Effects of SiC whiskers and particles on precipitation in aluminum matrix composites
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INTRODUCTION
ALUMINUM alloys reinforced with whisker or particulate SiC are attractive for applications requiring higher stiffness and strength than traditional aluminum alloys. Because the reinforcing phase is discontinuous, these "short fiber reinforced composites" (SFRC) are similar to conventional aluminum alloys in that they are nearly isotropic and can be processed and formed by traditional metalworking techniques. IlJ These advantages, coupled with a potential low cost, have led to rapid commercialization, and SFRC aluminum alloys are now available from three commercial sources. The original fabrication process was based on a powder metallurgy (PM) technique, I2j but an ingot metallurgy (IM) process has been developed so that both PM and IM materials are now available.[a] Unlike continuous filament composites, the properties of the matrix alloy of an SFRC play a significant role in the yield strength of the composite. To date, the matrix alloys of choice have been the age-hardenable alloys 2124, 6061, and 7475. The strength increment due to age-hardening is necessary in order to develop acceptable properties. For example, Arsenault found that adding 20 pct SiC to annealed 6061 increased the yield strength by 100 MPa, I41 but heat treating unreinforced 6061 from the annealed condition to T6 increased the yield strength by 220 MPa. 151 (The origihal, annealed value is 55 MPa. [51) Thus, the strength increment due to age-hardening is greater than or equal to the strengthening due to the introduction of SiC and must be considered when explanations for the mechanical behavior of these materials are sought. The strengthening whiskers or particles employed to make SFRC are typically high melting point, relatively inert phases such as SiC, B4C, and A1203, and they are not expected to modify significantly the overall chemistry of JOHN M. PAPAZIAN is Principal Staff Scientist with Grumman Corporate Research Center, A01-26 Bethpage, NY 11714-3580. Manuscript submitted November 13, 1987.
METALLURGICAL TRANSACTIONS A
the matrix aluminum alloy. To a first approximation, they are thought of as inert with respect to the age-hardening precipitation reactions. However, preliminary investigations have shown that there are effects of the reinforcing particles on precipitation in the matrix. Nieh and Karlak have shown that B4C particles affect the aging behavior of B4C reinforced 6061. Irj They found that the composite reached peak hardness in 3 hours at 450 K while a control PM alloy took l0 hours. They attributed this acceleration to the presence of high diffusivity paths in the composite, particularly the dislocations introduced by differential thermal expansion, and the particle matrix interfaces. Similar acceleratory effects of SiC in various aluminum alloy matrices are generally recognized, but specific results do not seem to have been published. Thus, the state of matrix precipitation is significant to the mechanical properties of SFRC, and the presence of the reinforcing phase influences the matrix precipitation,
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