THE EFFECT OF LOW MELTING POINT IMPURITIES ON THE PROPERTIES OF Al-Li ALLOYS

Transcription

1 THE EFFECT OF LOW MELTING POINT IMPURITIES ON THE PROPERTIES OF Al-Li ALLOYS D. Webster To cite this version: D. Webster. THE EFFECT OF LOW MELTING POINT IMPURITIES ON THE PROPER- TIES OF Al-Li ALLOYS. Journal de Physique Colloques, 1987, 48 (C3), pp.c3-685-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1987 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C3, suppl6ment au n09, Tome 48, septembre 1987 THE EFFECT OF LOW MELTING POINT IMPURITIES ON THE PROPERTIES OF A1-Li ALLOYS D. WEBSTER Kaiser Aluminum and Chemical Corporation, P.O. Box 877, Pleasanton, CA 94566, U.S.A. Abstract The effect of low melting point impurities, such as sodium and potassium, on the properties and microstructure of A1-Li alloys has been investigated. These impurities have been shown to occur as discrete lenticular particles along grain boundaries. In underaged and peak-aged conditions, the impurity particles are liquid and the toughness and ductility of alloys in this condition were found to be improved by lowering the test temperature below the freezing point of the impurity. In the overaged condition, the sodium and potassium occur as a solid phase and materials in this condition were found to follow the pattern of most aluminum alloys and show a decrease in Charpy impact toughness as the testing temperature was lowered. The addition of sodium and potassium to A1-Li alloys reduced toughness and ductility at room temperature in the range of compositions investigated (up to 476 ppm Na and up to 23 ppm K). Low melting point impurities were found to have little effect on the toughness or ductility of high purity aluminum. Introduction At the present stage of their development, A1-Li alloys generally compare unfavorably wi th conventional aluminum alloys in terms of toughness and ductility at equivalent strength levels. For example, A and A extrusions made from commercial ingots have significantly greater (2-3 times) impact toughness than identical A and A extrusions. There is evidence in the literature that this difference in toughness may be related to low melting point impurities such a sodium and potassium. Payne and Eynon, in 1957 British Patent , suggested that embrittlement of A1-Li alloys occurred if the sodium content was over 50 ppm. Vaynblat, et al, (Ref. 1) in 1976 found a 16% drop in toughness of A1-2 Li-0.13 Zr alloys when the sodium content was increased from 31 to 76 ppm. More recently, Vasudevan, et al, in 1982 (Ref. 2) observed linear decreases in toughness in A1-Li alloys containing up to 480 ppm sodium, and Webster (Ref. 3) observed toughness decreases in A1-Li alloys containing up to 434 ppm sodium and 23 ppm potassium. The addi tion of sodium and potassium promotes the grain boundary fracture mode in A1-Li alloys (Ref. 2) and these elements have been found by surface analysis to segregate to the grain boundaries (Refs. 2, 3, 4,,5). the aim of the present work is to identify the nature of the grain boundary segregation and the changes lt undergoes during aging. Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE Experimental Technique Most of the alloys evaluated in this work were from alloys cast into 127 mm diameter ingots and extruded to flat bar 58 x 14 mm. Four alloys with added Na and K were cast as both 127 mm diameter ingots, extruded to flat bar and as 100 mm x 25 mm ingots which were hot rolled to 6.4 mm plate. The alloys were melted under an argon atmosphere. Results When lithium metal is added to a molten aluminum alloy, there is an increase in sodium, potassium and hydrogen levels in the alloy. These elements are present in the lithium metal (53 ppm Na, 22ppm K and 280 ppm H). Additions of sodium, potassium and lithium greatly increase the hydrogen content of aluminum alloys. This is not surprising since hydrogen has a high solubility in all three alkali metals When the data are measured as atomic percentages the hydrogen content is equal to the cube root of the sum of the alkali metals. In all the alloys examined there is an excess of alkali metals above that required to form hydrides, even before the alloys are degassed. Small particles rich in Na and K were observed in trahsmission electron microscopy in all the alloys examined with added amounts of these elements. The particles were lenticular in shape with their longest dimension ( pm) along the grain boundary. In many cases there was a small particle, rich in Pb within of the lenticular Na-K particle, suggesting that this was acting as a nucleating agent for sodium and potassium segregation. The dihedral angle of the lenticular Na-K par icles at the grain boundaries was found to be in the range One of the smallest (0.3 pm) sodium-rich particles observed was found at a grain boundary triple junction in alloy 2BLKN and is shown in Figure 1. Inside this particle is a lead-rich particle. The EDX analysis from this particle indicates it is 81% Na, 19% K. A TEM examination of alloy 2BLK, which contains slightly more potassium than 2BLKN, but only 6% as much sodium, shows similar grain boundary particles rich in Na and K which also appear to have nucleated on lead particles. EDX analyses taken from the grain boundaries adjacent to the Na-K particles show only the presence of aluminum, indicating that the sodium and potassium are not present as a uniform grain boundary film but are concentrated in isolated particles. Because of the small volume of each sodium-rich particle, a quantitative analysis of the composition cannot be made by EDX in the STEM. However, the Na:K ratio can be obtained, and if the assumption is made that apart from occasional nucleating particles the sodiumrich particle contains only sodium plus potassium, then the composition of each particle and therefore its liquidus and solidus temperature can be estimated. This has been done for alloys 2BLK(llppmNa, 23ppm K), 2BLKN(476ppm Na, 15ppm K and 2BLKNC( lloppm Na) Figure 2. The composition of each particle is plotted on the Na- K phase diagram at the lowest temperature at which a liquid phase can exist in a particle of that composition. It can be seen that a liquid phase would be expected at room temperature in all three alloys and that potassium was identified in the grain boundary particles even when the amount was too low to be detected by bulk analysis (2BLKNC). In most of the sodium-rich particles observed after aging at 394K, small bubbles were observed moving slowly just inside the perimeter of the particle indicating that the phase is indeed liquid <Ref. 6>.

