13th International Conference on Fracture June 16–21, 2013, Beijing, China -9The TTT diagram suggests that, at 753 K used as one-step-aging temperature in the present study, Ni3(Ti, Mo) particles precipitates first and these particles grow accompanying the formation of fine Fe2Mo particles which leads to the peak hardening. Our microstructural investigation, rather, indicates that Ni3(Ti, Mo) and FeMo particles with much larger contents than that of Fe2Mo are attributable to the peak hardening at 753 K. Tewari et al. reported that S phase was initially formed at 673 K, which was followed by the precipitation of ω phase in 10 ks. Sha et al. [23] found ω phase (Ni47Fe12Co1Mo40) in Fe-Ni-Mo-Co model alloy aged at 687 K for 57 ks by using APFIM. They also showed that Fe7Mo6 particles were formed at this aging condition. According to these results, very fine particles of S phase, ωphase and Fe7Mo6 are considered to play a major role in the slow hardening at 673 K. The low-temperature boundaries of TTH curves suggests the possibility that age-hardening continues as the temperature is lowered below 673 K, and it requires longer time with the decrease in temperature. The extrapolation of the boundaries to 473 K, however, does not predict the hardness observed at this temperature. It is rather considered that the solute atoms absorbed at dislocations migrate along them to agglomerate as very fine clusters as the aging time is increased, as shown schematically in Fig. 11. Dissimilar to the aging at 673 K where lattice diffusion may also contribute to the growth of precipitates for prolonged duration, the transition from such clusters to precipitates is considerably retarded at 473 K. In fatigue tests at 473 K, however, dislocations encounter excess solutes atoms during their motion. This leads to the quick formation of solute atmosphere around the dislocation, which in turn impedes the dislocation motion and increases the resistance to fatigue cracking. It has not been made clear why the second-step-aging improves the fatigue strength in humid air. The specimens tested in 85%RH air showed that cleavage-like facets in several grains were formed at the initiation of cracks and the cracks propagated along lath boundaries, irrespectively of the aging conditions. On the other hand, the crack initiation in 25%RH air was associated with slip in grains, followed by the crack propagation along lath boundaries. These fractographic features obtained in the previous study [24] do not provide the information on the effects of second-step-aging on fatigue cracking. The present results, however, suggest that the increase in fatigue strength in humid air is also closely related with the microstructures formed by the second-step-aging. Fig. 10. TTH and TTT diagrams deduced from Fig. 11. Segregation and precipitation [21] and [22]. process during second-step-aging. S S+ ω+Fe2Mo Ni3(Ti,Mo)+ ω Ni3(Ti,Mo) Temperature (K) Aging time (ks) TTH TTT (b) initial stage of second-step-aging (a) one-step-aging (c) later stage of second-step-aging
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