Aging behavior of AlNp/2024Al composites
fabricated by squeeze casting
WU Gao-hui(武高辉)1, CHEN Guo-qin(陈国钦)1, JIANG Long-tao(姜龙涛)1,
LUAN Bai-feng(栾佰峰)1, N. KONO2, T. HAITANI2
1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;
2. Department of Metallurgical Engineering, Chiba Institute of Technology, Chiba 275-0016, Japan
Received 28 July 2006; accepted 15 September 2006
Abstract: 2024 aluminum matrix composites reinforced with different size AlN particles (0.5, 4 and 10 μm) were fabricated by the squeeze-casting technology. The aging behavior and microstructure of AlNp/2024Al composites were investigated by Brinell hardness measurement and transmission electron microscopy (TEM). The results show that the precipitation sequence of AlNp/2024Al composites is similar to that of the matrix alloy aged at 160 and 190 ℃，but the age hardening rate of composites is improved, and the AlN particles with large size promote the precipitation process more obviously, in comparison with smaller AlN particles. With increasing temperature, the precipitation processes are accelerated, and the time to reach the peak hardness is shortened. The acceleration of the formation of GP region and phase S′ in the composites is attributed to the interfaces (between particles and the matrix) and the high density of dislocations introduced by addition of AlN particles.
Key words: aluminum alloy; metal matrix composite; AlN particle; squeeze casting; aging behavior
1 Introduction
The AlN ceramic particle has been recognized as a promising reinforcement for aluminum matrix composites because of excellent combination of its high thermal conductivity, low thermal expansivity and high hardness[1-4]. For aluminum matrix composites, aging treatment has been widely used as an effective method to obtain stabilized microstructure and ultimate application properties[5-6]. Extensive researches [7-10] have been carried out on aging behavior of aluminum matrix composites with SiC and Al2O3 reinforcement, and many mature theory systems of the microstructure and strengthening mechanism were built. However, aging characteristics of high volume fraction (≥40%) AlN particle reinforced composites have so far received little attention.
In general, AlN particles reinforced aluminum matrix composites can be fabricated by pressure infiltration of liquid aluminum[11], powder metallurgy [12-14], partially or directly nitrided aluminum powders[15-16] , and in-situ reaction between Mg3N2 and Al. The previous study[17] showed that the contact angle between AlN and molten aluminum decreased with the increase of temperature, but above 850 ℃ the contact angle became less than that at 900 ℃ under 0.027 MPa. Therefore, pressure-assisted infiltration was a feasible technique to fabricate 50% (volume fraction) AlN/Al composites. The present study was concentrated on the aluminum matrix composites with different sized AlN particles (0.5, 4 and 10 μm) that were produced by squeeze casting, and their aging behavior and microstructure were investigated with Brinell hardness measurement and transmission electron microscopy (TEM).
2 Experimental
The reinforcements used in this work were angular-shaped AlN particles with nominal diameter of 0.5, 4 and 10 μm, and the volume fraction of the reinforcements were 50 %. The aluminum alloy was an Al-Cu-Mg alloy (2024Al) whose nominal compositions are listed in Table 1.
Table 1 Chemical compositions of 2024Al composites (mass fraction, %)
The AlNp/2024Al composites were fabricated by squeeze casting technology[18], as illustrated in Fig.1. Firstly, the AlN particles were filled and pressed into a mold to produce an AlN preform according to the given volume fraction. After that, the preform was pre-heated in a die. At the same time, the 2024Al was melt, degassed, cleaned in a graphite crucible and heated to 750-760 ℃. When the preform was heated to 550 ℃, the molten 2024Al was poured into the die and a vertical pressure up to 55MPa was applied immediately to force molten 2024Al to infiltrate into AlN preform completely. The pressure was maintained for about 3 min until the solidification was complete.
Fig.1 Fabrication process of AlNp/Al composites
The composites were solution treated at 495 ℃ for 1 h and water quenched at room temperature. After solution treatment, the alloy and composites were aged at 160 and 190 ℃ for periods up to 100 h. For comparison, the unreinforced matrix alloy was also treated as the craft mentioned above. An S-570 scanning electron microscope (SEM) was used to examine the microstructure of the composites. Brinell hardness (HB) tests were performed immediately after aging on an HBV-30 double-purpose tester with a 1 mm diameter ball indenter. A load of 300 N was applied and maintained for 30 s. Each hardness value was the average of at least five measurements. The dimensions of specimen are 10 mm× 10 mm×2 mm. Transmission electron microscope (TEM) foils were prepared by a standard combination of mechanical and ion-beam thinning using a Gattan-600 ion mill equipped with liquid-nitrogen cold stage to prevent heating during the ion-milling process. A Philips CM-12 electron microscope was used to characterize the microstructures at an accelerating voltage of 200 kV.
