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Distribution and segregation of
solute in Al
2
O
3
short fiber reinforced
Al-5%Cu alloy containing La by
squeeze casting
*LIU Zheng, LIU Lu-jun, LIN Ji-xing, and HU Yong-mei
(Faculty of Materials and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China)
B
ecause of the addition of the reinforcement, the temperature f i e l d , f l o w f i e l d , c o n c e n t r a t i o n f i e l d , a n d t h e thermodynamics and dynamics of crystal growth during the composite solidification as well as the structural morphology of the composite are changed in the processing of fiber reinforced Al matrix composite manufactured by liquid infiltration. The solute is subjected to hindrance from the fibers because the reinforcement acts as a barrier to solute diffusion ahead of the liquid/ solid interface in the fiber reinforced Al matrix composite [1-2].Therefore, a lot of solute segregated on, or some new phases are formed at the final location of solidification in the composite (e.g. fiber/matrix interface). The solute enrichment occurring around the reinforcement is also a main factor in promoting the interface reaction during the solidification. The solute segregation produced in the matrix will affect the final solidified structure and properties of the composite. It is seen that there is a need for research on solidification segregation of fiber reinforced Al matrix composite to improve the properties and to raise safety of the composite. So far, there has been some detailed foreign research [1-4] on
solute segregation, and several domestic literatures [5-6] have
mentioned and discussed this problem but little has been published on deeper study of the solidification segregation.
Some reports [7] indicate that the solubility of rare earths (RE)
in Al alloy is very low, and the solute redistribution in
solidification can be enriched at grain boundaries. Using this characteristic of RE and the lower electro-negativity of RE than that of other elements in the matrix, the RE can preferentially be adsorbed at the fiber/matrix interface, and that the enrichment of other element is lightened, so solute segregation is decreased. The solidified structures and solute segregation of Al2O3f /Al-5%Cu-La alloy composites manufactured by squeeze casting were observed using a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS), and the interaction between the La and the solute element in solidification segregation was analyzed.
1 Experimental procedures
The alumina short fibers (3-5 µm in diameter) used in the test consist of Al2O3 (80 wt-%) and SiO2 (20 wt-%). The main crystal phases of the fibers are α-Al2O3 and mullite. The matrix alloy is prepared by Al-5Cu alloy and Al-La master alloy. The composition of the alloy by weight is: 4.93 % Cu, 0.56 % La and the balance Al. Al2O3f /Al-5%Cu-La alloy composites were manufactured by squeeze casting. The fibers were made into preform, then dried and preheated. The preform was infiltrated by liquid Al alloy under a pressure of 80 MPa. After solidifying and cooling, the ingot casting of the composite containing 15 % by vol. fiber (Vf) was gained. The samples, made from the composites, were polished using standard metallographic practice, etched with 0.5 % aqueous solution of hydrofluoric acid and observed using a Philips XL-30 SEM. The phase structure and the composition of micro-area were analyzed on a Miniflex X-ray diffractometer and an EDAX electron probe.
Male, born in 1958, professor, Ph.D candidate. Research field: metal matrix composites.
E-mail: [email protected],
Received: 2007-01-10 Accepted: 2007-08-10 *LIU Zheng
Abstract: Al2O3 short fiber reinforced La-bearing Al-5%Cu alloy was fabricated by squeeze casting, and the
solidified structure and the solute segregation during alloy solidification were studied. The results indicated that La has been enriched near the interface which is favorable to improve the wettability between the fiber and Al alloy, but the RE-rich phase was not formed at the interface. At the end of the solidification of the composites, the change of the solute in the surplus liquid phase results in the type of matrix alloy being changed because of the selective crystallization, and the segregation at the interface is finally formed. There is no special influence by La on the Cu segregation in the matrix alloy.
Key words: composite; La; Al2O3 short fiber; Al-Cu alloy; solute segregation
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2 Results
There is no shrinkage porosity in the Al2O3f /Al-5%Cu -La alloy composites, and the fibers uniformly distribute in the matrix and combine well with the matrix, as shown in Fig.1. This indicates that the liquid aluminum alloy fully infiltrates the fiber preform during squeeze casting, and the preform is not obviously deformed or displaced.
Fig. 1 Distribution of the fiber in the composite
Figure 2 shows the solidified structures of the composites observed on an SEM. There are some of the second phases (gray stick) distributed at the fiber/ matrix interface and in dendrites around the fibers. This shows that the fibers cannot serve as propitious sites of heterogeneous nucleation for the primary phase in the matrix alloy, and are inclined to be surrounded by the surplus liquid melt causing the solute to be enriched. As there is a higher chemical potential at the interface than in the matrix, the interface is in the two phases of solid/ liquid state for a longer time during solidification of the composites. As viewed from thermodynamics and dynamics, certain chemical reactions may take place at the interface to form the obvious interface layer. The reactions may be the formation of aluminum-copper spinal [8]
and a reduction of silicon dioxide in the fiber [9]. These processes
occur at the end of solidification.
