Morphological effect on the mechanical
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I.
INTRODUCTION
T I T A N I U M aluminide intermetallic alloys based on Ti3A1 have received great attention in the past few years because of their strong potential for high-temperature applications. These materials, however, suffer from low ductility at low temperatures, tm~ which places a great limit on their engineering applications. Several approaches have been taken to improve the ductility of these materials, and it has been shown that the addition of Nb improves the ductility to some extent, t3] Beneficial effect of Nb addition comes from its effect on stabilizing the/3 phase, rendering the Ti3AI-Nb alloys as two-phase alloys. Recently, there have been several investigations on the effect of microstructure on the mechanical properties and creep in these classes of two-phase (c~2 + /3) alloys. ~4-HJThey have shown that fracture in these alloys occurs in a complex way involving the formation and linkage of microcracks ahead of the main crack tip. It has also been shown that the toughening mechanisms in these alloys include crack-tip blunting, crack shielding, crack deflection, and crack bridging. However, despite the enormous role of the various factors of the constituent phases, mechanical behavior of Ti3A1-Nb alloys has not been investigated based on the concepts of two-phase materials. In particular, the role of morphology (size, shape, and distribution) of the constituent phases (ce2 and 13) in Ti3A1 alloys is not yet clear. Previous work on dual-phase steel, t~2J whose structure consists of ductile ferrite and strong but brittle martensite, has shown that the mechanical properties and fracture behavior of dualphase steel are strongly influenced by the morphology of the constituents. Therefore, the aim of this investigation is to clarify the effects of the morphology on the NACK J. KIM, Associate Professor, and J.Y. KIM, Research Assistant, are with the Department of Materials Science and Engineering and Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-600, Korea. W.S. CHO, Chief of Materials Research Department, Technical Center, is with Kia Motors Corp., Seoul, Korea. Manuscript submitted April 27, 1992. METALLURGICAL TRANSACTIONS A
mechanical behavior of this technologically important Ti3A1-Nb alloy. II.
EXPERIMENTAL PROCEDURE
The material used in this investigation has a composition of Ti-13.5 wt pct A1-21.5 wt pct Nb (Ti-23.9 at. pct AI-11.1 at. pct Nb). The oxygen and nitrogen contents of the alloy were 520 and 72 ppm, respectively. The alloy was drop-forged in the a2 + / 3 phase field into the shape of pancake with a diameter of approximately 300 mm and a thickness of 20 mm. Tensile tests were performed using round specimens with 20-mm-gage length and 4-ram-gage diameter at 25 ~ with a strain rate of 4 • 10-4/S. In situ scanning electron microscopy (SEM) fracture observations were conducted on the polished flat surfaces of small double cantilever beam (DCB) specimens, such as the one described in our previous article, t~31and have been etched with
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