The effect of Ni on the creep deformation of Ti-6Al-2Sn-4Zr-2Mo

Creep deformation of the Ti alloy Ti-6Al-2Sn-4Zr-2Mo was studied over the temperature range 510 - 650degreesC at initial applied stress levels ranging from 69 - 414 MPa. Three heats of commercial Ti-6-2-4-2 each containing intentionally added trace levels of Ni were studied in order to gain a fundamental understanding of the influence of trace levels of this impurity upon high temperature deformation of this alloy. Both the magnitude of primary creep strain and steady - state strain rate are increased with increasing levels of Ni. The apparent stress exponent is independent of Ni content and within the experimental stress - temperature regime of this study, suggests that a dislocation process controls creep. The apparent activation energy for creep decreases with increasing Ni concentration and follows trends identical with those recently reported for Ti self - diffusion and Al substitutional diffusion in alpha Ti in the presence of trace levels of fast diffusing impurities. These results suggest that there is a strong correlation between creep and diffusion properties in this alloy and that creep is possibly dominated by deformation within the alpha phase. Based upon detailed TEM examination of deformation substructures, a new model for creep deformation of this alloy based upon the glide and climb of jogged screw dislocations is offered. We also show that this model is consistent with the trends regarding creep deformation of the alloy and Ti self-diffusion in alpha Ti.