Investigations into the slip behavior of zirconium diboride
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Christopher Weinberger Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 19104
William Fahrenholtzb) and Greg Hilmas Department of Materials Science & Engineering, Missouri S&T, Rolla, MO 65401
Mark L. Weaver and Gregory B. Thompsona) Department of Metallurgical & Materials Engineering, University of Alabama, Tuscaloosa, AL 35405 (Received 1 March 2016; accepted 28 April 2016)
The slip systems in ZrB2 flexural tested at 1000 °C and 1500 °C have been quantified. The dislocations in both samples were long and straight with a dislocation density of approximately 1013 m2. The structure of the dislocations as well as the low density is in agreement with a ceramic that is hard and brittle and dislocation nucleation and motion is restricted. The low temperature slip systems were found to include c-prismatic slip—1=3½0001ð1010Þ—and a-pyramidal slip—1=3½1120ð1101Þ whereas the elevated temperature sample revealed a-basal slip—1=3½1120ð0001Þ. Density functional theory Generalized Stacking Fault Energy curves for perfect slip were calculated and agreed well with geometric considerations for slip, including interplanar spacing and planar packing. Though basal slip has the lowest fault energy, the presence of the other dislocation types is suggestive that the activation barrier is not a hindrance for the temperatures studied and is likely activated to increase the number of plastic degrees of freedom.
I. INTRODUCTION
Several types of transition metal carbides, borides, and nitrides that have melting temperatures exceeding 3000 °C are categorized as ultrahigh temperature ceramics (UHTCs).1–4 These materials are also characterized with moderate to excellent oxidation resistance, high hardness, as well as a range in mechanical responses dependent upon their stoichiometry and the thermomechanical loading temperature. For example, tantalum carbides have been reported to exhibit ductility up to 33% with the deformation governed by the bonding type within the UHTC.5 De Leon et al.6 demonstrated how the local bond character within a UHTC unit cell regulates these slip systems that enables significant plasticity even in low symmetry crystals such as trigonal Ta2C. More recent work has shown how the availability of outer shell electrons in equivalent crystal structures modifies the preferred slip planes in determining the energy hierarchy for deformation.7 ZrB2 has received considerable attention because of its high hardness, relatively low density, and superior oxidation Contributing Editor: Eric Wuchina a) Address all correspondence to this author. e-mail: [email protected] b) Corresponding Editor Footnote: William Fahrenholtz DOI: 10.1557/jmr.2016.201
resistance in extreme environments.8 Consequently, this boride has been proposed as a candidate material for cutting tools,9 armor applications, structural materials for hypersonic vehicles,10 and even electrodes.11 Its high hardness is likely enhanced by its hexagonal P6/mmm symmetry, which limits the number of independent slip systems,
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