Electronic structure of planar faults in TiAl
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J. M. MacLaren Physics Department, Tulane University, New Orleans, Louisiana 70118
S. Rao Wright LaboratorieslMLLM, Wright Patterson Air Force Base, Ohio 45433-6533 (Received 27 September 1991; accepted 27 February 1992)
The mechanical behavior of intermetallic alloys is related to the mobility of the dislocations found in these compounds. Currently the effect of bonding on dislocation core structure and its influence on deformation behavior is not well understood. However, the unusual properties of these materials, such as the anomalous temperature dependence of flow stress observed in TiAl, are derived in part from the aspects of bonding that determine dislocation mobility. Several recent studies have suggested a particular relationship between directional bonding in TiAl and dislocation mobility. To understand better the flow behavior of high temperature intermetallics, and as a step toward bridging the gap between electronic structure and flow behavior, we have calculated the electronic structure of various planar faults in TiAl. The self consistent electronic structure has been determined using a layered Korringa Kohn Rostoker (LKKR) method which embeds the fault region between two semi-infinite perfect crystals. Calculated defect energies in stoichiometric TiAl agree reasonably well with other theoretical estimates, though overestimating the experimental (111) anti-phase boundary (APB) energy, found for Ti46Al54. We approximate the energy of the (111) APB for the Al-rich stoichiometry by calculating the energy of Al antisites near that defect plane. The calculated ( l l l ) A P B energy decreases by 6% in going from stoichiometric TiAl to Ti 46 Al 54 . The overall hierarchy of fault energies is found to be associated with the number of crystal bond states that are disrupted by the introduction of the fault plane. However, the hierarchy of fault energies is inconsistent with the traditionally accepted ordering. Changes in bonding taking place in the vicinity of the planar defects are illustrated through the density of states and charge density plots. A three body atomistic model is introduced to parameterize the bonding observed in TiAl. The L l 0 lattice (c/a = 1.00), within a second nearest neighbor three body model, yields a ( l l l ) A P B energy which is the sum of the complex and superlattice-intrinsic stacking fault energies.
I. INTRODUCTION TiAl is an attractive material for aerospace applications due to its high specific strength, high melting temperature, and favorable strength retention at high temperatures.1 Practical applications of this material are limited by its lack of ductility at room temperature. TiAl undergoes a brittle-to-ductile transition around 700 °C and the mechanism that mediates this transition is not well understood.2'3 The dislocation behavior that controls high temperature strength retention is also the subject of some controversy. Understanding and controlling these properties is critical in developing practical applications for intermetallic alloys. The mechanical propertie
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