High temperature deformation behavior in a 2124Al15VOL%SiCw composite

Scripta METALLURGICA et M A T E R I A L I A

Vol. 29, pp. 377-382, 1993 P r i n t e d in the U.S.A.

P e r g a m o n Press Ltd. All rights r e s e r v e d

HIGH TEMPERATURE DEFORMATION BEHAVIOR IN A 2124A1/15VOL%SiC~ COMPOSITE Jae-Hong Kirn, Dong Nyung Lee and Kyu Hwan Oh" Demartznent of Metallurgical Engineering, and Center for Advanced Materials Research Seoul National University, Shinrim-2-dong Kwanak-gu, Seoul 151-742 *Division of Materials, Korea Institute of Science and Technology, Seoul 130-650, Korea (Received (Revised

M a r c h 5, 1993) May 10, 1993)

Introduction One of the interesting p r o l ~ e s of AVSiC composites is a superplastic-like behavior or an extended ductility in a two-phase(solid+liquid) region(i,2). Mahoney and Ghosh(3) reported that a 7475A1/SiC~ (subscript p represents particulate) could be superplastically deformed up to 450% at 500°C. The matrix itself was a superpiastic material, which made it possible for the composite to he superplastically deformed. Wu and Sherby(4) reported a superplasticity in 202AA1/SiC~(subscript w represents whisker) under a thermal cycling condition, which was attributed to internal stresses. The above superplastic behaviors were obtained at very slow strain rates. On the other hand, Chokshi et oL(5) reported that elongation of a 2124A1/SiC~ increased with increasing strain rate up to 10-1sec-1 at 47b-°C. Nieh et a/.(1,2) reported an unusual superplasticity, more precisely, superplastic-like behavior that had not been discovered with earlier experimental results. They obtained elongation of 300% at 525°C above the solidus temperature(503°C) with an op~aum strain rate of 3xl0-*sec-1. The strain rate is a few hundreds to thousands times faster than that of conventional superpiasticity. Recently high strain rate superplasticities have been reported in A1/ShN4 composites(6-9) and mechanically alloyed A1 alloys(10-13). The superplasticity in the two-phase region was, however, little known except for some experimental results(7,10,11). Kim and Kim(14) observed that elongation of several kinds of A1/SiC~ composites increased near the eutectic temperature. To explain this behavior, they intreduced friction strength between whiskers and matrix and showed that the flow stress and the strain rate sensitivity of the composites were enhanced with increasing strain rate. The objective of this work is to obtain an optimum condition for hot working of a 2124A!/15%SiC~ and to suggest a nmdel which can qualitatively explain the extended duct~ty in the two-phase(solid and liquid

phases) region. Ex~rimental Procedures

The SiC whiskers and -44/~m 2124A1 alloy powders were wet-mixed and synthesized in an aluminum can to make a 135mm dia. billet. The billet was extruded by the ratio of 33 at about 450~C and then m-extruded by the ratio of 8.5 at 500°C. The final extruded composite was 8ram in diameter. It was machined to tensile specimens whose gauge length and diameter were 20ram and 4ram respectively. High temperature tensile tests were conducted in air over the temperature range of 450 to 550°C with 25°C interval. The cross head speed in the range of 0.5 to 500mm/min was used. The flow stress at each cross head speed was determined at peak stress, which was observed at the strain of 5 to 7%. The fracture surfaces were analyzed using scanning electron microscopy. Results Figure 1 shows elongation as a function of initial strain rate over the temperature range of 450 to 550°C. Despite of data scattering, it can he seen that below the solidus temperature(for cases at 450 and 475°C) the elongation tends to increase slightly with increasing strain rate, but above the solidus temperature the elongation increases very rapidly to a m a x i m u m value(about 80%) and then decreases with increasing strain rate. The peak elongations are located at a strain rate of 10-2 to 10-1sec-i. These results are similar to those of Nieh et a/.(1). According to their results, the 2124Al/20vol%SiC specimens in as-extruded state are poor in ductility and their elongations range from 30 to 40%, whereas they exhibit the m a x i m u m elongation of 300% at the strain rate of 3.3xl0-1sec-I when thermomechanically processed by rolling. The difference was attributed to microstructural homogeneity established during rolling process.

377 0956-716X/93 $6.00 + .00 Copyright (c) 1993 P e r g a m o n Press

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Figures 2 and 3 show that the peak true stress increases with increasing strain rate and decreasing temperature. The strain rate sensitivity starts to increase at a strain rate of about 10-2sec-1, when tested at 505, 525 and 550°C. However, little change in the strain rate sensitivity is observed for the specimens tested at 450 and 475°C. Figure 4 shows the surfaces fractured at 525°C. The fracture surfaces show a typical granular matrix. The granule size tends to decrease with increasing strain rate, An interesting finding is that hair-like fibers were observed on su~aces fractured at 525°C(Fig.4(b)) and 550°C(Fig.5(b)). The fibers were found not to be SiC whiskers and to have higher copper concentration(Fig.5(d)) than mean copper concentration of the matrix(Fig.5(c)). Judging from the long extension, the fibers are ~lieved to be formed from a high viscous material like chewing gum or glass. The viscosity of the material seemed to come from liquid and solid mixtures in interfaces between SiC and the matrix, because the solute is reported to be segregated in the interface(15,16). Initial Straln Rate, 10 "4 10 -'

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FIG.1. Elorq~ation-to-failure as a function of the cross head speed over the temperature range of 450 to 550°C.

