Fracture mechanisms of a 2124 aluminum

Fracture Mechanisms of a 2124 Aluminum Matrix Composite Reinforced with SiC Whiskers

YOUNG-HWAN KIM, SUNGHAK LEE, and NACK J. KIM

This study was aimed at investigating the effects of microstructure on the fracture behavior of a 2124 aluminum composite reinforced with SiC whiskers. Particular emphasis was placed on the role of matrix intermetallic particles, inhomogeneous distribution of whiskers, and whisker breakage in the fracture process. Various tests were conducted on the composite to identify the micromechanical processes that were involved in microvoid or microcrack formation. Detailed microstructural analyses showed that the aluminum matrix contained a significant amount of coarse manganese-containing particles of various sizes which could have been formed during composite processing. In situ scanning electron microscope (SEM) fracture study of the crack initiation and propagation processes clearly showed that these coarse particles fractured prior to matrix/whisker decohesion or whisker breakage, suggesting that the manganese-containing particles significantly accelerated crack initiation in the 2124 A1-SiCw composite. For a better matrix alloy for use in the composite, it is suggested that microalloying elements must be monitored to prevent the formation of the coarse intermetallic particles.

I. INTRODUCTION

SIC whisker reinforced aluminum (AI-SiCw) composites are generally known to exhibit high elastic mod- ulus and strength combined with light weight, providing a potentially wide range of applications, especially in the aerospace industry, tl,z,31 However, these gains are offset by a dramatic loss in ductility and fracture toughness. Such unfavorable fracture properties severely restrict a major development of A1-SiCw composites for structural applications. A fundamental understanding of the evolution of composite microstructure and its effects on the mechanisms of deformation and failure is essential to de- veloping better composite materials with improved fracture resistance.

A number of studies [2,4-61 have been reported con- cerning the mechanical properties and the fracture toughness of A1-SiCw composites. Tensile elongation of AI-SiCw composites is only 3 to 4 pct and their fracture

1/2 toughness is about 20 MPa. m at best, although there have been considerable efforts to improve these properties. One of the important topics relevant to these efforts is the study of the micromechanisms of fracture that might provide a means for rationalizing the microstructural origin of brittleness. As for 2124 A1-SiCw composites processed by hot pressing above the matrix solidus temperature followed by hot extrusion, major reasons for the brittleness have been suggested as follows: (1) void initiation at whisker ends due to severe stress concentration and resultant localized plastic f l ow; [7'8'9] (2) decohesion of whisker/matrix interface caused by separation of interface oxide layers; IT,l~ and (3) cracking of load carrying SiC whiskers which were damaged or degraded during composite processing.[12.~31 A few recent w o r k s I14-18] have also emphasized the effect

YOUNG-HWAN KIM, Graduate Student, and SUNGHAK LEE and NACK J. KIM, Associate Professors, are with the Center for Advanced Aerospace Materials, Pohang Institute of Science and Technology, Pohang, 790-600 Korea.

Manuscript submitted July 15, 1991.

of matrix microstructure on the deformation and failure behavior of these composites. However, the role of coarse matrix intermetallic particles in fracture processes has not yet been investigated in detail, although their presence in the microstructure on the powder metallurgy (PM) processed 2124 A1-SiCw composites has been reported in several studies. 17,1~

Most of the previous investigations in this field are based on the fine scale transmission electron microscope (TEM) observation of damaged microstructures and, ac- cordingly, provide no further information on the critical events in fracture process and interrelation between two or more different microfracture mechanisms. Recently, the techniques of dynamic fracture observation, which have been used in a few recent fracture studies for other A1-SiC composite systems, ttg,z~ provide more detailed and instructive information on the fracture initiation and propagation processes. Thus, in this study, the fracture processes have been analyzed by means of the scanning electron microscope (SEM) in situ fracture observation technique in order to elucidate a fundamental understanding of the relationships between the microfracture processes, the controlling microstructural features, and the associated composite processing methods for the 2124 A1-SiCw composite processed by a PM route. By using this technique, the fracture processes which include microcrack initiation and shear band formation within matrix can be scrutinized.

