Mechanisms of Ductility in CoTi and CoZr B2 Intermetallics
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UCTION
THE capacity of a material to accommodate extensive permanent deformation, or the lack thereof, plays an important role in determining its widespread use as a common engineering material. Intermetallics, in general, do not exhibit appreciable ductility.[1] The brittle behavior of B2 (CsCl) NiAl[2,3] and many other structurally similar intermetallics is attributed to the lack of independent slip systems that can accommodate arbitrary strains. The generalized criterion (attributed to Von Mises[4] and Taylor[5]) states that five independent slip systems are necessary for homogeneous polycrystalline ductility. In some B2 intermetallics, h111i 110 -type slip allows for five independent slip systems, as it does in bcc materials. However, it is well known that the favored slip mode for B2 intermetallics with high ordering energies, such as NiAl, is h100if011g; which corresponds to the glide of dislocations with a Burgers vector equal to the shortest lattice translation on the most closely packed planes, yet it only provides three independent slip systems.[6–8] While the behaviors discussed earlier are established ‘‘rules of thumb’’ for the behavior of B2 intermetallics, J.A. WOLLMERSHAUSER and C.J. NEIL, Graduate Research Assistants, and S.R. AGNEW, Associate Professor, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904. Contact e-mail: agnew@virginia. edu This article is based on a presentation given in the symposium ‘‘Neutron and X-Ray Studies of Advanced Materials,’’ which occurred February 15–19, 2009, during the TMS Annual Meeting in San Francisco, CA, under the auspices of TMS, TMS Structural Materials Division, TMS/ASM Mechanical Behavior of Materials Committee, TMS: Advanced Characterization, Testing, and Simulation Committee, and TMS: Titanium Committee. Article published online September 26, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
several highly ordered binary combinations are reported to accommodate significant plastic strains. These materials include combinations of rare earth and transition elements[9] such as YAg,[10] DyCu,[11] and YCu[12] as well as IVa-VIII intermetallic compounds, CoTi and CoZr, which are shown to accommodate room-temperature tensile strains of 2[13] and 17 pct,[14] respectively. It is interesting that the only active slip system unambiguously identified at room temperature in all of these compounds is h100if011g (cube-type slip). In CoTi, both single-crystal slip trace[15] and transmission electron microscopy (TEM)[16] studies find that dislocations of the type b ¼ h100i are the predominate dislocations regardless of crystal orientation. Although kink banding and associated serrations in the stressstrain curve are seen in single crystals oriented close to [100], cube dislocations are still characterized in the same samples[15] and polycrystal studies only find serrations and kinking at elevated temperatures.[17,18] Similar to CoTi, polycrystalline TEM studies of CoZr find the only active slip system to be h100if011g.[
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