4 Figure 1. TEM of Na-K particle in grain boundary triple junction of alloy 2BLKN.Dark particle contains Pb. FIG. 2. Na-K PHASE DIAGRAM SHOWING RELATIVE CONCENTRATIONS OF Na-K ElEASURED BY EDX FROM PHASES OBSFBVED BY 'TI31 WEIGHT PCT. SODlL?l

5 C3-688 JOURNAL DE PHYSIQUE In order to show the alkali metal distribution in alloys with only residual amounts, a more sensitive micro-analytical technique, the Cameca Ion Microprobe was used. The distribution of Na, K, Rb and Cs was examined on both polished samples and samples fractured intergranularly along the S-T direction. The K distribution in a l5oum diameter circle on a fracture surface of a semi commercial A alloy in the peak aged condition is shown in Figure 3. The Na and K distributions vary from particle to particle but the location of the high concentrations of each element are the same as indicated by the TEM work above. For the three large particles shown in Figure 3 the average Na and K atomic concentrations were about equal. The Cs levels were 24 times lower and the Rb levels were 300 times lower than the Na and K levels. In the case of the K and perhaps also the Cs and Rb there is enrichment in the particles compared to the bulk composition. FIGURE 3. PfYFASSIU? DISTRIBUTION ON SEIORT?EWEVERSE FRACI'LJRE SURFACE OF Al 8091 AUQY EXTRUSION AS SH@,JN BY THE ION.PUCROPROBE. TIE DARK AREAS ARE REGIONS OF HIGH POTASSIW4 CON~RATIONS. Na,Rb AND Cs WE ALSO CONCEJTRA"D IN THE SRIE NZEAS. Na AMD K LFVELS BY BULK ANALYSIS ARE 3 PPM AND 40.5 PPM RESPECTIVELY. THE NUMBEPS INDICATE TEE REXATIVE POTASSIUq C O N ~ T I O N S IN TI= YATIIIX AND IN THE PAF3ICLE CENTERS. The tensile and Charpy impact properties of underaged 6 mm A1-Li plate with various additions' of sodium and potassium are as follows. A t 295K there is a slight decrease in yield strength with impurity content and a significant decrease in ductility and toughness. The high potassium alloys have lower toughness than expected above 77K, based on the assumption that Na and K are of equal effectiveness in reducing toughness. A better correlation is obtained if the potassium content alone is assumed to be tha controlling factor. For the results at 295K this is a logical assumption since in all the alloys, Na which is the major alkali metal impurity, is a solid at 295K. In the complete absence of other alkali metals therefore no embrittlement should be observed at 295K. In this case the amount of

