HOT-EXTRUDED AND COLD-ROLLED TEXTURES OF THE MATRIX ALUMINUM IN DEFORMATION PROCESSED TWO-PHASE Nb/Al METAL–METAL COMPOSITES

In this article, the powder metallurgy technique combined with flat hot-extrusion and cold rolling processes was employed to fabricate 10 and 20vol.%Nb/Al metal–metal composite sheets. The hot-extruded and coldrolled textures of the matrix aluminum in these metal–metal composite sheets were investigated by three dimensional orientation distribution functions (ODFs) analysis. The results show that the extrusion mode and large second phase particulate metal, Nb, have strong influence on the development of the extrusion and cold rolling textures in composites’ matrix. The matrix Al forms -fiber textures after flat hot extrusion, where the components consist of B0-{011}h322i, S0-{124}h654i and C0-{113}h332i. After cold rolling process, only B0-{011} h322i changed to B-{011}h211i while the other components remained the same. The large particles in composites affect the matrix deformation in such a way that separates the distorted or bound zones from the deformation zones, which resulted in the final cold rolling deformation textures.


INTRODUCTION
The metal-matrix composites (MMCs) are usually referred to as the metals reinforced by another component. The reinforcement can be in the form of ceramic particles, whiskers, short fibers, or continuous fibers, so the MMCs can acquire combinative properties of metals and ceramics.
Metal-matrix composites can be obtained by heavy plastic co-deformation of powdermetallurgically prepared mixtures of two (or more) components, one of which is a metal, the other one may be metallic or nonmetallic. The process of co-deformation leads to an anomalous increase of the strength of the composite and it is accompanied by characteristic changes of the textures of both components (see e.g. Frommeyer and Wassermann, 1976;Bergmann et al., 1978;Wassermann et al., 1978). In recent years these deformation processed metal-metal composites (DMMC) or in situ metal-metal composites received much attention due to their anomalous strength and excellent thermal and electric conductivity (e.g. Russell et al., 1995;Raabe and Hangen, 1996;Han et al., 1999;Hong and Hill, 2000a;Xu et al., 2001).
Generally, a DMMC is composed of two immiscible metals or those having negligible solid solubility at up to its processing temperature, i.e. there is no intermetallic compounds within the binary systems. The starting material of the DMMC is usually a billet that has been prepared by casting or powder metallurgy (PM) technique and the subsequent large plastic deformation processing techniques, e.g. extrusion, swaging, drawing or rolling, could then be exerted to obtain the final metal-metal composite products. The Cu-Nb (e.g. Bevk et al., 1978;Brandao and Kalu, 1998;Sauvage, et al., 1999Sauvage, et al., , 2001, Cu-Cr (Jin et al., 1996), Al-Ti , Al-Nb (Thieme et al., 1993) are some of the representative composites and thus the most thoroughly investigated metal-metal systems.
For the DMMC finely dispersed with the second metal, the usual rule of mixture (ROM) is not applicable for characterizing their mechanical properties, since the experimentally measured tensile properties deviate strongly from the ROM (see e.g. Thieme et al., 1993;Russell et al., 1999). Besides the work-hardening effect caused by high degree of cold plastic deformation during composite fabrication, the other reason for the increase in tensile strength is generally believed to be resulted from the texture effect of the reinforcement and matrix metals, especially the textures in reinforcement metal (see e.g. Bevk et al., 1978;Russell et al., 1995Russell et al., , 1999. For example, in the Cu-X deformation processed composites (X refers to the bcc transition metal second phase insoluble in Cu), the second phase typically forms a h111i fiber texture often accompanied by a h001i fiber texture, two of the four h111i directions (the ½1 1 11 and the ½ 1 1 1 11 are positioned perpendicular to the center line and thus cannot slip. All slip is limited to the remaining two h111i directions (the ½ 1 1 11 and the [111] which lie opposite one another across the specimen center line, thus limiting the bcc phase to plane strain. The fcc Cu matrix, however, can readily deform axisymmetrically in either the h111i or h001i fiber texture, since these orientations possess three and four slip directions, respectively, to accommodate plastic flow. For cold-swaged Al-Ti  and Al-Mg composites (Xu et al., 1999) the microstructures are quite similar to that of Cu-X DMMC's. The h10 1 1 0i fiber texture was measured in the hcp Ti phase of the Al-Ti DMMC's, and no clear texture was observed in the hcp Mg second phase of the Al-Mg specimens.
The metal-metal composites in the form of wire could be obtained by processing techniques, such as drawing or swaying, etc., and presently there are many reports on the fabricating processes, microstructures, mechanical properties and strengthening mechanism for this kind of MMC (e.g. Thieme et al., 1993;Snoeck et al., 1998;Russell et al., 1999;Hong and Hill, 2000b;Chung et al., 2001). Concerning the sheet metal-metal composites, however, there is little appeared associating with the rolling technology. For pure fcc metals or alloys, their rolling textural characteristics have been well documented (e.g. Hu and Cline, 1988). However, for two-phase immiscible metal systems or large metal particulate reinforced metal-metal composites, there is a requirement to understand the textural development process in matrix or how they are affected with the existence of the second reinforcement metal during cold working process or recrystallization.
Therefore, in this study the rolling processing technique for manufacturing the MMCs sheet is briefly introduced and the emphasis is focused on the investigation of the deformation textural evolution of the matrix aluminum for the rolling processed two-phase Nb/Al metal-metal composite at stages of extrusion and cold rolling.

