The mechanical behavior of a hybrid metal matrix composite
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I.
INTRODUCTION
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hybrid reinforcement concept applicable to metal matrix composites is investigated. The concept entails development of a hybrid between a SiC particulate-reinforced A1 alloy and an unreinforced A1 alloy matrix, configured such that the reinforced material forms aligned rods within the alloy matrix [~j (Figure 1). This material is expected to exhibit elastic properties essentially the same as those for a homogeneous SiC.reinforced alloy having the equivalent volume fraction of p a r t i c l e s [2'31 but, otherwise, be superior, particularly with regard to the notch sensitivity. The latter expectation reflects the notion that cracks which form in the SiC-reinforced cylinders should arrest at the interface with the ductile alloy matrix. Hybrid composites having the requisite features have been fabricated tl~ and subjected to various mechanical measurements, as needed to critically examine this reinforcement concept. It is noteworthy that the present concept is diametrical to that wherein a composite containing a spatially homogeneous distribution of SiC particulates is produced, in order to disperse damage sites and thus enhance the mechanical properties. [4] 11.
MATERIALS
Two composite materials (designated A and B) have been investigated. Both materials incorporate aligned composite cylinders, 1.5 mm in diameter, consisting of 30 vol pct SiC particulates in a 6061 A1 alloy matrix, surrounded by an 80-/~m-thick annulus of the A1 alloy itself (Figure 1). The composite cylinders are embedded in a matrix of the same 6061 A1 alloy. The composite was consolidated by extrusion at 480 ~ Further characteristics of the macrostructure and microstructure are summarized in Figures 2 and 3.
F. ZOK, Assistant Professor, Materials Department, S. JANSSON, Research Engineer, Department of Mechanical and Environmental Engineering, and A.G. EVANS, Professor, Materials Department, are with the University of California, Santa Barbara, CA 93106. V. NARDONE, Senior Research Scientist, is with United Technologies Research Center, East Hartford, CT 06108. Manuscript submitted January 24, 1990. METALLURGICAL TRANSACTIONS A
Materials A and B differ only in the interfacial region between the metal annulus surrounding the composite cylinders and the adjacent alloy matrix. In material A, the composite cylinders had been acid cleaned prior to extrusion to promote "strong" bonding between the cylinders and surrounding metal matrix. In this case, no interracial layer was observed between the cylinders and the matrix (Figure 2(b)). In material B, the cylinders had been coated with a debonding agent to reduce the interfacial "strength." In this case, after extrusion, the interfacial region contained an essentially continuous oxide interracial layer - 2 /xm thick; the layer was observed after etching with Keller's reagent (Figure 2(a)). Higher magnification views reveal the SiC particle characteristics in the composite cylinders (Figure 3(a)) and also indicate the presence of porosity in the matrix interstices (Figure 3(b)). T
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