To further analyze whether or not, La forms an inter-metallic compound in the matrix of the composites, the composition of the micro-area in the composites was analyzed using an SEM with energy spectrum, and the BE imaging was used. The purpose of this is that the distribution of La in the composites is more distinctly analyzed. The BE image is shown in Fig.3. It is seen from the view that the grey matter with strip-like or elliptic-like shapes is the fiber, and the fiber presents as being dark grey due to its poor conductivity. In the observation on the solidified
Fig. 2 Solidified structure of the composite
structures, there are two kinds of solid phases in the matrix structure, where most of the bright solid phases are distributed around the fiber. Before checking on the bright white solid phases, it is conjectured that some heavy element may be enriched in the bright area according to the composition and the solidifying characteristics of matrix alloy.
Fig. 3 BE imaging of Al2O3f /Al-5%Cu-La
No elements with big atomic number other than La were added to the matrix in this test. It is known from the Al-La alloy phase diagram that La can form into varied compounds with Al, such as LaAl4, LaAl2, La3Al, and LaAl [7]. So it is theoretically
conjectured that these bright areas may be the La-rich areas. Some of the solid phases in Fig. 3, especially the bright solid phases, were selected to check the composition using a high energy electron beam from an energy spectrum diffractometer. It is found that these bright solid phases are indeed the result of enrichment by a heavy element, but there is less La to be checked. However, there exists La in the matrix from the energy spectrum map, as shown in Fig. 4. After EDS for the bright areas in the matrix, the distribution of La can be approximately obtained. It is found that there exists La in the composite in the form of accumulating and there is a tendency of La enrichment near to the interface, as shown in Fig. 4.
Fig. 4 EDS analysis of the composite
It can be seen from Fig. 4 that there is an energy spectrum line reflecting La, but it is very weak, similar to the past research results [10]. The line scanning of the elements in the same
micro-areas as in Fig. 3 was carried out using an electron probe, giving results as shown in Fig. 5. It shows that La in the matrix obviously distributes around the fiber and enrich at the interface. There is an obvious Cu enrichment as an alloy element from Fig. 5. This Cu enrichment is mainly achieved by liquid alloy taking binary eutectic reaction during solidification to form Al2Cu phase in Al-Cu binary alloy
[11].
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Fig. 5 Line scanning of elements in the composites
composite is a non-equilibrium process, the selective crystallization results in the solute concentration in the surplus liquid phase during the end of solidification increasing, so that the original solid solution alloy is changed into the hypo-eutectic alloy or even eutectic alloy. It can be calculated that the end temperature of solidification for Al-5%Cu alloy is about 580
based on the Al-Cu alloy equilibrium diagram. Under similar circumstances, it can be calculated that there is a lower volume fraction of solid phase in hypo-eutectic Cu alloy than in Al-5%Cu alloy during the last stage of the solidification temperature (580-600 ). That means more volume fraction for surplus liquid phase. Therefore, the eutectic transformation finally occurs in some of the surplus liquid phase, forming the Cu-rich phase during solidification of Al2O3f /Al-5%Cu alloy composite.
3 Discussions
Due to the strong affinity of Al for oxygen, the wettability between A12O3 short fibers and liquid alloy is poor in the A12O3 short fiber reinforced Al matrix composites. However, there are elements with smaller electro-negativity than that of aluminum in the liquid Al alloy (the electro-negativity of La is 1.1, that of Al is 1.5 [12]). As their adsorbing tendency at the fiber/matrix
interface is stronger than that of Al, they will be preferentially adsorbed on the interface, decreasing the interface energy. Consequently the wettability between the two phases is improved, and the adherence power is raised, so the wetting angle is reduced. On the other hand, as the surface active elements, La in Al alloy can reduce the surface energy of liquid Al alloy and decrease the wetting angle of liquid Al alloy on the solid, thus the wettability between the (matrix) liquid Al and the reinforcement is improved due to the formation of the new phases [10].
Due to the characteristics of RE, La will also enrich at the interface between the fibers and the matrix. From the view of thermodynamics, if RE elements meet the fibers or SiO2 with the surface covered non-uniformly and non-fully, a chemical reaction will take place, which would improve the wettability between the matrix and the fiber:
2La(s) + Al2O3(s)La2O3 (s) + 2Al (1)
4La(s) + 3SiO2(s) 2La2O3(s) + 3Si(s) (2)
The changes of standard free enthalpy for equation (1) and equation (2) being calculated, the values of the changes are both negative at the manufacturing temperature of the composite, which means that the two reactions both occur spontaneously.
