Effects of Gd addition, lamellar spacing and loading direction on creep behaviour of a fully lamellar Ti–44Al–1Mn–2.5Nb alloy

Abstract

A quaternary Ti–44Al–1Mn–2.5Nb (at.%) alloy and an equivalent alloy with 0.15 at.% rare earth element Gd (Ti–44Al–1Mn–2.5Nb–0.15Gd) were prepared by arc melting and casting. Both alloys were heat treated to obtain a fully lamellar microstructure with an average grain size of ∼270 μm. A controlled furnace cooling gave rise to an average lamellar spacing of the order of ∼250 nm whereas a fast cooling in air was used to produce much finer lamellar spacings of the order of ∼25 nm in both alloys. Tensile creep tests showed that for the materials with similar lamellar spacings, the secondary creep rate of the Gd-containing alloy was lower than that of the quaternary alloy, and for the same alloys, the materials with finer lamellar spacings exhibited a significantly lower secondary creep rate than those with coarser lamellar spacings. In tensile creep, the steady state was observed in the coarser lamellar materials, but not in the finer lamellar materials. In contrast, compressive creep tests showed that steady state creep existed in all the materials and those with finer lamellar spacings also displayed lower creep rates although the effect of Gd was not observed.