Relationships of fracture toughness and dislocation mobility in intermetallics
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11/9/03
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Relationships of Fracture Toughness and Dislocation Mobility in Intermetallics KWAI S. CHAN An analytical method has been developed and used to compute the Peierls–Nabarro (P–N) barrier energy, UP–N, for relevant slip systems in several intermetallics, including NiAl, FeAl, Nb-Ti-Al (B2), Ni3Al (L12), TiAl (L10), TiCr2, NbCr2 (C14, C15), Nb5Si3 (D8l), Mo5SiB2 (D8l), and Mo5Si3 (D8m). The P–N barrier energy and a generalized fault energy, F, are combined and used as a measure of dislocation mobility. Furthermore, a fracture model has been developed to describe the process of thermally activated dislocations moving away from the crack tip and to predict the corresponding fracture resistance. A ductility index defined in terms of the ratio of s/(UP–N F), where s is the surface energy, is used to correlate with the fracture toughness, KC, of individual intermetallics. The correlation indicates that fracture toughness increases with increasing values of s/(UP–N F), in accordance with the fracture model formulated based on thermally activated slip. The use of the fracture model for predicting the effects of slip behavior, temperature, and alloy additions on fracture resistance is demonstrated for selected intermetallics including NiAl, TiAl, Laves phase, and Nb5Si3.
I. INTRODUCTION
THERE has been considerable interest in developing new structural intermetallics for high-temperature applications.[1–11] Substantial efforts have been made to develop a fundamental understanding of the effects of alloy additions on the deformation and fracture mechanisms. Despite many new alloy developments, inadequate fracture resistance at ambient temperature remains one of the major obstacles for widespread applications of intermetallic alloys or in-situ composites in fracture critical components. Many intermetallics remain brittle because the relationships between deformation mechanisms and fracture resistance are poorly understood. Alloy additions to toughen brittle intermetallics have largely been explored by trial and error, mostly through empirical means that are labor-intensive and time-consuming. As a result, there has been an increasing trend to use computer-assisted approaches or computation-based methods to accelerate alloy development.[12–17] Brittle-to-ductile fracture transition is generally considered the result of a competition between cleavage fracture and emission of dislocation from the crack tip. The propensity to cleavage fracture is often measured in terms of the surface energy, s. In contrast, dislocation emission from the crack tip can be controlled by the nucleation of dislocations from the crack tip[18] (Figures 1(a) and (b)) or the mobility of dislocations[12,13,19,20] moving away from the dislocation nucleation sites either at the crack tip or other nontip sources (Figures 1(b) and (c)). The propensity of crack-tip dislocation nucleation can be described in terms of the unstable stacking energy (us)[18] (Figure 1(b)). On the other hand, dislocation mobility is represente
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