First-principles computations of mechanical properties of Ni 2 Cr and Ni 2 Mo
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I. INTRODUCTION
NICKEL-BASED alloys such as alloy 22 are candidate materials for potential applications as the outer container of the waste package for the disposal of high-level nuclear waste.[1] During fabrication processes and long-term storage, Ni-based alloy outer containers can undergo microstructural changes due to the formation of ordered Ni2(Cr, Mo) and topologically close-packed (tcp) phases. The Ni2(Cr, Mo) phases are orthorhombic (oI6), while the tcp phases have crystallographic structures that include hexagonal ( phase), orthorhombic (P phase and phase), and tetragonal ( phase).[2–5] These phases contain substantial amounts of Cr and Mo, whose depletion from the matrix can impact negatively the corrosion resistance of the Ni-based alloys. The precipitation temperature of tcp phases and the ordering temperature of Ni2(Cr, Mo) are in the range of 773 to 1073 K, and both reactions are sluggish.[2,3,4] Because of the slow reaction kinetics, the formation, morphological evolution, and properties of the Ni2(Cr, Mo) and tcp phases cannot be measured confidently using short-term tests over a reasonable time frame, but must be computed theoretically using first-principles computational methods.[6,7,8] Similarly, the mechanical properties of the Ni2(Cr, Mo) and tcp phases are largely unknown, as they have not been measured due to experimental difficulties in specimen preparation and heat treatment.[2–4,9,10] The effects of Ni2Cr ordering on the strain-hardening behavior of Ni-Cr alloys were studied sparsely in the 1960s.[2,9,10] Extremely slow cooling rates were required to induce ordering and the formation of Ni2Cr in Ni-based alloys. Some of the difficulties in specimen preparation associated with the slow K.S. CHAN, Institute Scientist, and Y.-D. LEE and Y.-M. PAN, Principal Engineers, are with Southwest Research Institute, San Antonio, TX 78238-5166. Contact e-mail: [email protected] This article is based on a presentation made in the symposium “Computational Aspects of Mechanical Properties of Materials,” which occurred at the 2005 TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA, under the auspices of the MPMD–Computational Materials Science & Engineering (Jt. ASM-MSCTS) Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
ordering kinetics were described in detail by these investigators.[2,9,10] The results of these deformation studies indicated that Ni2Cr ordering increased the work-hardening rate of Nibased alloys, because shearing of the ordered phase requires a pair of superdislocations separated by an antiphase boundary that hinders further dislocation motion,[9,10] which is a common hardening behavior and deformation mechanism of ordered intermetallics. In these early investigations, Ni2Cr was studied mainly as the strengthening phase in a Ni matrix.[10–13] Experimental evidence indicated that the Ni2Cr ordered precipitates were less than 500Å in size and appeared to be coherent with the matrix.[2] To the authors’ knowledge, there has been only one experimental study of the mechanica
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