13th International Conference on Fracture June 16–21, 2013, Beijing, China -7200 nm 200 nm (b) (a) in SA-P specimen tested in the humid air (Fig. 8(b)), while such abrupt growth does not appear in DA-P specimen. This result suggests that the second-step-aging suppresses the occurrence of abrupt crack growth in the humid air, which may lead to the larger resistance to fatigue cracking than is observed in SA-P specimen (Fig. 7). 3.5 Microstructures Fig. 9 shows the TEM images of microstructures in DA-U specimen. It is obvious from the bright field image that there are two types of precipitates having rod and spherical shapes, which are homogeneously distributed in grains. The analysis of diffraction patterns show that the rod-shaped particles are Ni3Ti, as is shown by the dark field image (Fig. 9(b)). The fine particles of spherical shape have much larger density than that of rod-shaped Ni3(Ti, Mo) particles. These features are common to the specimen one-step-aged to peak-hardening stage (SA-P) and the specimens two-step-aged at 673 K (DA series). The size and number of Ni3Ti particles do not seem to differ significantly between these specimens. The electrolyte extraction technique revealed that all of the specimens except SA-U1 specimen contained the particles of Ni3Ti, Ni3Mo, Ni3Al, FeMo and Fe2Mo [15]. In particular, the fractions of Ni3Ti, Ni3Mo and FeMo are much larger than that of Fe2Mo. However, the contents of these phases were much less in SA-U1 specimen. This suggests that the under-aging state of SA-U1 specimen contains much more solute atoms or finer precipitates of which content cannot be measured by the electrolyte extraction technique. Over-aging at 753 K in one-step-aging led to a drastic decrease in FeMo content and a marked increase in Fe2Mo instead, though the fractions of Ni3(Ti, Mo) particles increase very slightly. Thus it is suspected that FeMo particles dissolve and more stable Fe2Mo particles are formed at longer duration. On the other hand, no significant change was observed in the nature and contents of precipitates between SA-P and DA-P specimens. The similarity in the nature and contents of precipitates was also observed in DA-O specimen and the two-step-aged specimen in which the second-step-aging was conducted at 473 K. These results suggest that the static and dynamic hardening due to the second -step-aging at 473 K and 673 K are caused by very fine microstructures which are not identified by TEM observation and electrolyte extraction technique. Fig. 9. Microstructures in 350G steel two-step-aged to under-aging state (DA-U) . (a) Bright field image and (b) dark field image. 4. Discussion The present results clearly indicate that the hardness of one-step-aged maraging steels is increased by the addition of static aging at 473 K and 673 K. The rapid increase in fatigue strength within the duration of 0.4 ks at these temperatures also suggests that the microstructural change is promoted by the motion of dislocations. As mentioned in section 3.2, both of the static and dynamic agings are
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