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Microstructure and Mechanical Properties of Two Continuous-Fiber- Reinforced Zr-Based Amorphous Alloy Composites Fabricated By Liquid Pressing Process

July 17, 2008

By Lee, Sang-Bok Lee, Sang-Kwan; Lee, Sunghak; Kim, Nack J

The feasibility to fabricate the tungsten and STS-fiber- reinforced amorphous alloy matrix composites was verified by analyses of the thermal stress and cooling behavior between matrix and metallic fibers. Approximately 50 to 65 vol pct of fibers were homogeneously distributed inside the amorphous matrix, although the matrix of the STS-fiber-reinforced composite contained a small amount of crystalline phases. The compressive test results indicated that the tungsten-fiber-reinforced composite was not fractured at one time after reaching the maximum compressive strength of 2060 MPa, but showed some ductility as the compressive load was sustained by fibers. The STS-fiber-reinforced composite showed the maximum strength of about 1050 MPa, and its strength maintained over 800 MPa until reaching the strain of 40 pct. Both tungsten and STS fibers favorably affected the strength and ductility of the composites by interrupting the propagation of shear bands formed in the amorphous matrix, by dispersing the stress applied to the matrix, and by promoting deformation mechanisms such as fiber buckling. These findings confirmed the possibility to apply the continuous-fiber- reinforced amorphous alloy matrix composites to structural materials requiring excellent properties. DOI: 10.1007/s11661-008-9477-6

(c) The Minerals, Metals & Materials Society and ASM International 2008

I. INTRODUCTION

UNLIKE conventional crystalline alloys, amorphous alloys having excellent strength, stiffness, wear resistance, and corrosion resistance are vulnerable to abrupt fracture due to localized shear band formation.[1-9] In order to expand the applications of amorphous alloys to functional materials as well as structural materials, ways to overcome the shortcomings of poor ductility and fracture toughness are needed. Active studies on developing composites in which secondary phases or reinforcements are dispersed in an amorphous alloy matrix have been conducted. The ways to fabricate amorphous alloy matrix composites include the one in which amorphous alloys are partially crystallized to disperse nanocrystallines,[10,11] and the one in which crystalline particles are added to the amorphous melt.[12] Also included is the way to cast reinforcing fibers and amorphous alloys at the same time,[13,14] and the one in which dendritic crystalline phases are generated from the amorphous melt.[15]

When fabricating cast amorphous alloy matrix composites reinforced with continuous metallic fibers, it is critical to control reactions of fibers with the amorphous matrix and to develop the fabrication process technology with high reproductivity and reliability. This is because most metallic fibers (except refractory metal fibers) have high reactivity with the amorphous melt. The squeeze casting process, one of the conventional fabrication methods for composites, has merits such as high productivity and ease of near-net-shape fabrication, but has shortcomings of poor reliability, a requirement of high-pressure loading of 50 MPa or more in order to enhance the wettability between reinforcements and matrix, and easy collapse of preform patterns. It also experiences difficulties in fabricating thin plates because the flow resistance might abruptly increase as the partial crystallization occurs during solidification of the amorphous melt. To effectively fabricate amorphous alloy matrix composites reinforced with continuous fibers, it is, thus, necessary to introduce new-concept fabricating processes, one of which is a liquid pressing process,[16,17] using low pressure near the theoretically required minimum loading pressure. In this process, thin-plate-type composites can be readily fabricated, and the crystallization of the amorphous matrix can be prevented or minimized by rapid cooling of the melt.

In the present article, the feasibility for fabricating sound amorphous composites was predicted by investigating residual thermal stresses due to the difference in thermal expansion coefficients between the amorphous matrix and continuous fibers. Tungsten and STS 304 stainless steel continuous fibers were used for reinforcements; while the former with high strength and melting point shows excellent reaction resistance to the melted matrix, the latter shows high reactivity with the melted rmitrix, despite its excellence in corrosion resistance and ductility. Amorphous alloy composites, whose matrix was a Zr-based amorphous alloy, were fabricated by the liquid pressing process. Microstructures of the fabricated composites were analyzed, and their mechanical properties were evaluated by conducting compressive tests.

