Atomistic simulation of fracture in TiAl
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INTRODUCTION
THE intermetallic compound TiAl possesses a unique complex of properties that persist up to elevated temperatures. These properties include sufficiently low material density, high value of strength-ductility ratio, high elastic moduli, high oxidation resistance, low creep rate, and improved fatigue characteristics. These properties make TiAl alloys very attractive, particularly for structural applications in the aerospace and aeronautic industries, where, at certain temperatures, they might be capable of replacing heavier nickel-based superalloys. However, applications of TiAl alloys are restricted by their limited ductility at ambient temperatures.[1,2,3] An important mechanical characteristic of TiAl is the ‘‘anomalous’’ temperature dependence of the yield stress.[4] Unlike in pure metals and disordered metallic alloys, the yield stress increases up to a peak temperature (6507C to 7007C depending on the orientation and chemical composition) and then decreases with temperature. The positive temperature dependence of the yield stress has also been known for other ordered intermetallics. Despite many efforts taken, the microscopic mechanism of the anomalous behavior in TiAl remains unclear. The latter is equally true with regard to the limited ductility of TiAl at ambient temperatures; although the fracture mechanisms in TiAl have been the primary objective in a number of recent studies,[5– 8] a convincing explanation of this important phenomenon is still missing. Further improvement of the unique mechanical properties of TiAl and TiAl-based alloys requires the development of a deep fundamental knowledge of microscopic mechanisms of plastic deformation and fracture in this material. Atomistic computer simulations have proved to be very useful in providing a better insight in the mechanical behavior of intermetallic compounds in terms of dislocation reactions and crack propagation. JULIA PANOVA is a former Graduate Student, Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. DIANA FARKAS, Professor, is with the Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University. This article is based on a presentation made in the symposium ‘‘Fundamentals of Gamma Titanium Aluminides,’’ presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
The purpose of this work was to conduct an atomistic simulation study of crack propagation along various crystallographic planes in TiAl and to find the crack orientations that favor the brittle mode of the fracture. It was also important to correlate the cleavage mechanisms as revealed by atomistic simulations with macroscopic mechanical properties of TiAl. In our simulations, we used molecular statics with interatomic interactions described by embedded atom potentials. The technique is briefly reviewed in Se
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