It has been reported that in unreinforced aluminum alloys, the coarse intermetallic particles act as major void initiation sites, since they cleave at early stages of plastic deformation of the alloys, tzu Once initiated, voids generally grow by large plastic strain in the adjacent ductile matrix, leading to a dimpled fracture surface containing the fractured particles. However, the effect of the matrix intermetallic particles on the fracture behavior of the A1-SiC composites has remained unexplored in spite of its crucial importance in the toughness improvement. Therefore, in the present investigation, the role of the matrix intermetallic particle in the fracture initiation processes of the 2124 A1-SiCw composite, which has been

METALLURGICAL TRANSACTIONS A VOLUME 23A, SEPTEMBER 1992--2589

underestimated until now, was brought into focus. Based on direct observation of the fracture process, effect of these particles in a whisker reinforced composite was compared to that in an unreinforced alloy in order to elucidate the changes in their specific role in the fracture process due to the presence of reinforcing whiskers. Furthermore, the importance of whisker cracking in the fracture process was discussed in detail. Finally, the correlation between the formation mechanism of the coarse intermetallic particles and the processing method was discussed, and then a possible way to improve ductility and toughness of the A1-SiCw composite was suggested.

I I . EXPERIMENTAL

The material used in this study was a 2124 AI-SiCw composite which was manufactured by a powder metallurgy technique and contained 15 vol pet reinforcing SiC whiskers. An unreinforced 2124 aluminum alloy, processed by the same PM route, was also studied as a baseline material for comparison purposes. These materials were purchased from Advanced Composite Materials Corporation, Greer, SC, in the form of a 1.3 • 12.7 cm extruded rectangular bar. The samples were so- lutionized at 504 ~ water quenched, and aged at 177 ~ for 4 hours (peak aging conditionl~4J).

The evolution of microstructure in response to the processing route and aging treatment was thoroughly char- acterized. Microstructures were analyzed using an SEM and a TEM. Thin foil TEM specimens were prepared by sectioning and mechanically polishing to a thickness of about 80 p,m. Discs of 3-mm diameter were then ion- milled to perforation using a liquid nitrogen cold stage, and the thin foils were examined with a TEM operated at 200 KV.

Tensile tests were performed using longitudinally ori- ented specimens at room temperature. Threaded end tensile specimens were used with a 25.4-mm gage length and a 6.35-mm gage diameter. The stress-strain behavior was determined in an Instron test machine using a con- stant crosshead deflection rate of 0.5 mm/min . The fractured tensile specimens were sectioned parallel to the tensile axis, polished, and examined in an SEM. Frac- ture surfaces were also analyzed by scanning electron microscopy and Auger electron spectroscopy (AES).

In conjunction with fractography, an in situ SEM fracture test 122,23~ of a small double cantilever beam (DCB) specimen was conducted to reveal the microstructural factors controlling the fracture behavior in the A1-SiCw composite. The specially designed wedge-loading stage shown in Figure l(a) was inserted into the vacuum chamber of an SEM, thereby replacing the standard SEM sample stage. A thin DCB specimen (Figure l(b)) was metallographically polished and mounted on the stage. A wedge was driven into the notch of the specimen to initiate and propagate a crack. In order to prevent the specimen from buckling or twisting during loading, the wedge was held in a guide with two adjustable ledges. Precise control of the loading ann under high torque could be achieved by using an external gear system. The load applied to the specimen was monitored by a load cell. After in situ fracture tests, fracture surfaces of the DCB

(a)

(b)

Fig. l - (a) The wedge loading stage used for in situ fracture observation, which was inserted into the vacuum chamber of an SEM replacing the standard sample stage. (b) The shape and dimensions of a thin double cantilever beam specimen (unit: mm).

specimens were examined to compare the fracture behavior observed on the polished surface with that of the interior area.

