Fracture toughness of gamma-base titanium aluminides
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I.
INTRODUCTION
T I T A N I U M aluminide-base intermetallic compounds are light in weight and possess excellent high-temperature properties, tl,2,3J However, their low room-temperature ductility and toughness restrict any practical application. The fracture characteristics of gamma-base titanium aluminides is the subject of many ongoing researches, t4-91 Single-phase (7) titanium aluminides were found brittle compared with the two-phase (a2 + T) materials and exhibited very low fracture toughness. E3I Fracture toughness tests carried out by Kampe e t al. t41 have shown the increase in fracture toughness with an increase in volume percent of lamellar grains at room temperature. Mitao e t a l . 151 have studied the influence of lamellar lath orientation on the fracture toughness. Their results show the strong dependence of fracture toughness on the lamellar lath orientation to crack plane. When the lamellar lath was parallel to the crack plane, a very low fracture toughness (~10 MPam ~ was obtained. The fracture toughness was very high (~35 MPam ~ when the angle between lamellar and crack plane was very high. Addition of ternary elements such as chromium t6,71 and niobium t8,9~ was found to have influence on the mechanical properties and fracture behavior of titanium aluminides. Chromiumalloyed material investigated by the authors was found to have high intermediate (RT to 600 ~ temperature fracture toughness compared with the binary equiaxed microstructure material, t71 From the past investigations, it is understandable that the high-temperature fracture behavior may depend on the microstructure and alloying addition. In the present study, a simple test procedure, using a single specimen with side grooves, was followed to estimate the fracture toughness at room and elevated temperatures. A comprehensive study clearly delineating the R. GNANAMOORTHY, formerly Graduate Student, Nagaoka University of Technology, is Research Associate, Institute for Materials Research, Tohoku University, Sendal 980, Japan. Y. MUTOH, Professor, is with Nagaoka University of Technology, Nagaoka 940-21, Japan. N. MASAHASHI and Y. MIZUHARA, Researchers, are with Nippon Steel Corporation, Kawasaki 211, Japan. Manuscript submitted March 16, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
influence of microstrncture, alloying addition, and processing route on the room- and high-temperature fracture toughness is presented. Fracture behavior at room and elevated temperatures is investigated and correlated with the fracture toughness.
II. T E S T M A T E R I A L S AND EXPERIMENTAL PROCEDURE Cast and heat-treated binary gamma-base titanium aluminide, Ti-46 at. pct A1 (hereafter Ti-46A1-HT), and ternary niobium-alloyed Ti-47 at. pct A1-3 at. pct Nb (hereafter TiA1Nb-HT) were used. Subsequently, isothermal forged Ti-50AI and Ti-47 at. pct A1-3 at. pct Cr (hereafter Ti-50AI-ITF and TiA1Cr-ITF, respectively) were also used. Ingots were prepared by plasma arc melting of high-purity materials. The materials were cast into ingots of 80-mm diameter
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