Forced chemical mixing at Cu-Nb interfaces under severe plastic deformation
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Yinon Ashkenazy Department of Materials Science and Engineering, University of Illinois, Urbana Champaign, Urbana, Illinois 61801; and Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem, Israel
Pascal Bellon Department of Materials Science and Engineering, University of Illinois, Urbana Champaign, Urbana, Illinois 61801
Jian. Wang Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 23 March 2012; accepted 2 April 2012)
Forced chemical mixing during severe plastic deformation was investigated at Cu-Nb face-centered-cubic (fcc)/body-centered-cubic (bcc) interfaces using molecular-dynamics simulations. Three Cu-Nb interfaces were considered, with either Kurdjumov-Sachs or Nishiyama-Wassermann orientation relationship (OR) between fcc and bcc phases. Forced mixing of a spherical bcc-Nb precipitate in fcc-Cu was also studied for comparison. Deformation was imposed by shape-preserving cycles using two different modes, biaxial compression and biplanar shearing to investigate the effects of strain path. For biplanar shear, the chemical mixing rate is strongly dependent on structure of the interface, with the Kurdjumov-Sachs OR and a (111)Cuk(110)Nb habit plane being particularly resistant to mixing. During compression, no such dependence was found. Influences of interface diffuseness and roughness on stability were also investigated. The simulations show the interface mixing is inversely related to interface shear strength during shear deformation, but dominated by dislocation-glide through the Cu phase and subsequent absorption at Cu-Nb interfaces during compression deformation.
I. INTRODUCTION
Severe plastic deformation (SPD) is commonly observed to force chemical mixing and homogenization of immiscible two-phase alloys.1–3 In systems with coherent, coexisting phases it had been suggested that such forced mixing derives from the random glide of dislocation on slip planes,4 resulting in a mixing that is ballistic in character and independent of temperature.5 Molecular dynamics (MD) simulations have provided quantitative support for this approximation for fcc-fcc, two-phase systems with low-to-moderate positive heats of mixing and small differences in elastic constants of the constitutive elements.6,7 In these systems, slip can transfer across phase boundaries, which then become both rough and diffuse in response.4,8,9 Subsequent MD simulations by Lund et al.,10 Vo et al.,8 and Delogu11 have shown that mechanical alloying in coherent alloy systems with very large positive heats of mixing a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.106 J. Mater. Res., Vol. 27, No. 12, Jun 28, 2012
or with large differences in elastic constants is not entirely ballistic, but rather it is guided by the thermochemical and mechanical properties of the alloys and it is temperaturedependent. These recent simulations are now able to explain a number of SPD experiments on coherent systems. For example, Cu-Ag alloys, which have a small positive heat of m
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