Investigation of the precipitation kinetics in an A16061 ... - Science Direct

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering A237 (1997) 12-23

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Investigation of the precipitation kinetics in an A16061/TiB2 metal matrix composite C. Bartels a,,, D. R aabe '~, G. Gottstein ~, U. Huber b a Institut fiir Metallkunde und .~Ietallphysik, Kopernikusstr. 14, R WTH Aachen, 52056 Aachen, Germany b Daimler Benz AG, Forschung und Technik, Abt. F4K/M, Willy Messerschmitt-Str., 85521 Ottobrunn, Germany

Received 6 January 1997; received in revised form 21 March 1997

Abstract The aging kinetics of AA6061 ( A 1 - M g - S i - C u ) based metal matrix composites (MMCs) containing TiB2-particles were investigated using hardness measurements, texture determination and electron microscopy. The samples were produced by an in-situ process. Two industrially manufactured in-situ composites with an AA6061 matrix and TiB2-particles (3.4 vol.%, 6.8 vol.% TiB2) were studied at two different aging temperatures (160~ 250~ and compared to a similarly processed non-reinforced AA6061 alloy. An increasing volume fraction of TiB a correlated with changes in the aging response of the composites. Samples subjected to three aging times were selected for microstructure analysis in the TEM (10 rain, 200 rain, 1365 rain, aging temperature 250~ Special emphasis was laid on the investigation of the growth kinetics of metastable needle shaped matrix precipitates, which cause the increase in hardness. The TEM examinations of the MMCs substantiated that precipitate growth was accelerated in the presence of TiB2 particles as compared to the uureinforced AA6061. These differences in kinetics were related to microstructural changes induced by the presence of the ceramic TiB 2 particles. 9 1997 Elsevier Science S.A. Keywords: Aging kinetics; At based MMCs; On-situ MMC

1. Introduction The addition of ceramic particles such as TiB z, SiC, or AlzOa to a n a l u m i n u m based matrix does not substantially change the density o f the material. However, it usually leads to a significant rise in strength and thus, to an i m p r o v e d strength-to-weight ratio, which is a m a j o r issue in optimizing advanced weightsaving metal matrix composites (MMCs) for structural applications. In contrast to the conventional manufacturing techniques where pre-existing ceramic particles are incorporated into the a l u m i n u m matrix, a competitive new class of processing routes is b a s e d on the in-situ form a t i o n of particles during m a n u f a c t u r e of the material. The response of these in-situ formed aluminum based M M C s to aging t r e a t m e n t and to therrno-me-

* Corresponding autttor. 0921-5093197/$17.00 9 1997 Elsevier Science S.A. All rights reserved. PII S0921-5093(97)00104-4

chanical processing is of prime importance for its structural performance. This study focuses on the aging kinetics o f such MMCs. F o r this purpose two industrially manufactured in-situ composites with an AA6061 matrix reinforced with TiBz-particles (3.4 vol.%, 6.8 vol.% TiB~) were investigated and compared to a similarly processed non-reinforced AA6061 alloy using hardness measurements. F o r relating the observed hardness development to microstructural changes, transmission electron microscopy (TEM), optical microscopy, Xray macrotexture measurements and microtexture determination using the electron back scatter diffraction (EBSD) method in the scanning electron microscope (SEM) were carried out. Special attention was paid to the identification of significant microstructural differences between the M M C s a n d the non-reinforced reference material, to account for the differences in aging kinetics.

C. Barrels et al. / Materials Science and Engozeering A237 (1997) 12-23

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2. Experimental

2.2. Microhardness measurements

2. I. Materials

The extruded material was sectioned perpendicular to the extrusion direction into slices o f 3 mm thickness. These samples were solution treated for 1 h at 560~ and subsequently quenched in ice water. The surface of the slices was ground with SiC-paper and then mechanically polished using waterbased diamond suspensions (6 lam, 3 lain). The aging treatment was carried out in an oil bath at two different temperatures: 160~ and 250~ The elapsed time was 488.83 h (29 330 min) for an aging temperature of 160~ and 22.75 h (1365 rain) for an aging temperature o f 250~ In order to follow the decomposition kinetics o f the matrix aging was interrupted after various lengths of time and the microhardness of all three samples was measured using a Vickers hardness testing machine with a load of 200 g. For the determination of the average hardness value at each time ten data points were used neglecting the smallest and the largest ones, respectively.

