Effects of microstructure on the fracture toughness of Ti 3 Al-based titanium aluminides
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I.
INTRODUCTION
THE potential of titanium aluminides, based on Ti3Al, for high performance applications is mainly limited by their poor ductility at room temperature and their lack of oxidation resistance at temperatures above 600 7C. It has been found that fracture toughness of Ti3Al-based alloys at room temperature varies markedly with microstructure,[1–4] and high values are known to be associated with the retained b phase. It is noted that the volume fraction of b phase may vary the crack initiation site under tensile tests.[1] The distribution of b phase is also important for a low volume fraction of b phase,[1,5] since a continuous film of b phase around a2 is suggested to lead to a relatively superior fracture toughness in Ti-24Al-11Nb (at. pct). Any role of film thickness for a given volume fraction was unclear in this previous work. However, in many microstructures, the b phase in Ti3Al-based alloys appears as discontinuous individual laths. In such cases, the effects of the volume fraction and size of b laths on ductility have not been reported previously. In this present article, both Ti-23Al-11Nb-0.9Si (at. pct) and Ti-23Al-9Nb-2Mo-1Zr-1.2Si (at. pct) Ti3Al-based alloys have been investigated. The microstructures obtained under different heat treatment conditions have been characterized in terms of both the volume fraction and the size of b laths. Fracture toughness values of these microstructures were measured experimentally at room temperature. Attention is focused on whether the volume fraction or the size (width) of the b phase can be considered to play a controlling role in determining the fracture toughness of these materials.
X. WU, Research Fellow, and P. BOWEN, Professor, are with the School of Metallurgy and Materials, The University of Birmingham, Birmingham, B15 2TT, United Kingdom. Manuscript submitted September 17, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The materials used in this present study are based on the intermetallic compound Ti3Al and form part of an alloy development program being carried out at DRA Aerospace Division (Pystock). The compositions of the alloys under investigation here are Ti-23Al-11Nb-0.9Si (at. pct) and Ti23Al-9Nb-2Mo-1Zr-1.2Si (at. pct). Both alloys were melted by consumable electrode vacuum arc melting and cast in 50-kg ingots. These ingots were then extruded at a temperature of 1180 7C (within the b phase field) at an extrusion ratio of 12.3:1, to a bar diameter of 50 mm. These bars were then sectioned into a length of 50 mm and then heat treated (conditions employed here are shown in Table 1). Solution treatments were carried out in both the b and a2 1 b phase fields and followed by aging at 800 7C (an earlier aging treatment at the temperature of 625 7C is also included). These treatments were designed to alter the morphology and content of the a2 and b phases. In particular, attention is focused on the size of the b laths and the volume fraction of the b phase. An image analysis system (IAS) was used to measure the
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