Attempt to ductilize a lightweight intermetallic compound by controlling crystal structure and microstructure

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An attempt has been made by modifying crystal structure and microstructure to improve the low room temperature ductility, a serious drawback, of titanium trialuminide (Al_3Ti) base intermetallics which are lightweight and have excellent resistance to high temperature oxidation. Ingots of Al_3 (Ti_<1-y>, X) was produced by argon arc melting, where X=Sc, Er or Lu, and y=0.25,0.5 and 0.75. Each ingot was found to consist of L1_2 and D0_<22> phases at y=0.25 by X-ray diffraction. The ductility (plasticity) of these ingots was assessed by compression test in comparison with that of monolithic Al_3Ti compound composed of D0_<22> single phase and produced in the same way. The results showed that the Sc addition raise the plasticity to about 3% while Er and Lu added alloys have virtually nil plasticity, being the same as the Al_3Ti. The increase in the Sc addition, y=0.5 and y=0.75, brought about further increase in the plasticity to 6% and 11%, respectively. To eliminate the solidification defects and solidification segregation, hot working was applied which had rarely carried out in trialuminide base alloys. Hot isothermal free forging at 1175ﾟC by 55% in a vacuum of about 10^<-3> Pa was found to be made successfully for Al_<74> Ti_<26> and Al_<65> Cr_9 Ti_<26> alloys resulting in a completely recrystallized structure with practically no inhomogeneity arising from solidification. This forging increased the plasticity of the chromium modified alloy from 4% to 9%. An Al_6 TiLi ingot, which was produced by induction melting in an attempt for further reduction in the weight of Al_3 Ti by replacing half of titanium atoms by lithium, was found to contain no L1_2 phase and to have the same tetragonality of the D0_<22> lattice as the Al_3 Ti binary compound.