3 Results and discussion
3.1 Microstructure observation
Fig.2 shows SEM microstructure of the as- fabricated AlNp/2024Al composites. The AlNp/Al composites are fully infiltrated and no matrix-rich channels are found. AlN particles distribute homo- geneously in the composites, without any particles clustering. The composites are dense and macro- scopically homogeneous, free from common cast defects such as porosity and shrinking cavities, because pressure is applied during the solidification of AlNp/Al composites.
Fig.2 Microstructure of AlNp/Al composite
3.2 Aging behavior
Age-hardening curve of 2024Al and A1Np/2024A1 composites with different particle sizes are shown in Figs.3 and 4. The hardnesses of the four materials increase monotonically with aging time before reaching the maximum and then gradually decrease. The precipitation sequence of A1Np/2024Al composites is similar to that of the matrix alloy. Due to the addition of AlN particulates, the hardness of composites is obviously higher than that of matrix alloy. The composites reach the maximal hardness after 6-9 h, while the matrix alloy requires 9-11 h, indicating accelerated age- hardening in the AlNp/Al composites compared with the unreinforced 2024Al alloy. Fig.5 gives the effect of aging temperature on the peak-aging time for the composites as well as unreinforced alloy. It can be observed that the peak-aging time decreases with increasing particle size of AlN. The AlN particles with large sizes promote the precipitation process more obviously, in comparison with smaller AlN particles. The peak-aging time decreases with increasing aging temperature for both the alloy and composites, and the age-hardening kinetics becomes faster as the aging temperature increases. Under experimental temperatures, the time to achieve the maximal hardness is shorter in the composites as compared with the monolithic alloy.
Fig.3 Age-hardening curve of 2024Al and composites with different particle sizes (at 160 ℃)
Fig.4 Age-hardening curve of 2024Al and composites with different particle sizes (at 190 ℃)
Fig.5 Effect of aging temperature on peak aging time for 2024Al and composites
3.1 TEM behavior
As shown in Fig.6, a few dislocations exist in the 2024Al matrix alloy and some precipitates are also observed. While, in Fig.7, a lot of dislocations and precipitates exist on the interfaces of composites. Fig.8 shows the TEM micrographs of 2024Al and AlNp/ 2024Al composites aged at 160 ℃ for 6.8 h. It can be seen that the precipitates S′ existing in the 2024Al alloy matrix are fewer with a fine needle or clava shape. For comparisons of the three sized composites within the Figs.8(b), (c), (d), the precipitates S′ of phases in the matrix of composites becomes larger, the amounts also increase. Among these, the 10 μm AlNp/2024Al composite attains the peak aging and the phases S′ precipitate fully. In general, the residual stresses and high density dislocations as well as interfaces introduced by the presence of high volume fraction AlN particles provide favorable locations for non-uniform nucleation of phase S′, which reduces the nucleation incubation period and accelerates the nucleation precipitation. On the other hand, high density dislocation makes Cu and Mg atoms diffuse faster, which reduces the formation energy of phase S′ and promoted phase S′ precipitation. So the age-hardening velocity of AlNp/2024Al composites was faster than that of the monolithic 2024 alloy. In addition, the dislocation density existing in the matrix around the particles increases with increasing particle size. Therefore, the AlN particles with large sizes promote the precipitation process more obviously, in comparison with small sizes of AlN particles. And the precipitation process speeds with increasing particle size.
Fig.6 Microstructure of 2024Al alloy as-cast
Fig.7 Microstructure of AlNp/2024Al composite as-cast
Fig.8 Microstructures of 2024Al and AlNp/2024Al composites: (a) 2024Al aged at 160 ℃ for 6.8 h; (b), (c), (d) AlNp/2024Al composites with AlNp sizes of 0.5, 4, 10 μm respectively aged at 160 ℃ for 6.8 h
4 Conclusions
1) Due to the addition of high volume fraction of AlN particles, the hardness of the composites is obviously improved. The precipitation sequence of AlNp/2024Al composites is similar to that of the matrix alloy, but the age hardening rate of composites is improved, and the AlN particles with large sizes promotes the precipitation process more obviously, in comparison with small sizes AlN particles.
2) With increasing temperature, the precipitation processes are accelerated, and the time to reach the peak hardness is shortened.
3) The acceleration of the formation of GP region and the activation energy of phase S′ decreases in the composites, which attributes to the interfaces and the high density of dislocations introduced by the addition of AlN particles.
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(Edited by LONG Huai-zhong)
Foundation item: Projects(59771014; 50071019) supported by the National Natural Science Foundation of China
Corresponding author: CHEN Guo-qin; Tel: +86-451-86402372-5058; E-mail: chenguoqin@hit.edu.cn
