The effects of dwell time on material behavior in titanium alloy IMI 829

A near-alpha titanium alloy was studied for its fatigue behavior under dwell time conditions at room temperature. The history of work related to the dwell time effect in titanium alloys was presented.The alloy studied was IMI 829 (88.7% Ti, 5.04% Al, 3.24% Sn, 1.94% Zr, 0.25% Si, 0.25% Mo, 0.19% Nb, 0.0048% Fe, 0.033% H, 0.030% S, and 0.023% Cr). The specimens used in this study were smooth surface cylindrical specimens with 1/4", 1/2", and 11/16" diameter and 1" gage length. All the fatigue tests and dwell time tests were conducted under load control and with an R ratio of zero.The test results unequivocally demonstrated the adverse dwell time effect on the fatigue behavior of this near-alpha titanium alloy. The fatigue life was reduced by 5 to 45 times under dwell time conditions compared to the pure fatigue conditions. The fatigue life debit was found to be consistently more severe for the smaller specimens than for the larger specimens. The time dependent plastic strain recorded during the tests indicated that creep at room temperature certainly played an important role in the material behavior under dwell time conditions. An extensive fractographic examination of all the specimens tested under different conditions revealed that there was a change of mechanism in both the crack nucleation and the crack propagation process. Cleavage was identified to be the mechanism for the crack nucleation and for the early stage of crack propagation under dwell conditions.It is believed that the anisotropy of the microstructure and large prior beta grain size caused uneven distribution of the plastic deformation inside of the alloy. It also was shown that the fatigue lives under dwell conditions did not directly correspond to the total time to failure obtained with creep rupture specimens. The fracture surface morphology also was different between the two kinds of tests. An explanation of these differences was given.Based on the information generated in this study and that in the literature generated by other researchers, a model is proposed to rationalize the material behavior under dwell fatigue conditions. The model incorporates creep at room temperature and the role played by the interstitial hydrogen atoms.