An investigation of the creep behavior of an ordered Nb–11Al–41Ti–1.5Mo–1.5Cr intermetallic alloy

A preliminary investigation of the high temperature creep behavior of an ordered Nb–11Al–41Ti–1.5Mo–1.5Cr (in at.%) alloy has been performed over the temperature range 650–760°C at initial applied stress levels ranging from 69 to 275 MPa. The material has been forged then direct aged for 25 h at 750°C to produce a microstructure consisting of a fine distribution of orthorhombic (O) phase within an ordered (B2) beta matrix. Creep experiments performed in the longitudinal and transverse orientations reveal that the creep resistance in the longitudinal orientation is greater than that in the transverse orientation. Creep experiments performed in the longitudinal orientation revealed normal primary transient strain, apparent stress exponents ranging from 2.2 to 2.3 and an apparent activation energy for creep of 285 kJ mole−1. Creep experiments performed in the transverse orientation revealed an inverted primary transient strain, apparent stress exponents ranging from 3.1 to 3.4 and an apparent activation energy for creep of 323 kJ mole−1. The present results indicate that the apparent stress exponents and activation energy obtained from longitudinal creep data appear to be consistent with values recently reported for orthorhombic titanium aluminide alloys where grain boundary sliding was suggested as rate controlling for creep. The inverted primary transient strain along with stress exponents close to three suggest that creep in the transverse orientation is controlled by the viscous glide of dislocations consistent with Class I creep behavior. At present, the reason for the change in creep mechanism with orientation remains unclear. A tentative explanation has been offered in terms of the possible role of texture.