Ti 5 Si 3 whisker in-situ reinforced TiAl alloys
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CONSIDERABLE attention has been concentrated on the development and evaluation of gamma titanium aluminides, due to their attractive properties for high-temperature structural application.[1–4] Most of TiAl alloys consist primarily of a high volume fraction of ␥ phase with a small volume fraction of ␣2 phase. Extensive investigations were carried out to study the effect of alloying elements such as V, Cr, Nb, Mn, Mo, W, and Ta[5–12] on the microstructures and properties of TiAl alloys. It was found that the addition of Mn, Cr, and V can effectively improve the ductility of two-phase TiAl alloys at room temperature,[8,9,10] while Mo and Nb improve the high-temperature strength.[4,11] Recently, the addition of W was found to refine the microstructure and enhance the creep resistance of TiAl alloys.[13,14,15] The aforementioned elements are classified as solid solution– strengthening alloying elements. The mechanical properties of the two-phase TiAl alloys could be further improved via thermomechanical processing by controlling the microstructures.[16] Furthermore, the dispersion-strengthening alloying elements such as Si were of great interest.[17,18] The creep resistance of TiAl alloys was significantly improved by a 0.1 to 0.5 at. pct addition of Si.[17] The addition of large amounts of Ti5Si3 phase to the TiAl alloy was considered to be an effective method to improve the properties of the TiAl alloy. The addition of 2.7 pct Si to the ternary alloy resulted in a high volume fraction of Ti5Si3 particles by powder metallurgy.[19] This can be regarded as a Ti5Si3 particle–reinforced TiAl composite. On the other hand, the addition of about 3.0 at. pct Si to ternary Ti-Al-Si alloys produced a microstructure of Ti5Si3 whiskers in a ␥ matrix by ingot FU-SHENG SUN, Principal Senior Engineer, and CHUN-XIAO CAO and MING-AO YAN, Scientists, are with the Beijing Institute of Aeronautical Materials, Beijing 100095, P.R. China. SEUNG-EON KIM, Senior Researcher, and YONG-TAI LEE, Principal Senior Researcher, are with the Korea Institute of Machinery and Materials, Changwon, Kyungnam, Korea. Manuscript submitted August 6, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
metallurgy.[20] The cast microstructure and compressive properties have also been evaluated in Ti-Al-Si alloy systems. It showed a promising method to obtain a Ti5Si3 whisker–reinforced TiAl alloy, although all of the properties of high-Si-bearing TiAl alloys were unsatisfactory due to the brittleness of Ti5Si3 whiskers. The mechanical properties of TiAl alloys can be improved by particle, whisker, and fiber strengthening, as in conventional titanium alloys. For the whisker-reinforced TiAl alloys, an attractive candidate for TiAl alloys is the Ti5Si3 phase. A fibrous morphology of Ti5Si3 phase can be introduced into the TiAl alloy by conventional ingot metallurgy. In addition, the interface of ␥ and Ti5Si3 phases is thermochemically stable at high temperatures. Compared to SiC fiber–reinforced TiAl composites, the Ti5Si3 whiskers can readily be formed in-situ d
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