In Section 3.1 we identified stress, temperature and grain size/microstructure as the three important factors which determine the extent of creep defor­mation that a material experiences. However there are examples where two materials with similar compositions, grain sizes and second phase distributions might creep at vastly different rates under a given stress and temperature. Such anomalous behavior has been observed in titanium alloys by Mishra et al." who found that alloys with nominally similar compositions crept at sig­nificantly different rates. It was found that the presence of trace elements such as Fe and Ni degrade the creep properties of the titanium alloy. Even though these elements are present only in the order of ppm, they influence the dif­fusion rates to an extent as to bring about significant changes in creep rates. The activation energy for diffusion in the higher Fe/Ni containing alloys was found to be smaller and vice versa. Mishra et al. suggest that the Fe/Ni appear to dissolve interstitially and form foreign atom-vacancy pairs which play a sig­nificant role in accelerating the diffusion kinetics of the titanium.