is more stronger than that of the n e a r - I l l 3] crystals. It is generally accepted that the serrated yielding due to dynamic strain aging is caused by the elastic interaction of dislocations with solute atoms in the solid solution. It is
Vol. 31, No. 11
also believed that there is no much difference in concentration of solute atoms in the A1-Li solid solution after peakaging and overaging. The main difference in microstructures between the two aged conditions is the sizes of 6' precipitates. As indicated in Fig.5, the coarse 6t precipitates in the overaged specimen promote multiple slip which is different from the planar deformation in the peakaged specimen with smaller fit precipitates (see Fig.4). Furthermore, as compared the curve a and the curve b in Figs.1 and 2, respectively, the  orientated specimen which is characterized by planar deformation shows more strong P - L effect. Apparently, the more planar deformation the stronger P - L effect. Therefore, it is suggested that the shearing of 6t precipitates by dislocations rather than the elastic interaction of dislocations with solute atoms or dynamic strain ageing gives rise to the observed discontinuous (serrated) yielding in A1-Li alloy.
Concluding remarks Pcakagcd and overaged binary A1-Li alloy single crystals exhibit excellent ductilities at temperatures ranging from 296K to 90K. An anomalous temperature dependence of the yield strength was observed for aged specimens. This phenomenon is considered to be caused by the special L12 structure of 6' precipitates. The mechanical properties of the monocrystalline A1-Li alloy depend strongly on the crystallographic orientation of the specimen, which can be explained in terms of the dislocation interaction and its end products. The intensity of serrated yielding in monocrystalline A1-Li alloy is significantly affected by temperature, crystallographic orientation and the aging condition. The shearing of 6' precipitates by dislocations is suggested to be the main cause of the observed discontinuous yielding.
References 1. R.C. Dorward, Scripta Metall., 20(1986)1379. 2. J. Glazer, S.L. Verzasconi, R.R. Sawtcll and J.W. Morris, Mctall. Trans., 18A(1987)1695. 3. Y.B. Xu, L. Wang, Y. Zhang, Z.G. Wang and Z.Q. Hu, Metall. Trans., 22A(1991)723. 4. D. Webster, Mctall. Trans., 18A(1987)2181. 5. K.T. Vcnkatcswara Rap, H.F. Hayashigatani, W. Yu and R.O. Ritchic, Scripta Metall., 22(1988)93. 6. Y. Miura, R. Yusu, M. Furukawa and M. Ncmoto, Proc. 4th A1-Li Conference(G. Champier, B. Dubost, D. Miannay and L. Sabetay eds.,), C3-549, 1987, Paris. 7. C. Lall, S. Chin and D.P. Pope, Mctall. Trans., 10A(1979)!323. 8. N. Behnood and J.T. Evans, Acta Metall., 37(1989)687. 9. W. Liu, Master Degree Thiscs, '~The Deformation Behavior in M o n o - and Polycrystalline A1-Li alloys", 1992, Institute of Metal Research, Academia Sinica, Shenyang, China(in Chinese).
Vol. 31, No. 11
M O N O C R Y S T A L L 1 N E AI-Li A L L O Y
b+.PABI0--2BOK C: P A S 3 - - 9 0 I
Displacement, mm Fig.1. The load-displacement curves of peakaged A1-2.2%Li alloy single crystals in tensile testing at various temperatures. The orientation of the tensile axis is close to 
Displacement, mm Fig.2. The load-displacement curves of peakaged A I - 2 . 2 % L i alloy single crystals in tensile testing at 296K and 245K. The orientation of the tensile axis is close to .
Vol.31, No. 11
Z2 ~ i !
Fig.3. The load-displacement curves of overaged AI-2.2%Li alloy single crystals in tensile testing at various temperatures. The orientations of the tensile axis are shown in the standard triangle.
Fig.4. SEM micrographs showing the surface slip traces of peakaged single crystals after tensile deformation. (a) near-J113] crystal deformed at 296K, (b) near-Ill3] crystal deformed at 245K and (c) near- crystal deformed at 296K.
Fig.5. SEM micrographs showing the surface slip traces of overaged single crystals after deformation. (a) near- crystal deformed at 296K and (b) near- crystal deformed at 140K.