Tensile and Fatigue Behavior of Aluminum Oxide Fiber Reinforced Magnesium Composites: Part II. Alloying Effects
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I.
INTRODUCTION
THE important effect that matrix composition and heat treatment have on off-axis properties has been demonstrated in a number of metal matrix composite systems.l'2 Generally, these variables have not been found to influence axial properties. Although some attempts have been made to correlate matrix composition and heat treatment with corresponding changes in the toughness of the composite, ~ no quantitative correlations have been made with the local decohesion stress. In addition, effects of microstructural changes in the matrix have not been successfully separated from residual stress and interracial effects. A recent study (presented in Part I) of the fatigue and tensile behavior of a model material (FP AlzO3 fibers in a commercially pure Mg matrix) indicated that off-axis fatigue and tensile properties were suppressed because of the combination of a low fiber/matrix interfacial strength and a soft matrix. Under either cyclic or static loading the drastic drop in fatigue and tensile properties with angle was found to correspond with a change in failure mode from flat fracture across the fibers under axial loading conditions to interfacial and/or matrix failure along the fiber orientation for off-axis loading. One of the conclusions of the study was that alloy additions designed to increase the strength of the fiber/matrix interface and/or the strength of the matrix should improve off-axis properties. It has also been found that the transverse strength is dependent on matrix composition in FP/Mg composites. 3 It was not known whether this phenomenon was purely a matrix strengthening effect or due to a subtle change in interfacial structure. This paper reports the initial results of a study of the influence of matrix composition on the micromechanisms of fracture and the tensile and fatigue properties of A1203 fiber reinforced magnesium composites. This work and earlier work on the model material (Part l) provide the foundation for a continuing program designed to develop quantitative relationships between matrix microstructure, composition, and properties; fiber orientation and volume percent; and fatigue crack growth behavior in these materials through R.A. PAGE and J.E. HACK, Senior Research Metallurgists, R. SHERMAN, Research Metallurgist, and G.R. LEVERANT, Assistant Director, are all with the Department of Materials Sciences, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78284. Manuscript submitted September 8, 1983. METALLURGICALTRANSACTIONS A
direct observations of the effects of these variables on the strain field at the tip of a growing crack.
I1.
EXPERIMENTAL
The effect of matrix alloy content was investigated by comparing the fatigue and tensile properties of two materials which differed only in the composition of the matrix. Commercially pure magnesium (CPMg) was utilized as the baseline matrix material while ZE41A, a magnesium alloy with approximately 4.25 wt pct Zn, 0.5 wt pct Zr, and 1.25 wt pct rare earths, was chosen to represent alloyed matrix materials. B
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