Fracture characteristics of Al-4 pct Mg mechanically alloyed with SiC
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I.
INTRODUCTION
A L U M I N U M alloys reinforced with silicon carbide particles show an increased modulus, due to the higher modulus of the silicon carbide, and often exhibit an increased flow stress, depending on the matrix alloy and its treatment in the manufacturing process and any subsequent heat treatment given the composite. Three manufacturing processes are currently used for manufacturing these composites: (1) silicon carbide is mixed with a molten aluminum alloy, which is subsequently cast, and perhaps further worked; (2) silicon carbide is mixed with rapidly solidified aluminum alloy powder, which is then consolidated into a billet using powder metallurgy forming processes; and (3) mechanically alloyed aluminum alloy powders are mechanically "alloyed", or mixed, with silicon carbide and formed using P/M manufacturing methods. Each of these processes results in composites with different properties, and it is not known if this is due solely to the manufacturing process or if there are other causes, such as the size and distribution of the silicon carbide particles. Although these composites show increases in modulus and yield strength, they have values of fracture toughness less than unreinforced alloys and lower than desirable for many structural uses. The research reported here was undertaken to examine the origins of low fracture toughness in these composites, and to suggest ways of increasing this property. Composites made by all manufacturing processes, and having approximately equal volume fractions of silicon carbide are being studied. This paper reports on our investigation of the mechanically alloyed, powder metallurgy product designated as IN-9052 containing 15 vol pct SiC manufactured by the NOVAMET subsidiary of International Nickel Company. Composition of the matrix material was given as follows: ~A1-4Mg- 1C-0.9 O (wt pct). Similar results will be published later for composites formed by other processes. *IN Is a trademark of the INCO family of companies. D. L. DAVIDSON is Institute Scientist with the Department of Materials Sciences, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78284. Manuscript submitted July 24, 1986.
METALLURGICAL TRANSACTIONS A
The goals of this work were to measure the physical and chemical characteristics of this material, and to determine how these factors control mechanical properties of the composite, especially fracture. Both fatigue and fracture toughness were studied, and a method was devised for computing the fracture toughness from measured physical properties.
II.
MECHANICAL CHARACTERIZATION
A. Fatigue and Fracture Toughness Two single edge notched specimens 19 • 2.8 mm were machined from an as-received extrusion of cross section 44 • 9.5 mm. The gage sections of these specimens were hand polished using standard metallographic preparation procedures, and then lightly etched. A single, 0.5 mm wide slit was cut into each specimen, and then they were cycled at a stress ratio (R) of 0.1 until a crack initiated and grew away from the influ
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