Phase Stability and High Temperature Tensile Properties of W doped gamma-TiAl
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Phase Stability and High Temperature Tensile Properties of W doped gamma-TiAl Keizo Hashimoto1, Hirohiko Hirata1 and Youji Mizuhara2 1
Dept. of Materials Science & Eng. Teikyo University,
Utsunomiya, 320-8551, JAPAN 2
Advanced Technology Research Labs. Nippon Steel Corp.,
Hikari, 743-8510, JAPAN ABSTRACT Tungsten (W) doped γ-TiAl is one of promising alloys among many other proposed TiAl base alloys, for the purpose of structural applications at elevated temperatures. Ingots of W doped γ-TiAl were produced by plasma arc melting, followed by homogenizing heat treatment and isothermal forging to control their microstructures. The phase stability of W doped γ-TiAl has been studied quantitatively, using the specimens quenched from 1273 K. Equilibrium compositions of consisting phases were analyzed by means of EDS analysis in a TEM . An isothermal cross section of the Ti-Al-W ternary phase diagram at 1273K has been proposed based on the experimental observations. Small amounts of W addition (< 1at%) to Ti-48at%Al cause a phase shift from α2+γ to α2+β+γ, which suggests that W is the strongest β stabilizer among transition metals, such as Cr and Mo. Mechanical property measurements of W doped γ-TiAl show that the high temperature tensile strength has been improved by the W addition. Relationships between the microstructures and the mechanical properties of W doped γ-TiAl have been discussed. 1. INTRODUCTION Gamma titanium aluminides (γ-TiAl) have been studied extensively, since they have been considered as a candidate material for advanced jet engines, automobile exhaust valves, turbo-chargers and so on. Alloy design of γ-TiAl has progressed in developing advanced alloy compositions and microstructures. Many researchers have reported that the mechanical properties of γ-TiAl are improved by micro alloying [1-4]. The first generation of γ-TiAl developed by S.C.Huang [5,6] is Ti-48at%Al-2.0at%Cr-2.0at%Nb (48-2-2), which is a castable material having moderate ductility at room temperature. Both Cr and Nb belong to the transition metal group in the periodic table and are also β phase stabilizing elements in Ti. Other reported third alloying elements to γ−TiAl such as V, Mn, Ta, Mo, and W are also β phase stabilizing elements in Ti. On
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the other hand, it is known that Al stabilizes the α phase of Ti. Consequently, phase stability of α, β and γ phases is one of the most crucial parameter for developing the optimized phases and microstructures of γ-TiAl base alloys. In order to assess the third element additions in γ-TiAl, it is necessary to consider the Ti-Al-X ternary phase diagrams, phase stability and microstructure of γ-TiAl which change depending on the compositions and the process temperatures [7,8]. Recently, various types of W doped γ-TiAl have been developed [9-11]; however, the phase stability of the Ti-Al-W system has not been fully understood. The present report, therefore, aims at presenting experimental data on Ti-Al-W ternary systems.
Based on the experimental
observations, a Ti-Al-W ternary phase di
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