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I.

INTRODUCTION

TITANIUM aluminides (both c~2-Ti3AI and y-TiA1) are very attractive structural materials for potential application in the aerospace industry due to their low density, high specific strength and modulus, and good oxidation and corrosion resistance, tt,2m However, a serious handicap in using these alloys is their low ambient temperature ductility. There have been several attempts in recent years to improve the ambient temperature ductility of these aluminides by the following methods: (1) grain refinement via nonequilibrium processing methods such as rapid solidification, mechanical alloying, or innovative heavy deformation methods; (2) modification of the crystal structure to a more symmetric cubic lattice to achieve the minimum number of slip systems required for plasticity (for example, formation of the ordered body-centered cubic (bcc) (B2) phase in Ti3A1 alloys by addition of niobium and other bcc metals has been shown to increase the ductility); and (3) changing the nature and proportion of constituent phases by heat treatment. These methods have resulted in significant improvements for the a2-Ti3A1 and have led to some improvements in the y-TiAl compoundJ ~m Mechanical alloying (MA) is a process of repeated welding, fracturing, and rewelding of powder particles in a dry high-energy ball mill which was initially developed in the late 1960s to produce oxide-dispersion strengthened nickel- and iron-based superalloys. [4,5J However, in recent years, this technique has been extensively exploited to produce a variety of stable and metastable crystalline and quasicrystalline intermediate and amorphous phases, t6'7] Further, the grain size decreases continuously with milling time, and consequently, the MA powder may have nanometer-sized grains, t8,91 This is expected to improve the ductility/ GUO-HAO CHEN, formerly Graduate Student, University of Idaho, is currently employed at Branson Ultrasonics Corporation, Danbury, CT 06810. C. SURYANARAYANA and F.H. FROES, Professors, Department of Metallurgical and Mining Engineering, are also Associate Director and Director, respectively, of the Institute for Materials and Advanced Processes, University of Idaho, Moscow, ID 83844-3026. Manuscript submitted July 27, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A

fabricability of the alloy due to the increased tendency of the material to deform plastically. There have been a number of investigations in recent years on the MA of titanium alloys. [l~ Even though many reports on the structural evolution of phases in binary Ti-A1 alloys have been reported, H1-22] only a few reports are available on the results in ternary Ti-AI-Nb alloys.j23-29] Formation of a supersaturated solid solution of AI in Ti and an amorphous phase have been reported in MA binary Ti-A1 alloys. [t51 In the ternary Ti-A1-Nb alloys, a B2/bcc phase formed after the solid solution but before the formation of the amorphous phase.t25] In all alloys studied, the crystalline phases had nanometersized grains. Addition of 12 to 22 at. pct* Nb to Ti3