On the role of matrix creep in the high temperature deformation of short fiber reinforced aluminum alloys

The present study investigates creep of short fiber reinforced metal matrix composites (MMCs) which were produced by squeeze casting. Two types of aluminum alloys were used as matrix materials with class I (alloy type) and class II (metal type) creep behavior. The creep behavior of the resulting MMCs is similar both in terms of the shape of the individual creep curves as well as with respect to the stress dependence of the minimum creep rates. Their creep behavior can be rationalized on the basis of a coupling of elementary deformation processes. There is a stress transfer to the fibers by the formation of a work hardened zone. A recovery process in which dislocations move along the fiber to the fiber ends where they shrink and annihilate counteracts this strain hardening process. Once fibers and/or subfibers reach critical stresses they break; this results in multiple fiber breakage. It is the coupling of these elementary processes which governs creep of short fiber reinforced aluminum alloys. This microstructural creep scenario controls creep of short fiber reinforced aluminum alloys irrespective of whether the isolated matrix material shows class I or II creep behavior. The results of this study also show that fiber reinforcing results in a significant increase of creep strength. In this respect the matrix material is important because the creep strengths of the MMCs scale with the creep resistance of the matrix materials.