The influence of SiC particulates on fatigue crack propagation in a rapidly solidified Al-Fe-V-Si alloy
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INTRODUCTION
THE need for high-temperature structural aluminum alloys possessing superior specific strength and stiffness, primarily for aerospace applications, has led to the development of several new alloys and their composites. The addition of transition elements such as Fe, V, Ce, Ti, Zr, and Hf increases the elevated temperature strength of aluminum alloys through dispersionhardening mechanisms, tl'2] Unfortunately, ingot metallurgy processes are limited by the solid solubility of the alloying elements, and the desired mechanical properties cannot be achieved. These solubility limits can be exceeded by the use of rapid solidification processes such as gas atomization, melt spinning, or splat cooling. The end products of these rapid solidification processes are very fine grained, nonequilibrium powders which must be consolidated through the use of powder metallurgy techniques such as hot isostatic pressing (HIP), sintering, or roiling, t3'4J Because the primary applications for such dispersionstrengthened aluminum alloys are in aerospace structures where fatigue crack propagation resistance of the material is critical due to the cyclic nature of the loading, it is necessary to understand whether high-temperature strength has been achieved in these materials at the expense of fatigue resistance. Unfortunately, Gray et al. demonstrated that fine-grained materials can exhibit higher crack growth rates at a given AK (or, equivalently, low values of threshold stress intensity factor, T.J. SUTHERLAND, formerly Graduate Research Assistant, University of California-Davis, is Machinery Engineer, Chevron USA Products Company, Richmond, CA 94802. P.B. HOFFMAN, Graduate Research Assistant, and J.C. GIBELING, Associate Professor, are with the Division of Materials Science and Engineering, University of California-Davis, Davis, CA 95616. Manuscript submitted April 16, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
AKth) compared to coarse-grained alloys owing to both intrinsic and extrinsic factors, tSl In particular, such materials do not develop significant levels of roughnessinduced crack closure or plasticity-induced closure as a consequence of their fine grain sizes and high strengths. Further, the crack path tortuosity that serves to dissipate energy in coarse-grained materials, thereby reducing the crack growth rates, is lacking in the fine-grained alloys. Several authors have shown these concepts to be valid for high strength aluminum alloys. For example, Bretz et al. studied alloy 7091 in both the coarse- and finegrained conditions, t6] They observed that the coarsegrained material exhibited a higher AKth in part due to an increased level of closure but also due to increased amounts of crack deflection and branching. In an extreme case, Minakawa et al. observed a complete lack of crack closure and no influence of load ratio, R, on crack growth rate in mechanically alloyed IN-9021 with an ultrafine average grain size of 0.2 /xm. tT] More recently, Jata and Walsh have observed fairly low values of AK,h and li
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