Deformation and Fracture Characteristics of a Gamma Titanium Aluminide at High Temperatures
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DEFORMATION AND FRACTURE CHARACTERISTICS OF A GAMMA TITANIUM ALUMINIDE AT HIGH TEMPERATURES V. SEETHARAMAN*, S. L. SEMIATIN#, C. M. LOMBARD# AND N. D. FREY* * UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432. #Wright Laboratory, Wright-Patterson AFB, OH 45433. *Battelle Memorial Institute, 505 King Avenue, Columbus, OH 43201.
ABSTRACT The hot workability of a y titanium aluminide alloy was investigated using hot tension tests in the temperature range 900-1350OC and for deformation rates varying from 0.0001 to 5 s-I. One series of tests was performed on cast + HIP'ped material and was designed to characterize fracture behavior during primary ingot break-down. Another series of tests was conducted on extruded and heat treated material in order to investigate the fracture processes occurring during secondary processing. The deformation and fracture characteristics were analyzed to assess the influence of microstructural features as well as the relative volume fractions of the y and a/a 2 phases.
INTRODUCTION Deformation processing of y titanium aluminides constitutes a crucial step in the successful development and application of these materials. High strength and resistance to plastic deformation exhibited by y titanium aluminides at high homologous temperatures make them very attractive for high temperature service, but drastically reduce their hot workability. In general, the hot working regime in which the material can be processed with adequate control over both microstructural evolution, and nucleation and propagation of cracks, is quite narrow. This is especially true for cast materials with relatively coarse grain size, which are prone to cracking and fracture under the influence of tensile stresses. Even though primary hot working processes such as forging or extrusion used for the break-down of the ingot structure involve nominally compressive states of loading, secondary tensile stresses can be generated due to geometrical, frictional or thermal effects. Examples of failures induced by tensile stresses include free surface bulging and fracture in open die forging, edge cracking during rolling and nose fracture during canned extrusion [1,21. Tensile deformation and fracture behavior of y or near y titanium aluminides has been extensively investigated. The majority of these studies has been devoted to the evaluation of the alloys under potential service conditions, i.e., up to - 8000C. Notable exceptions are the studies by Lipsitt et al. 131, Huang and Hall 141 and by Krishnamurthy and Kim 151. While these studies extended the test temperatures up to 1000°C, the strain rates used were typically in the range of 10-4 - 10-3 s-. The only published study on the tensile behavior of a near y alloy over a wide range of temperatures and strain rates relevant to hot working was performed by Nobuki et al. 16,71. These researchers evaluated the temperature and strain rate dependence of flow stress and fracture strain of a two-phase alloy containing fine, equiaxed grains of y and cx2 phases obtained by isothermal fo
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