Correlation between Activation Volume and Pillar Diameter for Mo and Nb BCC Pillars
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1185-II07-04
Correlation between Activation Volume and Pillar Diameter for Mo and Nb BCC Pillars A. S. Schneider1,Ω, B. G. Clark2, C. P. Frick3, and E. Arzt4 1
Max Planck Institute for Metals Research, Heisenbergstrasse 3, 70569 Stuttgart, Germany Sandia National Laboratories, Albuquerque, NM 87185, USA 3 University of Wyoming, Mechanical Engineering Department, 1000 East University Avenue, Laramie, WY 82071, USA 4 INM - Leibniz Institute for New Materials and Saarland University, Campus Building D2 2, 66123 Saarbrücken, Germany 2
ABSTRACT Compression tests with varying loading rates were performed on [001] and [235] oriented small-scale bcc Mo and Nb pillars to determine the contribution of thermally activated screw dislocation motion during deformation. Calculated activation volumes were shown to be in the range of 2 - 9 b3 and by further examination were found to decrease with pillar diameter. This suggests that the kink-pair nucleation of screw dislocations is enhanced by surface effects in the micron and submicron range. INTRODUCTION Compression tests on focused ion beam (FIB) machined, single crystal metal micropillars have shown that flow stress is inversely related to pillar diameter [1-6]. Although the fundamental mechanisms which govern the size effect are still under debate, a consistent scaling relationship for the flow stress σy with pillar diameter d on the order of σy ∝ d-0.6 – d-1.0 has been found for several face-centered cubic metals [1-5]. Because fcc and bcc metals differ fundamentally in their dislocation processes it is perhaps not surprising that bcc metals show a different size scaling on the order of σy ∝ d-0.2 – d-0.4 [7-8]. The weaker size dependence of bcc metals has been attributed to the low mobility of screw dislocations in bcc metals leading to either conventional dislocation-dislocation interactions [7] or kinetic pile-ups of screw dislocations in the vicinity of dislocation sources [8]. To elaborate on the role of thermally activated screw dislocation motion in the deformation of small-scale bcc metal pillars, compression tests on Mo pillars were performed at various loading rates in a previous study [8]. Calculated activation volumes were found to be in the range of 2 - 6 b3, where b is the Burgers vector, which is in good agreement with activation volumes measured for thermally activated kink-pair nucleation of screw dislocations in bulk bcc Mo [9]. In this study the relationship between pillar diameter and calculated activation volumes are examined in more detail for Mo, and compared to new results from Nb pillars. EXPERIMENTAL For this study, [001] and [235] oriented Mo and Nb pillars with diameters between 300 nm and 3000 nm were FIB machined on the surfaces of bulk Mo and Nb single crystals using a Dual BeamTM FIB. Both orientations for each material were obtained from the same bulk single
crystal by electron discharge machining after the orientations were first determined by Lauediffraction. For each orientation several diameters were investigated and for each diameter a
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