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I.

INTRODUCTION

THE attractive features of g TiAl intermetallic alloys include their high specific strength, good oxidation resistance, burn resistance, and high-temperature strength as compared to these properties for conventional superalloys.[1,2] Although much has been learned about their roomtemperature properties, the understanding of high-temperature deformation and damage mechanisms in these alloys is far from complete.[1] In particular, a thorough understanding of the influence of microstructural variables such as grain size, phase morphology, a2 volume fraction, and lamellar spacing is still lacking. Both single-phase g and dual-phase a2 1 g forms of TiAl alloys are finding growing application. In the twophase category, dual-phase equiaxed and fully lamellar (FL) forms are promising candidates for high-temperature structures. The creep behavior of these materials has been investigated by several researchers.[1–6] The preliminary results suggest that the FL microstructure is generally stronger than the dual-phase and single-phase equiaxed microstructures at elevated temperatures. However, the behaviors of FL and equiaxed TiAl with the same phase ratio and grain size have not been compared. Such a comparison is needed to distinguish differences in creep behavior resulting from the volume fraction of a2 from those due to a difference in phase morphology. In addition to experimental studies, investigations using numerical models offer an effective means for developing a better understanding of the role of microstructure in controlling creep behavior. AlANIRBAN CHAKRABORTY, Graduate Student Researcher, and JAMES C. EARTHMAN, Associate Professor, are with the Department of Materials Science and Engineering, University of California, Irvine, CA 92697-2700. Manuscript submitted May 23, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

though numerical simulations of time-independent plastic deformation and fracture behavior of TiAl alloys have been performed previously,[7] a numerical study of creep behavior has not been reported for these materials. It is generally assumed that dislocation-facilitated creep is the dominant mechanism of high-temperature deformation in TiAl alloys. However, grain boundary and lath boundary sliding are mechanisms that can also have a profound effect on creep deformation and damage, particularly in materials that possess a relatively low dislocation mobility. Furthermore, Hazzledine and co-workers[8,9] have proposed that certain misorientations of adjacent g /g lamella in TiAl alloys can give rise to a lath boundary sliding phenomenon similar to grain boundary sliding (GBS). Grain boundary sliding elevates the overall creep strain rate of the material by raising the stresses in regions deforming by dislocation creep. One way to characterize the effect of GBS on overall creep strain rate and stress distribution is to determine the stress enhancement factor, f. If the deformation inside the grains is described by a power law of the form

~ !

n

se εz e 5 εz 0 s0

[1]

then the creep