II. EXPERIMENTAL

The Zr-based amorphous alloy used for the fabrication of amorphous alloy composites reinforced with continuous fibers was an “LM1″ alloy, which is a commercial brand name of Liquidmetal Technologies (Lake Forest, CA). Its chemical composition is Zr^sub 41.2^Ti^sub 13.8^CU^sub 12.5^Ni^sub 10.0^Be^sub 22.5^ (at. pct).[18] Tungsten continuous fibers and STS 304 stainless steel continuous fibers were used as reinforcements. Scanning electron micrographs of the Zr-based amorphous master alloy and continuous fibers are presented in Figures 1(a) through (c). Diameters of tungsten and STS fibers are about 100 and 110 [mu]m, respectively (Figures 1(a) and (b)). In the amorphous master alloy, fine polygonal crystalline particles sized by approximately 3 to 10 [mu]m are distributed in the amorphous matrix (Figure 1(c)). These crystalline phases are identified to be fee phases (lattice parameter: 1.185 nm),[19] and their volume fraction is approximately 3 to 4 pct. Representative physical properties and room-temperature tensile properties of the amorphous master alloy and continuous fibers are summarized in Table I.

Figure 2 illustrates a schematic diagram of the liquid pressing process[20] used to fabricate amorphous alloy matrix composites. The mold interior is sized by 60 x 60 x 6 mm. A preform of continuous fibers and amorphous master alloy plates were inserted into the mold, degassed, and vacuumed. The mold was heated to 870 [degrees]C, held for 5 minutes, and then pressed under a pressure of about 10 MPa. Pressing was accompanied with water cooling so that the solidified matrix could readily form amorphous phases.

The fabricated composites were sectioned, polished, and etched in a solution of 70 mL H2O, 25 g CrO^sub 3^, 20 mL HNO^sub 3^, and 2 mL HF for optical and scanning electron microscope (SEM) observations. Phases formed on the composites were analyzed by X-ray diffraction (XRD). The composites were machined into cylindrical specimens of 2phi x 3 mm, 4phi x 6 mm, and 5phi x 7.5 mm in size, and room- temperature compression tests were conducted on these specimens at a strain rate of 1.8 x 10^sup -4^ s^sup -1^. Compressively fractured specimens were observed by an SEM after the test.

III. RESULTS AND DISCUSSION

A. Solidification Behavior in Fabrication of Zr-Based Amorphous Alloy Composites

The cooling behavior of the Zr-based amorphous alloy was interpreted on the basis of the shape and size of the mold used. The software used for the calculation was ProCAST (ESI Group, France), which is a commercial solidification analysis program. The casting space of a mold with a thickness of 5 mm was designed to be sized by 60 x 60 x 6 mm. which was the same size as the experimentally fabricated composites. Figure 3 shows the results predicted from the interpretation of the cooling behavior. The cooling rate of the mold region in direct contact with the coolant water is calculated to be about 500 [degrees]C/s, while the rates of the side and the center regions of the composite are about 50 [degrees]C/s and 10 [degrees]C/ s, respectively. The cooling rate of the composite is slower than that of the mold because the composite is located at the center of the mold and because the thermal conductivity of the amorphous matrix is about 1/10 of general metals. Because the slowest cooling rate at the center of the composite is much greater than the critical cooling rale (approximately 1 [degrees]C/s to 2 [degrees]C/ s) of the Zr-based amorphous alloy, the fabrication of the Zr-based amorphous composites by the liquid pressing process can be confirmed.

B. Thermal Stress Analysis of Composites Reinforced with Tungsten and STS Continuous Fibers

One of the structural shortcomings of the fiber-reinforced composites is the residual stress between fibers and matrix. In the present article, the residual stresses of fibers and matrix were calculated by using the finite element method. Based on the calculated results, the presence or absence of defects inside the composites was investigated. The boundary conditions were set to be infinite in the fiber lengthwise, while fibers (diameter, approximately 50 to 55 [mu]m length, 500 [mu]m) were surrounded by the amorphous matrix (radial thickness, approximately 50 to 55 [mu]m). It was assumed that material properties were constant in the analyzed temperature range, and that the boundary between fibers and the matrix was completely bonded.