SJrain Rote, sec FIG.2. Peak true stress as a function of the true strain rate over the temperature range of 450 to 550°C.

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Discussion

17mh et aL(2,17), who first observed the superplastic-like hehavior, reviewed the extended ductility in A1/SiC composites and some Al alloys. They ~ied to explain these behaviors by one of the conventional superplastic theories, whereby they attributed the superplastic region shifted to a higher strain rate region to refinement of grain size. The model, to some degree, explains the superplasticity i n the corc~sites. Very recently, they treated the composites as a semi-solid materia~ and demonstrated that strain rote sensitivities of the composites ranged from 0.39 to 0.6 depending on the SiC volume fraction(18), The data that they cited were, however, those at a high tempemture(700°C for the Ai-6.SSi/SiC composites) and at very high shear strain rates( ~ =200-1000sec-Z). The strengthening mechanism of the composite is based on the following two approaches. One is the shear lag theory(19), and the other is the dislocation enhancement theory(20). At high temperatures, as in this work, the latter is not a probable mechanism. The former is based on the load transfer from matrix to whiskers, so that properties of the interfaces play a vital role. The strength a~ of the composite with the fiber volume fraction V/is, based on the mixture rule for short fiber composite, approximated by oc = (I-V/)o= + VtL~/D

(i)

where om is the strength of matrix, L and D are the length and diameter of fiber, and x is the shear stress at the matzix-fiher interface, Slightly above the solidus temperature liquid, more precisely, slurry preferentially forms at the interface between whiskers and matrix due to solute segregation(15,16). The whiskers can lose the ability to support the load, because the liquid phase at the interface cannot transfer the load to whisker effectively(~0). Therefore the flow stress of the composite becomes inferior to that of the matrix= During deformation of the composite, the matrix deforms while the whiskers do not deform(Fig.6). A velocity gradient in the slurry, which exists between the whiskers and the solid matrix, forms. If the whisker is absent, this slurry moves at the same speed as the matrix. In this case, there will be no velocity gradient in the slurry layer. Although there are m a n y laws which describe fluid motions, the slurry is assurr~ to follow Newton's law of viscosity. Mcon(21) reported that Al alloy slurries behaved like a non-Newtonian fluid and that there seemed to exist two limiting Newtonian behaviors in the very low and high limits of shear rates.

Here, the expression of "very low limit c~ shear rate" in his thesis means that ~ << 100sec-i. Then shear stress effect can he ~ s e d as r = ~ du/dh where r ,

(2)

~, u and h are shear stress, viscosity, relative deformation velocity and thickness of the slurry

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FIG.6. DeformaUon behaviors (a)in presence of whiskers, and (b)in absence of whiskers.

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layer respectively. The matrix moves at the relative velocity of r ~ ~L

381

L z to whiskers, so eq.(2) is expressed as

"E/h

(3)

where ~ is the strain rate of the specimen. Substitution of eq.(3) into eq.(1) yields oc = ( 1 - V 1 ) o m + V / ~ L 2 / ( D h )

(4)

For the 2124A1, the quantity of liquid at 525°C is calculated to be about 3% of the whole matrix. If 5 ~ m long, 0.5/z m diek whiskers are homogeneously distributed and the liquid layer forms around the whiskers due to the higher solute segregation, the thickness of the liquid layer is calculated to be about 0.02/~ r~ It is interesting to note that the solute segregation zone at the SiC/2xxxA1 interface is about 0.02urn thick(16). The thickness of the slurry zone may be a little thicker and is assumed to be 0.05~ m in calculation regardless of temperattLre within the range of 505 to 550°C. The peak stress of matrix o~ may be estimated from the data at lower strain rates in which contribution of the second term of the righ-hand side of eq.(4) may be negligible. The peak stresses at the strain rate of 4.17x10-4sec-1 are assumed to be the values of ore(l-V1) with VI being 0.15. From the slopes of curves at lower strain rates in Fig.2, om can be assumed to vary proportionally with ~0.n. The best fitting of the data in Fig.2 in accordance with eq.(4) yields ~(505°C) = 5.88x106poise, n(525°C) = 3.52x106poise, n(550°C) = 3.23x106poise. Even though the calculated viscosities have been obtained based on rough assumptions, the order of magnitude does not seem to be far from realistic values. A typical viscosity of liquid metal is about 1 to 2centi-poise, whereas that of solid is over 10mpoise(22). If solid' and liquid coexist in the form of slurry, the viscosity increases steeply with increasing solid fraction(23). In the glass working range(viscosity ~104~108poises) the glass can be easily formed to any shape by processes such as drawing, blowing and mUing, but it has sufficient rigity to support its o w n weight for a short period of time(7_A). As already mentioned in the section of results, the hair-like fiber observed on the fracture surface m a y be associated with the viscous slurry. It can be seen from Fig.7 that the increase in strain rate sensitivity with increasing strain rate can be attributed to the strength of matrix-fiber interface, which in turn gives rise to the increase in elongation. At a much higher strain rate, we have to consider two competing effects. One effect is the interface hardening(due to increased k) and the other is a softening(due to decreased n and increased h) due to an increase in tem~mture of specimen caused by deformation in adiabatic condition. If a specimen of 10MPa in flow stress is assumed to be subjected to the strain of 0.5 then the temperature increase is estimated to be a few degrees. The latter effect seems to dominate as the specimen temperature increases and in turn give rise to the decreases in elongation. This may be case of Fig.1.