The m a t e r i a l investigated in this study was processed by a n e w in-situ technique developed by London and S c a n d i n a v i a n Metallurgical [1]. During stirring, the salts K 2 T i F ~ and KBF 4 are added to molten aluminum. In the m e l t the following reactions are supposed to take place a n d to lead to the formation o f TiB2 particles in the m o l t e n material. 2 K B F 4 + 3 A1 = A1B2 + 2 KA1F4 3 K 2 T i F a + 13 A1 = 3 TiAI 3 + K3A1F 6 + 3 KA1F 4 A1B2 + TiAI~ = TiBz + 4 AI T h e s e TiB 2 particles have a hexagonal morphology with a d i a m e t e r of up to 1.5 vtm and a thickness of less than 0.5 l-tin. When particle formation is completed, the reaction products KAIF 4 and K3A1F6 have to be removed f r o m the melt. Subsequently, alloying elements can be a d d e d and the melt can be cast. C e r a m i c particles in composites processed by this in-situ t e c h n i q u e are much smaller than in most other c o m m e r c i a l composites manufactured via a conventional m e l t processing route. In o r d e r to study the influence of the ceramic particles o n t h e aging kinetics of an AA6061 matrix, two c o m p o s i t e s containing 3.4 vol.% and 6.8 vol.% TiB 2 were c h o s e n and compared to an AA6061 alloy without TiB 2 b u t with a similar processing path. In Table 1 the c o m p o s i t i o n s of the investigated materials are listed. The n o n - r e i n f o r c e d material served as a reference. All samples were hot extruded to rods at a temperature o f 430~ u s i n g an extrusion ratio of 8:1 and a die angle o f 60 ~. Table 1 Chemical analysis of the investigated materials, given by the producer

6061 (wt.%) 6061+3.4vol.% TiB2 6061+6.8 vol.% TiB2 (wt.%) (wt.%) Si B Ti Fe Cu Mn Zn Mg Pb Sn Cr Li Ni V K Ca Sr Zr

0.63 O.Ol 0.03 0,14 0.27

Max: 14.4

~



Levels 1.00 2.00 4.00 7.00 10,00



b)

~ MaxI

10.1

>

/. "~-,,~"'~---~



Levels 1.00 ZOO 4,130 7,00

10,00



c) S~~ Max: 4.94





Levels 1.00 2.00 4.00



Fig. 8. Inverse polefigures for the extrusion direction in: (a) AA6061 + 6.8 vol. %, (b) AA6061 + 3,4 vol. % TiB2 and (c) AA6061.

Both, the hardness measurements and the growth rate of the precipitates showed an accelerated aging response of the particle reinforced materials as compared to the kinetics observed in the non-reinforced reference sample (Figs. 1, 2 and 6). This result is in accord with previous studies where similar effects were observed on samples with larger volume fractions of ceramic particles [6-11]. The hardening effect in 6XXX alloys at 160~ and 250~ is due to the formation and growth of metastable needle shaped precipitates. Although a number of studies have been conducted on the formation sequence of such metastable precipitates in A1-Mg-Si alloys, some details such as their exact composition and structure are still under debate [11-20]. In a recent investigation [12] the following precipitation sequence was proposed: The incipient stages of aging are characterized by the formation of silicon clusters followed by the development first of GP I and then of GP II zones. The latter zones are referred to as /?" precipitates. Subsequently, /3' and finally the stable Mg2Si (fl phase) precipitates appear. In other investigations a somewhat different sequence of the first diffusional decomposition steps was suggested, namely, the clustering of magnesium [13], of magnesium and silicon together [13-15], as well as the clustering of vacancies [13]. While different structures were suggested for the/3" phase [16-18], the hexagonal, semicoherent structure of fl' as identified by Jacobs [16] was confirmed by the studies of Lynch et al. [17] and of Matsuda et al. [19]. Besides the /3' phase, another phase referred to as B' was found in A1-Mg-Si alloys [20]. It is assumed that the needle shaped precipitates observed in the current study (Figs. 3 and 5) are of the fl' or B' type both of which show the same needle shape morphology. Moreover the characteristic angle between needles grown into different crystallographic directions is 90 ~ which corresponds to the observation that the needles are aligned along the During the in-situ process the reaction products KA1F4 and K3AIF6 were formed. If these salts are not entirely removed from the melt after the formation of TiB 2 further reactions with the subsequently added alloying elements are conceivable. TMs some magnesium may have been absorbed by these impurity phases and therefore, does not contribute to age hardening any more. Moreover, the level of excess silicon in the matrix is altered when magnesium is partly absorbed. However, the presence of excess silicon was reported to accelerate age hardening [32] and thus can contribute to a change in aging kinetics. 4.2. Crystallographic textures