Figures 4(a) and (b) present the residual stress states at fiber/ matrix interfaces. In the tungsten-fiber-reinforced composite, because the thermal expansion coefficient of tungsten fibers is smaller than that of the matrix, the compressive stress applies in all the directions of fibers (Figure 4(a)). Because the thermal contraction of the matrix is disrupted by tungsten fibers, the compressive stress takes place in the radial direction of fibers at the fiber/matrix interface. In the hoop direction of fibers, the tensile stress starts applying, but is reduced further away from the interface. In the longitudinal direction of fibers, the stress is a tensile one, and is about the same as that in the hoop direction. It is almost constant at about 97 MPa, irrespective of the location. In the STS-fiber-reinforced composite, because the thermal expansion coefficient of STS fibers is larger than that of the matrix, the tensile stress applies in all the directions of fibers and in the radial direction of the matrix (Figure 4(b)). The compressive stress applies in the hoop direction of the matrix and in the longitudinal direction of fibers. The compressive stress in the longitudinal direction of fibers is calculated to be about 147 MPa. Because the multidirectional stresses of matrix and fibers in the tungsten- and STS-fiber-reinforced composites are much smaller than the strengths of the matrix and fibers, it can be predicted that no defects are formed inside the composites and at the fiber/matrix interfaces. C. Microstructure of Continuous-Fiber-Reinforced Composites

Figures 5(a) and (b) show XRD patterns of the two composites reinforced with tungsten and STS fibers, respectively. Typical halo patterns of amorphous alloys and peaks of continuous fibers are observed without peaks of other crystalline phases in both composites. This indicates that the composite matrices are mostly composed of amorphous phases.

Figures 6(a) through (d) are SEM micrographs of the two composites. In the tungsten-fiber-reinforced composite, tungsten continuous fibers are homogeneously distributed in the matrix, and the volume fraction of fibers is about 65 pct as analyzed from the cross-sectional picture of the composite (Figure 6(a)). Crystalline phases are rarely observed in the amorphous matrix, and defects formed by misinfiltration or reaction products formed by interfacial reaction at fiber/matrix interfaces are hardly found (Figure 6(b)). Figures 5(a) and 6(a) and (b) show a successful fabrication of the amorphous composite reinforced with tungsten fibers by the liquid pressing process. This is because tungsten fibers with high-melting points are thermally stable against the melted amorphous alloy. Because tungsten fibers are hardly diffused into the amorphous melt during the liquid pressing process, they hardly cause the composition changes of the amorphous matrix, and thus, the amorphous forming ability of the Zr-based amorphous alloy can be maintained in the composite matrix. Thermal deformation, which arises from the difference in thermal expansion coefficients between fibers and matrix, cannot be observed, as shown in Figure 6(a). This is consistent with the thermal stress analysis data of Figure 4(a).

In the STS-fiber-reinforced composite, STS continuous fibers are relatively homogeneously distributed in the matrix, and their volume fraction is about 50 pct (Figure 6(c)). Pores, defects, or reaction products are not found. The volume fraction in the surface region of the composite plate of 6 mm in thickness is somewhat lower than that of the center region, although the overall distribution is relatively homogeneous. In the matrix, dark and bright regions, together with fine polygonal crystalline particles, are observed (Figure 6(d)). These fine polygonal crystalline particles are sized by approximately 2 to 3 [mu]m, and their volume fraction is about 7 pct. The dark and bright regions might be formed by the partial crystallization according to the phase separation of the amorphous melt during solidification. The bright regions indicate amorphous phases, while the dark regions show dendritic crystalline phases. The volume fraction of these dendritic crystalline phases is about 50 pct. According to the XRD patterns of Figure 5(b), only typical halo patterns of amorphous phases and peaks of STS fibers are observed. This might be because peaks are not visible in XRD patterns when the volume fraction of crystalline phases is so small (less than several percentages) or when the phases are fine. As STS 304 fibers have lower melting points and are less thermally stable than tungsten fibers, some elements in STS fibers are diffused when they come in contact with the amorphous melt of 870 [degrees]C, thereby leading to the composition change of the melt. Because the amorphous forming ability is highly sensitive to alloy composition changes,[23-25] the changes in the matrix composition result in the reduced amorphous forming ability and the formation of dendritic crystalline phases. When atoms such as Fe infiltrate into the matrix by diffusion, the critical cooling rate of the matrix becomes faster than the cooling rate during the composite fabrication, thereby causing the partial crystallization in the matrix. Therefore, in order to maintain the matrix as amorphous phases in the STS-fiber- reinforced composite, it is required either to use the faster cooling rate during the composite fabrication or to control the diffusion or reaction at fiber/matrix interfaces.

Though the composites fabricated by the liquid pressing process contain a small amount of crystalline phases, they do not have defects or pores formed by misinfiltration. As the hydrostatically applied pressure in the liquid pressing process of this article overrides the theoretical pressure required for infiltration, the amorphous melt sufficiently infiltrates into the fiber preform, and pores formed by the solidification contraction are eliminated. This can also be applied to the fabrication of large-scale amorphous composites reinforced with two-or three-dimensional continuous fibers as well as one-dimensional fibers.