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Conclusions The elongation of the 2124Al/15vol%Si2C~,~ composi~ slightly above the solidus temperature reached the maximum value at the strain rate of 10- to lO-Zsec- . As the temperature of specimen increased above the solidus temperature, the flow stress gradually decreased without a sharp drop. Above the solidus temperature the strain rote sensitivity increased from about 0.1 to 0.3 with increasing strain rate. The hair-like fibers were found on the surfaces fractured at temperatures above the solidus temperature, which are believed to be formed from a viscous slurry. Mechanical behavior of the composite above the solidus temperature was explained by the shear stress devdoped in the viscous slurry layers in the interfaces between the whiskers and matrix.

This work has been partially supported by Korea Science and Engineering Foundation through Research Center for Thin Film Fabrication and Crystal Growing of Advanced Materials, Seoul National University. P,efea-e~ices 1. T.G.Nieh, C.A.Henshall and J.Wadswcxth, Scripta Metall., 18, 1405 (1984) 2. J.Wadwortl% C.A.Hensahll and T.G.Nieh, High Strength Powder Metallurgy Aluminum Alloys, ed. G.J2-LIdm-nan eta/., p.137, TMS-AIME, Warrendale (1985) 3. M.W.Mahoney and A.K.Ghosh, Metall. Trans., 18A, 653 (1987) 4. M.Y.Wu and O.D.Sherby, Scripta Metall., 18, 773 (1984) 5. A.H.Chokshi, T.G.Nieh, J.Wadsworth and A.K.Mukherjee, Strength of Metals and Alloys, ed. P.O.Kettuen et aL, p301, Pergamon Press, Oxford, (1988) 6. M.Mabuchi, I~I-Iigashi, Y.Okna~ S.Tanimura, T.Imal and K.Kubo, Scripta Metall. Mater., 25, 2003 (1991) 7. T]mai, MaMabuchi, Y.Tozawa and M.Yamada, J. Mater. Sci. Lett., 9, 255 (1990) 8. M.Mabuchi and T.Imai, J.Mater. Sci. Lett., 9, 761 (1990) 9. M.Mabuchi, K.Higashi, K.Inoue and S.Tanimura, Scripta Metal[ Mater., 26, 1839 (1992) I0. K.I-Iigashi, T.Okada, T.Mukai and S.Tanimur~ Scripta Metall. Mater., 25, 2053 (1991) 11. K.I-Iigashi, T.Okata, T.Mukal, S.Tanimura, T.G.Nieh and J.Wadworth, Scripta Metall. Mater., 26, 185 (1992) 12. T.R.Bieler, T.G.Nieh, J.Wadsworth and A.ICMukerjee, Scripta Metall., 22, 81 (1988) 13. T.G.Nieh, P.S.Gilman and J.Wadsworth, Scripta Metall., 19, 1375 (19@5) 14. T.S.Kim and T.H.Kim, J. Korean Inst. Metals, 28, 443 (1990) 15. S.KNutt and R.W.Carpenter, Mater. Sci. Eng., 5, 169 (1985) 16. M.Strangwood, C~a,.Hippsley and J.J.Lewandowski, Scripta Metall. Mater., 24, 1483 (1990) 17. T.G.Nieh and J.Wadworth, Mater. Sci. Eng., A147, 129 (1991) 18. T.G.Nieh and J.Wadsworth, J. Metals, Nov., 46 (1992) 19. V.C.Nardone and K.M.Prewo, Scripta Metall., 20, 43 (1986) 20. R.J.Arsenault and N.Shi, Mater. Sci. Eng., 81, 175 (1986) 21. H.K.Moon, Rheological Behavior and Microstmcture of Ceramic Particulate/Aluminum Alloy Composites, p.85, Ph.D. Thesis, Mass. Inst. Tech. (1990) 22. D.Turnbull, Trans. MetaU. Soc. AIME, 221, 422 (1961) 23. D.B.Spencer, R.Mehrabian and M.C.Flemings, Metall. Trans., 3, 1925 (1972) 24. C.R.Barrett, W.D.Nix, A.S.Telelman, The Principles of Engineering Materials, p.335, Prentice-Hall Inc., New Jersey (1973)