The effect of trace levels of Ni upon the intermediate-temperature creep behavior of Ti-6Al-2Sn-4Zr-2wt%Mo was investigated. Experiments were performed in tension at 510 to 565C, using stresses of 138 to 413MPa. Two heats of commercial-grade Ti-6Al-2Sn-4Zr-2Mo, with Ni levels of 0.006 or 0.035wt%, were studied. The high-Ni material exhibited higher primary creep strains and minimum strain rates than those of lower-Ni material. Stress exponents of 5 to 7 and 4 to 6 were found for the high-Ni and low-Ni materials, respectively. At 565C, a transition to a low-stress region with a stress exponent of about unity was found for both materials. At all stress levels, the apparent activation energy was lower for the high-Ni material. The apparent activation energy was found to be in excellent agreement with those reported for lattice self-diffusion in α-Ti in the presence of fast-diffusing impurities. The results also suggested that creep in the higher-stress regime was controlled by dislocation motion within the α-phase. It was proposed that trace levels of Ni in the α-phase accelerated self-diffusion and therefore increased the rate of dislocation climb; thus leading to the higher creep rates which were observed in high-Ni material.

Creep Behavior of Ti–6Al–2Sn–4Zr–2Mo - I. The Effect of Nickel on Creep Deformation and Microstructure. R.W.Hayes, G.B.Viswanathan, M.J.Mills: Acta Materialia, 2002, 50[20], 4953-63