I I I . R E S U L T S

A. Microstructure

Figure 2 is a typical microstructure of the 2124-T6 aluminum composite reinforced with 15 vol pct SiC whiskers. SiC whiskers are preferentially aligned along the extrusion direction. Whisker-depleted regions whose widths are on the order of 10 to 4 0 / x m are commonly observed in the plane longitudinal to the extrusion direction, while their thicknesses are reduced to 5 to 10 ~m, as can be seen in the transverse and short- transverse planes. This large scale inhomogeneity is one of the important microstructural factors which strongly

2590--VOLUME 23A, SEPTEMBER 1992 METALLURGICAL TRANSACTIONS A

affects the mechanical properties of the composite, 115,2~ although it was not treated in detail in this study. A TEM micrograph of the matrix is shown in Figure 3. The grain size of the matrix ranges from 0.5 to 1 /xm, and most whiskers are observed to be connected with grain boundaries, indicating that whiskers had quite a desired effect of pinning the grain boundaries. Other fine precipitates, e.g., S ' phase (A12MgCu), were also dispersed throughout the matrix.

In addition to these fine precipitates, coarse and irregularly shaped particles are present in the composite material studied, as shown in Figure 4(a), although PM processing usually results in refinement of intermetallic

particles. They show various sizes ranging up to 20/zm. Even though their distribution is not distinctive, it can be noted that very coarse particles are mainly located along the boundaries between whisker-rich and whisker- depleted regions (Figure 4(a)) and that relatively finer particles with few micron sizes reside within whisker- rich regions. In what follows, these are referred to as coarse and fine particles, respectively. Energy dispersive

Fig. 2--Typical microstructure of the 2124-T6 aluminum composite reinforced with 15 vol pct SiC whiskers. (b)

Fig. 3 - - T E M micrograph of the 2124-T6 aluminum composite reinforced with 15 vol pct SiC whiskers.

Fig. 4 - - ( a ) SEM micrograph of the 2124-T6 aluminum composite reinforced with 15 vol pct SiC whiskers, showing coarse and irregularly shaped particles. (b) EDS spectra showing excess manganese content on these particles. (c) SEM micrograph of unreinforced 2124 alloy showing the particles identical to those in the reinforced composite.


spectroscopy spectra obtained from these particles indicate the presence of excess manganese in the coarse particles, as shown in Figure 4(b), suggesting that their nature is probably A120Mn3Cu2.17'141 Figure 4(c) is an SEM micrograph of the unreinforced 2124 aluminum alloy, showing manganese-containing particles of various sizes. In this case, they are more spherical and smaller than those in the reinforced composite. Their distribution is somewhat homogeneous without any preferential loca- tion throughout the matrix.

B. Mechanical Properties

Room-temperature tensile properties are shown in Table I. The ultimate tensile and yield strengths of the 2124 A1 composite reinforced with 15 vol pct SiC whiskers are significantly higher than those of the unreinforced alloy, while the composite exhibits lower ductility. Figure 5 shows a fracture surface of the composite tensile specimen, showing mostly a transgranular ductile rupture mode, although this composite exhibits limited ductility on a macroscopic scale. SiC whiskers are often found inside dimples, suggesting that the whiskers are preferential sites for void nucleation. The observation of stereo pairs revealed the dimples to be shallow, which is consistent with the low plasticity level. The dimple sizes of the composite are generally between 1 and

Table I. Room-Temperature Tensile Properties of the 2124 T6 Ai-15 Volume Percent

SiCw Composite and the Unreinforced Matrix Alloy

Material

Tensile Yield Tensile Strength Strength Elongation (MPa) (MPa) (Pct)

2124 T6 Al-15 vol pct 704 440 3.7 SiCw

2124 T6 A1 512 378 7.0

2 p,m, which is similar to the measured value of the interspacing between SiC whiskers, 2 .2/xm. This effect can be directly related to the toughness results, t6~ since the whiskers play an important role in initiating fracture. A few isolated cleavage-like flat fracture facets together with internal secondary microcracks are also detected, as indicated by the arrow in Figure 5. The fraction of the cleavage facets covers approximately 10 pct of the total fracture surface area. These facets were analyzed by EDS and AES to contain a greater amount of manganese than the matrix, suggesting that the brittle fracture mode probably occurs at coarse manganese-containing intermetallic particles.

