Creep behavior of Ti–6Al–2Sn–4Zr–2Mo: I. The effect of nickel on creep deformation and microstructure

The effect of trace levels of Ni on the intermediate temperature creep behavior of the alloy Ti–6Al–2Sn–4Zr–2Mo (wt%) has been investigated. Creep experiments were performed in tension over the temperature range 510–565 °C at stress range 138–413 MPa. Two heats of commercial grade Ti–6Al–2Sn–4Zr–2Mo with Ni levels of 0.006 and 0.035 wt% were studied. The high Ni material uniformly exhibited higher primary creep strains and minimum strain rates than the lower Ni material. Stress exponents in the range 5–7 and 4–6 were obtained for the high Ni and low Ni material respectively. At 565 °C a transition to a low stress region with a stress exponent equal 1 is found for both materials. At all stress levels, the apparent activation energy was lower for the high Ni material. The apparent activation energy is in excellent agreement with those reported for lattice self-diffusion in -titanium in the presence of fast diffusing impurities. The results also suggest that creep in the higher stress regime is controlled by dislocation motion within the -phase. We suggest that trace levels of Ni in the -phase accelerate self-diffusion therefore increasing the rate of dislocation climb leading to the higher creep rates observed in the high Ni material. In Part II, direct evidence in support of dislocation-based creep being important in both low and high stress regimes is presented.