Deformation mechanisms in the O phase

The intermetallic Ti<SUB>2</SUB>AlNb has a ternary ordered, orthorhombic structure (which is a slightly distorted form of the hexagonal D0<SUB>19</SUB>, Ti<SUB>3</SUB>Al phase) and is the major constituent of the 'orthorhombic' alloys of the Ti-Al-Nb system. We explore in this paper the mechanical behaviour of this intermetallic over a wide range of strain rates and temperatures. Thermal activation analysis of the flow behaviour indicates that at low temperatures the flow behaviour is controlled by a strong thermally activated process with small activation volumes typical of a Peierls-type barrier. Dynamic recovery occurs at higher temperatures, but a specific rate-controlling mechanism for flow remains to be identified. The creep of the intermetallic has also been examined in a temperature-strain-rate regime well removed from the DSA regime. Stress exponents decrease slowly from 7 to 5 with increasing temperature and the dislocation structure consists of a three dimensional network linked by attractive junctions. A network model of creep, as proposed for example by McLean, may be appropriate to describe the creep of this intermetallic. The effect of Nb content and a variety of quaternary additions on the mechanical behaviour of the intermetallic has also been evaluated. Nb is shown to strengthen the intermetallic through the thermal component of the flow stress, while Si is found to have the strongest solid solution strengthening effect at low temperatures. Zr and oxygen additions enhance the dynamic strain ageing effect at intermediate temperatures. The Nb content does not affect the creep strength of the intermetallic