Figure 6 shows the composite microstructure beneath the fracture surface of a tensile specimen. Voids appear to nucleate at whisker ends by the decohesion of whisker/ matrix interfaces when the whiskers are roughly parallel to the tensile axis. Once nucleated, voids grow mainly toward the matrix in the direction of the tensile axis because of the considerable plastic deformation occurring in the matrix.

C. In Situ Fracture Observation

A series of SEM micrographs (Figures 7(a) through 7(c)) shows the crack initiation processes near a blunt notch tip of the 2124 Al-15 vol pct SiCw composite. Figure 7(a) illustrates the microstructure near the notch tip whose direction is perpendicular to the extrusion direction. At the initial loading stage, coarse manganese- containing particles were cleaved first to form microcracks without any severe deformation of the matrix (Figure 7(b)). These initial microcracks became larger in the direction normal to the wedge loading as the sur- rounding aluminum matrix underwent plastic deformation, but they did not produce long discontinuous cracks because of the adjacent ductile matrix material. Instead, the opening action of the microcrack was accompanied by a locally high stress concentration within the near tip region where an intense strain localization associated with

Fig. 5 - SEM fractograph of the 2124-T6 aluminum composite reinforced with 15 vol pct SiC whiskers tested at room temperature, showing fibrous fracture and the presence of whiskers inside dimples. As indicated by the arrows, a few isolated fiat facets with secondary microcracks are also found.

Fig. 6 - - S E M micrograph of the round tensile specimen sectioned parallel to the tensile axis to reveal the microstructure beneath the fracture surface. The material is 2124-T6 a luminum reinforced with 15 vol pct SiC whiskers fractured at 25 ~ The tensile axis is vertical in the micrographs.


Fig. 7 - - ( a ) through (c) A series of SEM micrographs near a blunt notch tip in the 2124-T6 aluminum composite reinforced with 15 vol pct SiC whiskers, showing crack initiation processes.


the nearby whisker ends occurred, as shown by the arrow labeled C in Figure 7(c). Voids were rarely found on the surface due to severe shear deformation of the matrix, whereas the fracture surface of tensile specimens revealed mainly dimpled ductile fracture mode caused by void initiation and growth. Figure 7(c) also shows the shear bands formed in an area of high stress concentration adjacent to the notch tip. These shear bands were highly localized and observed to be initiated mainly at the microcracked finer manganese-containing particles and whisker ends, labeled F and W in Figure 7(c). Re- sidual pores (R in Figure 7(c)) associated with a couple of unbound whiskers also acted as initiation sites of the strain localization, but their contribution might be ig- nored due to their rareness in the whole microstructure. These localized shear bands were then connected to each other as the load was increased. Thus, fracture took place in a zigzag pattern along lines of intense localized shear.

In the case of the unreinforced alloy whose microstructure also contains manganese-containing particles in the matrix, these coarse particles fractured again at the early stage of loading and shear bands were preferentially formed along these cracked particles (Figure 8). This indicates, as expected, that the manganese-containing particles can significantly reduce the ductility of the unreinforced alloy. Thus, it is evident that the low ductility of the PM processed AI-SiC~ composite is originated, in large part, from the inherently brittle nature of the coarse matrix constituent particles. It has been well known that the PM processed 2XXX aluminum alloys generally display strength-toughness combinations superior to those shown by the conventional ingot metallurgy (IM) 2XXX alloys. [24j These improvements are mainly attributed to the rapidly solidified microstructures. However, it should be noted here that if the consolidation process is carried out at a temperature above the solidus temperature, the rapidly solidified microstructure cannot be retained and coarse intermetallic phases may be formed which significantly deteriorate ductility and toughness. Thus, this unreinforced alloy cannot be simply compared to standard commercial aluminum alloys.

