SYNTHESIS AND CHARACTERISATION OF TITANIUM ALUMINIDES FOR SPACE APPLICATIONS

Abstract

Titanium aluminides belong to the class of ordered intermetallics, which are in the advanced stage of development for aerospace applications. The main attributes of titanium aluminides are excellent modulus and strength retention to elevated temperatures coupled to low density and good high temperature oxidation resistance. Presently, there are two types of titanium aluminides of interest namely y and a2 aluminides, which are based on TiAI (Ti- 49 to 66 at% Al) and Ti3A1 (Ti- 22 to 39 at% Al) stoichiometry respectively. The strength retention in the a2 aluminides is upto a temperature of 973K whereas y alum inides retain strength to 1273K temperature. However, these aluminides require coating to protect against oxidation above a temperature of 923K for a2 and 1173K for y. Thus out of the two aluminides, y aluminides exhibit better high temperature capability both in terms of strength, modulus and oxidation resistance. Additionally, y aluminides have lower density and hence, excellent specific strength (strength to density ratio) and specific modulus (modulus to density ratio) at ambient as well as elevated temperatures. However, these aluminides suffer from low ambient ductility and toughness. Extensive work is progressing globally to encounter these limitations by way of alloying, thermo mechanical treatments and by using advanced near net shape processing techniques to alleviate need for extensive mechanical working. Various alloys with core composition centered around Ti-48at% Al have been designed with alloying additions of elements like Cr, Mn and V to improve the ductility and elements such as Ta, Mo, W and Nb to achieve better oxidation resistance. Few of such promising y aluminide alloys for thermo-structural applications are Ti-48A1-2Cr-2Nb, Ti-46.6A1-1.4Mn- 2Mo and Ti-47A1-1Cr-1V-2.5Nb.