Interpretation of the creep behavior of nanocrystalline Ni in terms of dislocation accommodated boundary sliding
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NANOCRYSTALLINE (nc) materials are those with grain sizes in the range 1 to 100 nm. Because of their small grain size, features such as grain boundaries, junction lines, and nodes have significant volume fractions and influence properties far more strongly than in more conventional microstructures.[1,2,3] Nc-materials have novel physical, chemical, biological, and mechanical properties that may have important engineering uses. As a consequence, they are attracting wide attention in materials research. Over the past several decades, notable progress has been made not only in documenting reliable data on the creep behavior of coarse-grained and micrograined materials but also in rationalizing their creep characteristics in terms of deformation processes. By contrast, data on the creep behavior of nc-materials not only are limited in scope[4] but also are obtained under questionable conditions. Four examples for the purpose of illustrating this problem are provided. First, the total strain measured in creep experiments conducted on nc-materials in several investigations did not exceed 0.01 in most cases.[5–11] With such small strains, it is difficult to ascertain whether the steady-state creep is reached. Second, creep strains measured in a very recent investigation[12] on creep in electrodeposited (ED) nc-Ni were large (.0.1). However, creep tests in that investigation were performed under constant compression load, a condition that leads to a continuous decrease in applied stress as the area of the specimen increases during deformation. Third, in several cases, creep findings were inferred from approximate measurements under the condition of significant grain growth.[9] The approximate determination of the stress exponent under this condition raises the question FARGHALLI A. MOHAMED, Professor, and MANISH CHAUHAN, Graduate Research Assistant, are with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697. Contact e-mail: [email protected] Manuscript submitted March 30, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
of whether the findings reflect the true creep behavior of the material. Fourth, creep characteristics were occasionally determined[12] from creep curves in which the strain rate was decreasing with time, i.e., no true steady-state stage was achieved. Finally, there are no systematic measurements for the average grain size as a result of creep at various stresses. According to the creep study performed by Kottada and Chokshi,[12] ED nc-Ni with an initial grain size of about 30 nm exhibited a grain size of about 40 nm after compression creep at 373 K and 2000 MPa. Yin et al.[11] reported that the initial grain size of 27 nm became 40 to 50 nm after creep testing at 473 K and 200 MPa. In both studies,[11,12] no additional measurements for the average grain size were reported at other creep stresses. The lack of experimental data that would characterize creep and deformation-induced grain size growth in nc-materials under the conditions of large strains and constan
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