Thus, through the reaction between enriched La at the interface and the surface of the fiber, La can further improve the wettability between the liquid alloy and the fiber, and at the same time improve the combination of the interface.
The literature [7] has pointed out that the solubility of La in liquid Al is only 0.05 % at 915 K, and the atomic radius of La is relative large (about 0.1877 nm). Thus, there are two possibilities: La is only segregated at the interface or it forms metallic compounds enriching La during solidification in theory.
During the investigation on the distribution of La in Al2O3 fiber
reinforced hypo-eutectic and eutectic Al-Si alloy, it is found that La enriches at the interface between the fiber and the matrix [13-14]
to form the compounds which grow on the surface of the fiber
[14]. That means that La has a tendency to segregate at the interface.
From the view of reactive dynamics, the elements which can enrich in the matrix near the interface will hinder the reactive dynamics to a certain extent. According to the analysis above mentioned, there is the small possibility to produce the reaction at the interface.
The location of La distributing in the composites has been determined. However, further study and analysis is still needed to determine in what form or in what kind of compound the La exists in the composites.
Because of the squeeze casting of Al2O3f /Al-5%Cu alloy composite being a non-equilibrium process, and the solute equilibrium distribution coefficient K0<1, when the α-Al phase crystallizes, a part of Cu enriches at the solidifying front of the crystallizing α-Al phase. When the part of Cu diffuses into the liquid phase under the effect of convection, the diffusion of the Cu will be stopped by the fiber. Because the reinforcement (e.g. Al2O3 fiber) acts as a barrier to solute diffusion ahead of the liquid/solid (e.g. α-Al phase/liquid alloy) interface [1], it is difficult
to mix the solute elements uniformly in the liquid phase. The solute concentration in the local liquid phase near the fiber rises, presenting hypo-eutectic composition or even eutectic composition.
The distribution of solute element satisfies the law of mass conversation yet during solidification. The law can be expressed as the following equation [15]:
It can be seen that the distribution, or change, of local solute composition is affected by solute diffusion, liquid phase flow, and liquid phase solidification, in equation (3). During the initial stage of composite solidification, because of liquid alloy flow strengthening the mass transfer between the solid phase/liquid phase interface, the solute concentration in the solid crystallized (e.g. α-Al) is on the low side. During the end stage of solidification, as the fraction of the solid phase is increasing, the apparent viscosity coefficient and apparent yield stress of the alloy in the solid/liquid phase zone increase; and the liquid phase flows with difficultly. Some investigation [16] shows that when
the fraction of solid increases to 0.6 - 0.73 in Al-4.5%Cu alloy, the liquid in the two-phase zone stops flowing.
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is larger than some volume before the finish of solidification, the surplus liquid phase cannot flow, that is Ulx= 0. So equation (3) can be expressed as:
Equation (4) can be seen as a special expression of Fick’s Diffusion Law in the end stage of solidification of the composite. Thus, in the last stage of the solidification, the solute transfer at the solid phase/liquid phase interface is mainly carried out by diffusion, and the solute concentration in the surplus liquid phase may reach C0/K0 (C0 being the original composition of alloy) before the finish of solidification. When the solidification finishes, the surplus liquid phase which is solute rich will change into the solid phase at the fiber/matrix interface or between the dendrites near the fiber.
The segregation produced in the composite causes the non-uniform distribution composition which results in the difference of structure in the composite. This may alter the properties of the composite and may affect its use, suggesting that it should be reduced or eliminated. On the other hand, segregation of the alloy elements at the interface may cause suitable interface reactions [9, 17-19], which can promote wetting between the metal
and the ceramic [20-21]. Judged by thermodynamics, the chemical
reaction occurs spontaneously within the system accompanied by an energy reduction for the whole system. In Al2O3f / Al-5%Cu alloy composite, the interface reaction caused by element segregation is favorable to reduce the solid phase/liquid phase interface energy and promote combination between the fiber and the matrix. How to control and use the interface segregation during the manufacture of the composite is still pending further study.
4 Conclusions
(1) Solidification of Al2O3f /Al-5%Cu alloy composite is a non-equilibrium process. At the end of solidification, selective crystallization causes part of the surplus liquid phase to change from solid solution alloy into hypo-eutectic alloy, resulting in alloy elements segregating at the interface.
(2) La can enrich at the fiber/matrix interface which is favorable to improving wettability between liquid Al alloy and Al2O3 short fibers. There are not any inter-metallic compounds enriching La to be found at the interface.
(3) The segregation of alloy elements at the interface brings about the formation of the new phase and the interface reaction, which is favorable to combination between the fiber and the matrix.
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