6 liquid present will be controlled entirely by the second major alkali metal impurity i.e. potassium which when in the presence of sodium forms phases which are liquid below 295K. To be consistent the charpy data for 373K should be plotted against the sum of Na+K since this temperature is above the melting point of Na. When this is done the number of lines on the plot is reduced to two since the 373K data becomes an extension of the 295K data <Fig. 4). In order to FIG.4.EFFECT OF LIQUID IMPURITY LEVELS,K AT 77K AND Na+K AT 3733 ON CHARPY VALUES OF Al-Li ALLOYS WITH ADDED Na+K. fully evaluate the effect of Na and K on the toughness of A1-Li alloys, it is desirable to be able to evaluate an alloy where these elements are extremely low. This is not easy to do by ingot metallurgy, however, this condition can be approached in alloys made by powder metallurgy. Two alloys 1611 and 1614 were atomized at 1200K into -100 mesh [<I49 pm] particles. The high temperature and the small particle size would be expected to enable almost all the sodium and potassium to evaporate. Specimens were aged at temperatures up to 533K and tested at room temperature and 77K. The temperature dependence of toughness for these alloys was quite different from ingot metallurgy alloys containing higher Na and K. The powder alloys showed a decrease in toughness at 77K under all aging conditions <Fig. 5>. Discussion The above results show that the addition of lithium to aluminum results in an increase in the undersirable impurities Na, K and H. A l l three of these elements are concentrated in the grain boundaries <Ref. 3>, and the K and to a lesser extent the Na, reduce the toughness of A1-Li alloys at temperatures above approximately 200K. The temperature dependence of impact touqhness and ductility, of

7 JOURNAL DE PHYSIQUE FIG.5 TOUGHNESS AS A FUNCTION OF AGING TEMPERATURE FOR POWDER METALLURGY EXTRUSIONS 1611 AND 1612 TESTED AT 295K and 77K. alloys with added Na and K, and alloys with only residual amounts of these elements, differ only in the magnitude of the effect <Ref. 3). It has been shown that these elements occur as isolated grain boundary particles rather than continuous grain boundary films. These liquid phases can act as grain boundary cracks, which cause extensive local deformation in the grain boundary region. Impurity particles containing only Na and K are liquid down to 260K according to the phase diagram. The alloys investigated in this work show an increase in toughness down to at least 200K. This suggest that other impurity elements, such as the Cs and Rb observed above in the ion microprobe, are associated with Na and K. In the ternary system, Na-K-Cs, a liquid phase can exist down to 195K and the solidus is lowered further in the presence of mercury. In most A1-Li alloys, a continuous increase of toughness would be expected between the freezing point of Na and the Na-K-Cs eutectic. This would produce a marked similarity to the actual toughness-temperature relationships observed <Refs. 3,5>. The toughness of A1-Li alloys increases markedly as the test temperature is decreased, while the toughness of conventional high strength aluminum alloys decreases at low test temperatures. The result of this is that the strengthtoughness combination of A1-Li alloys at cryogenic temperatures is equal to, or in many cases superior to, that of conventional aluminum alloys. This appears to create a problem for alternate theories for the embrittlement of A1-Li alloys such as strain localization due to the ordered A1 Li phase. Strain localization theories do not predict an increase in toughness at cryogenic temperatures nor an absence of embrittlement at these temperatures. Conclusions 1. The addition of up to 476 ppm Na and 23 ppm K to A1-Li alloys reduces their impact toughness and ductility but has little influence on strength. 2. Na and K reduce toughness and ductility by forming discrete lenticular grain boundary particles which are liquid at room temperature. These liquid phases promote intergranular failure. The solidification of the liquid phases at temperatures down to about 200K results in an increase in toughness and ductility.

8 Acknowledgements. The author would like to thank the following personnel of Kaiser Aluminum & Chemical Corporationls Center for Technology in Pleasanton, California; D.A. Nguyen for outstanding transmission microscopy, R. Hintze for the development of improved analytical techniques for alkali metals and also D. Wentz, R.C. Dickinson and K.S. Dayton. In addition the expertise of Charles Evans and Ron Fleming of Charles Evans and Associates in Redwood City, California was invaluable when using the ion microprobe. The author is a consultant for Advanced Material Development at Verde Vista Lane, Saratoga, California 95070, USA. (408) References 1. Y. M. Vaynblat, B. A. Kopeliovich and Y. G. Gollder Fiz. Metalloved, V42, No. 5, pp , A. K. Vasudevan, A. C. Miller and M. M. Kersker, "Proceedings of the Second International Aluminum-Li thium Conference," published by AIME 1984, pp D. Webs ter, "Proceedings of the Third International Aluminum- Lithium conference," published by Institute of Metals, London 1986, pp J. A. Wert and J. B. Lumsden, Scripta Met, V19, No. 2, 1985, pp D. Webster, Metal Progress, V125, No. 5, April 1984, pp D. Webster, New Scientist, V114, No. 1556, April 16, 1987, PP