The Effect of Carbon and Thermal Exposure on the Tensile Behavior of Ti-48Al-1V (at%)
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THE EFFECT OF CARBON AND THERMAL EXPOSURE ON THE TENSILE BEHAVIOR OF Ti-48Al-IV (at%)
WILLIAM T. DONLON AND W.E. DOWLING, Jr. Ford Motor Company, Research Laboratory, S-2065, P.O. Box 2053, Dearborn, MI 48121
ABSTRACT Room temperature tensile properties of Ti-47.5A1-1V-0.2C (at%) and Ti-48.2Al-1V-.06C (at%) alloys were measured after thermal exposure at 775'C (in air and vacuum) to evaluate the influence of carbide precipitation and environment. Both alloys possess =2% tensile ductility in the unexposed condition. Thermal exposure of fully machined tensile samples consistently reduces the ductility of the 0.2 carbon alloy to =0.5%. Exposure of the low carbon (0.06) alloy in vacuum results in no ductility loss, while exposure in air reduces the ductility to 1.3%. Yield strengths are unaffected by thermal exposure and are 450 MPa and 300 MPa for the 0.2 and the low carbon alloys, respectively. The pre-exposure ductility is recovered for tests in which thermally exposed machined tensile samples had = 10gum of their surfaces removed by polishing. Thermal exposure prior to machining results in no change in tensile behavior. SEM and TEM examination of thermally exposed surfaces show that below the oxide surface layer, a layer close in composition to Ti2 Al, having a simple cubic (a 0 =6.85A) structure is present. Beneath this layer, Ti3 AIC carbides are observed in the 0.2 carbon alloy. The density of these carbides is observed to decrease away from the surface. No carbides were observed in the low carbon alloy. Although carbon significantly enhances the yield strength of this alloy it also makes it much more susceptible to embrittlement from thermal exposure. INTRODUCTION Recent research on alloy development and microstructural optimization of two-phase (y' + C12) Ti-Al alloys has produced material with consistently greater than 2% room temperature tensile ductilityl, . Maintaining these properties3 after repeated thermal exposures has not yet been 5 6 demonstrated. Precipitation of carbides ,, and/or the formation of various surface phases during thermal exposure, can alter mechanical properties. The purpose of this study is to evaluate the effect of thermal exposure on microstructure and the tensile properties of a Ti-48A1-1V (at%) alloy as a function of carbon content. EXPERIMENTAL PROCEDURE Castings (0.2m dia x 0.9m) with nominal compositions of Ti-48Al-1V but with different carbon contents were acquired from TIMET after hot isostatic pressing. The composition of these castings as determined by TIMET are shown in Table I. TABLE I 0.2 C Alloy Low C Alloy
Aluminum 47.5 48.2
Material Chemistry (at%) Vanadium 1.1 1.1
Carbon 0.21 0.06
Oxygen 0.13 0.15
The 0.2 C ingot was sectioned into 80 mm tall mults and the low C ingot was sectioned into 150 mm tall mults. Both alloys were isothermally forged at 1150*C to thicknesses of 19 mm and 44 mm, respectively. Different multiple step heat treatments and cooling rates were utilized to produce microstructures with a range of 0 2 and (y + ca2 ) lath contents for both castin
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