EXPERIMENTAL
In this study the typical powder metallurgy method was employed to fabricate the starting billets of DMMCs at first and the hot extrusion and cold rolling technologies were then used to produce MMCs sheet.
The raw materials are Al and Nb powders. The matrix Al powder has a mean particle size of 45 mm and a purity of 99.7%, and the Nb particulate powder used as reinforcement possess an average size of 300 mm (99.9% purity). The Al and Nb powders (90 : 10 and 80 : 20, in volume percentage) were mechanically mixed with a V-type blending machine for 10 h. After being consolidated at room temperature with a pressure of 700 MPa for 1 min, the cylindrical composite billets of 40 mm in diameter were extruded into 10 Â 4 mm slab composites at 773 K with an extrusion ratio of 31 : 1. The multipass cold rolling process was finally used to produce composite plate at room temperature in a two-roller mill.
Texture measurement (specimen's central layer) of the composites was conducted on a fully automatic X-ray diffractometer (RINT-2000, Japan). The texture specimens were prepared by mechanical grinding and then electrolytically polished to a final thickness of about 0.15 mm. As the X-ray diffraction peaks of Al {111} and Nb {110} are totally overlapped, the planes of Al {200}, {220}, and {311} were selected to measure its complete pole figures by Schulz's reflection method combined with Deck's transmission method. The normalized pole figures data were obtained after compensating the absorption and defocusing and deduction background. The ODFs calculated by series expansion method (L max ¼ 22) were presented as constant '2 sections. Figure 1 shows the experimentally measured constant '2 ODF sections of the matrix aluminum for the extruded 10 and 20 vol.%Nb/Al metal-metal composites at 773 K.