IV. DISCUSSION

The analysis of the fracture mechanism of the A1-SiCw composite material is quite complicated, because a large number of microstructural features which are mutually interdependent play an important role in the fracture process. These features have been generally classified into the following three categories: (1) features governing matrix properties, e.g. , aging precipitates, dislocations, and fine dispersoids in matrix; (2) characteristics of the whisker/matrix interface, e.g. , interfacial precipitates, oxides, and reaction products; and (3) size and shape factors of SiC whiskers, e.g. , aspect ratio, surface roughness, and corner profile of whiskers. In this study, however, it has been found that another microstructural factor such as coarse interrnetallic particles significantly affects fracture behavior of the composite.

The in situ fracture technique was very useful in studying fracture behavior of the 2124 A1-SiCw composite by providing information on the process zone ahead of a notch and by relating microstructural parameters to the fracture process. It was found that microcracks were formed first at the coarse particles both in the reinforced and the unreinforced alloys, because they were fractured at relatively low stress levels. Between these cracked particles, the deformation becomes localized into intense shear bands and the local stress concentration significantly influences neighboring whisker ends, as shown in Figure 9. The strain localization at this stage is quite similar to that formerly observed in aluminum composite reinforced with SiC particulates. 12~ The sharp corner profile of whisker ends is also thought to act as a stress concentrator which can lead localized deformation re- sponsible for void initiation. [7,8.9] Failure will take place along these shear bands, and the fracture surface will contain equiaxed dimples and the cleavage-like facets caused by coarse manganese-containing particles. If the finer manganese-containing intermetallic particles are also distributed mainly within the whisker-rich region, the high strains within the shear bands can cause microvoids to

Fig. 8 - - S E M micrograph near a blunt notch tip in the unreinforced 2124 aluminum alloy, showing the crack initiation process.

Fig. 9 - - S E M micrograph showing a microcrack formed at a coarse Mn-containing particle in 2124 T6 aluminum alloy reinforced with 15 vol pct SiC whiskers. Localized shear and whisker cracking due to local stress concentration are also shown near the microcrack.


form at the particle-matrix interfaces or at the microcracks induced by the finer intermetallic particles. Then voids will form around these finer particles within the shear bands. From the above results, it is clear that the most important event in the fracture initiation process of the A1-SiC composite is the brittle cracking of the coarse manganese-containing particles.

It has been reported recently that the coarse manganese- containing constituent particles were formed when powder processed 2124 A1-SiCw composites were consolidated above the solidus temperature.I~~ According to Mondolfo t25~ the temperature of the invariant reactions which brought about the formation of the coarse particle is in the range 547 ~ to 626 ~ It can be deduced that these particles are formed preferentially at the boundaries between the solid and the liquid matrix, since all the reactions involve both the liquid and the solid phases. During hot pressing, the liquid phase infiltrates into the whisker agglomerates to form the whisker-rich region, whereas the solid phase remains unmixed with whiskers. Thus, coarse manganese-containing particles are formed by active reactions along the boundaries between the whisker-rich and the whisker-depleted regions. This pe- culiar distribution of the coarse particles seems to remain mostly unaltered even after a subsequent hot extrusion process. In the case of the unreinforced alloy, the similar formation mechanism can be assumed, but the particles are distributed homogeneously by hot extrusion because of the relatively easier plastic deformation of the matrix without reinforcements.

The present study has provided evidence of the importance of the coarse manganese-containing particles in determining mechanical properties of the AI-SiC composite. However, the results indicate that much of the phenomenology is not so readily explained and must be developed through the interpretation of the actual microfracture processes including whisker cracking. Our investigations of fracture surfaces show that they are populated by dimples of sizes corresponding to whiskers in all cases. This suggests that interfacial decohesion at whisker ends or whisker breakage leads to the formation of microvoids. The microvoid formation at whisker ends is commonly accepted as a major fracture initiation process by a number of analytical 191 and phenomenologi- cal t7,81 studies. On the other hand, informaion on the whisker cracking is still lacking, retarding comprehen- sive understanding of the role of SiC whiskers in fracture process.

