Pressure-Assisted Reactive Synthesis of Titanium Aluminides from Dense 50AI-50Ti Elemental Powder Blends E. PARANSKY, E.Y. GUTMANAS, I. GOTMAN, and M. KOCZAK In the present research, dense 7-TiA1 based intermetallic samples were fabricated by reactive syn- thesis of fully dense elemental 50 at. pct A1-50 at. pct Ti powder blends. Two different processing routes were attempted: thermal explosion under pressure (combustion consolidation) and reactive hot pressing. In both approaches, relatively low processing or preheating temperatures (<900 ~ were used. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The application of a moderate external pressure (<250 MPa) during synthesis was shown to be enough to accommodate the negative volume change associated with TiA1 formation from the elemental components and, thereby, to ensure full density of the final product. Microstructure and phase composition of the materials obtained were characterized employing X-ray diffraction and scanning electron microscopy with energy dispersive analysis. It was found that at elevated temperatures (e.g., 900 ~ the equiatomic 50AI-50Ti alloy lies beyond the homogeneity range of the 3,-TiA1 phase in the Ti-A1 binary and contains, in addition to T-TiA1, Al-rich Ti3A1. Mechanical properties of the materials synthesized were evaluated in com- pression tests at different temperatures and by microhardness measurements. Due to its very fine microstructure, the Ti-A1 material synthesized via reactive hot pressing exhibited superplastic be- havior at temperatures as low as 800 ~ I. INTRODUCTION WITH the growing need for improved high perform- ance materials for advanced engineering applications, in- termetallic alloys have emerged as the most promising substitutes for conventional superalloys. Among all the in- termetallics, titanium aluminide alloys based on 3,-TiA1 are receiving the most attention due to a very favorable com- bination of the low density and good mechanical properties coupled with the high melting temperature and excellent oxidation resistance. Ultimately, bringing these attractive intermetallic materials into use largely depends upon the availability of practical processing routes. Conventional casting of TiAl-base intermetallics is difficult due to their relatively high melting temperature and the extreme reac- tivity of Ti. More importantly, however, cast TiA1 alloys are usually not suitable for subsequent forming as a result of inhomogeneities and segregations in the solidification microstructure combined with the lack of ductility.tl] At the same time, powder metallurgy methods capable of provid- ing near-net-shape parts seem to be a more attractive proc- essing alternative. Over the past 2 decades, significant progress has been made in the area of powder processing of intermetallics. Rapid solidification techniques, such as E. PARANSKY, Graduate Student, and E.Y. GUTMANAS, Professor, are with the Department of Materials Engineering, Technion, Haifa 32000, Israel. I. GOTMAN, Postdoctoral Fellow, formerly with the Department of Materials Engineering, Drexel University, is with the Department of Materials Engineering, Technion. M. KOCZAK, Professor, is with the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. This article is based on a presentation made in the "In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics" symposium, held February 12-16, 1995, at the TMS Amaual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees). melt spinning, inert gas atomization, and mechanical alloy- ing, have been extensively used to produce nickel-alumi- num and titanium-aluminum intermetallic powders, r2,31 These prealloyed powders are consolidated to full density employing hot pressing, hot extrusion, or hot isostatic pressing (HIP)t4L--methods that usually require tempera- tures in excess of the material service temperature. Such temperatures can result in the undesirable TiAI grain growth and, more importantly, in the interaction between TiAI and reinforcement phases in the cases where TiAI acts as a matrix for a composite material. Thus, reactions be- tween TiA1 and high strength continuous SiC fibers (e.g., SCS-6) at temperatures as low as 1100 ~ have been shown to result in catastrophic deterioration of the fiber proper- tiesY] In order to maintain a desirable microstructure (and, thereby, to obtain enhanced behavior in a compacted part), the time-temperature thermal exposure during consolidation should be minimized. A different approach to the processing of intermetallics, in general, and of aluminides, in particular, is reactive syn- thesis from elemental powders. One of the important ex- amples of this approach is self-propagating high temperature synthesis (SHS). [6'7'8] The term SHS, or com- bustion synthesis, is used to describe processes in which initial reagents, when ignited, spontaneously transform into products, due to the exothermic heat of reaction. The SHS reaction can proceed as a layerwise or volumetric combus- tion, the latter usually referred to as "thermal explosion." The technique is extremely attractive due to the self-gen- eration of energy and short processing cycles, and it can be useful in producing both intermetallic powders and sintered bodies. Various SHS-related techniques developed for the synthesis of dense intermetallics, in general, and TiAI, in particular, have been recently discussed by Dunand in a review paper.t9] Dense TiA1 (p > 95 pct) was successfully synthesized from elemental powders t1~ or foils[8,16,t71em- 2130--VOLUME 27A, AUGUST 1996 METALLURGICALAND MATERIALS TRANSACTIONS A