RESULTS AND DISCUSSIONS
One can observe from Fig. 1(a) and (b) that there formed the hot-extrusion textures within the matrix aluminum of the Nb/Al composite at extruding temperature of 773 K. The extrusion texture can be described by superposition of orientations, whose ODFs in the Euler space are also depicted by the texture '' 0 -tube'' similar to the cold rolling textures in fcc metals. For the present flat-extruded composite specimen, this texture tube -fiber contains the components of B 0 -{011}h322i, S 0 -{124}h654i, and C 0 -{113}h332i, which is totally different from the typically extruded axial fiber textures in fcc metals resulted from axisymmetric extrusion deformation and similar to the result for flat extruded aluminum alloy reported by Inoue (2002).
It is well known that the textures in a cold-rolled fcc metals can be represented by a -fiber, i.e. B-{011}h211i, S-{123}h634i and C-{112}h111i, which is produced by the plane strain compressive deformation during cold rolling process. The specimens under the present extrusion condition will be co-compressed or contracted in the normal directions of the flat plane, while elongated along the longitudinal or extruding direction. In such a case, the deformation state is close to the plane strain compression behavior without widening, which would lead to the 0 -fiber texture and also slightly deviate from the normal -fiber. For both the 10 and 20 vol.%Nb/Al composites, the similar texture components were observed existing in the matrix Al and only the difference in the orientation density could be noticed. The component B 0 {011}h322i appeared in the composite is very close to the Brass component {011}h211i with 8 mutual rotation about the normal direction of {011} plane. As for the other components, S 0 and C 0 , they both are not far from the standard orientations of the S and C components. One can also observe that there is a weakening tendency with increasing the volume percentage of the reinforcement Nb. Besides the 0 -fiber, there were no other strong texture components within the extruded matrix aluminum of the composite.
As we know, the Cube recrystallization textures {001}h100i usually occurs during annealing for high or medium stacking fault energy fcc metals. Due to the absence of Cube texture in the present case, however, we can deduce that no recrystallization or dynamic recrystallization took place during hot-extrusion process.
The 10 and 20 vol.%Nb/Al metal-metal matrix composite sheets were obtained by multipass rolling at room temperature along the extruded direction and the textures of the matrix Al were revealed by representation of the ODFs shown in Figs. 2 and 3, respectively. From Fig. 2 it can be seen that their orientation densities tend to be enhanced with increasing the total cold rolling reductions, especially at lower volume content of the reinforcement (e.g. 10 vol.%Nb). On the other hand, the locations of the main texture components change with the strengthening of the -fiber texture.  both 10 and 20 vol.%Nb/Al composites. As the CCR reached up to 80% or more, the unstable Goss moved towards the orientation B in the -fiber and a typical fcc cold rolling -fiber texture formed within the matrix Al of the metal-metal matrix composites. Although there is somewhat a subtle difference in the texture details for the two-phase metal-metal matrix composites, undoubtedly this arises from the presence of the second phase particulates in the matrix, which will affect the microplastic deformation of the matrix during the cold rolling process. That is to say, the second phase particulate, especially those large-sized particulate will give rise to heterogeneity of the deformation region that lead to their independent deformation and rotation of the grains around the particulate.
Concerning the plastic deformation of the ceramic particulate reinforced aluminum composites (SiC/Al), Humphreys et al. (1990Humphreys et al. ( , 1991 emphasized that the rotations within the deformation zones in polycrystals are related to both strain and the slip systems active in the matrix. For the present composites, the Nb particulate is much harder than Al particles and also has a relatively rather large size ($ 300 mm) such that the bound zones can be formed. In addition to the formation of deformation zones, large particles are also likely to disturb the slip pattern in the matrix, resulting in a lensoid distortion of the substructure around the particle. A lensoid distortion of the substructure generally originates from the inhomogeneous deformation in matrix by multislip around the large particles with the periphery matrix shape changed in the lens-like form, which is caused by the crystal rotation around the particles. Examples of the deformation features associated with SiCp in low volume fraction aluminum composites were experimentally provided by Humphreys (1991). As a model for metal-metal composites shown in Fig. 8, larger particles can produce distorted zones and they can readily be restricted to deform around the particles. Therefore the texture in particles containing two-phase composites is not fully developed and sometimes deviates from the position of the typical orientations.
The , and orientation lines depicted in Figs. 4-7 clearly featured the texture evolution during processing of hot extrusion and cold rolling deformation for the Nb/Al composites. As the locations of the -fiber orientation lines in extruded 10 and 20 vol.%Nb/Al composites are almost the same, they were plotted together in Fig. 6(b). The -fibers at various processing states in Fig. 4(a) and (b) show that the texture orientation distribution has a scattering and no other but only B 0 {011}h322i is remarkable at plate-extruded state. By increasing the cold rolling reduction, the B 0 {011}h322i gradually transforms into stable B{011}h211i and there are random components within it. A minor {011}h100i component in rolled specimen comes from the shear strain exerted during cold rolling deformation due to the existence of Nb particulates.
-fiber line refers to the all orientations corresponding to '1 ¼ 90 , '2 ¼ 45 and ¼ 0 $ 90 . As shown in Fig. 5, it is obvious that only near Copper C 0 {113}h332i is dominant and stable before and after cold rolling deformation, which resulted from the lensoid distortion of the slip planes and substructure around the particulates.

CONCLUSIONS
The texture evolution of the 10 and 20 vol.%Nb/Al metal-metal matrix composite were studied by analyzing the experimentally measured ODFs at states of extrusion and cold rolling deformation. The following conclusions can be drawn: 1. The hot-extruded texture of the matrix aluminum in Nb/Al metal-metal matrix composites can be represented by a -fiber containing the components of  B 0 -{011}h322i, S 0 -{124}h654i, and C 0 -{113}h332i, which is close to the -fiber in a cold-rolled pure fcc metal. 2. When this composite was subjected to cold rolling deformation, the -fiber texture in the matrix Al will be enhanced where the orientation components consist of B{011}h211i, S 0 -{124}h654i, and C 0 -{113}h332i.  3. By increasing the cold rolling reduction, the textures become stronger and get more stable at B, S 0 and C 0 . The second-phase particulate existing in the composites weaken the deformation texture and this effect depends mainly on the volume fraction of the particulate employed. 4. Large particulates in the metal-metal composites strongly affect the plastic deformation of the matrix Al in such a way that they separate the matrix into deformation zones and bound zones around the particulates. The texture deviating from the normal -fiber (B-{011}h211i, S-{123}h634i, and C-{112}h111i) will be produced in the inhomogeneous deformed microstructures.