In recent years, Mahon et a l . 1131 have suggested that the common form of damage is the cracking of SiC whiskers in a direction normal to their long axis, which is dependent on the degree of interfacial precipitation. During the in situ observations in this study, however, cracked whiskers are scarcely found at the notch tip region until the final unstable fracture stage, as can be seen in Figures 7(a) through (c). Furthermore, several whiskers (B in Figure 7(c)) located perpendicular to shear bands seem to restrain flow localization of the adjacent matrix and subsequently inhibit the propagation of the shear band. Bridging of unfractured whiskers is fre- quently found along the shear bands. It is also observed from Figure 9 that most of the cracked whiskers are af- fected by microcracking of the coarse intermetallic particles when they are connected directly with the cracked

whiskers or are apart at a short distance. Indeed, the intermetallic particles fractured first and the cracks they caused substantially developed the local stress concentration within the high triaxial stress field of a sharp crack tip, leading to cracking of neighboring whiskers. There- fore, this type of whisker cracking should be placed under the category of the fracture initiation behavior of the coarse intermetallic particles in the A1-SiC composite.

There is only limited information available on the effect of the PM processing of the AI-SiC composite in spite of the fact that the processing technique is a critical issue of enhancing mechanical properties. It is evident that if the conventional billet consolidation temperature is selected between the solidus and the liquidus temper- atures of the matrix alloy, the formation of coarse intermetallic particles is difficult to avoid during the conventional PM processing. Thus, one of the major ad- vantages of PM processing, i .e . , particle size refinement, appears to be negated by processing above the solidus temperature. Perhaps the simplest way to improve mechanical properties of the 2124 AI-SiCw composite is to control the formation of coarse intermetallic particles during the composite processing. As an ex- ample, manganese can be removed from the 2124 aluminum matrix, since in the composites, the presence of reinforcing SiC whiskers and the PM processing route are thought to prevent substantial grain growth, while manganese is usually present in commercial IM aluminum alloys to form fine dispersoids which contribute to refining grains as well as strengthening.

V. CONCLUSIONS

By investigating the correlation between microstructural factors and the fracture behavior of A1-SiCw composites reinforced with SiC whiskers, the following conclusions can be drawn. 1. Microstructural analyses of 2124-T6 AI-SiCw com-

posites indicate that coarse and irregularly shaped manganese-containing particles exist mainly along the boundaries between whisker-rich and whisker-depleted regions in addition to fine precipitates.

2. The fractographic results show that the presence of the brittle cleavage-like fracture facets originated from the coarse manganese-containing intermetallic particles which might have been formed during the composite PM processing, although fracture occurs mostly by the ductile dimpled mode.

3. From the in situ SEM observation of fracture processes of the composite, coarse intermetallic particles are cleaved first to form microcracks at relatively low stress levels and act as local stress concentrators which facilitate the void formation at the nearby whisker ends.

4. It is suggested that a possible way of improving the fracture toughness and ductility of the present PM processed 2124 A1-SiCw composites is to control the matrix composition to inhibit the formation of the coarse intermetallic particles during the semisolid consolidation processing.

A C K N O W L E D G M E N T S

This work was supported by the Research Institute of Industrial Science and Technology (RIST) under Contract

METALLURGICAL TRANSACTIONS A VOLUME 23A, SEPTEMBER 1 9 9 2 - - 2 5 9 5

No. 1-205-F. The use of the RIST Technical Service Center Facility is also gratefully acknowledged. This article is dedicated to the late Professor Doo Young Lee of POSTECH.

