Structural variations in strained crystalline multilayers
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H. Jonsson Department of Chemistry, University of Washington, BG-10, Seattle, Washington 98195 (Received 14 February 1994; accepted 22 April 1994)
We present a computer simulation study of thin crystalline multilayers constructed from two fee solids with differing lattice constants and binding energies. Initially the two solids have the same orientation, and the interface is perpendicular to the common [100] direction. We then minimize the energy of the system at zero temperature or equilibrate it at a finite temperature. Both materials are described by Lennard-Jones interatomic potentials. A novel technique for analyzing local atomic ordering, common neighbor analysis, is used to identify structural characteristics in these systems. As we gradually vary the lattice mismatch between the two solids, several structural changes are observed in the layers of smaller atoms after energy minimization. At a mismatch larger than 14%, the layers transform into the hep structure, while at smaller mismatches extended structural defects are generated. At elevated temperatures, the hep structure is transformed back to fee, and the structure defects disappear.
I. INTRODUCTION The properties of solids in very thin alternating layers are of interest both from a theoretical as well as a practical point of view. Theoretically, it is important to fully define thermodynamic properties of systems only several atomic layers thick, where conventional continuum methods may not apply. Practically, there is a growing interest in the properties of layered nanoscale materials in areas such as electronic devices and interfacial adhesion in composite materials. Recent investigations of possible enhancement of mechanical properties for composition-modulated superlattices, i.e., the socalled supermodulus effect, ^ have also generated a great deal of interest in metallic multilayers. Due to the small size and complicated geometry, experimental study of the structural properties of these materials is difficult. Computer simulation, on the other hand, is well suited for this kind of study. The nanoscale dimensions of these systems provide an excellent opportunity for applying simulation methods such as molecular dynamics which, due to the large amount of computation involved, have to be used on systems that are either naturally or artificially reduced to a manageable size. Structural phase transitions in single crystals due to external stress have been studied extensively.7"10 Milstein and Farber studied the fcc-to-bcc phase transition under [100] tensile loading based on theoretical arguments.7 Using their powerful extension of the molecular-dynamics simulation technique, Parrinello and Rahman were able to demonstrate the possibility of the fec-to-hep transition under compression.8 Further 2190
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 8, Aug 1994
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studies of fee metals such as Th, Ag, Pt, and Sr, using a Morse potential,9'10 examined the stability of their stressfree bec phases, as well as the path of th
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