Toughening mechanisms in titanium aluminides
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INTRODUCTION
TITANIUM aluminides that are o f engineering significance include Ti3AI- and TiAl-base alloys, t~,2~ Stoichiometric Ti3A1 (a2) and TiA1 ('y) compounds are o f limited potential f o r structural applications, because they exhibit l o w tensile ductility and fracture toughness at ambient temperature33'4~ By macroalloying, ductility and fracture toughness can be imparted in Ti3AI- and TiAIbase systems by increasing dislocation and twinning activities, t5-12~ More importantly, macroalloying in many instances leads to a two-phase microstructure containing an intermetallic matrix (Ti3AI o r TiA1) with one o r more secondary phase(s), t6-1°'~3-15j The second phase might be a ductile, metallic phase (e.g., body-centered cubic (bcc) /3 o r B2 in Ti-24A1-1 1Nb) or another intermetallic phase that might o r might not be ductile (e.g., Ti3A1in lamellar TiAI alloys). The Ti3Al-base alloys developed to date are two-phase systems with compositions based on Ti-(23 to 25)AI-(10 to 18)Nb-X(V, Mo, Cr, o r V), where X ranges from nil to a few percent.J7] Typical compositions o f Ti3Al-base alloys are Ti-24AI- 11Nb, Ti-25A1-10Nb-3V- 1Mo (super a2), and Ti-25Al-17Nb-lMo. In comparison, current TiA1 alloys have typical compositions based on Ti-(46 to 52)A1(1 to 10)M,~8'9"1°1 where M represents at least one element from V, Cr, Mn, Nb, Ta, W , and Mo. Both single- and two-phase alloys are possible in this range o f compositions, ts'9'w] Like conventional c~ + fl T i alloys, two-phase titanium aluminides can be heat-treated to a variety o f microstructures./~,7-m,~6'171 A s reviewed in several recent articles, t~,7a6,~7] microstructures in ~2 alloys include equiaxed o~z + "/3," basketweave, and Widmanst~itten microstructures either with o r without primary a2- The second, "/3" phase can be the b c c / 3 phase,E7,~6'~8J the ordered B2 phase, t16,171 or the ordered orthorhombic O phase,u9~ depending on alloy systems and heat treatments. The m e t a stable w-type phaset7,16,2°1 can also be formed in some K.S. CHAN, Staff Scientist, is with the Materials and Mechanics Department, Engineering and Materials Sciences Division, Southwest Research Institute, San Antonio, T X 78228-0510. This article is based on a presentation made in the symposium "QuasiBrittle Fracture" presented during the T M S fall meeting, Cincinnati, OH, October 21-24, 1991, under the ausprices of the T M S Mechanical Metallurgy Committee and the A S M / M S D Flow and Fracture Committee. METALLURGICAL TRANSACTIONS A
alloys after quenching. Three types o f microstructures can be attained in TiAl-alloys: t7-l°] (1) equiaxed gamma, (2) fully lamellar, and (3) duplex microstructures. The equiaxed gamma microstructure consists entirely o f 3' grains in single-phase alloys or predominantly 5' grains with small amounts o f grain boundary alpha-two particles in two-phase alloys. The fully lamellar microstructure consists entirely o f lamellar colonies (i.e., grains) that are comprised o f alternating layers o f a2 and y platelets aligned according to the {111}~//(0
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