Loading Rate-Dependent Mechanical Properties of Bulk Two-Phase Nanocrystalline Al-Pb Alloys Studied by Nanoindentation
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INTRODUCTION
THE deformation behavior of nanocrystalline materials has been fascinating as it distinctly differs from that of coarse-grained materials.[1,2] In structural materials of engineering importance with micron sized characteristic length scales (grain size) in three dimensions, mobile dislocations located in the interiors of the grains play a dominant role, governing the plasticity.[3] In single phase materials, as the grain size is refined to less than 100 nm, the larger fractions of grain boundaries and other interfaces also will contribute to the plastic deformation in a significant way.[4–9] As grain size (d) is reduced, the strength increases in polycrystalline solids following Hall–Petch relation.[10,11] However, when the grain size approaches £100 nm, the Hall–Petch line will have a different slope, than that in 100 nm £ d £ 1000 nm range, albeit a positive number. With grain size further decreasing to about 10 nm, the Hall–Petch line exhibited SREEDEVI VARAM, Doctoral Student, and KOTESWARARAO V. RAJULAPATI, Assistant Professor, are with the School of Engineering Sciences and Technology, University of Hyderabad, Gachibowli, Hyderabad 500046, India. Contact e-mail: kvrse@ uohyd.ernet.in K. BHANU SANKARA RAO, Ministry of Steel Chair (Govt. of India), is with MGIT, Hyderabad. RONALD O. SCATTERGOOD, KORUKONDA L. MURTY, and CARL C. KOCH, Professors, are with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 276957907, USA. Manuscript submitted February 21, 2014. Article published online July 11, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
a negative slope suggesting that the strength decreases in materials with these very fine grain sizes approaching the amorphous limit.[4] This is termed as Inverse Hall– Petch effect.[12] The interesting and superior mechanical properties of nanocrystalline materials have thrown a grand challenge to the scientific community in the form of several uncertainties and unanswered questions, while unfolding the underlying deformation mechanisms.[5] Finer grain size yielded higher strength values in nanocrystalline single-phase materials, but at the expense of ductility.[13] Therefore, acceptable ductility levels should also be available along with strength in these materials for potential structural applications. Strain rate sensitivity (SRS) and activation volume are important parameters in a plastic deformation process.[14] SRS is a qualitative indicator of ductility of a given material, with higher SRS value means more ductility.[15] While suggesting strategies to improve ductility of nanocrystalline materials, Koch[16] indicated that incorporation of a second phase could delay the onset of localized deformation under tensile loading conditions. Although extensive research investigations have been carried out to explore the mechanical behavior of various single-phase nc materials, studies that address the influence of a nano-sized second phase dispersions in a nanocrystalline matrix with an average grain size less than 100 nm a
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