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I. INTRODUCTION

SCIENTIFIC and technological interest in nanostructured materials has engendered the development of a number of synthesis techniques that are capable of producing structural materials with grain sizes in the 10- to 500-nm range in bulk quantities.[1] In order to enhance the fundamental understanding of the nature and origin of mechanical deformation processes in nanostructured materials, a number of investigations on the tensile mechanical performance of nanostructured materials have been reported.[2–9] The strength increase that accompanies a decrease in grain size is widely described in terms of the empirical Hall–Petch relationship (

o  k  d1/2).[10,11] A high strength is generally observed in most nanostructured materials, as expected.[1,12] However, in spite of their attractive high strength, low ductility is frequently reported in nanostructured materials and it appears that the ductility decreases with decreasing grain size.[13–16] This compromises their potential as structural materials. The low ductility in nanostructured materials is often attributed to the lack of intrinsic dislocation activity when grain sizes are in the nanoscale range, since the uniform deformation (u) is related to the average distance of dislocation movement (L) in the form of u  m  b  L where m is the density of mobile dislocations and b the Burgers vector. Occasionally, high ductility has been reported in nanostructured materials,[17,18] but this appears to be attributed to the existence of subgrains or stacking fault/microtwins in nanocrystalline grains rather than the dislocation activities in real nanocrystalline grains. The fracture toughness is expected to increase with decreasing grain size since the stress intensity factor is proportional to the material strength.[19,20] However, inspection of all the existing reports on fracture toughness measured by an indentation approach or drilling a hole in nanostructured or ultrafinegrained materials[21–25] reveals that there is a lower value of B.Q. HAN, Assistant Researcher, and E.J. LAVERNIA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. F.A. MOHAMED, Professor, is with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697. C.C. BAMPTON, Manager, is with the Rocketdyne Division, Boeing, Canoga Park, CA 91309. Contact e-mail: [email protected] Manuscript submitted June 19, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

fracture toughness, as indicated in Table I. In several of these studies, the measured low fracture toughness was attributed to the existence of processing flaws, less-than-optimum powder consolidation practices, or poorly bonded interparticle regions, at smaller grain sizes.[21,26] Low fracture toughness measured using plane strain crack initiation toughness was observed in a cryomilled ultrafine-grained pure Al composite containing 3 vol pct Al2O3 dispersoids.[27] Most recently, low fracture toughness of