D. Compressive Properties of Continuous-Fiber-Reinforced Composites

Figures 7(a) and (b) and 8(a) and (b) show compressive stress- strain curves of the 2phi x 3-mm-sized compressive specimen and SEM micrographs of the fractured compressive specimen, respectively, of the two composites. In the tungsten-fiber-reinforced composite, fracture does not take place at one time after reaching the maximum strength of 2060 MPa, but proceeds as the loading is sustained by continuous fibers (Figure 7(a)). This is one of the typical characteristics of continuous-fiber-reinforced composites.[26] In the side of the compressive specimen, some buckling is observed, and tungsten fibers are bent while sustaining the loading (Figure 8(a)). Shear bands are formed in the maximum shear stress direction in the matrix, cracks initiate, the crack propagation is interrupted by continuous fibers, fiber/matrix interfaces are separated, and the fracture proceeds. In the STS-fiber-reinforced composite, the maximum strength is 1050 MPa, which is lower than that of the tungsten-fiber-rein forced composite (Figure 7(b)). Fracture does not take place until reaching the strain of about 40 pct, while keeping the strength over 800 MPa. As in typical amorphous alloys, the compressively fractured specimen shows the progression of fracture at the maximum shear stress direction (about a 45 deg angle against the compressive loading direction) (Figure 8(b)). Figure 9 shows an SEM micrograph of the cross-sectional area of the compressively fractured specimen of the STS-fiber-reinforced composite. Many cracks initiate in the matrix around the main shear crack propagated at a 45 deg angle against the loading direction, and STS fibers are bent and unfractured. According to this deformation of STS fibers, the STS-fiber-reinforced composite shows the large ductility, while maintaining the strength over a certain level.

The reasons behind the higher maximum compressive strength of the tungsten-fiber-reinforced composite (2060 MPa) than that of the amorphous master alloy (1920 MPa) include the following. (1) Tungsten fibers have higher strength than the amorphous master alloy (Table I). (2) The residual tensile stress of about 100 MPa present in the matrix along the longitudinal direction of fibers plays a buffering role of the compressive stress during the compression test (Figure 4(a)). In the STS-fiber-reinforced composite, the maximum compressive strength is 1050 MPa, which is still lower than the 1300 MPa expected from the rule of mixtures, despite considering that the maximum strength of STS fibers is lower by one-third than that of the amorphous master alloy. This is associated with the presence of the residual compressive stress of about 150 MPa in the matrix along the longitudinal direction of fibers, together with the fiber buckling that might negatively influence the maximum compressive strength, although the detailed investigation is not conducted in the present study. The residual compressive stress works for additional compressive loading during the compressive test and for initiating the fracture at the lower strength than the matrix’s maximum strength. These results indicate that the residual thermal stresses and the fiber buckling formed in the cast composites significantly affect mechanical properties of the composites, and that the thermal stress analysis data of this article should be seriously considered when fabricating the cast composites.

