Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders
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I.
INTRODUCTION
II.
EXPERIMENTAL
T I T A N I U M aluminide alloys based on the intermetallic compound y-TiA1 are considered as potential materials for high-performance airframe and gas turbine applications. They possess low densities, high melting temperatures, good elevated temperature strength, and elastic modulus retention, l~'21 However, their practical applications are largely hindered due to poor workability and low ductility at ambient temperatures. To tackle this problem, numerous studies have been conducted, t3-91 It has been reported that the ductility of two-phase (y-TiA1 and a2-Ti3A1) alloys can be improved by adding alloying elements such as Cr and Mn and by microstructural control. As a result, a room-temperature tensile fracture strain of about 3 to 4 pct has been achieved in recent years. 13-71 The problem with the poor workability of titanium aluminides can be solved via a reactive powder manufacturing route. 18,91By using elemental powders as starting materials and conducting reactive sintering of preconsolidated specimens at the last processing step, this route enables preparation of near netshaped titanium aluminide parts. In the present article, two TiAI(Si) alloys, made from elemental powders, are investigated. Silicon was added with the primary aim of achieving grain refinement and dispersion hardening. It has been reported that Si alloying can improve the room=temperature ductility 171and the creep and oxidation resistance of TiA1 at high temperatures. I~~ The investigated alloys were prepared in the same way, in order to study the influence of Si addition on the microstructure and high-temperature tensile properties.
A. Alloy Preparation
G.-X. W A N G is with the Department of Materials Science and Engineering, Zhejiang University, 310027 Hangzhou, China. B. D O G A N and M. D A H M S are with the Institute of Materials Research, GKSS Research Center, 21502 Geesthach, Germany. F,-Y. HSU and H.-J. KLAAR are with GFE, Technical University of Aachen, 52056 Aachen, Germany. Manuscript submitted March 24, 1994.
Both alloys were examined under an optical microscope (OM), a scanning electron microscope (SEM), and a transmission electron microscope (TEM). Microanalysis data were obtained in TEM using an X-ray energy dispersive spectrometer (EDX). The sizes of Ti fibers, y grains, and Tis(Si, A1)3 particles, as well as the
METALLURGICAL AND MATERIALS TRANSACTIONS A
Elemental titanium and aluminum powders, as well as powder of an Al-12 wt pct Si alloy, with powder sizes smaller than 150 k~m and purities of about 99.8 to 99.9 pct, were mixed in air to the desired compositions of Ti-46.4 at. pct AI-1.4 at. pct Si (Si poor) and Ti-45 at. pct A1-2.7 at. pct Si (Si rich). Figure 1 shows the positions of both alloys in the Ti-A1-Si system. In the diagram, both alloys lie in the y + ( p h a s e region at 1200 ~ The powder mixtures were pressed at room temperature under a 1000 kN load to green compacts of 50-mm diameter. These compacts were further cold extruded at room temperature to 15-ram-diamete
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