the high cycle fatigue life of aged nitac 14b at 825 $degree$ c in vacuum is strongly dependent upon frequency and at high frequencies, surface finish. crack initiation controls fatigue life at all frequencies, although the mechanism changes with frequency fiber cracking is especially prominent at low frequencies, as is dislocation penetration of $gamma$ $prime$ rosettes. the results are consistent with a creep-fatigue interaction at low frequencies such that cracks are initiated at the interior, and with surface-dominated crack initiation at high frequency. fatigue lives at low frequencies are consistent with creep rupture data theoretical analysis of cyclic deformation behaviour shows that decrease of frequency causes internal matrix stresses to relax and to increase stresses in the reinforcing phase. this situation corresponds to transition from fatigue to creep loading conditions the alloy composition is: ni-3. 93 wtco, 4. 2cr, 3. 16mo, 4. 52w, 6. 84re, 5. 5al, 9. 01tac, 0. 27c and 0. 01b. (edited author abstract) 18 refs