In order to investigate the specimen size effect on compressive properties, the compressive specimens of 4phi x 6 mm and 5phi x 7.5 mm in size were prepared from the STS-fiber-reinforced composites, tested, and compared with the case of the 2phi x 3-mm- sized specimen. An optical micrograph of the cross-sectional area of the 4phi x 6-mm-sized specimen is shown in Figure 10. The volume fraction of fibers in the surface area is somewhat lower than that in the center region. The fiber volume fractions were measured, and the results are 55 and 53 pct for the 4phi x 6-mm-sized and 5phi x 7.5-mm-sized specimens, respectively. The maximum strengths for the 4phi x 6-mm-sized and 5phi x 7.5-mm-sized specimens were measured after the three specimens of each size condition were tested, and the average strengths are 1101 and 1040 MPa, respectively. Here, fracture did not take place until reaching the strain of about 40 pct, as in the 4phi x 6-mm-sized specimen. The maximum strengths of the 2phi x 3-mm-sized, 4phi x 6-mm-sized, and 5phi x 7.5-mm-sized specimens are 1050, 1101, and 1040 MPa, respectively. Considering that the 2phi x 3-mm-sized specimen contains more fibers than the 4phi x 6-mm-sized or 5phi x 7.5- mm-sized specimen, the maximum strengths of the three sized specimens might be similar. These results imply that the compressive specimen size hardly affects the compressive properties of the STS- fiber-reinforced composites. The cross-sectional areas of the compressivcly fractured specimen of the two 4phi x 6-mm-sized specimens are shown in Figure 11. Cracks initiate in the matrix around the main shear crack propagated at about a 40 deg angle against the loading direction, and STS fibers are bent and unfractured. A number of fiber/matrix interfacial separations and curved fibers are also observed around cracked areas. This is because fibers first form shear bands at the shear direction and show a considerable amount of ductility, whereas the amorphous matrix shows the abrupt fracture along shear bands. The present results in which amorphous alloy matrix composites reinforced with metallic continuous fibers were fabricated by the liquid pressing process not only help to understand the formation process of microstructures better, but also confirm the possibility to eventually overcome shortcomings of low ductility while maintaining advantages of amorphous alloys. When amorphous alloys are reinforced with metallic continuous fibers by the liquid pressing process, composites with excellent interfacial bonding between matrix and fibers can be successfully fabricated without pores or defects. Fibers interrupt the propagation of shear bands formed in the amorphous matrix, disperse the stress applied to the matrix, and promote deformation mechanisms such as fiber buckling, thereby favorably affecting the strength and ductility of the composites. The tungsten-fiber-reinforced composite shows an increase in the compressive strength and ductility over the amorphous alloy, while the STS-fiber-reinforced composite shows a dramatic enhancement in ductility up to 40 pct despite some reduction in strength. Because of the excellence of these composites in high strength, ductility, and fracture toughness, new possibilities are open to be applied to structural materials requiring excellent properties. In order to make further improvements in microstructures and properties of the amorphous alloy matrix composites, continuous studies are required to select and develop new fibers and matrix alloys, to establish liquid pressing process parameters, and to elucidate mechanisms involved in improving strength, ductility, and fracture toughness.

IV. CONCLUSIONS

In this study, Zr-based amorphous alloy matrix composites reinforced with metallic continuous fibers were fabricated by the liquid pressing process, and their microstructures and compressive properties were investigated to reach following conclusions.

1. The feasibility to fabricate amorphous alloy matrix composites without forming pores or defects due to thermal deformation was verified in this article, based on the analyses of the thermal stress and cooling behavior between matrix and metallic fibers.

2. Using the liquid pressing process, Zr-based amorphous alloy matrix composites reinforced with metallic continuous fibers were successfully fabricated without pores or defects, and showed excellent fiber/matrix interfaces. About 65 vol pct of tungsten fibers were homogeneously distributed inside the amorphous matrix in the tungsten-fiber-reinforced composite. In the STS-fiber- reinforced composite, about 50 vol. pct of STS fibers were distributed in the amorphous matrix containing a small amount of crystalline phases.

3. According to the compressive test results, the tungsten-fiber- reinforced composite was not fractured at one time after reaching the maximum compressive strength of 2060 MPa, but showed some elongation as the compressive load was sustained by tungsten fibers. The STS-fiber-reinforced composite showed the maximum strength of about 1050 MPa, and its strength maintained over 800 MPa until reaching the strain of 40 pct.

4. Both tungsten and STS fibers favorably affected the strength and ductility of the composites by interrupting the propagation of shear bands formed in the amorphous matrix, by dispersing the stress applied to the matrix, and by promoting deformation mechanisms such as fiber buckling. These findings confirmed the possibility to apply them to structural materials requiring excellent properties.

ACKNOWLEDGMENTS

This work was supported by the fundamental research funds of Korea Institute of Machinery and Materials “Development of Amorphous Matrix Composites Reinforced with Ductile Metallic Fiber” and by the National Research Laboratory Program (Grant No. M10400000361- 06J0000-36110) funded by the Korea Science and Engineering Foundation (KOSEF. The authors are grateful to Mr. Kyuhong Lee, Postech, for his help on the microstructural analysis of the fabricated composites.

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SANG-BOK LEE and SANG-KWAN LEE, Senior Researchers, are with the Composile Materials Laboratory. Korea Institute of Machinery and Materials. Changwon 641-010. Korea. Contact e-mail: shlee@postech.ac.kr SUNGHAK LEE and NACK J. KIM. Professors. Center for Advanced Aerospace Materials, and Materials Science and Engineering, arc with Pohang University of Science and Technology. Pohang 790-784. Korea.

Manuscript submitted April 12, 2007.

Article published onlined February 21, 2008

Copyright Minerals, Metals & Materials Society Apr 2008

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