Figs~ 8 and 9 demonstrate that the presence of TiB2 particles in the material influences the recrystallization characteristics. The non-reinforced AA6061 alloy in the hot extruded state and in the solution treated state (subsequent to extrusion) shows a texture that is typical of primary recrystallization in aluminum when nudeation occurs at transition bands [33]. This interpretation is supported by the corresponding misorientation distribution (Fig. %), which shows a major contribution of large' angle grain boundaries. In contrast, the textures of the composites are in both states (extruded and solution treated) similar to hot-extrusion textures rather than to recrystallization textures [34]. While the MMC macrotextures indicate ordinary plastic deformation during extrusion, their grain morphology is more similar to that of the recrystallized non-reinforced alloy. The grains of all samples are slightly stretched parallel to the extrusion direction. However, the MMC samples have a much smaller grain size (Fig. 10). The non-reinforced material reveals an average grain length of 84/~m and an average width o f 45 lira. In both types of MMCs (3.4 vol.%, 6.8 vol.% TiB2) the average grain length amounts to 4.5/~m and the average width to 3.2 ~tm. The grain size in the composites seems to be independent of the particle content, at least by practical means. This discrepancy between the observed macrotexture, which indicates plastic deformation, and the corresponding microstructure indicates the formation of new grains by recovery. This is supported by the large number of low angle grain boundaries (Fig. 9a,b) and the high dislocation density observed in the composites

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C. Bartels et a l . / Materials Science and Eng#zeerhzg A237 (1997) 12-23

(a)

4. T E M studies show that the presence o f TiB2 particles accelerates the growth o f the metastable needle shaped precipitates in the AA6061 matrix which are responsible for hardening. 5. The effect of accelerated aging in TiB 2 containing AA6061 is interpreted in terms of the higher dislocation densities observed in these MMCs. It is suggested that pipe diffusion stimulates accelerated aging particularly at lower temperatures while the influence of bulk diffusion prevails at elevated temperatures. 6. The observed high dislocation density in the M M C s is attributed to thermal stresses as a consequence of different coefficients o f thermal expansion a n d to the suppression of primary recrystallization during the initial hot-extrusion process and subsequent heat treatment. 7. In the M M C s magnesium was partly a b s o r b e d by salts which remain as impurities in the matrix after the formation o f the TiB2 particles. It is suggested that this amount of magnesium does not contribute to age hardening and affects the level of excess silicon which also influences the aging kinetics.

Acknowledgements Fig. 10. Grain structure of the (a) composite AA6061+ 3.4 vol. % TiB2 and (b) the alloy AA6061 alloy. (Fig. 7a,b,c). Thus, it is conceivable that the high dislocation density observed in the composites might to some extent stem not only from differences in the thermal expansion of the co-existing phases but also from the extrusion process.

The authors are grateful to the technical division of L o n d o n and Scandinavian Metallurgical for providing the material for this study. We wish to thank Vincent Pitrou for measuring the microtexture and for preparing the metallographic sections for the optical microscope.

References 5. Conclusion The aging kinetics of two AA6061/TiB 2 MMCs (3.4 vol.%, 6.8 vol.% TiB2) were investigated and compared to the aging of a non-reinforced AA6061 reference specimen. The main conclusions are: 1. Even small volume fractions of TiB 2 particles (3.4 vol.%, 6.8 vol.%) significantly alter the aging kinetics of an AA6061 + TiB2 composite. 2. A n increasing volume fraction of TiB2 shifts the maximum hardness to shorter aging times. This effect is more pronounced at a low aging temperature (160~ as compared to an elevated aging temperature (250~ 3. The hardness drop following the maximum value correlates with the TiBz content. It becomes more distinct with an increasing particle content.

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