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I.

INTRODUCTION

Gamma titanium-aluminide alloys exhibit a remarkable combination of density-normalized strength, stiffness, and fatigue resistance to temperatures approaching 1000 7C, which are attractive properties for designing high-temperature structures.[1,2,3] Research over the last 25 years led to an emerging class of alloys exhibiting a revolutionary balance of mechanical properties.[3] However, only in the last 7 years have systematic studies shown the mechanisms governing the mechanical behavior of such systems, especially for the two-phase alloys of commercial interest. Nearly all aspects of the deformation mechanisms active in TiAl alloys were reevaluated in this period, and while basic studies continue, there now exists a common basis for examining the behavior of two-phase systems. When gamma alloys are processed to exhibit a fully lamellar (FL) microstructure with a grain size below '300 mm, the overall balance of properties is more suitable for high-temperature structures; notably, the combination of yield strength, fracture toughness, and creep resistance is markedly better than that exhibited by the duplex or nearly gamma microstructural forms.[4–7] Further, Kim and DENNIS M. DIMIDUK, Research Leader for Structural Materials, is with the Materials Directorate, Wright Laboratory, Wright-Patterson AFB, OH 45434-7817. PETER M. HAZZLEDINE, Principal Scientist, TRIPLICANE A. PARTHASARATHY, Senior Scientist, and MADAN G. MENDIRATTA, Director of Materials Sciences Division, are with UES Inc., Dayton, OH 45432. SRIRAM SESHAGIRI, Visiting Scientist, is with Systran, Inc., Dayton, OH 45432. 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. 37—VOLUME 29A, JANUARY 1998

Dimiduk[4] and Kim[5,8] demonstrated that for selected FL processing conditions, the apparent grain size dependence of the flow stress was remarkably higher than that for single-phase or DP-structured gamma alloys. Specifically, thermomechanically processed (TMP) alloys exhibit an apparent Hall–Peter (HP) slope, ky, of nearly 5 MPa=m for the FL microstructure in the grain size range from '1500 to '300 mm, which compares to a ky of about unity measured for single-phase materials in previous studies.[9–12] Also noteworthy is the observation of yield strength as a function of lamellar spacing for hard-mode deformation of PST crystals, which shows a ky , 0.5 MPa=m dependence.[13] The studies suggested that further grain refinement could lead to unprecedented levels of strength for gamma alloys. Using alloys prepared from powder metallurgy (PM) methods, Liu et al.[14] demonstrated yield strength (sy) improvements of .500 MPa beyond the yield strengths of large-grained FL alloys, although the strengths are not compared for a constant-alloy composition. More recently, Kim[15,16] demonstrated that refinement of the FL grain size led