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METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 28A, JUNE 1997—1357Effects of Microstructure on the Fracture Toughness ofTi 3 Al-Based Titanium AluminidesX. WU and P. BOWENThe influence of microstructure on the fracture toughness of Ti-23Al-9Nb-2Mo-1Zr-1.2Si (at. pct)and Ti-23Al-11Nb-0.9Si (at. pct) Ti 3 Al-based alloys has been investigated. Basket-weave microstruc-tures comprising different volume fractions of a 2 and retained b phases were produced by systematicheat treatments. Besides the volume fraction of the retained b phase, the average size of the b lathshas also been used to characterize these microstructures. The toughness of both alloys was examinedat room temperature, and the brittle transgranular fracture modes were found to be controlled bymicrostructure. However, the toughness is not determined solely by the volume fraction of the re-tained b phase, and a linear relationship has been obtained between the fracture toughness and theaverage size of the retained b laths. It appears therefore that the toughness of Ti 3 Al-based alloys atroom temperature is controlled primarily by the width of retained b laths rather than by the retainedb volume fraction.I. INTRODUCTIONT HE potential of titanium aluminides, based on Ti 3 Al,for high performance applications is mainly limited by theirpoor ductility at room temperature and their lack of oxi-dation resistance at temperatures above 600 7C. It has beenfound that fracture toughness of Ti 3 Al-based alloys at roomtemperature varies markedly with microstructure, [1–4] andhigh values are known to be associated with the retained bphase. It is noted that the volume fraction of b phase mayvary the crack initiation site under tensile tests. [1] The dis-tribution of b phase is also important for a low volumefraction of b phase, [1,5] since a continuous film of b phasearound a 2 is suggested to lead to a relatively superior frac-ture toughness in Ti-24Al-11Nb (at. pct). Any role of filmthickness for a given volume fraction was unclear in thisprevious work. However, in many microstructures, the bphase in Ti 3 Al-based alloys appears as discontinuous indi-vidual laths. In such cases, the effects of the volume frac-tion and size of b laths on ductility have not been reportedpreviously.In this present article, both Ti-23Al-11Nb-0.9Si (at. pct)and Ti-23Al-9Nb-2Mo-1Zr-1.2Si (at. pct) Ti 3 Al-based al-loys have been investigated. The microstructures obtainedunder different heat treatment conditions have been char-acterized in terms of both the volume fraction and the sizeof b laths. Fracture toughness values of these microstruc-tures were measured experimentally at room temperature.Attention is focused on whether the volume fraction or thesize (width) of the b phase can be considered to play acontrolling role in determining the fracture toughness ofthese materials.X. WU, Research Fellow, and P. BOWEN, Professor, are with theSchool of Metallurgy and Materials, The University of Birmingham,Birmingham, B15 2TT, United Kingdom.Manuscript submitted September 17, 1996.II. EXPERIMENTAL PROCEDUREThe materials used in this present study are based on theintermetallic compound Ti 3 Al and form part of an alloydevelopment program being carried out at DRA AerospaceDivision (Pystock). The compositions of the alloys underinvestigation here are Ti-23Al-11Nb-0.9Si (at. pct) and Ti-23Al-9Nb-2Mo-1Zr-1.2Si (at. pct). Both alloys were meltedby consumable electrode vacuum arc melting and cast in50-kg ingots. These ingots were then extruded at a tem-perature of 1180 7C (within the b phase field) at anextrusion ratio of 12.3:1, to a bar diameter of 50 mm. Thesebars were then sectioned into a length of 50 mm and thenheat treated (conditions employed here are shown in Table1). Solution treatments were carried out in both the b anda 2 1 b phase fields and followed by aging at 800 7C (anearlier aging treatment at the temperature of 625 7C is alsoincluded). These treatments were designed to alter the mor-phology and content of the a 2 and b phases. In particular,attention is focused on the size of the b laths and the vol-ume fraction of the b phase.An image analysis system (IAS) was used to measure thearea fraction of some phases in the different microstruc-tures. Metallographic sections taken from all orientationsconfirm that the area fraction of phases measured can beconsidered to be similar to the volume fraction of phasespresent, and for convenience, the term ‘‘volume fraction’’is used throughout the text.Fracture toughness tests were carried out broadly in accor-dance with BS5447:1977. [7] The samples with a length of 50mm, width of 10 mm, and thickness of 10 mm were pre-cracked to lengths between 3.5 to 4.3 mm (i.e., an (a/W) ratioof 0.35 to 0.43) in accordance with the general requirementsof BS5447:1977. The tests were performed on a 50-kN ser-vohydraulic machine, manufactured by ESH Testing Limited(Leys Road, Brockmoor, Brierley Hill, West Midlands, DY53UT, U.K.), under three-point bending at room temperaturewith an overall span of 40 mm. The loading traces againstclip-gage displacements were recorded. Fracture toughness,K IC , values were deduced from measured K Q values when con-ditions satisfied the requirements specified in BS5447:1977.