REFERENCES

1. A.P. Divecha, S.G. Fishman, and S.D. Karmarkar: J. Met., 1981, pp. 12-17.

2. D.L. McDanels: Metall. Trans. A, 1985, vol. 16A, pp. 1105-15. 3. M.G. McKimpson and T.E. Scott: Mater. Sci. Eng., 1989,

vol. A107, pp. 93-106. 4. R.J. Arsenault: J. Compos. Technol. Res., 1988, vol. 10,

pp. 140-45. 5. C.R. Crowe, R.A. Gray, and D.F. Hasson: in Proc. 5th Int. Conf.

on Composite Materials, W.C. Harrigan, Jr., J. Strife, and A.K. Dhingra, eds., TMS-AIME, Warrendale, PA, 1985, pp. 843-66.

6. K. Cho, S. Lee, Y.W. Chang, and J. Duffy: Metall. Trans. A, 1991, vol. 22A, pp. 367-75.

7. S.R. Nutt: in Interfaces in Metal-Matrix Composites, A.K. Dhingra and S.G. Fishman, eds., TMS-AIME, Warrendale, PA, 1986, pp. 157-67.

8. S.R. Nutt and J.M. Duva: Scripta Metall., 1986, vol. 20, pp. 1055-58.

9. S.R. Nutt and A. Needleman: Scripta Metall., 1987, vol. 21, pp. 705-10.

10. T.G. Nieh, R.A. Rainen, and D.J. Chellman: in Proc. 5th Int. Conf. on Composite Materials, W.C. Harrigan, Jr., J. Strife, and

A.K. Dhingra, eds., TMS-AIME, Warrendale, PA, 1985, pp. 825-42.

11. L.-J. Fu, M. Schmerling, and H.L. Marcus: in Composite Materials: Fatigue and Fracture, ASTM STP 907, H.T. Hahn, ed., ASTM, Philadelphia, PA, 1986, pp. 51-72.

12. C.R. Harris and F.E. Wawner: in Processing and Properties for Powder Metallurgy Composites, P. Kamar, K. Vedula, and A. Ritter, eds., TMS-AIME, Warrendale, PA, 1988, pp. 107-16.

13. G.J. Mahon, J.M. Howe, and A.K. Vasudevan: Acta Metall. Mater., 1990, vol. 38, pp. 1503-12.

14. T. Christman and S. Suresh: Acta Metall., 1988, vol. 36, pp. 1691-1704.

15. T. Christman, A. Needleman, S.R. Nutt, and S. Suresh: Mater. Sci. Eng., 1989, vol. A107, pp. 49-61.

16. T. Christman, A. Needleman, and S. Suresh: Acta Metall., 1989, vol. 37, pp. 3029-50.

17. J.M. Papazian and P.N. Adler: Metall. Trans. A, 1990, vol. 21A, pp. 401-10.

18. A. Levy and J.M. Papazian: Metall. Trans. A, 1990, vol. 21A, pp. 411-20.

19. M. Manoharan and J.J. Lewandowski: Acta MetaU. Mater., 1990, vol. 38, pp. 489-96.

20. D.L. Davidson: Metall. Trans. A, 1991, vol. 22A, pp. 113-23. 21. G.T. Hahn and A.R. Rosenfield: MetaU. Trans. A, 1975, vol. 6A,

pp. 653-68. 22. J. Hong and J. Gurland: Metallography, 1981, vol. 14, pp. 225-36. 23. B.C. Kim, S. Lee, D.Y. Lee, and N.J. Kim: Metall. Trans. A,

1991, vol. 22A, pp. 1889-92. 24. H.G. Paris and D.J. Chellman: in Rapidly Solidified Powder

Aluminum Alloys, ASTM STP 890, M.E. Fine and E.A. Starke, Jr., eds., ASTM, Philadelphia, PA, 1986, pp. 527-44.

25. L.F. Mondolfo: Aluminum Alloys: Structure and Properties, Butterworth's, London, 1976, pp. 505-08.


Documents

Fracture mechanisms of a 2124 aluminum